Coatings: Materials, Processes, Characterization and Optimization (Materials Forming, Machining and Tribology) 3030621626, 9783030621629

Table of contents :
Preface
Contents
About the Editors
Part IProcesses
1 Friction Stir Processing: A Novel Way to Produce the Surface Composite Coating
1.1 Introduction
1.2 FSP Working Parameters
1.3 Composite Materials
1.4 Surface Coating
1.5 Surface Composite Coating by FSP
1.6 Previous Work in the Surface Composite Coating by FSP
1.7 Effect on Microstructure After FSP
1.8 Improvement in Material Properties After FSP
1.9 Future Scope
1.10 Conclusion
References
2 Microwave Processing of Engineering Materials
2.1 Basics of Material Processing via Microwave
2.1.1 Heating Mechanisms
2.1.2 Types of Heating
2.2 Processing of Composite Materials Using Microwaves
2.2.1 Polymer Matrix Composites
2.2.2 Metal Matrix Composites
2.2.3 Ceramic Matrix Composites
2.3 Other Microwave-Assisted Fabrication Techniques
2.3.1 Joining
2.3.2 Surface Coatings and Claddings
2.4 Synthesis of Special Purpose Materials Using Microwaves
2.4.1 Reaction Synthesis of Ceramics
2.4.2 Synthesis of Nanomaterials
2.5 Future Trends
References
Part IIApplications
3 Application of Edible Coatings and Packaging Materials for Preservation of Fruits-Vegetables
3.1 Introduction
3.2 Edible Coatings and Films
3.2.1 Biomolecule Based Edible Coatings
3.2.2 Protein Derived Edible Coatings
3.2.3 Lipid-Based Coatings
3.2.4 Other Alternatives in Edible Coatings
3.2.5 Need for Edible Coatings
3.2.6 Challenges in Creating Edible Coatings
3.3 Packaging Materials
3.3.1 Need for Packaging Materials
3.3.2 Types of Packaging Materials
3.4 Chemical Preservatives
3.4.1 Traditional Food Preservatives
3.4.2 Acidulants
3.4.3 Gaseous Food Preservatives
3.4.4 Antioxidants
3.4.5 Flavour Additives
3.4.6 Sweeteners
3.5 Conclusion
References
4 Corrosion Resistance of High Entropy Alloys
4.1 Introduction
4.2 Background of HEAs
4.3 Corrosion Resistance of HEAs
4.3.1 Corrosion Resistance of HEAs in Chloride Environment
4.3.2 Corrosion Resistance of HEAs in Acidic Environments and the Role of Alloying Elements
4.3.3 Electrochemical Response of High Entropy Alloys and Pitting Potential
4.3.4 High Temperature Corrosion Resistance of HEAs
4.3.5 Surface Coatings of High Entropy Alloys for the Enhancement of Corrosion Resistance
4.4 Conclusions
References
Part IIICharacterization
5 Characterization and Processing of PMMA/SiO2 Nanocomposite Films and Their Applications
5.1 Introduction
5.1.1 Overview of Poly(methyl methacrylate)
5.1.2 Nanoparticles
5.1.3 Polymer Thin Film Processing
5.1.4 Synthesis of Nanoparticle Infused PMMA
5.2 Materials
5.2.1 Aerosil Silica Nanoparticles
5.2.2 Commercial Polymer
5.2.3 Lab Synthesized PMMA
5.3 Materials Processing Techniques
5.3.1 PMMA/SiO2 Synthesis
5.3.2 Fabrication of Thin Films
5.4 Experimentation
5.4.1 Scanning Electron Microscopy (SEM)
5.4.2 Transmission Electron Microscopy (TEM)
5.4.3 Gel Permeation Chromatography (GPC)
5.4.4 Optical Microscopy (OM)
5.4.5 Thermogravimetric Analysis (TGA)
5.4.6 Tensile Analysis
5.5 Results and Discussion
5.5.1 Scanning Electron Microscopy
5.5.2 Transmission Electron Microscopy
5.5.3 Gel Permeation Chromatography
5.5.4 Optical Microscopy Analysis
5.5.5 Thermogravimetric Analysis of PMMA/SiO2
5.5.6 Tensile Test Analysis
5.6 Conclusions
References
6 Characterization of Coatings Through Indentation Technique
6.1 Relevance of Instrumented Indentation in Materials Research
6.2 Nanoindentation Test
6.3 Working Principle of Nanoindenter
6.4 Interpretation of Load-Displacement Curve (P–h)
6.5 Indentation Crater Profile
6.6 Contact Stiffness
6.7 Berkovich Indenter
6.8 Oliver-Pharr Procedure to Determine the Coatings Mechanical Properties from the Instrumented Indentation Response
6.9 Dao’s Reverse Analysis
6.10 Indentation Size Effects Model
6.11 Strain Rate Sensitivity of Materials
6.12 Conclusions
References
Part IVSimulation and Optimization
7 FE-RSM Modeling of Wire Drawing of Brass-Plated Steel Wire
7.1 Introduction
7.2 FE Simulation of Wire Drawing Process
7.2.1 Formulation of the Model
7.2.2 Material Geometry and Meshing
7.2.3 Simulation and Results
7.3 Empirical Modeling of Wire Drawing Process
7.3.1 Design of Experiments
7.3.2 Development of the Empirical Models
7.3.3 Validation of the Developed Models
7.4 Effects of Process Parameters
7.5 Conclusion
References
8 Optimization of Process Parameters for AA6063 Alloy Friction Surfacing on Mild Steel
8.1 Introduction
8.2 Experimental Work
8.2.1 Material
8.2.2 Working Ranges of Process Parameters
8.2.3 Design of Experiments
8.2.4 Friction Surfacing
8.3 Coatings Characterization
8.3.1 Visual Examination
8.3.2 Ram Tensile Test Setup
8.3.3 Electron Probe Micro Analysis
8.4 Results and Discussion
8.4.1 Coating Tensile Strength
8.4.2 Electron Probe Micro Analysis
8.4.3 Bend Ductility
8.4.4 Torque
8.5 Conclusions
References
Index

Citation preview

Materials Forming, Machining and Tribology

Kaushik Kumar B. Sridhar Babu J. Paulo Davim   Editors

Coatings Materials, Processes, Characterization and Optimization

Materials Forming, Machining and Tribology Series Editor J. Paulo Davim, Department of Mechanical Engineering, University of Aveiro, Aveiro, Portugal

This series fosters information exchange and discussion on all aspects of materials forming, machining and tribology. This series focuses on materials forming and machining processes, namely, metal casting, rolling, forging, extrusion, drawing, sheet metal forming, microforming, hydroforming, thermoforming, incremental forming, joining, powder metallurgy and ceramics processing, shaping processes for plastics/composites, traditional machining (turning, drilling, miling, broaching, etc.), non-traditional machining (EDM, ECM, USM, LAM, etc.), grinding and others abrasive processes, hard part machining, high speed machining, high efficiency machining, micro and nanomachining, among others. The formability and machinability of all materials will be considered, including metals, polymers, ceramics, composites, biomaterials, nanomaterials, special materials, etc. The series covers the full range of tribological aspects such as surface integrity, friction and wear, lubrication and multiscale tribology including biomedical systems and manufacturing processes. It also covers modelling and optimization techniques applied in materials forming, machining and tribology. Contributions to this book series are welcome on all subjects of “green” materials forming, machining and tribology. To submit a proposal or request further information, please contact Dr. Mayra Castro, Publishing Editor Applied Sciences, via mayra.castro@springer. com or Professor J. Paulo Davim, Book Series Editor, via [email protected]

More information about this series at http://www.springer.com/series/11181

Kaushik Kumar B. Sridhar Babu J. Paulo Davim •

•

Editors

Coatings Materials, Processes, Characterization and Optimization

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Editors Kaushik Kumar Department of Mechanical Engineering Birla Institute of Technology Jharkhand, India

B. Sridhar Babu CMR Institute of Technology Hyderabad, Telangana, India

J. Paulo Davim Department of Mechanical Engineering University of Aveiro Aveiro, Portugal

ISSN 2195-0911 ISSN 2195-092X (electronic) Materials Forming, Machining and Tribology ISBN 978-3-030-62162-9 ISBN 978-3-030-62163-6 (eBook) https://doi.org/10.1007/978-3-030-62163-6 © Springer Nature Switzerland AG 2021 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

The editors are pleased to present the book Coatings—Materials, Processes, Characterization and Optimization under the book series Management and Industrial Engineering. Book title was chosen as it depicts upcoming trends in the industrial world for various critical applications. This book is a compilation of different aspects of the same. Treatment of surface is a established provider of advanced materials processing and coating technologies for a wide range of applications in the automotive, aerospace, oil and gas, semiconductor, missile, power, electronic, biomedical, textile, petroleum, petrochemical, chemical, steel, power, cement, machine tools, construction industries and many more. The development of a suitable high-performance coating on a component fabricated using an appropriate higher mechanical strength metal/alloy offers a promising method of meeting both the bulk and surface property requirements of virtually all imagined applications. This book focuses on the recent developments in the coating processes, sub-processes and emphasizes on processes with the potential to improve performance quality and reproducibility. The book demonstrates how application methods, environmental factors and chemical interactions affect each surface coating’s performance. In addition, it provides analysis of the latest polymers, carbon resins and high-temperature materials used for coatings and describes the development, chemical and physical properties, synthesis, polymerization, commercial uses and characteristics for each raw material and coating. The coverage further includes the characterization techniques to select the right ones to solve the coating problems, and optimization study to identify the critical coating parameters would be needed in order to ensure a robust process. The main objective of the book is dedicated to deal with the interesting field of coatings, preparation and characterization of the same and elaborates on application of optimization techniques for sustainability and effectivity. In view of the changing scenario, this book has parts providing general introduction, characterization, applications, simulation and optimization towards the subject called Coatings. This book will serve as a knowledge databank by providing state-of-the-art descriptions of the corresponding problems and advanced methods for solving them for industrial fraternity. v

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The chapters in the book have been categorized in four parts, namely Part I: Processes; Part II: Applications; Part III: Characterization; and Part IV: Simulation and Optimization. Part I contains Chaps. 1 and 2, whereas Part II has Chaps. 3 and 4; Part III with Chaps. 5 and 6 and Part IV contains Chaps. 7 and 8. Part I starts with Chap. 1 which provides an overview of friction stir processing (FSP). This is a novel way to produce the surface composite coating and also provides a simple solution to enhance the surface properties of the material by producing a surface composite coating over the base material. So, with the application of FSP, various properties like hardness, strength, corrosion, wear, microstructure, etc., can be enhanced. The chapter hence provides an understanding of composite materials and surface composite coatings. The information about FSP and how surface composite coating is prepared by FSP has been discussed in detail. The chapter concludes with the future scope of FSP. Chapter 2 provides another method for surface treatment and surface modification, i.e. microwave processing. This method apart from being quite effective is also cleaner and environment-friendly. The chapter provides a detailed explanation of microwave heating and its application in the processing of various composites of engineering importance highlighting the suitability of microwaves in joining and surface treatments of materials. The chapter also elaborates on the preparation of ceramics and nano-materials using microwaves. The chapter culminates with a discussion on future perspectives and applications of this technology in processing and fabrication of other engineering materials as coating. Chapter 3, the first chapter of Part II, enlightens the readers with application of edible coatings and packaging materials for preservation of fruits and vegetables. Fruits and vegetables are particularly perishable commodities as they contain 80–90% of water by weight. Edible coatings are thin films made applied to the exterior surface of a substance, which offers protection against external moisture, oxygen and pathogens. Various components commonly used in the manufacture of edible coatings include polysaccharides, proteins, lipids, composites and resins. The present chapter discusses the use of different edible coatings, preservatives and packing methods as carriers of functional ingredients on fresh fruits and vegetables to maximize their quality and shelf life. Moreover, it also elaborates on recent developments in the application of antimicrobials during packaging to increase the functionality of foods. Chapter 4 describes corrosion resistance of high-entropy alloys. High-entropy alloys (HEAs) are categorized as alloys containing at least four major elements. This chapter presents a brief account of the formation of high-entropy alloys and corrosion resistance property and infers that the unusual corrosion resistance properties of HEAs might be due to the local distortion/disordered chemical environment. So deposition of HEAs with their superior corrosion resistance onto the surface of the materials leads to the formation of corrosion-resistant layer or coating, and this has found wide applications in aviation industries. The chapter also describes thermal spray technology which is currently being used for the deposition of HEAs.

Preface

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Chapter 5, which starts the part on characterization, talks about characterization and processing of PMMA/SiO2 nanocomposite films and their applications. In this chapter, authors have synthesized polymethyl methacrylate (PMMA) using its monomer methyl methacrylate. The synthesized polymer was spun into a thin film using the Laurell Spin Coating System along with infused aerosil silica nanoparticles. The thin film was further tested for its thermal properties via thermogravimetric analysis, and mechanical properties were investigated using tensile testing. The infusion of the nanoparticles at varying weight ratios has shown improvement in both mechanical and thermal characterizations for the polymer. In Chap. 6, an innovative technique, indentation technique was discussed for characterization of coatings. Instrumented indentation tests are most promising, reliable and easy non-destructive testing procedures in the material research, and these procedures are extended to characterize the coatings developed on the surface of the substrates. Indentation tests are conducted at different length scales, i.e. macro- to nano-levels. The indentation test data is used to determine different mechanical properties of the coatings. This chapter gives different numerical procedures or analytical models used to evaluate the elasto-plastic deformation behaviour of coatings by using indentation data. Chapter 7, the commencing chapter of the last section of the book, i.e. Part IV, concentrates on FE-RSM modelling of wire drawing of brass-plated steel wire. It provides a brief description of the concept of wire drawing, which is a material deformation process in which the cross section of the wire or rod is reduced by pulling the wire or rod through a single or a series of converging dies. In this chapter, a three-dimensional elasto-plastic finite element (FE) model of wire drawing process is developed using ANSYS®. The brass-plated high carbon steel wire is used as the wire material, while the polycrystalline diamond is used as the die material. The von Mises yield criteria, associative flow rule and isotropic work hardening are implemented in the plasticity model. The FE model predicts the drawing stress on wire and die, in response to the selected wire drawing process parameters. The results of the FE model are used to feed the experimental matrix designed to develop the empiric models using the surface response method (RSM). Models are validated using test results and found to be consistent within the range of the parameters studied. The effects of the wire drawing parameters on drawing stress are also investigated and discussed. The concluding chapter of the section and the book, Chap. 8, concentrates on optimization,, i.e. optimization of process parameters for AA6063 alloy friction surfacing on mild steel. The main aim of the present chapter is to achieve the optimal relationship between the friction surfacing process parameters and the process response that can be utilized for AA6063 aluminium material coating on the mild steel. To optimize the process parameters, factorial experimental design approach was implemented in which rotational speed of the consumable rod, axial load and substrate traverse speed parameters were predominantly influencing the friction surfacing process response. After the experimental result analysis, it was concluded that the sound coating was developed at (i) high rotational speed and

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with low axial load and traverse speed (ii) lower rotational speed and with higher axial load and traverse speed. First and foremost, we would like to thank God. It was His blessings that this work could be completed to our satisfaction. You have given the power to believe in passion, hard work and pursue dreams. We could never have done this herculean task without the faith they have in you, the Almighty. We are thankful for this. We thank our families for having the patience with us for taking yet another challenge which decreases the amount of time we could spend with them. They were our inspiration and motivation. We would like to thank our parents and grandparents for allowing us to follow our ambitions. We would like to thank all the contributing authors as they are the pillars of this structure. We would also like to thank them to have belief in us. We would like to thank all of our colleagues and friends in different parts of the world for sharing ideas in shaping our thoughts. Our efforts will come to a level of satisfaction if the professionals concerned with all the fields related to coatings get benefitted. We owe a huge thanks to all of our technical reviewers, Editorial Advisory Board Members, Book Development Editor and the team of Springer Nature for their availability for work on this huge project. All of their efforts helped to make this book complete, and we could not have done it without them. Last, but definitely not least, we would like to thank all individuals who had taken time out and help us during the process of editing this book; without their support and encouragement, we would have probably given up the project. Jharkhand, India Hyderabad, India Aveiro, Portugal

Kaushik Kumar B. Sridhar Babu J. Paulo Davim

Contents

Part I

Processes

1 Friction Stir Processing: A Novel Way to Produce the Surface Composite Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shalok Bharti, Nilesh D. Ghetiya, and Kaushik M. Patel 2 Microwave Processing of Engineering Materials . . . . . . . . . . . . . . . . Padmakumar A. Bajakke, Vinayak R. Malik, Prakash Mugali, and Anand S. Deshpande Part II

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Applications

3 Application of Edible Coatings and Packaging Materials for Preservation of Fruits-Vegetables . . . . . . . . . . . . . . . . . . . . . . . . D. Manojj, M. Yasasve, N. M. Hariharan, and R. Palanivel 4 Corrosion Resistance of High Entropy Alloys . . . . . . . . . . . . . . . . . . K. Ram Mohan Rao Part III

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Characterization

5 Characterization and Processing of PMMA/SiO2 Nanocomposite Films and Their Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 Abiola Gaines, Deepa Kodali, Shaik Jeelani, and Vijaya Rangari 6 Characterization of Coatings Through Indentation Technique . . . . . 139 B. Sridhar Babu and Kaushik Kumar Part IV

Simulation and Optimization

7 FE-RSM Modeling of Wire Drawing of Brass-Plated Steel Wire . . . 153 Anup Kr. Pathak, Aditya Singh, Gitanshu Raj, Milind, and Bappa Acherjee

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8 Optimization of Process Parameters for AA6063 Alloy Friction Surfacing on Mild Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 B. Vijay Kumar and B. Sridhar Babu Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

About the Editors

Kaushik Kumar, B.Tech. (Mechanical Engineering, REC (Now NIT), Warangal), MBA (Marketing, IGNOU) and Ph.D. (Engineering, Jadavpur University), is presently an Associate Professor in the Department of Mechanical Engineering, Birla Institute of Technology, Mesra, Ranchi, India. He has 19 years of Teaching and Research and over 11 years of industrial experience in a manufacturing unit of Global Repute. His areas of teaching and research interest are composites, optimization, non-conventional machining, CAD/CAM, rapid prototyping and quality management systems. He has nine patents, 35+ book, 30 edited book, 55 book chapters, 150 international journal publications, 22 international and one national conference publications to his credit. He is on the editorial board and review panel of seven international and one national journals of repute. He has been felicitated with many awards and honours. (Web of Science core collection (102 publications/ h-index 10+, SCOPUS/h-index 10+, Google Scholar/h-index 23+). B. Sridhar Babu, Professor and Dean (IIIIC), has completed B.E. with Mechanical Engineering from Kakatiya University, M.Tech. with Advanced Manufacturing Systems from JNTUH University and Ph.D. with Mechanical Engineering from JNTUH University. He has 22 years of teaching experience, out of which 10 years of experience in CMR Institute of Technology itself. He is Fellow of the Institution of Engineers (I), Kolkata, and also Member of ISTE, IAENG and SAE India. He has published 55 papers in various international/national journals and international/ national conferences. He is author of 06 text books. He received Bharath Jyothi award for his research excellence from India international friendship society, New Delhi, India. He is reviewer for various international journals and conferences and guided more than 75 B.Tech. and M.Tech. projects. His research interests include manufacturing, advanced materials, mechanics of materials, etc. He is a guest editor for Proceedings of 1st International Conference on Manufacturing, Material Science and Engineering (ICMMSE’19), Materials Today—Proceedings (Scopus and CPCI Indexed) and AIP Proceedings (Scopus Indexed); guest editor for SN applied Science Springer journal; guest editor for eight edited books.

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About the Editors

J. Paulo Davim is a Full Professor at the University of Aveiro, Portugal. He is also distinguished as honorary professor in several universities/colleges in China, India and Spain. He received his Ph.D. degree in Mechanical Engineering in 1997, M.Sc. degree in Mechanical Engineering (materials and manufacturing processes) in 1991, Mechanical Engineering degree (5 years) in 1986, from the University of Porto (FEUP), the Aggregate title (Full Habilitation) from the University of Coimbra in 2005 and the D.Sc. (Higher Doctorate) from London Metropolitan University in 2013. He is Senior Chartered Engineer by the Portuguese Institution of Engineers with an MBA and Specialist titles in Engineering and Industrial Management as well as in Metrology. He is also Eur Ing by FEANI-Brussels and Fellow (FIET) of IET-London. He has more than 30 years of teaching and research experience in Manufacturing, Materials, Mechanical and Industrial Engineering, with special emphasis in Machining & Tribology. He has also interest in Management, Engineering Education and Higher Education for Sustainability. He has guided large numbers of postdoc, Ph.D. and master’s students as well as has coordinated and participated in several financed research projects. He has received several scientific awards and honors. He has worked as evaluator of projects for ERC-European Research Council and other international research agencies as well as examiner of Ph.D. thesis for many universities in different countries. He is the Editor in Chief of several international journals, Guest Editor of journals, books Editor, book Series Editor and Scientific Advisory for many international journals and conferences. Presently, he is an Editorial Board member of 30 international journals and acts as reviewer for more than 100 prestigious Web of Science journals. In addition, he has also published as editor (and co-editor) more than 150 books and as author (and co-author) more than 15 books, 100 book chapters and 500 articles in journals and conferences (more than 280 articles in journals indexed in Web of Science core collection/h-index 58+/10500+ citations, SCOPUS/h-index 62+/13000+ citations, Google Scholar/h-index 80+/22000+ citations).

Part I

Processes

Chapter 1

Friction Stir Processing: A Novel Way to Produce the Surface Composite Coating Shalok Bharti , Nilesh D. Ghetiya , and Kaushik M. Patel

Abstract Surface properties are important for a particular application such as marine, aerospace etc. In these applications, the required properties of the material depends only upon its surface properties. Therefore the enhancement in surface properties of the material is sufficient for such applications. Friction stir processing (FSP) provides a simple solution to enhance the surface properties of the material by producing a surface composite coating over the base material. Various properties like hardness, strength, corrosion, wear, microstructure, etc. can be enhanced by using FSP. An approach is made in this chapter to provide an understanding of composite materials and surface composite coatings. The information about FSP and how surface composite coating is prepared by FSP is discussed in detail. Previous work on FSP along with its effect on various properties and microstructure of the material has been presented in this chapter. Finally, the future scope and conclusion have been discussed in the end. Keywords Friction stir processing · FSP · Coating · Composite · Surface modification · Material processing

1.1 Introduction Friction stir processing (FSP) is the variant of Friction stir welding (FSW) which was developed in 1991 by “The welding institute (TWI)”. FSW is used to join the similar or dissimilar material with the help of a rotating tool which consists of the pin and a shoulder. The friction between the rotating tool and surrounding material S. Bharti (B) · N. D. Ghetiya · K. M. Patel Department of Mechanical Engineering, Institute of Technology, Nirma University, Ahmedabad, Gujarat 382481, India e-mail: [email protected] N. D. Ghetiya e-mail: [email protected] K. M. Patel e-mail: [email protected] © Springer Nature Switzerland AG 2021 K. Kumar et al. (eds.), Coatings, Materials Forming, Machining and Tribology, https://doi.org/10.1007/978-3-030-62163-6_1

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Fig. 1.1 Working of FSW

produces the friction which helps to convert the material into a solid-state phase. The stirring action of the tool helps in the movement of the semi-solid material from one side towards the other, creating welding between the two materials. This technique has been used in several applications like aerospace, automobiles, etc. where the joining of two materials with different properties is an essential part of the application. Recently it has begun to use in air or immersed medium to tackle the problem of microstructure softening [1, 2]. Figure 1.1 shows the working of FSW. FSP works on a similar principle of FSW. Instead of joining the two materials, the rotating tool is passed over the material to improve the property of the material. Various properties like microhardness, wear, microstructure, etc. can be enhanced by this technique. FSP was first produced to achieve superplasticity into the material but with time, it has been used to provide enhanced material properties and surface composite coatings. Different process parameters of FSP helps the material to achieve enhanced properties. Figure 1.2 shows the working of FSP. Composite materials are the materials in which the two or more materials are added or mixed to enhance the properties of the base material. In it, the reinforcement material helps to achieve the enhanced properties. There are various types of composites that can be developed depending upon the type of material used. In composite material, the whole base material is converted into the composite material which proves costlier than the surface composite coating. In surface composite coating, only the required surface of the base material with particular depth is converted into the composite material and below the surface remains unchanged. FSP is one of the best methods to achieve such a surface composite coating. The reinforcement particles are inserted into the base material and then the rotating tool helps to mix the reinforcement particles and surrounding materials with the help of heat which produces due to the friction.

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Fig. 1.2 Schematic of FSP

A large amount of literature is available discussing the surface composite coating by FSP and how it enhances the properties of the material. In this paper, the working principle of FSP and how it helps to produce surface composite coating is discussed. Previous work in this field is presented in the tabular form along with its effect on different material properties. Future trends and conclusionsare discussed in the end.

1.2 FSP Working Parameters FSP works on the basic principle of Friction Stir Welding (FSW) which was developed “by “The Welding Institute (TWI)” in 1991 [3]. FSP consist of a rotating tool which involves an assembly of pin and shoulder. The workpiece is placed over the backing plate and clamped on the bed of the FSP machine. The rotating pin is provided with an axial force and descended into the workpiece so that the tool pin comes in contact with the workpiece and is further inserted till the shoulder comes in contact with the workpiece. Figure 1.3 shows the pin and shoulder in FSP tool used in the process. After inserting the rotating tool into the workpiece, the tool is provided with a traverse speed which helps the tool to move in the transverse direction. This rotating and traverse action of the tool produces the heat due to the friction and helps the material to undergo intense plastic deformation and dynamic recrystallization [4]. Due to the heat produced during the process, the workpiece material converts to the semisolid state. Tool rotation helps in the movement of this semi-solid material from the advancing side of the workpiece toward the retreating side of the workpiece. This stirring action helps in the microstructure refinement and thus enhancing the material properties.

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Fig. 1.3 Tool pin and tool shoulder in FSP tool

The tool rotating speed and the traverse speed plays an important role in FSP. These speed helps the process to maintain the amount of heat produced during the process. More tool rotating speed produces more heat due to friction and vice versa whereas, on the other hand, more traverse speed reduces the contact time between the tool and the shoulder and hence reduces the amount of heat. In order to produce the material with enhanced properties, the amount of heat should be optimum. If the amount of heat produced is less, then the microstructure deformation will not take place whereas if the heat production is more, then the microstructure softening will take place. Therefore optimum speeds should be chosen during the process. Another important parameter that should be taken care of during the process includes tool tilt angle, tool dimensions, pin profile, and the number of passes. These parameters play an important role to enhance the material properties during FSP. Tool tilt angle is the angle by which the tool is tilted towards the workpiece. Tool tilt is necessary during the process because it is due to this angle that the movement of semisolid material is possible in the material. Mahallawy et al. [5] in their study of FSP to produce Al 1050/SiC surface composite suggested that the tool tilt angle should be between 2° and 4° otherwise cavity defects can be developed in the workpiece [6, 7]. Figure 1.4 shows the schematics of the tool tilt angle during FSP. FSP tool consists of the pin and the shoulder. The dimensions of this pin and shoulder are important parameters to decide during the process. The height and Fig. 1.4 Tool tilt angle

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diameter of the tool pin require to be in accordance with the dimension of the material to be processed. In general, the pin height and pin diameter should be equal. Shoulder diameter decides the area of processing. Therefore it should be in accordance with the required area of the processing. Figure 1.5 shows the sample dimension for the tool dimension. It should be noted that the end of the tool should be in according to the chuck holder of the FSP machine. Pin profile is the shape of the pin used for the process. Various pin profiles have been developed and used for the FSP. It was found that the pin profile helps in the easy movement of the material during the process. Some of the pin profile used for the process includes a cylindrical pin, threaded pin, square pin, triangular pin profiles, etc. Some of these pin profiles are shown in Fig. 1.6. Various other parameters like plunge depth, axial force, clamping design, etc. play an important role during the process. To enhance the properties and microstructure

Fig. 1.5 Dimension of a tool being used in FSP

Fig. 1.6 Different pin profiles for FSP

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of the workpiece material, the selection of process parameters should be selected carefully. The process parameters should be selected in such a way that the heat generated during the process should be optimum and the easy movement of material could take place.

1.3 Composite Materials Sometimes the property of single material is not sufficient for particular applications. Therefore the concept of composite materials helps to enhance the property of such material by combining the two or more materials and getting the resultant material with combined enhanced properties of all the material [8]. In composites, there are two basic terms for the material. One is matrix material (base material) and the other is reinforcement material. The material whose properties are needed to be improved is called the base material whereas the materials which are added to the base material to enhance its properties is called reinforcement material [9]. The composite technology is not new. In fact, wood, bones, rocks, sand, etc. are natural composite materials that exist in nature. The composite materials are light in weight and have enhanced mechanical and tribological properties [10]. Based on the matrix material, composite materials can be divided into three categories: Metal matrix composite (MMC), Polymer matrix composites (PMC), and Ceramic matrix composite (CMC). Whereas depending upon the type of reinforcement particles, composite can be divided into two categories: Particulate composite and Fibrous composites. Figure 1.7 shows the classification of composite materials depending upon the type of matrix material. Composites have proved itself a better method to obtain a material with less expensive and enhanced properties [11, 12]. Nowadays composites are used in various industries, some of which include automotive, aerospace, construction, etc. [13–15]. In aerospace industries, MMCs are used in Housing covers, rotors, electronic panel boards, external bodies, etc. whereas on the other hand, composites are used in engine parts, brake calipers, body, etc. in automotive application [16]. Compositesare also used in Military applications, Turbine blades, Railway applications, etc. Various MMCs have been developed and used in different applications. Singh et al. [17] developed a self-lubricating Al-SiC-nAl2 O3 -WS2 Nanocomposite. They found that the composite containing 5% of WS2 showed lower wear as well as Friction. The density of the composite increased by increasing the quantity of the reinforcement which occurred due to the filling of gaps in the composite by reinforcement particles (WS2 ). They found an increase in the hardness with the addition of WS2 as compared to the composite having no WS2 . Slathia et al. [18] Produced a novel hybrid composite by using aluminum 2024 as the base material and Zirconia and Graphite powder as the reinforcement. They developed the composite by mixing the reinforcement powders in steps and studied the properties like micro-hardness, compressive strength, and density. They found that the hardness of the composite decreased by increasing the quantity of graphite

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Fig. 1.7 Types of composite materials based on matrix material type

powder whereas the compressive strength of the composite increased by the addition of the solid lubricants. They proposed the application of the composite in the aerospace and automobile industries. Wang et al. [19] studied the composite of the Aluminum matrix using Graphene Nano-sheets as reinforcement. They successfully produced the metal matrix composite using Aluminum as base material and Graphene Nano-sheets as the reinforcement material and found an increase in the tensile strength of 249 MPa in composite material as compare to 125 MPa in the base material. They found that the increase in the mechanical properties was due to grain size refinement, stress transfer, and dislocation strengthening during the composite formation. Hassan et al. [20] studied the effect of hardness in Al–Si–Fe/SiC composite. They heat-treated and quenched the composite samples with different aging time intervals. They used 5–25% of SiC in producing different grades of composites. They concluded an increase in the hardness value with the increase in the SiC quantity and a decrease in the hardness value with the increase in the aging time interval. Raja and Sahu [21] studied the effect of hardness on Al-B4 C composite by using the fabrication method of the powder metallurgy route. They used different volume fraction of reinforcement ranging from 5 to 20% and found better distribution and bonding of matrix and reinforcement in the composite microstructure. They also found an increase in the hardness value of the composite having a reinforcement fraction of 20%. Natarajan et al. [22] produce MMC of A356/25SiCp and studied the effect of wear on the composite. They found the application of the composite in discs in the brake system in automobile industries. They studied the friction and wear mechanism on a pin on disc machine at different load, sliding velocities, and sliding distance and found

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that the fabricated composite material had better wear resistance as compared to the conventional material in automobiles. They also found a decrease in the coefficient of friction in composite material with an increase in applied load. Sachit et al. [23] studied the effect of mechanical properties on the composite with Aluminum LM4 as base material and SiC of different particle sizes as reinforcement. They found an increase in the mechanical properties with a decrease in the reinforcement size whereas on the other hand with the increase in the reinforcement particle size, the wear properties of the composites increased. Various other composites materials have also been developed by different researchers. Some of which are shown in Table 1.1. Table 1.1 Combination of matrix and reinforcement particles

Matrix

Reinforcement

Reference

AlSi7 Mg2

Silicon carbide particle (SiC-p)

[24]

AA7075

Al–SiC powder

[25]

A359

20SiCp

[26]

A356

Al2 O3

[27]

A413

Flyash/B4 C hybrid

[28]

Aluminum 6061

SiC/ZrO2

[29]

LM6

Flyash and rice husk ash

[30]

Inconel 625

TiB2

[31]

Al7075

Al2 O3

[32]

Al2024

B4 C

[33]

Al7075

SiCnp

[34]

AA 7075

B4 C and graphite

[35]

Al2024

SiC

[36]

Al 6061

20SiCp

[37]

SS316

TiN

[38]

AZ31 Mg alloy

Reduced graphene oxide

[39]

Al7075

TiC

[40]

Al6061

E glass fibers and micro Titanium particles

[41]

Stainless steel 2205

Al2 O3 , TiS2 , and Fe powders

[42]

Aluminium AA430

SiC + MgO

[43]

2024 AA

Al2 O3

[44]

AZ31 Mg

SiC@r-GO

[45]

356 AA

TiB2

[46]

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Fig. 1.8 Surface composite coating by a FSP[47] b friction surfacing[48]

1.4 Surface Coating In composite materials, the base material and reinforcement materials are mixed with each other and the complete material is converted into the composite material. But sometimes only the surface of the material is required to have improved properties and below surface material does not plays an important role in the application. In such applications, only the surface of the material is required to enhance to make a surface composite coating. There are various ways by which the surface composite coating can be achieved over the material, some of which include laser cladding, friction surfacing, electrohydrodynamics, FSP, etc. Figure 1.8 shows the surface composite coating by FSP and friction surfacing. With the improvement in FSP technology, the concept of surface composite coating become popular. Various researchers used this technique to enhance the surface properties of the material. It helped to improve the microstructure, mechanical, and other properties of various materials.

1.5 Surface Composite Coating by FSP FSP has been used for producing the surface composite coating since the early 2000s. The technique providesa simple solution to coat the surface of the base material with the surface composite coatings. Many researchers have used this technique to modify the surface of the material and enhance its mechanical, tribological, and microstructural properties. Mishra et al. [49] used SiC as the reinforcement and aluminum 5083 alloys as the base material to produce surface composite via FSP. They observed refined grain size and improved micro-hardness after the process. Similarly, Morisada et al. [50] performed FSP to produce a surface composite coating on AZ31 magnesium alloy by using SiC as the reinforcement particles. They observed that the FSP helped to improve the mechanical as well as microstructural properties of the composite material after the process. In the same way, many other researchers used FSP on different materials by using various reinforcement particles to enhance its properties. Some of the studies are shown in Table 1.2.

12 Table 1.2 Combination of base and reinforcement materials to form composite in FSP

S. Bharti et al. Base material

Reinforcement

Reference

AZ91

SiC

[51]

AZ91

SiO2

[52]

AZ91

Al2 O3

[53]

AZ31

Al2 O3

[54]

Al

TiC

[55]

AA1016

MWCNT

[56]

AA1100

Ni

[57]

AA2024

Al2 O3

[58]

AA5052

SiC

[59]

AA5083

Cu

[60]

AA6061

SiC

[61]

AA6082

Al2 O3

[62]

Cu

SiC

[63]

Cu

B4 C

[64]

Ti

SiC

[65]

Steel

TiC

[66]

Polymer

Nano-clay

[67]

In order to produce the surface composite coating via FSP, firstly the reinforcement particles are embedded into the surface of the base material. After inserting the reinforcement particles into the base material, the rotating tool is inserted into the material and is provided with an axial force. The friction between the rotating tool and surrounding materials generates friction which helps to produce enough heat to convert the material into a semi-solid state. The rotating action of the tool helps to move and mix the semi-solid base material and reinforcement particles. The tool is then provided with a traverse speed which helps to convert the seam into surface coatings. Figure 1.9 shows the mechanism to produce surface composite coating by FSP. There are various methods by which we can insert the reinforcement particles into the base material. Some of these methods include spraying method, layer coating method, hole drill method, groove method, etc. These methods provide simple solutions to insert the reinforcement into the material for producing surface composites. Out of these methods, the most popular methods are the hole drilled method and groove method. In the hole drill method, several holes with required depth and diameter are drilled into the surface of the base material. The reinforcement particles are then inserted in the holes and FSP is applied over the surface. Whereas in the groove method, a simple V-shaped slot is cut in the base material with the required width and depth. The reinforcement is embedded into the slot and FSP is applied over the surface to produce a surface composite. The hole method and groove method is preferred over other methods because the uniform distribution of reinforcement

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Fig. 1.9 Surface composite coating by FSP

particles inside the material is also possible in these methods. Figure 1.10 shows the hole drill and slot method. When FSP is applied over the material’s surface and the tool is traversed only once, then this FSP is known as the single-pass FSP. Sometimes the single-pass FSP is not sufficient to provide enough heat and uniform distribution of the reinforcement particles. Therefore researchers use more than one pass to produce the surface composite coating with uniform distribution of reinforcement particles. This type of FSP is known as multi-pass FSP. Multi-pass FSP includes 2 passes, 3 passes, and

Fig. 1.10 a Hole drill method [68]. b Slot method

14 Table 1.3 Different number of passes used in FSP

S. Bharti et al. Material

Number of passes

Reference

Al 1050/SiC

2 and 3

[69]

AZ31

2

[70]

Al 6061/B4 C

4

[71]

AA 7005/TiB2 -B4 C

3

[72]

ZK 60/Nano-hydroxyapatitle

3

[73]

AA1050/Al2 O3

2

[74]

AA7075/Graphite

2

[75]

Cu/CNT

2

[76]

Al5083/Graphene oxide

3

[77]

AA2014/SiC

4

[78]

more depending upon the material and reinforcement particles. Many researchers used different numbers of passes during FSP, some of them are listed in Table 1.3. FSP has provided a simple solution to produce surface composite coating over various base material and enhanced various properties. The enhanced properties depend upon various process parameters and the method by which reinforcement has been added into the material. Multi-pass FSP has also helped to provide material with surface composite with enhanced properties. To get a good quality surface composite coating, one must keep the optimum process parameters while performing FSP.

1.6 Previous Work in the Surface Composite Coating by FSP Since the development of FSP, various studies have been done in order to improve the various properties of the material. Researchers investigated the effect of surface composite coating on properties like hardness, microstructure, corrosion, wear, etc. In some studies, the improvement in material properties depends upon the optimum process parameters and the type of reinforcement material used. Various studies on aluminum and magnesium alloys suggest that the surface composite coating helps to increase the hardness of the material. Various other materials like copper and titanium have also been used in the process. The trend in the materials to be processed by FSP is shown in Fig. 1.11. FSP was developed to achieve the super-plasticity into the material. With the passage of time surface composite coating was beginning to develop using this technique. Nowadays researchers began to use the concept of hybrid reinforcement particles and in situ composite to produce a better surface composite coating with enhanced properties. A lot of work has been done to improve this technique and various studies have been done to improve the properties of the material. Some of the studies are shown in Table 1.4.

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Steel

Titanium

Copper

Magnesium

Aluminum 0%

10%

20%

30%

40%

50%

60%

70%

80%

Fig. 1.11 Use of material trends in FSP [79]

There are various other studies also which showed a significant increase in material properties after FSP. The improvement in material properties depends upon the chosen process parameters and therefore it is important to choose the optimum combination of process parameters.

1.7 Effect on Microstructure After FSP FSP is a thermomechanical process which helps to enhance the microstructural property of the material. FSP helps to refine the grain size of the material and hence improve its properties. The stirring action of the tool helps to provide dynamic recrystallization in the material which helps in the grain refinement. The reinforcement particles in the base material undergo nucleation which leads to the pinning effect under the action of the rotating tool in FSP. In FSP generally, four types of regions/zones are formed in the material. These include the nugget zone, thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ), and unaffected zone. Figure 1.12 shows the different zones in the material after FSP. The nugget zone is the region in which the maximum plastic deformation takes place. This is the region where the grains undergo intense plasticity and maximum heat is experienced in this region. Due to the stirring action of the tool and friction between the tool and surrounding materials, the heat is generated in the different regions. Due to the maximum heat experience in the nugget zone, this region undergoes maximum recrystallization. Whereas in the other hand, TMAZ does not experience much heat as compared to the nugget zone due to which the grains in this region do not refine as much as the nugget zone and also the amount of plastic deformation in this region is less. The HAZ experiences the microstructure

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Table 1.4 Previous studies in FSP Material

Studies

AZ31/SiC

SEM, TEM, optical Refined microstructure microscopy, microhardness was observed. Microhardness was enhanced from 48 to 69.3 Hv. FSPed sample showed better microstructure and mechanical properties

Result

References

AZ61

SEM, optical, hardness, wear, fracture

AZ31

SEM, TEM, optical Nano grained structure microscopy, microhardness was achieved after FSP. Multi pass FSP helped to achieve better microhardness

[70]

5A06Al/SiC

SEM, optical microscopy, microhardness

FSP helped to achieve uniform distribution of reinforcement particles. Enhanced mechanical properties was achieved after FSP

[81]

Cu/25% AlN + 75%BN

SEM, optical microscopy, microhardness, wear, tensile

FSP increased the [82] hardness as well as wear resistance of the FSPed samples. However the ductility of material decreased after FSP. Strength was increased and COF decreased after FSP

Al 6061/B4 C

SEM, optical microscopy, microhardness, wear

The wear resistance and [71] hardness of the material was improved after FSP. Uniform distribution of reinforcement particles was achieved after FSP

AA 7005/TiB2 -B4 C

To enhance the ballistic resistance of AA7005 by FSP

Microhardness and ballistic resistance was improved after FSP

[50]

FSPed samples shower [80] better microhardness and strength as compare to base material. Microstructure was improved and ductility was achieved

[72]

(continued)

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Table 1.4 (continued) Material

Studies

Result

References

Z91 magnesium-lithium alloy

Microstructure, tensile, corrosion, texture

Tensile strength was enhanced after FSP. Microstructure was refined. Yield strength decreased after FSP. Corrosion resistance increased in FSPed samples

[83]

AA1050/Al2 O3

SEM, optical microscopy, microhardness, wear

Multi pass FSP improved the homogenous dispersion of reinforcement particles. Tensile properties and wear resistance increased after FSP

[74]

AA5083/SiC

Optical microscopy, microhardness

Surface composite [49] coating was successfully fabricated after FSP. Microhardness was also enhanced after FSP

AA5052/graphene oxide

Raman spectroscopy to confirm MMC in stir zone, thermal conductivity, quasi-static tensile load, atomic force microscopy

Thermal conductivity [84] increased by 15% at 230 degree, ductility was increased and tensile strength was slightly decreased after FSP

Fig. 1.12 FSP zones

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change only due to heat and the least plastic deformation takes place in this region. The unaffected zone experience lest heat and hence do not undergo much change in microstructure. The shape and size of these zones mainly depend upon the heat experienced by the zones during the process and also depends upon the type of materials being used. In case of the surface composites, FSP helps to uniformly distribute the reinforcement particles into the base material. The stirring action of the tool helps to distribute the particles homogeneously among the substrate material. This uniform distribution helps to enhance the mechanical as well as the tribological properties of the material. Many researchers observed the uniform distribution of the reinforcement particles in their study. In the previous study by Bharti et al. [85] on the study of aluminum 5052 by using ZrO2 as the reinforcement particles, the uniform distribution of the reinforcement particles was observed which further helped to improve the hardness of the surface composites. Bourkhani et al. [74] in their study on FSP of AA1050 alloy used Al2 O3 as the reinforcement particles and found a uniform distribution of reinforcement particles after FSP. Similarly, Qin et al. [73] studied FSP on ZK60 magnesium alloy by using Nano-hydroxyapatite (nHA) of 100 nm particle size as the reinforcement particles and found that the single-pass FSP was unable to provide the uniform distribution of the reinforcement particles. However, the second pass FSP helped to provide the uniform distribution of the particles into the material Fig. 1.13 shows the uniform distribution of different reinforcement particles in aluminum alloy after FSP. The microstructure of the processed material or surface composite mainly affects by the parameter chosen during the FSP. The microstructure of the material affects with the amount of heat produced during the process. Therefore the parameters of FSP should be selected in such a way that the amount of heat remains in the optimum condition. Generally, the amount of heat required during the process should be between 0.5t and 0.9t i.e. the melting temperature of the base material. The heat should not be less than 0.5t because in such condition the grain hardening or freezing will occur which do not let the grains to get refine whereas if the temperature is more than 0.9t then the grain softening will take place. Therefore the amount of heat should be in between the range where grain refinement and recrystallization could take place. Therefore proper tool rotating, tool traverse speed, and other parameters should be selected for an optimum process with enhanced material properties.

1.8 Improvement in Material Properties After FSP FSP plays an important role to enhance the material’s properties by producing surface composite coating. The reinforcement particles help the material to enhance the mechanical, tribological, and other properties of the material. The surface composite act as the coating over the surface of the material and helps to enhance the surface properties of the material. The enhanced material properties after FSP proved its application in automobile, aerospace, marine, etc. industries. Several properties

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Fig. 1.13 Uniform distribution of Si particles in aluminum after FSP [86]

like mechanical, tribological, microstructural, and corrosion resistance has been improved by FSP surface composite coating. FSP affects the mechanical properties of the base material. The surface composite coating helps to make the coating which increases the properties like hardness and tensile strength. FSP helps to uniformly distribute the hard reinforcement particles which further enhance the hardness of the surface composite [87]. Various researchers used FSP to produce surface composite coating and enhance the hardness property of the material. Some of which are listed in Table 1.5. Similarly, with the enhancement of hardness property, the surface composite coating helps to enhance the tensile property of the material. Some of the studies related to enhancement in tensile properties after FSP are shown in Table 1.6. With the increase in the hardness property of the material, FSP also helps to enhance the properties like wear and coefficient of friction (COF). The hard reinforcement particles resist the friction between the two surfaces and thus material loss due to friction reduces. This results in improved wear resistance as well as COF. Various studies have been performed by different researchers to enhance the tribological properties after FSP. Some of them are listed in Table 1.7.

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Table 1.5 Effect of FSP on hardness Material

Hardness before FSP

Hardness after FSP

Reference

AZ61Mg/SiO2

60

105

[88]

Cu/SiC

70

90

[63]

Al2024/Al2 O3

90

230

[58]

AZ91/Al2 O3

63

110

[89]

5A06 AluminumAlloy/Al84.2Ni10La2.1amorphous

80

97

[90]

AA5083/Cu particles

79

136

[60]

AA1050/TiC

30

55

[91]

Ti/SiC

160

534

[65]

AZ91/SiC

63

92

[92]

AZ91/Al2 O3

63

110

[89]

Table 1.6 Effect of FSP on ultimate tensile strength Material

Ultimate tensile strength before FSP

Ultimate tensile strength after FSP

Reference

AZ31/ZrO2

160

258

[93]

AA336/Si

159

371

[94]

Al5083/CNT

230

396

[95]

A356/Si

169

251

[96]

5A06/Al84.2Ni10La2.1 amorphous

340

410

[92]

Al5083/Tungsten

296

404

[97]

Al5083/CeO2 + SiC

272

314

[98]

AA6082/TiC

254

326

[99]

Al 6061/SiC-Gr

219

295

[100]

A1016/CNT

90.3

190.2

[56]

Table 1.7 Effect of FSP on wear resistance Material

Weight loss before FSP

Weight loss after FSP

Reference

A356/SiC/MoS2

45 mg

30 mg

[101]

A356

0.216 µm/s

0.014 µm/s

[102]

Al2024 plate

37.7 mg

25 mg

[103]

AA5083

8.2 g/km

2.5 g/km loss/mm3

20 volume

[104] loss/mm3

Al 6061–SiC/Gr

90 volume

A6082/Al2 O3

38 mg

13 mg

[106]

[105]

Al6061

0.190 ± 0.006 mm3

0.083 ± 0.002 mm3

[107]

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FSP plays an important role to enhance the corrosion resistance of the material. It is due to these improved properties that the FSP found its application in marine industries. The surface composite coating helps the base material to enhance the corrosion resistance of the material. There are various studies that verify the enhancement of material properties after FSP. Researchers observed enhanced mechanical, tribological, etc. properties after FSP. These enhanced properties after FSP makes it an industry-friendly technique.

1.9 Future Scope FSP provided a novel method to develop the surface composite coating on the required materials. Various surface composites have been successfully fabricated by using FSP. Since the development of this technique in 1999, various researchers put their efforts to improve this technique and provided improved materials with enhanced mechanical, tribological, microstructural properties. But still, a lot of improvement is needed to develop this technique as an industry-friendly technique. Some of the future aspects of this area are as follows. • FSP is used to fabricate the MMCs by various researchers [24, 108–110]. An effort should be made to develop other composites by FSP such as PMCs, CMCs, etc. Some of the researchers [111, 112] used FSP to develop PMCs but the research is in the preliminary stage and more study is required to produce composited by FSP. • A new technique for adding reinforcement into the base material can be developed. • New processing parameters can be developed to produce surface composites without defects and enhanced properties. • An effort should be made to develop new pin profiles for the FSP tool so that the material movement can be achieved easily. • Since the FSP is a new and costly technique, efforts should be made to reduce its processing cost and make it suitable for industrial applications. These are some of the future trends that can be developed in the future. These aspects will make FSP suitable for many industries and prove its importance in various applications.

1.10 Conclusion Many researchers proved FSP as a successful technique to develop surface composite coating over the surface of various materials. This technique provides FSPed material with enhanced properties and microstructure. The stirring action of the tool helps to attain superplasticity and provide equiaxed refined grain structures. The technique

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provides a novel method to produce surface composites. Some of the conclusions made by the study are: • FSP can be successfully applied over various materials like aluminum, copper, steel, copper, etc. • Various researchers used different reinforcement particles like Al2 O3 , SiC, TiC, etc. and found that the FSP helped in the uniform distribution of the reinforcement particles into the base material. • FSP tool provides the stirring action which helps in the dynamic recrystallization and superplasticity in the material. • The tool rotating and tool traverse speed plays an important role in the generation of heat. Therefore the combination of tool speeds should be selected carefully. • The process parameters like tool dimensions, method of adding reinforcement into the base material, plunge depth, axial force, etc. play an important role in the microstructure refinement and thus enhancing the properties of the materials. Therefore these parameters should be selected carefully. • The composite coated material by FSP proved better corrosion resistance and mechanical properties in various studies. Therefore the use of FSP in industrial applications should be encouraged. Even though the FSP has developed remarkably since its development, the process still needs more understanding and research. The costly nature of the process is the reason that most industries do not want this technique for industrial applications. Besides this limitation, FSP proved its importance in various studies by enhancing mechanical, tribological, corrosion resistance, and microstructural properties. The composite coated material by FSP provides better strength and appearance as compared to other techniques.

References 1. N.D. Ghetiya, K.M. Patel, Welding speed effect on joint properties in air and immersed friction stir welding of AA2014. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 231, 897–909 (2017). https://doi.org/10.1177/0954405417690555 2. N.D. Ghetiya, K.M. Patel, Numerical simulation on an effect of backing plates on joint temperature and weld quality in air and immersed FSW of AA2014-T6. Int. J. Adv. Manuf. Technol. 99, 1937–1951 (2018). https://doi.org/10.1007/s00170-018-2632-3 3. W.M. Thomas, E.D. Nicholas, J.C. Needham, M.G. Murch, P. Templesmith, C.J. Dawes, GB patent application no. 9125978.8. Int. Pat. Appl. No. PCT/GB92/02203 (1991) 4. Z.Y. Ma, Friction stir processing technology: a review. Metall. Mater. Trans. A 39, 642–658 (2008). https://doi.org/10.1007/s11661-007-9459-0 5. N.A. El-Mahallawy, S.H. Zoalfakar, A. Abdel Ghaffar Abdel Maboud, Microstructure investigation, mechanical properties and wear behavior of Al 1050/SiC composites fabricated by friction stir processing (FSP). Mater. Res. Express 6, 096522 (2019). https://doi.org/10.1088/ 2053-1591/ab2ce2 6. V. Infante, C. Vidal, Tool and welding design, in Advances in Friction-Stir Welding and Processing (Elsevier, Amsterdam, 2014), pp. 199–240. https://doi.org/10.1533/978085709 4551.199

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Chapter 2

Microwave Processing of Engineering Materials Padmakumar A. Bajakke, Vinayak R. Malik, Prakash Mugali, and Anand S. Deshpande

Abstract Quick, efficient and cost-effective processing methods grab attention in the current manufacturing scenario. At the same time, they are expected to be cleaner and environment-friendly. Microwave processing and fabrication of engineering materials seem to be a promising technique meeting both these demands. This chapter provides a detailed explanation of Microwave heating and its application in the processing of various composites of engineering importance. Further, it also highlights the suitability of Microwaves in joining and surface treatments of materials. In the end, a separate section is dedicated to the preparation of ceramics and nano-materials using Microwaves. The chapter culminates with a discussion on future perspectives and applications of this technology in processing and fabrication of other engineering materials. Keywords Microwave · Processing · Composites · Ceramics · Coatings · Cladding · Joining Brief Background After Faraday discovered electromagnetic induction in 1831, Maxwell started working on Faraday’s concept and in 1864 presented the theory of electromagnetics and anticipated the existence of unseen electromagnetic waves. During the 1880s, Heinrich Hertz proved the existence of the unseen electromagnetic waves, which possess a longer wavelength than that of the light source. Hertz also depicted that, substances started to reflect radio waves at ~450 MHz microwave frequency. In the 1890s, Jagadish Chandra Bose furthered Hertz’s experiments and found the same result even for a higher microwave frequency range of 60–120 GHz. P. A. Bajakke · V. R. Malik (B) · A. S. Deshpande Department of Mechanical Engineering, KLS Gogte Institute of Technology, Belagavi, Karnataka 590008, India e-mail: [email protected]; [email protected] Visvesvaraya Technological University, Belagavi, Karnataka, India P. Mugali Enerzi Microwave Systems Pvt. Ltd, Belagavi, Karnataka, India © Springer Nature Switzerland AG 2021 K. Kumar et al. (eds.), Coatings, Materials Forming, Machining and Tribology, https://doi.org/10.1007/978-3-030-62163-6_2

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The first application of the microwave concept was for the development of radar systems. However, the need for commercialization and on-ground application evolved during World War II to improve the radar detection of enemy submarines and aircraft. Further, an important milestone in the application of microwave technology was found in heating, the most familiar being a domestic microwave oven. The other heating applications like processing of composite materials, fabrication techniques and synthesis of special-purpose materials using microwave energy are discussed in the following sections.

2.1 Basics of Material Processing via Microwave Microwaves are a form of electromagnetic (EM) waves and energy is released in a waveform that travels at the speed of light. The EM waves hold magnetic and electric fields oscillating and propagating at right angles to each other with the frequency between 0.3 and 300 GHz and wavelength ranging from 0.001 to 1 m. For beginners in this field, the word microwave might limit their thoughts to a domestic microwave oven. In recent years the thermal treatment using microwave has been extended from food processing to advanced engineering materials. Figure 2.1 gives an overall view of materials, classified based on their processing temperatures (T °C) range [1].

Fig. 2.1 A pictorial view representing categorized microwave processing of materials depending on their operating temperature range

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2.1.1 Heating Mechanisms The principle of microwave heating mainly depends on how the substance responds to alternative magnetic and electric fields. The electrical and magnetic properties are permittivity, dielectric loss, permeability and magnetic loss. The microwave power absorbed in a material indicates the energy conversion within a heated material. Hence, the microwave power absorption capacity of any material is greatly affected by the penetration depth of the radiations. However, the interaction of microwave depends upon the target material. The electric field is responsible for the penetration of microwaves into a non-metallic material. The magnitude of microwave field strength drops by an amount of 1/e from the target material surface. Equation 2.1 represents the penetration depth (d). The power density (Dp ) of the microwave field reduces to a value of 1/e at a certain distance from the material surface and is half of the penetration depth as given in Eq. 2.2. The bulk metals exhibit negligible microwave penetration, restricted to surface itself and are termed as skin depth (d). Here the microwave penetration is influenced by the magnetic field and is expressed mathematically by the Eq. 2.3. 1/2 1/2 1/2   1 + (tanδ)2 −1 d = 2π f μ0 μ ε0 ε |2

(2.1)

where μ0 is the permeability of free space (μ0 = 4π × 10−7 H/m), ε0 is the of free space (ε0 = 8.854 × 10−12 F/m) and tanδ is the loss tangent  permittivity   ε |ε . D p = d/2  d=

ρ π f μ0 μ

(2.2) (2.3)

where ρ is the resistivity of metal (μmm) and f is the frequency of microwaves (GHz).

2.1.1.1

Heating in Non-magnetic Materials

The non-magnetic constituent (like Al, Cu, water, ceramics and polymer) will respond to the electric field alone. Here dipolar and conduction losses are the two main loss mechanisms. The conduction losses dictate in metallic materials, while the dipolar losses dominate in dielectric insulators. Conduction loss Conduction loss is found considerably in pure metals, metal matrix composites, and semiconductors (like Fe, Cu, Al, Ni and Si). Such substrates possess unrestricted

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Fig. 2.2 The heating mechanism in conduction loss [4]

electrons (Fig. 2.2a), which start mobilizing with a certain velocity (v) in the path of the external electric field (E) (Fig. 2.2b). An enormous current (Ii ) is induced due to the rapidly weakened field inside the material (Fig. 2.2c). Hence magnetic field (Hi ) is induced within the material, which is developed opposite to the direction of the external magnetic field. A force is generated on moving electrons by the induced magnetic field that drags conducting electrons in the opposite direction with equivalent speed. Hence electrons acquire kinetic energy and hinder the movement of other electrons. This is ascribed to the interaction forces at the molecular level, and other forces like elastic, frictional and inertial. This phenomenon is repeated by the oscillating electric field and produces a uniform and volumetric heating inside the material (Fig. 2.2d) [2, 3]. Dipolar loss The dielectric insulator materials (such as water, food, ceramics, ceramic/polymer matrix composites) when brought under the external electric field generate dipoles and cause dipolar loss. It occurs due to the agitation of the molecular dipoles produced by oscillating electric field while processing of such materials (Fig. 2.3). The frictional, inertial, molecular interaction and elastic forces constrain repetitive variations in the orientation of molecules and accelerate kinetic energy. The bulk heating takes place due to an increase in the kinetic energy and causes a quick rise in the temperature of the target material [5, 6].

Fig. 2.3 The heating mechanism in dipolar loss [4]

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Heating in Magnetic Materials

Both electric and magnetic fields are effective in microwave processing of magnetic substrates (like Iron, Cobalt and Nickel) and affect the heating mechanism. The motion of free electrons is influenced by the electric field while the orientation of domains, domain wall and electron spin is imparted by the magnetic field. Along with conduction losses additional magnetic losses encountered are residual losses (electron spin resonance and domain wall resonance), hysteresis, and eddy current [1, 3, 7]. Residual losses The key contributors to residual loss are the presence of an external magnetic field, which disrupts electron spin or displacement of domain walls. Further, electron spin and domain wall mechanisms are responsible for resonance phenomena that occur in the magnetic material [1, 3, 7]. Electron spin loss Electron spin loss mechanism is mainly concerned with ferromagnetic materials and is also termed as Ferromagnetic resonance. The spinning electrons engender the net magnetic moment, which is coupled with angular momentum (m) (Fig. 2.4a). An uneven field on an individual electron establishes an internal magnetic field (He ) (Fig. 2.4b). The energy gained can be maintained at the smallest amount in the nonappearance of an external magnetic field (H) as the internal magnetic field and the angular momentum will be parallel and align with each other. Consider a ferromagnetic substance that is brought under the microwave shower and the external magnetic field (H) is applied perpendicular to the internal magnetic field (He ). It gives rise to a torque (τ), which acts in the direction of the external magnetic field (H) on the angular moment (m) with an angle of precession (θ) (Fig. 2.4b). As the microwave field is shut off, the energy is converted and dissipates into thermal energy and angular moment (m) gets spiralled within until it is parallel and aligns with the internal magnetic field (He ) (Fig. 2.4c). Hence the oscillatory magnetic field repeats this phenomenon at a faster rate and uniformly heats the material (Fig. 2.4d) [1, 3, 7].

Fig. 2.4 Mode of heating due to electron spin loss [4]

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Fig. 2.5 The heating mechanism in domain wall resonance loss [4]

Domain wall resonance loss The material experiences zero net magnetic field in the absence of an external magnetic field (H) (Fig. 2.5a). Under the influence of the external magnetic field, the domains which are not arranged along the path of the magnetic field undergo contraction (Fig. 2.5b). Conversely, other domains that are primarily arranged along the path of the magnetic field tend to expand. This phenomenon reverses with the change in the path of the external magnetic field (Fig. 2.5c). The domain walls regain their original shapes due to the detached external magnetic field (Fig. 2.5d). Thus, the expansion and contraction of domain walls in the existence of waving magnetic field experience inertia and frictional effects. Hence these are termed to be the key factors for uniform heating and heat dissipation in bulk magnetic substrates (Fig. 2.5e) [1, 3, 7]. Hysteresis loss The applied external magnetic field induces the disturbance in the orientation of magnetic domains and causes hysteresis loss. The magnetic moment and the magnetic domain are interrelated because each electron has anisotropy field of the grains and a large quantity of revolving electrons in the domain. In bulk magnetic materials total magnetic effect of the material will be zero due to the irregular orientation of the domains (Fig. 2.6a). As the material comes under the action of the external magnetic field (H), the domains align into the pathway of the external magnetic field (Fig. 2.6b). Though the pathway of the external magnetic field is changed, the domains realign in the pathway of the field (Fig. 2.6c). Further, when the supply of the magnetic field is stopped, the domains regain the original positions leading to the unmagnetized condition (Fig. 2.6d). The oscillatory magnetic field traveling around a loop imparts a hysteresis aspect and uniformly disperses heat energy (Fig. 2.6e). The constituents influencing uniform heating are impurities, intrinsic properties of the material, porosity and grain size [1, 3, 7]. Eddy current loss Any conducting substrate under the effect of varying magnetic field experiences eddy current loss (Fig. 2.7a). When the material comes in contact with the external magnetic field (H), every domain at the periphery is induced with closed-loop eddy current (Fig. 2.7b). Net effects of individually induced eddy currents form a resultant eddy current (Iie ). The total magnetic field is reduced due to increased external magnetic field (H) at the loop which in turn induces an opposing magnetic field (Hi )

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Fig. 2.6 The heating mechanism in hysteresis loss [4]

Fig. 2.7 The heating mechanism in eddy current loss [4]

(Fig. 2.7c). When the external magnetic field (H) on the loop decreases, the resultant eddy current (Iie ) induces a magnetic field (Hi ) and increases the net magnetic field (Fig. 2.7d). The changes occurring in the path of induced current make the energy to dissipate as a heat source (Fig. 2.7e). Thus the material heats up quickly and uniformly as the oscillating magnetic field repeats the mechanism [1, 3, 7].

2.1.2 Types of Heating Considering the coupling ability of a material to microwaves, the method of heating is selected for a given scenario. Typical methods adopted are discussed in this section. Direct microwave heating The materials which readily absorb microwave energy such as foodstuffs, rubber products, metallic particulates and ceramics can be exposed and heated directly in microwave. In such kinds of materials microwaves easily penetrate and generate heat. Sometimes overheating of the material leads to the formation of hotspots, which can be ascribed to inherent temperature gradients while processing. Processing of ceramics via direct heating leads to non-uniformity in material properties and result in cracking due to their thermal instabilities [8–10]. Selective microwave heating Selective microwave heating is another form of direct microwave heating. Here selected region of the material is exposed to the microwaves and the rest of the

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Fig. 2.8 Microwave susceptive mold [19]

material is covered with masking material. This type of heating finds application in bulk metal joining [11, 12]. Hybrid microwave heating It is quite difficult to heat the materials that do not absorb the microwaves. For enhancing the heating capabilities, a special arrangement was made like: a low microwave-absorbing material was applied or covered or brought in contact with a highly microwave-absorbing material such as susceptor and masking material. Placement of susceptor in the vicinity of the target material (material to be heated) accomplishes heating as follows—(a) self-heating of the susceptor, (b) the transfer of heat from hot susceptor to the microwave passive material and (c) as the target material reaches a critical temperature, starts absorbing microwave. It happens as the microwave coupling of material improves with increasing temperature and critical temperature is a temperature above which material becomes microwave responsive [13]. Hybrid microwave heating assists in joining or bonding of bulk metals [14–18]. Figure 2.8 illustrates the case of microwave curing of multidirectional carbon fiber reinforced polymer [CFRP] matrix. Curing with the help of microwaves is quite difficult due to low penetration and heating. Li et al. addressed this problem with the adoption of indirect microwave heating. Here a microwave susceptive mold was fabricated to cure CFRP [19].

2.2 Processing of Composite Materials Using Microwaves The microwave absorption capacity of composite material is governed by the extent of the coupling of its major constituents, i.e. matrix and reinforcement. Either or any one of the constituents could exhibit higher dielectric loss. The constituent possessing high dielectric loss generates heat by microwave absorption and transfers this heat to other constituents via conventional modes. For enhancing the microwave absorbing ability, the conductive reinforcements possessing higher dielectric loss were added into the matrix material [20]. The processing of various composite materials via microwave irradiation is discussed in subsequent sections.

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2.2.1 Polymer Matrix Composites The microwave treating of polymer matrix composites (PMCs) relies on the temperature, characteristic properties of constituent materials, dipole structure and frequency of processing. While processing of PMCs, constituent material with higher dielectric loss dictates absorption capacity. For example, when the polymer matrix is reinforced with poor conductive fibers like natural fibers, glass and aramid then the dielectric properties of the polymer matrix facilitate the absorption of microwave power. However, allotropes of carbon are high conductivity materials that quickly adhere to microwave power and suppress matrix. Microwave processing may alter the chemical structure, which disturbs the dielectric properties of the polymer matrix. The thermoset polymers are subjected to crosslinking, which changes viscosity and internal network structure. The thermoset matrix effectively couple with microwave power at room temperature. Further, increased temperature and viscosity limit the dipole orientation in the oscillatory electric field. It causes a higher rate of cross-linking and reduces microwave absorption in the matrix material [21–23]. However, the thermoplastics are non-reactive to microwave at room temperature due to their poor dielectric property and a higher degree of crystallinity [24]. The thermoplastic is transparent to microwave if the degree of crystallinity is >45% and is ascribed to restricted dipoles alignment in the electric field [25]. CFRP composites are one of the advanced composite materials which find wide applications in aerospace structures and automobile parts with low weight, high strength, better fatigue and a lower rate of corrosion [26–28]. The conventional manufacturing method of CFRP composites is the autoclave curing technique. Here the heating mechanism leads to high energy consumption due to the long cure cycle [29, 30]. Currently, the autoclave curing technique is being replaced by microwave curing technology due to its quick, selective and bulk heating mechanism [31]. Li et al. [19] compared the curing of carbon fiber embedded epoxy composite with thermal curing (TC) and indirect microwave heating (IMH). The IMH resulted in reduced time for curing and microwave power consumption by an amount of 42.1% and 75.9%, respectively. Further, mechanical characterizations like interlaminar shear strength (Fig. 2.9a, b) and compressive strength (Fig. 2.9c, d) of IMH specimens found to be 7.2 and 4.8% higher than TC. It was attributed to an improved compaction effect at the surface of the composite material. Among the five different samples tested, the average values of compressive strength were 402.61 MPa (TC) and 421.96 MPa (IMH) and interlaminar shear strength were 46.71 MPa (TC) and 50.09 MPa (IMH). In microwave curing, the operating power is one of the key factors that influence the resulting properties of the material. On the same lines, Singh and Zafar [32] investigated microwave curing of coir reinforced high-density polyethylene composite at different microwave power levels. The study revealed that the composite samples processed with 360 W resulted in enhanced tensile strength of ~29.5 MPa and flexural strength of ~37.74 MPa. The tensile failure depends on the interfacial strength between fiber and matrix/fiber. Here, the increased tensile strength at 360 W can be

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Fig. 2.9 a Compression test setup, b compressive test results of multidirectional carbon fiber epoxy composite, c interlaminar shear strength test setup and d interlaminar shear strength test results of multidirectional carbon fiber epoxy composite [19]

ascribed to better interfacial bonding between the molecules. As power decreases, the degree of interfacial bonding increases. Hence the material gains better mechanical properties. The better flexural strength at 360 W can be attributed to molecules experience oscillatory motion for a longer period. It tends to decrease the free oscillatory energy associated with the interface of matrix/fiber. Hence, composite material treated at lower microwave energy will absorb the maximum load. Figure 2.10 demonstrates the correlation between interfacial bonding and temperature as a function of microwave power. The matrix and reinforcement material govern the heating ability in any composites. As polymers are low dielectric loss materials, microwave heating results in non-uniform and partial heating of the composite material. This problem is tackled by Pal et al. [33] by reinforcing SiC powder into the epoxy resin matrix. The study aimed at investigating the effect of SiC as a filler material in the resulting dielectric properties of the composite. The SiC/epoxy composite was fabricated with varying wt% of SiC as 10, 20 and 30 wt% and heated in the microwave at 2.45 GHz, 300 W for 20 min. Both dielectric constant and dielectric loss increased with higher SiC

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Fig. 2.10 Effect of microwave power on interfacial bonding and temperature [32]

content. The interfacial polarization between matrix/reinforcement is responsible for the enhanced dielectric properties. The variation in the dielectric properties of matrix and reinforcement, build a charge at the interface. Hence, the increased content of SiC build-up more charges and thereby enhances the dielectric properties of the composite. Figure 2.11 shows a plot of dielectric constant and dielectric loss as a function of varying SiC contents. Fig. 2.11 Illustrating the result of varying SiC wt% on dielectric constant and dielectric loss [33]

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2.2.2 Metal Matrix Composites The prime factors influencing microwave processing of metal matrix composites (MMCs) include prior history of powder (manufacturing method, size and shape), amount of reinforcement, target temperature and individual properties of matrix/reinforcement [34–36]. The MMCs comprises of base metal powder often reinforced with ceramic powder, which possesses a high dielectric loss factor [37]. The microwave absorption in metal powder depends on powder size, which in turn defines skin depth (penetration depth of microwave). As the particle size of the matrix and the reinforcement reduces microwave absorption is coupled with an increased rate of heating. Here processing of MMCs via microwave follows the powder metallurgy route. During processing, the microwave energy is first absorbed by the reinforcements with high dielectric loss and start heating metal matrix material that finally yields sintering. The reinforcement particles at the micro and nano level possess a higher surface area and which act as a susceptor. It enhances the rate of diffusion and reduces porosity [38]. Further, the mismatch in the coefficient of thermal expansion of the metal matrix and the reinforcement particles result in better heating [39]. The authors are actively working on the synthesis of MMCs and surface treatment via friction stir welding/processing [40–43]. In Copper MMCs, the biggest challenge is to obtain better mechanical properties by retaining the highest possible thermal or electrical conductivity. Pouyani and Rajabi worked on the same lines by reinforcing nano ZrB2 powder in the copper substrate. The microwave-assisted sintering leads to improved mechanical properties like compressive strength, density, hardness, flexural strength, abrasion resistance, thermal and electrical conductivity. Further, the optimum reinforcement level was reported to be 12 wt% [44]. Aluminum is one of the significant candidates for MMCs due to their low density, better corrosion resistance, good workability, high electrical and thermal conductivity. Rajabi et al. [45] dispersed nano ZrO2 particles into the aluminium matrix. The experiments conducted with varying reinforcement levels from 3 to 15 wt% and sintered via traditional muffle furnace and microwave furnace. The sintering time was reduced using microwave heating. The increasing content of ZrO2 decreases the relative density (Fig. 2.12a), which was ascribed to the high volume of porosity and agglomeration due to increased wt% of ZrO2 . Enhanced compressive strength (Fig. 2.12b), and microhardness (Fig. 2.12c) was found with 6 wt% of ZrO2 . The aluminum matrix and ZrO2 differ in thermal expansion coefficient which induces stress, increases dislocation density and hence higher compressive strength and microhardness is obtained. The bending strength (Fig. 2.12d) reduced with a higher content of ZrO2 , which was attributed to increased brittle ceramic particles, acts as a source of cracking and reduces flexibility.

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Fig. 2.12 a Relative density, b compressive strength, c microhardness and d bending strength as a function of zirconia percentage in microwave-assisted sintering of Al/ZrO2 composite [45]

2.2.3 Ceramic Matrix Composites In ceramic matrix composites (CMCs), the matrix material is ceramic and reinforcement may or may not be ceramic powder. Here the microwave processing depends on powder size, dielectric properties of constituent materials, the critical temperature of ceramics and frequency of processing. Ceramic particulates possess lower dielectric loss and act as transparent materials when exposed to microwaves at room temperature. However, as the material reaches a critical temperature, a rapid increase in dielectric loss results in effective heating. Adopting hybrid microwave heating for processing of CMCs overcomes the drawback of cracking due to non-uniform heating associated with conventional sintering techniques. The addition of secondary particles having higher dielectric loss into the matrix owing to lower dielectric loss will surely enhance the heating capabilities. Such secondary particles function as a high source of energy as it acts like a localized susceptor by absorbing microwave energy. The reinforcement particles conduct the heat to matrix material unless it reaches a critical temperature. Once the critical temperature is attained, both the constituents directly absorb microwave energy and uniform heating lead to higher densification. The authors infer that volumetric heating is a function of particle size of constituent materials. The whisker type of reinforcement is also preferred as it enhances adhesion between matrix/reinforcement interfaces [46]. Manshor et al. [47] sintered ZTA-TiO2 -Cr2 O3 at a microwave power of 2.45 GHz, temperature range from 1200 to 1400 °C and soaking time of 5–20 min. The specimen treated at 1350 °C with a soaking cycle of 10 min resulted in better mechanical properties like high density, enhanced hardness (1803.4 HV), improved fracture toughness

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(9.61 MPa m1/2 ). Microstructural analysis revealed that samples treated at 1200 °C (Fig. 2.13a) and 1250 °C (Fig. 2.13b) indicated an inhomogeneous grain distribution and high porosity. For 1300 °C (Fig. 2.13c), notable grain growth and densification were observed. Further, 1350 °C (Fig. 2.13d) sample accounted for negligible porosity, very compact grain refinement and identical microstructure was seen. From this work, it can be concluded that sintering below 1350 °C was insufficient to attain higher densification and grain growth. Hajiaboutalebi et al. [48] synthesized SiC-CNT nanocomposite with varying amounts of CNT (1, 0.5, 2, 5, 10 vol.%) via microwave sintering. The results depicted that hardness of 0.5 vol.% CNT (908 VNH) was 23% higher than the hardness of the parent SiC sample. Greater than 0.5 vol.% CNT declined the hardness and was attributed to clustering and poor interfacial bonding among the constituents. For the same 5 vol.% of CNT, the highest fracture toughness of 9.24 Mpa m1/2 and bending strength of 630 MPa was observed. The shorter sintering time resulted in toughening and strengthening of CNT within the matrix which lowered the grain growth. The increased strength was also attributed to hindered crack propagation and bridging of the SiC matrix by CNT. The fracture surfaces of SiC-CNT nanocomposite are shown in Fig. 2.14. The microstructure of parent SiC material is reflected in

Fig. 2.13 Microstructures of microwave treated specimens at various temperatures a 1200 °C, b 1250 °C, c 1300 °C and d 1350 °C with 10 min of soaking time [47]

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Fig. 2.14 Fractographs of the SiC-CNTs nanocomposite embedded with varying vol.% of CNTs a 0 vol.%, b 0.5 vol.%, c 2 vol.%, d 5 vol.% and e 10 vol.% [48]

Fig. 2.14a. 0.5 vol.% of CNT was appropriately dispersed in the SiC matrix with less porous sites (Fig. 2.14b). It was attributed to crack bridging as a toughening mechanism between SiC and CNT and thereby results in improved properties. Whereas, increased content of CNT (Fig. 2.14c–e) exhibit clustering of CNTs with maximum porous sites accounting for declined properties.

2.3 Other Microwave-Assisted Fabrication Techniques With the use of microwave hybrid heating, novel fabrication techniques have emerged into the field of bulk metal joining, surface coating and cladding. In most of the traditional welding processes, the microstructural changes happen even beside the weld

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region. Whereas, joining via microwave coupled with hybrid heating reduces the alteration of microstructure near the weld nugget to a greater extent. In microwave heating, dipole rotation and ionic conduction mechanisms are responsible for the transfer of electromagnetic energy at a molecular level. Hence, the microwave is always associated with an energy translation. Microwave hybrid heating has also exhibited its application in surface engineerings such as coating and cladding. Microwave coating and cladding overcome the major problems associated with conventional surface coating and cladding methods like porosity and thermal distortion. The microwave-assisted fabrication techniques—joining, surface coating and cladding are discussed separately below.

2.3.1 Joining Due to the inherent benefits of lower processing time and minimum energy requirements associated with microwave heating, attempts are being made to join materials of engineering importance using this approach. Initial trials were attempted by Siores and Rego on microwave joining of plastics, metals and ceramics using custom-built setups [49]. Since most of the ceramic and plastics couple with microwave, they do not require intermediate materials like bonding agents/priming agents. For instance, for the joining of plastics, focused microwave energy helps in softening the material at a seam region. Applications of light pressures promote diffusion resulting in the bonding of two plastic pieces. The challenge lies in the joining of bulk metals as they reflect the microwaves and possess a low dielectric loss. However, if the metal is reduced to powder form, it becomes responsive to microwaves. The size of the powder should be approximately equivalent to the skin depth. The skin depth could be calculated mathematically from the Eq. (2.4).  δ=

ρ π f μr μo

(2.4)

where, δ, f and ρ are skin depth (μm), frequency of microwaves (GHz) and resistivity of metal (μmm) respectively. μ is the magnetic permeability with μr and μo being relative and absolute permeability. Joining with metallic powders was initially attempted by Srinath et al. [14] and was exhibited for joining of 4 mm thick copper plate of commercial grade. The copper powder was placed in the region of the expected joint mixed with a certain type of epoxy resin. A hybrid microwave heating concept was adopted with charcoal powder as a susceptor to facilitate preliminary heating at the joint. Further, to avoid arcing of bulk metallic copper it was covered with refractory brick material. Graphite was placed in between charcoal powder and powder mixture of copper and epoxy resin. Under the exposure of microwaves, charcoal heats up rapidly and transfers the heat by conventional mode to the copper powder mixture. The metallic powder at elevated temperature starts coupling with

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Fig. 2.15 2D diagram representing microwave hybrid heating for metal joining [14]

microwave and it ultimately results in fusing of the joint. Figure 2.15 shows the line diagram of hybrid heating adopted for metal welding.

2.3.2 Surface Coatings and Claddings Prominent research work is being carried out in surface engineering to achieve tribological coatings with tailor-made properties on engineering material surfaces without altering the properties of the bulk material. The maximum reported catastrophic failure of the engineering substrates is probably due to oxidation, corrosion and wear which propagate from the surface. Hence, it is of prime importance to modify the surface by functional coating and extend the service life of the component. It can be achieved by conventional surface modification techniques like coating/cladding, carburizing, nitriding and cyaniding. However, coating/cladding is widely preferred via plasma spraying, chemical vapor deposition, thermal spraying, physical vapor deposition, laser and so on. These processes yield defects due to higher operating temperatures like thermal distortion, porosity and residual stresses, which are unavoidable. Further, the rapid cooling rate during solidification cause cracks. In recent years researchers have explored and arrived at an alternative and emerging technology named microwave coating and cladding process. It involves hybrid microwave heating with rapid material processing, greater deposition efficiency and reduces the level of porosity. A few of the research findings are discussed below. The feasibility of cladding CoMoCrSi on commercially pure Titanium grade-2 was attempted by hybrid microwave heating to improve the sliding wear resistance. During the process, several oxide phases were formed, such as Co3 O4 , MoO3 , MoO2 , Cr2 O3 and CoO. These oxide phases acted as a protective shield for the clad surface and resulted in a decrease in material loss during sliding conditions. X-ray diffraction pattern as represented in Fig. 2.16 depicts the presence of tribo-oxide films on

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Fig. 2.16 X-ray diffraction spectrum of CoMpCrSi microwave cladded worn surfaces at different test temperatures [50]

CoMoCrSi microwave cladded worn surfaces at different test temperatures [50]. Industries look for low cost and simple processing coating techniques. Among thermal spraying, flame spraying is one variant widely used. However, flame sprayed coating results in increased porosity up to 20%, poor cohesion between the splats and poor bonding with the material. Due to the presence of microcracks and micro porosities in the coated surface tend to debond, spalling and nucleate for corrosion. To overcome this problem Zafar and Sharma [51] adopted hybrid microwave heating as a post-processing technique for the flame sprayed Ni-based coating. The microwave post-processing effectively healed microcracks and micropores and resulted in a homogenized microstructure in the coated surface. Figure 2.17 illustrates

Fig. 2.17 Pictorial representation of densification mechanism including a flame-sprayed surface and b microwave post-processed surface [51]

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the densification mechanism involved in flame-sprayed surface and post-processing via microwave.

2.4 Synthesis of Special Purpose Materials Using Microwaves There is a huge requirement for microwave dielectric materials in the industries concerned with mobile multimedia systems, intelligent transport systems and ultrahigh-speed wireless to establish local area networks. Earlier, various wetchemical techniques were adopted in the synthesis of rare-earth aluminates. During the process, harsher synthetic materials, dangerous solvents and surfactants are used, which are not eco-friendly. Over the past decade, researchers have overcome the drawbacks of conventional reaction synthesis of ceramic materials by adopting microwave technology. The synthesis of nanomaterials by conventional methods is now assisted by microwave heating. This technique has benefits like rapid volumetric heating, increased reaction rates, energy-saving and enhanced production efficiency. The synthesis of such special purpose materials via microwave energy is elaborated in below sub-sections.

2.4.1 Reaction Synthesis of Ceramics Two or more mixed solid phases can be sintered with microwave energy to synthesis a single and stable ceramic compound. During the process, microwave develops a reaction mode between the constituent phases, which lead to reaction synthesis of ceramic material. Compared to conventional heating, microwaves offer the benefit of rapid and more effective heating, sintering and crystallization of ceramics. Wang et al. [52] defined the simplest method (marked as MCT1 in Fig. 2.18) for the solid-state reaction synthesis of MgTiO3 -CaTiO3 ceramics. The constituent powders MgO, CaCO3 and TiO2 were ball milled for four hours and treated at various temperatures (1200–1400 °C). Whereas in the traditional method (marked as MCT2 in Fig. 2.18) the major constituent powders MgTiO3 and CaTiO3 were synthesized separately and then mixed to prepare MgTiO3 –CaTiO3 crystalline phase. Both methods resulted in an increasing trend of grain size with increasing the sintering temperature. Further, the porosity level found to be decreased at 1300 °C with a bulk density of 3.83 g/cm3 . Among the two reaction pathways, MCT1 was preferred in the industrial production of MgTiO3 –CaTiO3 ceramics. Synthesis of Rare-earth aluminates via conventional processing techniques was associated with operating at a higher temperature for a long time. Researchers have explored microwave-assisted synthesis for faster reaction kinetics at a reduced time. Microwave-assisted synthesis of rare-earth aluminate ceramic particles has been

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Fig. 2.18 The process diagram of two reaction pathways MCT1 and MCT2 [52]

investigated. Here the increasing particle size distribution was the major factor that improved the flow rate [53].

2.4.2 Synthesis of Nanomaterials The material behavior at the nanoscale is quite different from both bulk and molecular levels. It can be ascribed to the increased number of surface atoms to the number of bulk atoms. As a result of this, the number of unsatisfied valence atoms in the nanoparticles increases. Further, the reactivity of powder dictates sintering kinetics and is attributed to the higher surface area of more reactive nanomaterials. It is a great challenge to sinter nanopowders and obtains a high density with retaining parental nanostructure in the sintered part. It is because nanopowders are highly reactive and tend to agglomerate and coarsen during sintering. In conventional sintering, the green compact part requires soaking at sintering temperature for several hours. It, in turn, results in coarsening and grain growth. However, microwave processing accomplishes sintering in a much shorter time and thereby minimizes further grain growth. Hence there exists a curiosity to study the superior properties encountered in the nanomaterials [54, 55]. Various microwave-assisted synthesis like combustion, hydrothermal and solgel methods have been reported in the synthesis of hierarchical and complex nanostructures. The property and application of the nanoparticles are dependent on their morphology [56]. The hydrothermal method associated with microwave technique found to be effective for the preparation of complex hierarchical

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Fig. 2.19 TEM images of CuO/TiO2 nanocomposite [58]

micro/nanostructured TiO2 . The material exhibits unique structural properties that find typical applications in photocatalytic degradation of water pollutants, dyesensitized solar cells, gas sensing, and lithium-ion batteries [57]. Ashok and Rao [58] prepared CuO/TiO2 nanocomposite via microwave-assisted synthesis for humidity sensor applications. The transmission electron microscopy revealed the formation of nanocomposite tubes highlighted in Fig. 2.19. The ionic liquid being at room temperature initiated the formation of nanostructured materials. The average diameter and average length of CT-5 was found to be 60 nm and 100 nm respectively. Whereas CT8 exhibited smallest diameter and longest length (30 and 800 nm). Hence the results indicate that material adsorption property is enhancing. Further, nano composite tubes possess free moving electrons and protons which increase conductance and corresponding sensitivity also increases.

2.5 Future Trends This chapter provides critical insights into material processing by microwave emphasizing on heating mechanisms and types of heating. Further, microwave processing of various materials, fabrication techniques and synthesis of special-purpose materials are discussed. Most of the work reported a drastic decline in cycle time

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and power consumption. The volumetric heating and faster reaction kinetics influenced the processed materials to gain superior quality in terms of reduced porosity, increased bulk density, phase purity, homogeneous microstructure and enhanced mechanical/functional properties. The mode of heating is said to be reinvented with the microwave energy. The use of microwave energy in various heating applications will dramatically reduce energy consumption, which in turn reduces the formation of hazardous gases and help in a cleaner environment. Microwave processing has opened new research avenues into the various fields like waste material processing/management, rubber vulcanization, food grains disinfestation, glass melting, metal casting and so on. The waste materials exist in the form of sludge, solid waste, medical waste and e-waste have become the bane of every town. Microwave processing of these materials has a lot of significant advantages over other methods. As rubber is a bad conductor of heat, any heating methods rely on its conductivity and score a lower conductivity point as compared to microwave heating. The volumetric heating of microwave enables faster heating rate. The use of insecticides as a disinfestation technique leads to severe health hazards. Microwave technology is found to be novel, effective and state-of-the-art technology to treat food grains in post-harvest management. The distinguished advantages are; chemical-free treatment, enhanced shelf life, unaltered food nutrients. Glass melting with microwave energy is found to be energy efficient with shorter processing time. In the microwave treated zinc borate and barium borosilicate, OH concentration was reduced effectively. It can be a potential area of study to minimize the OH concentration in the various glass matrix. The different glass matrix compositions melted in a microwave furnace resulted in lower volatilization loss. Hence a detailed study may be carried out to minimize the volatilization loss for different compositions at different temperatures. Controlled microwave processing can eliminate dendritic microstructure associated with conventional casting processes. Acknowledgements Authors are grateful to the affiliated institute and an industry partner—Enerzi Microwave Systems Private Limited, Belagavi, Karnataka, India for establishing the Center of Excellence for Industrial Microwave Application (The readers can refer to the website https:// www.enerzi.com for wide range microwave furnaces being manufactured to serve various engineering applications). One of the authors (Mr Padmakumar A. Bajakke) thank the Minority Welfare Department, Directorate of Minorities, Government of Karnataka, India, for granting fellowship vide registration No. DOM/FELLOWSHIP/CR-15/2018-19 for pursuing Ph.D.

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Part II

Applications

Chapter 3

Application of Edible Coatings and Packaging Materials for Preservation of Fruits-Vegetables D. Manojj, M. Yasasve, N. M. Hariharan, and R. Palanivel

Abstract Fruits and vegetables are particularly perishable commodities as they contain 80–90% of water by weight. Several methods have been employed to protect and increase the shelf life of fresh goods during packaging, transport and storage. Edible coatings are thin films made applied to the exterior surface of a substance, which offers protection against external moisture, oxygen and pathogens. The various components commonly used in the manufacture of edible coatings includes polysaccharides, proteins, lipids, composites and resins. The packaging of fresh fruits and vegetables is an essential step to protect against further contamination, damage and excess moisture loss. Bags, trays, sleeve packs, boxes, cartons and palletized containers are the generally used packaging materials for convenient handling and transportation of fresh products. Various types of films made of polyethylene, polyester, polyvinyl, cellulose and aluminum are currently used in packaging as moisture-resistant materials. Preservatives are compounds added to food substances to prevent the deterioration of quality and spoilage induced by the growth of micro-organisms or unwanted chemical changes from decomposing. The same is also incorporated in fruits-vegetables through physical and chemical modes which prolong the shelf-life of the product even further. The present work discusses the use of different edible coatings, preservatives and packing methods as carriers of functional ingredients on fresh fruits and vegetables to maximize their quality and shelf life. Furthermore, recent developments in the application of antimicrobials during packaging to increase the functionality of foods have been elaborated. Keywords Fruits · Vegetables · Edible coatings · Packaging · Preservation D. Manojj · N. M. Hariharan (B) Department of Biotechnology, Sree Sastha Institute of Engineering and Technology (Affiliated to Anna University), Chennai, Tamil Nadu 600123, India e-mail: [email protected] M. Yasasve Department of Biotechnology, Sri Venkateswara College of Engineering (Autonomous—Affiliated to Anna University), Sriperumbudur Tk., Tamil Nadu 602117, India R. Palanivel Department of Mechanical Engineering, Shaqra University, Dawadmi, Riyadh 11911, Saudi Arabia © Springer Nature Switzerland AG 2021 K. Kumar et al. (eds.), Coatings, Materials Forming, Machining and Tribology, https://doi.org/10.1007/978-3-030-62163-6_3

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3.1 Introduction Nutritional-Health is an important domain about technology, especially in the food industry. Fruits and vegetables play a crucial role in sustaining an individual’s health by supplying the necessary macro and micronutrients [1]. Thus, they are the most sought-after products in the market. Consumers are willing to taste new fruits and vegetables even without considering its quality which leads to nutritional loss in overtime and then it continues to decay. This is an important barrier in the food and agriculture industry [2]. Fruits and vegetables start to ripen and decompose as soon as they are cut from the plant at a faster rate, also this becomes a problem in the availability fresh-cut ready-to-eat fruits and vegetables that shorten their shelf life with a loss in nutrition, undesirable look and their palatability [3]. To cease this biological process of ripening after harvesting, many technological methods are used especially in terms of preservation like physical and chemical methods [4]. Also, packaging plays a vital role in the preservation of post-harvest products, which protects them not only from the external environmental exposure but also helps in slowing the ripening process. Several advancements have been made in packing industry with the emerging nanotechnology methodologies recently [5]. Packaging is an essential step because the harvested product is not the final that reaches the consumer. It needs to undergo a series of processing steps to reach the consumer as a finished product [6]. There is a high possibility for them to undergo degradation during these long steps. Thus, preservatives are added intentionally to increase their shelf life along with the packaging [7]. Over the last decade, edible coating as a packaging medium has gained a lot of momentum over reducing the enzymatic and non-enzymatic deterioration of food products such as browning and fruit softening, particularly after cutting. Wide ranges of formulation are incorporated with antimicrobial, anti-oxidant, flavouring agents and others to increase the nutritional content and the aesthetic appeal of the product [8, 9]. This work mainly elaborates on the use of different kinds of edible coatings, preservatives and packing materials used in the fruits-vegetables industry.

3.2 Edible Coatings and Films The edible coatings and films for the preservation of bio-products and other consumables that are easily perishable, has been in use for a very long time since 1990s [10, 11] and had undergone a lot of technological advancements in the recent decade. These coatings are made of very thin layers of biologically derived products that are edible [12] and it may include compounds like starch and starch-based composites, protein composites, and plant-based composites. Modern advancements combine nano-materials and other natural antimicrobial agents to control the microbial growth which causes damage [13]. The effectiveness of these films depends highly on their wettability, gas exchange property, stability and their ability to firmly adhere to the

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Fig. 3.1 Edible coating as a protective barrier

surface being coated [14]. The coatings are primarily used in increasing the shelf life of products, by creating a protective layer that acts as a barrier, forming a modified atmosphere. This barrier formation reduces susceptibility to degrading factors like oxidation, attack of pathogens, loss of moisture, and damage to surfaces as represented in Fig. 3.1. The common modification of these horticulture products has already been made in the market and has been greatly discussed like FreshSeal™, NatureSeal™, SemperFresh™ and others that greatly impact the transportation and preservation of the products [14, 15]. The coating for these products usually involves the techniques like immersion of products in large volumes of emulsions or air spraying in large scale and manual hand coating in small scale industries. In case of films, they are developed as thin membranes in industrial scales. Though these coatings provide a protective layer they essentially do not preserve the products, and it only act as an additional protective layer to increase the shelf life. This film formation technique works effectively only when it combines with the conventional preservation methods [16]. The different type of coatings from different derivatives is represented in Fig. 3.2.

3.2.1 Biomolecule Based Edible Coatings Biomolecules are complex compounds composed of proteins, carbohydrates, lipids and other natural products formed by living organisms [17]. These biomoleculebased derivatives can be classified into polysaccharides, protein complexes and lipid complexes. Polysaccharides have high hydrophilicity and proved a barrier against the drying.

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Fig. 3.2 Types of edible coatings

3.2.1.1

Polysaccharides

Polysaccharides are naturally occurring carbohydrate long chains, which are highly stable and easily degradable. Since these are a major part of human diet, they are edible and do not exhibit any toxicity. The plant-based carbohydrates are again classified into two broad groups as storage polysaccharides and cell wall polysaccharides [18].

3.2.1.2

Starch

This is an abundantly available storage complex of carbohydrate that is present in almost all the plant-based materials like tubers, legumes, and other vegetables. Starchbased coating forms transparent or a semi-opaque coating that does not contribute any taste, colour or odour [19]. When coated they maintain the integrity of fruits, carbon dioxide concentration, and reduce respiration and oxidation upon the surfaces. Also, it has a good oxygen barrier that is essential in increasing the shelf life [20]. Many starch derivatives include dextrin and pullulan, which are both used in the packaging industry and have demonstrated excellent results in food preservation [21]. It is the most economical and fastest way to make the coating edible. Commercial products include FreshSeal™ and Semperfresh™ are some of the examples that uses starch as their main ingredient [16]. An earlier study had reported that grapes coated with the blend of starch, gelatin and glycerol presented better appearance even after 3 weeks of storage under refrigerated conditions compared to control sample which is illustrated in Fig. 3.3 [22].

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Fig. 3.3 Processed red crimson grapes a control sample; b Coated with Modified Waxy corn starch and Gelatin plasticized with glycerol [22]

3.2.1.3

Cellulose and Pectin

Like starch, cellulose is a material that exists naturally and is often derived from plants. It forms the cell wall which makes up the skeleton of the plant cell. Similar to starch they are colourless, odourless and tasteless, but unlike starch and its derivatives, cellulose is water-insoluble, rigid and tough [23]. Due to these qualities, cellulose in its native form cannot be used as coating medium. Though it is time consuming and a non-cost-effective process, cellulose was pre-processed by dissolving it in certain chemical compounds earlier. Recent developments in processing of cellulose made the conventional method easier and compound derivatives like carboxymethyl cellulose, methylcellulose, hydroxypropyl methylcellulose, and hydroxypropyl cellulose are now used as edible coatings and film to protect the fruits and vegetables. Compared to the starch-based coatings, cellulose derivatives proved better protection against oils, water and fat corrosion with a considerable amount of rigidity and flexibility [24]. Some of the commercial examples for this incorporation include Prolong, Tal Prolong, etc. [16]. Pectin on the other is also a component of the cell walls of fruits and vegetables that can be water solubilized and re-casted into thin films upon solidification. Researchers have shown results that are similar to increase the shelf life of the products [25] (Fig. 3.4).

3.2.1.4

Carrageenan, Alginates and Agar

The compounds carrageenan, alginates and Agar are derived from seaweeds. Alginate is derived from brown seaweed, to obtain alginic acid, guluronic monomers and salts. It is dissolved and re-casted to form a very thin property less film upon drying. It forms a good barrier against oils, water vapour and other fat bases [26]. It also acts

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Fig. 3.4 Commercial polyethylene wax and carboxymethyl cellulose/chitosan bilayer edible coatings on mandarin orange [24]

as a good oxidation retardant. When cast as a wet film it acts as a sacrificial layer to the product that prevents drying and retains moisture to a certain extent [27]. Carrageenan is derived from red seaweed family, which is combined with several other compounds to enhance the preservation property [28]. Agar is also a similar compound as that of carrageenan derived from red seaweed is similar in properties. They are also mixed with other preservative compounds like antimicrobial agents to form a protective film [29] which in turn increases the shelf life of products.

3.2.2 Protein Derived Edible Coatings Compared to the polysaccharide-based edible coatings proteins are not much preferred due to their brittleness and poor properties. They are mainly chosen for their nutrition content along with their moderate preservative property [30]. Some of the protein-based edible coatings include Corn-zein, Casein, gluten and soy proteins.

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Fig. 3.5 Appearance of mushroom covered with Chitosan-zein film with various concentration [33]

3.2.2.1

Casein

Casein is a milk product of animal origin, used in the processing of foods. Owing to the presence of hydrophobic and hydrophilic ends, it is used as an emulsion base in the creation of edible films. The resultant film is less textured and clear and is extremely water-soluble. Owing to the weak properties of pure casein [31], a combination of compounds in the preparation of such films forms a composite that is generally favored.

3.2.2.2

Corn-Zein

Zein is a protein, and it is main derivative of labyrinths. For its extensive properties, corn zein is often favored to generate edible films as opposed to others. It has a rough, smooth texture and very strong adhesive properties. It also has a good oxygen barrier that prevents oxidation in easily perishable fruits and vegetables like tomato and apples. It is insoluble in water and thus it has to be processed with alcohol and glycols to dissolve and cast them into a film which prevents the products from oxidation of enzymes, microbial growth, senescence, and the loss of nutritional quality [32, 33] (Fig. 3.5).

3.2.2.3

Soy Proteins

Soy protein isolates (SPI) are derived from soybeans. It is added with some complex additives and plasticizers to enhance their properties. SPI are good oxidation retardants compared to other coatings, and also it has shown some effective preservation coatings on apricots [34].

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3.2.3 Lipid-Based Coatings Lipids are structural molecules of living cells, and it includes biomolecules like fats, oils and wax complexes. Lipid-based coatings have good water vapour barriers and are highly water repellent. This property aids in maintaining the product’s moisture content but it is brittle [35]. Some of the common lipid coatings are made of bee wax, carnauba wax, and resins.

3.2.3.1

Wax-Based Coatings

The carnauba wax is derived from the palm tree, which is used as an edible coating due to hydrophobic and water repellent nature that provides moisture loss. This coating is mostly used for fruits and certain vegetables. Bee wax is another alternative to carnauba produced by bees. It is used in combination with other coatings to preserve the integrity of the fruits [36, 37].

3.2.3.2

Resin-Based Coating

Shellac wax is another compound similar to bee wax that comes under the category of resins produced by lac beetle that has a similar property to bee wax. It is glossier but is not deemed safe for consumption as it requires pre-processing steps to make it edible [38] (Fig. 3.6).

3.2.4 Other Alternatives in Edible Coatings 3.2.4.1

Herbal Coatings

Alovera gel is yet another recently explored edible coating that provides good moisture retention and anti-microbial property. Other herbal coatings from turmeric, tulsi, garlic, ginger, neem are being reviewed [39]. These herbal based coatings are good in retarding microbial growth compared to other edible films and coatings. Herbal coatings are environmentally benign and it is simple to prepare and process. Another notable quality of herbal coating is rich nutritional content along with the medicinal properties that are being coated on the product. The main drawback of this coating is that they do alter or add some flavours which are inherent [40].

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Fig. 3.6 Effects of commercial waxes and synthesized organoclay-carnauba wax emulsion coatings on the visual appearance of orange fruit [37]

3.2.4.2

Essential Oils

Essential oils are derivatives of plants that possess high antimicrobial, antioxidant and other medicinal properties. Due to these properties, these are extensively used in food preservation. The essential oils contain a phenolic compound that renders antioxidant activity. Similarly, the presence of volatile compounds in the oils provides antimicrobial properties against a variety of pathogens [41]. Some of the common essential oils used in preservation includes clove, lemon, sage, rosemary and others.

3.2.5 Need for Edible Coatings Edible coatings do extend the shelf life of the food products to an equal range to that of synthetic coatings. Comparatively, these are much more environment friendly and less expensive to produce on a larger scale basis. Moreover, they do not pose any health hazards which makes them a popular choice. The products are allowed to consume along with the edible coatings as it has some nutritive value which is quite advantageous.

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3.2.6 Challenges in Creating Edible Coatings The key consideration of developing edible coating is to increase the adhesive properties of the coatings, particularly to prevent the loss of moisture content on the surface of freshly cut. In most cases, edible coatings are known for their hydrophobic nature, repelling water surfaces that lead to poor control of moisture and oxygen barriers. Most of the coatings are not robust enough to withstand much tension. It also deals with the wettability of the coating sheet. Without proper wettability and adhesivity, the products being coated will not provide a good shelf life. Another important aspect is to impart the texture quality and taste quality to the coatings that makes them more aesthetically pleasing to the consumers [42]. New technologies like nanoemulsions are being included in the mix, that raises concern over the cytotoxicity and biomagnifications in the body. Despite all this, edible coatings are good and economic alternative to synthetic and plastic packing technologies.

3.3 Packaging Materials 3.3.1 Need for Packaging Materials The main aim of the packaging and preservation material is to increase the shelf life and transport of the product to the customer in pristine condition for consumption. The ideal challenge is to make sure that the fruits and vegetables are protected from the external damage through bruising in transport and handling, contamination, relative humidity, and reducing moisture loss [43]. Since the coating materials only contribute to a certain extent thus, packing plays a vital role in the preservation of the quality and the appearance of the fruits. Temperature and airflow control are yet another domain for serious consideration in the packing since the process of ripening depends on it and also there is the release of volatile vapours and chemical compounds that make it undesirable for consumption if not handled properly [44, 45]. Therefore, the packing of products must be done with utmost care and foresight.

3.3.2 Types of Packaging Materials The type of packaging entirely depends on the purpose it is about to serve and the product involved. Simple and effective packing like using a cloth or polythene bag is sufficient for small quantity and also for immediate consumption rather than storing. But in case of long-distance transport, and for storing larger quantity this technique is not feasible [46]. Similarly, easily perishable fruits and vegetables can’t be transported using very hard packing material. The different types of packaging materials currently used are listed in Fig. 3.7.

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Fig. 3.7 Types of packaging materials

3.3.2.1

Consumer Packing

These involve the category of primary packing and secondary packing, also known as unit packing. This is the easiest and most reliable way to prepare items for immediate distribution. Often involves the way of packing that the consumer is finally given with. It is the most cost-effective and mass-produced materials for common household usage are packed with and it is easily disposed. Bags This forms the primary packing category. Bags may be made of either plastic (polythene) or plant-based (cloth, paper and others) materials. High-grade polythene bags are used for refrigeration purposes where the product needs to be kept in a cold environment. This provides two functions; one is to give a little thermal resistance and the other as storing for pre-processed products. The best example is a bag of fresh frozen peas or corn [47]. Low-grade polythene bags with ventilation holes, cloth and paper-based bags are mainly used for short term transport purpose [48]. This also includes the netted bags that offer ventilation to the produce like onions and potatoes. Trays and Sleeve Packing The main purpose of trays and sleeve packing is for display, short transport and storage that falls under the secondary packing category. Here the functionality is to immobilize the produce with a transparent or an open face for the consumer to view with at the same time providing a packing solution [49]. The trays are made of polythene, polyvinyl chloride, cardboard, and other materials with an open face on one side that is wrapped with stretch wrap or a shrink wrap that forms the secondary packing category along with the sleeves. In a sleeve packing an additional layer is covered with a box-like design that slides out which provides strength and integrity to the immobilized product. Even in this design, ventilation holes are present depending on the product’s breathing needs [50]. Examples include a tray of dry plumes, baby corn and others with a tight wrap.

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Clam Shell Packing Clamshell packing is named after its design structure that is made of a single unit with two halves just like a clamshell. This method of packing is the modification of tray and sleeve packing which is more secure and tamper-proof comparatively. The purpose and function of clamshell is same as that of the tray and sleeve packing and even made out of similar materials. A greater advantage in this type of packing is that all sides of container can be made visible or transparent for viewing by the consumer if made by PVC materials. This is primarily used for berries and other small fruits-vegetables. In case of paper and cardboard, they are used to pack ready-to-eat consumer items, often in the fast-food chain [51, 52]. Tin Boxes and Cans Tin Boxes and Cans are the common choice in packing food products that provide a very long shelf-life with only one storage option necessary. These have a solid protective shield for the goods within them, along with a changed atmosphere for longer protection of the substance that is not exposed to the external environment [53].

3.3.2.2

Transport Packing

This is a tertiary level of packing that mainly focuses on the transport of large quantities of the unit product for longer distances and time. These are large and sturdy compared to the primary and secondary packing materials. The transport packing category includes the following: Wooden Boxes Pre-processed and thoroughly dried plywood and other types of wood materials are used in this type of packing that usually requires a heavy-duty protection and impact resistance for a substantial load both in terms of quantity and weight for having a long-term storing purpose. These are mostly preferred for products that can withstand a lot of surface friction and aberration from tight packing with a longer shelf life like in case of coconuts [54]. Corrugated Boxes Corrugated boxes are made of thin sheet materials that are lined over each other with air pockets that provide a little bit of flexibility and cushioning to the products placed within them. These are used for transport and storing for a short duration of time in days that do not require heavy-duty protection. (i) Fibreboards and Cardboards: These are made of plywood, wood pulp and wood fibres that are comparatively less strong than wooden boxes and provide a little bit of flexibility. These are used again for long transport with a longer time for a commodity that is less rigid, especially in the case of fruits like apples. These packing materials has

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a relatively less moisture absorbance capacity to a certain extent. Despite their flexibility and corrugation for cushioning, an additional layer of cushioning is required for each unit as the box can’t withstand impacts or heavy loads [55, 56]. (ii) Plastic Corrugated Boxes: These are used as a replacement for cardboard and fibreboards for their ability to be reused multiple times and their cost-efficiency. They also have a low wright density compared to the fibre or cardboard but offer a higher mechanical strength comparatively. Also, they have excellent properties in terms of water resistance [57]. Plastic Crates Plastic Crates are the new alternative for wooden crates to transport the products in larger quantities easily. These are made of high-density polyethylene that provides fairly similar resistance property to that of wooden boxes. In some cases, the crates are even specifically designed as a replacement for the wooden boxes as these have higher reusability and cost-effectiveness. But unlike wooden boxes, they can’t be dismantled to make alterations as they are made out of mold castings [58]. Sacks Sacks are made of jute and woven plastic and are mostly used in the food industry for the transportation of raw produces, especially in vegetables that have relative resistance to aberrations like in case of potatoes, carrots and others. These are highly cost-effective, less space occupying and easily disposable packings compared to the other large packing materials [59]. Palletization and Unitization These are strategic packing options that are used in bulk transportation which involves machine handling to reduce cost. Unitization is the process in which multiple small units of the product are bulked over each other usually in large crates or boxes and are secured with reinforcements [60]. When higher quantities of unitized products are required for shipment, palletization is used. Palletization is the process of stacking large quantities of Unitized units over a pallet (a wooden framework) with reinforcers to hold them in place that has space provision for the use of mechanical handles in carrying them. This greatly reduces the requirement for larger spaces, organizing and sorting process in bulk transport. These techniques are followed in air or ship transport [61].

3.3.2.3

Active Packing

Active packing is a new technological innovation that can interact with the modified atmosphere inside the packing by altering the inner composition of gases like oxygen, carbon dioxide, and ethylene [62]. The mechanism depends on the coating provided or the packing with an additional material which has the property of interacting with

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Fig. 3.8 Use of nanotechnology in active packaging [63]

a particular portion. For example, using desiccants like silica gels or zeolite inside a packing helps in moisture control. Others include metal chelators and oxygen scavengers that are used in extending the shelf life of sensitive products [63] (Fig. 3.8).

3.3.2.4

Anti-microbial Packing

This is a form of active packaging that includes, coating the packaging material with an antimicrobial agent to minimize the growth of microbes and has the potential to prevent food spoilage [64]. This packing methodology is under study and experimentation is required for the deeper understanding of the toxicity and diffusivity of agents into the products being packed with like in the case of coating with nanomaterials [65]. Popular methods like modifying the atmosphere with alternate gases like carbon and sulfur dioxide in fruit packings or nitrogen flushing have helped in reducing the growth of microorganisms [66].

3.3.2.5

Other Packaging Materials

Cushioning materials are used in protecting the bulk products when packed in a crate or containers from aberrations caused due to excessive movements or pressure. The natural materials used for cushioning includes paddy straw, paper shreds, and coir

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(coconut fibres) [67]. Synthetic options like foam nets, bubble wraps and cardboard liners are also used. Vacuum sealed bags are another popular option in both homes and industries. These are devoid of oxygen and it prevents the growth of microorganisms. It works even better when the products are stored under refrigerated conditions [68]. Though this may provide substantial protection against most of the microorganisms, anaerobic microorganisms may grow under these conditions [69].

3.4 Chemical Preservatives Preservatives are the substances that delay the growth of microbes without necessarily destroying the nutrients or prevent the reduction in the quality during manufacture and distribution. They are naturally occurring or synthetic substance which is added to the products such as foods, biological samples, etc., to prevent decomposition caused by microbial growth or undesirable chemical reactions [70, 71].

3.4.1 Traditional Food Preservatives Sugar and salt are the widely used preservatives added to the edible food-stuffs from ancient times. Salt is applied or added directly to the food which increases the osmotic pressure levels, thus preventing the growth of microorganisms. Salt will induce dehydration by absorbing the food’s main water content; therefore, it inhibits microbial multiplication. Also, ionized salt-producing chloride ions will interfere the action of proteolytic enzymes produced by the harmful food microbes [72]. The sugar preservation process is measured by the ratio between the overall quantity of sugar in the finished product and the concentration of sugar in the liquid form. Sixty percent of sugar concentrations in the finished product guarantee food preservation. The water activity in the food material is reduced due to the addition of sugar; thus, inhibiting the growth of bacteria and yeast [73]. Packaged fruit products (Example: citrus and angelica) are usually cooked to the point of crystallization in sugar, and the resulting product is then stored dry.

3.4.2 Acidulants Benzoic acid belongs to the sodium salt group, which is one of the most commonly used chemical food preservative. Sodium benzoate is an antimicrobial preservative that is added to acidified foods, such as fruit juices, sauerkraut, pickles, etc., which mainly prevents the yeast growth. Sodium benzoate is carcinogenic and has also been liable for aggravating the health problem of asthma patients which leads to further complications [74].

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In general, sorbic acids are added to the food at low quantities which contributes to regulate antimicrobial activity. The World Health Organization (WHO) suggests sorbic acid as a non-toxic food preservative found in a wide range of fruit-vegetable products for the prevention of yeast/mould growth [75]. Furthermore, lactic acid produced in food fermentations reduces the pH levels, contributing to unfavourable conditions for the development of spoilage microbes such as putrefactive anaerobes and acid-producing bacteria [76].

3.4.3 Gaseous Food Preservatives Sulphur dioxide (SO2 ) and sulphites are chemical preservatives added to fruit juices for controlling the microbial growth. The different dissolved sulphites in water contain 50–65% active sulphur dioxide, which reduces the pH values contributing to maximum antimicrobial activity [77]. Besides, sulphur-containing preservatives inhibit enzymatic browning and keep products fresher for a longer period. In recent years, the authority of Food and Drug Administration (FDA) has prohibited the use of sulphur preservatives because of the serious side effects caused to many consumers [78]. In many countries, carbon dioxide (CO2 ) also known as “dry ice” is used as an additive for the storage and transport of food products at low temperatures. Gaseous carbon dioxide generally inhibits the growth of psychrotrophic microorganisms and prevents fruit and vegetable spoilage. An artificial environment consisting of the right proportion of oxygen-carbon dioxide delays ripening of fruits as well as retarding mould and yeast growth [79].

3.4.4 Antioxidants In the prevention of rancidities in foods and fats, antioxidants are beneficial. Fats that are exposed to light, moisture, heat or heavy metal ions become activated and oxidize to peroxide. The widely used antioxidants are Butylated Hydroxy Anisole (BHA), Butylated Hydroxy Toluene (BHT), propyl gallate, Natural/Synthetic tocopherols (Vitamin E) ascorbic acid (vitamin C) and lecithin [80, 81].

3.4.5 Flavour Additives Natural flavours of food are hardly used because the methods to obtain the quantities needed are expensive. Flavour consistency or chemical composition cannot be standardized and depends on seasonal availability. Therefore, the artificial flavours are required if the demand for flavouring agents in our food supply needs to be

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fulfilled. The most widely used flavouring agents are esters-c pentyl acetate which has a banana flavor, and aldehyde such as benzaldehyde which is cherry-flavored [82, 83].

3.4.6 Sweeteners Sweeteners may be categorized as nutritious or non-nutritious to improve the flavor of certain foods. Nutritive sweeteners contain calories because they are metabolized by the body to produce energy [Example: sucrose, glucose (dextrose, fructose, invert sugar and high fructose syrup)]. Non-nutritious sweeteners like carbohydrates do not provide calories because they aren’t metabolized. Aspartame is categorized as non-nutritional, but metabolic since the amounts of aspartame alone are exceedingly low and aspartame is weeded to produce sugar that is similar to sucrose [84, 85].

3.5 Conclusion A new era of edible coatings is under development, which aims to incorporate, integrate and/or monitor the release of active compounds using nanotechnological solutions such as nanoencapsulation and multi-layers in a new generation of comestible coating products. Nano-technologies are currently used to improve food nutrition by using nanoscale additives, nutrients and bioactive compound delivery systems. The vision for the future of packaging is one in which the material can eventually act as an integrated device that combines both modern and traditional products, across the supply chain.

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Chapter 4

Corrosion Resistance of High Entropy Alloys K. Ram Mohan Rao

Abstract High Entropy Alloys (HEA) composed of at least four major elements are the non-conventional alloys exhibiting exotic properties due to their unusual design/structural constitution. Hence these alloys have drawn the attention of industries for its application as aerospace, nuclear materials. In this presentation the formation of high entropy alloys and their corrosion resistance properties are focused. Corrosion is the destructive phenomena of materials which is responsible for the loss of materials worth billions of dollars. Hence, the design of materials with highly corrosion resistance property is warranted. The unusual corrosion resistance properties of HEAs might be due to the local distortion/disordered chemical environment. The deposition of HEAs with their superior corrosion resistance onto the surface of the materials leads to for the formation of corrosion resistant layer. Thermal spray technology is a cost-effective and efficient technology currently receiving much attention for the deposition of HEAs for the corrosion/oxidation resistance properties. This article is focused on first the HEAs basics, effects of various alloying elements and the HEA materials with enhanced corrosion resistance. Finally, various HEA coatings deposited by a versatile thermal spray technology for the corrosion/oxidation resistance were summarized. Keywords Coatings · Corrosion · High entropy alloy · Microstructure · Potentiodynamic polarization · Thermal spray

4.1 Introduction The traditional alloys contain mainly one or two constituting elements which impart the desirable properties whereas the trace elements inclusion tunes the desired structure and enhance the desired properties [1, 2]. Simply on the basis of the principal component, one can talk about the family of the alloy system. For example, steel is the alloy containing iron as the principal component and other alloying additions K. Ram Mohan Rao (B) Department of Chemistry, GITAM Institute of Science, GITAM (Deemed to be University), Visakhapatnam, Andhra Pradesh, India e-mail: [email protected] © Springer Nature Switzerland AG 2021 K. Kumar et al. (eds.), Coatings, Materials Forming, Machining and Tribology, https://doi.org/10.1007/978-3-030-62163-6_4

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of several tenth percentages of elements like carbon, manganese, chromium etc. A new approach had broken this conventional trend of designing the alloys which was proposed by a group of workers who introduced the new concept of designing the alloys with exotic properties [3–7]. Professor Brian Cantor and Jien-Wei Yeh worked on these alloys independently and published the work on multiprincipal alloys containing with nearly equiatomic compositions (5–35 at.%) [8–10]. Yeh given the name ‘High Entropy Alloys (HEA)’ to these new type of alloys because the configurational entropy of the system stabilizing the solid solution in the melt increases with the increase in the number of principal elements [11]. Miracle et al. found that the contribution of the configurational entropy Sconf. is maximum in the high entropy of mixing Smix and hence on this basis defined these alloys as High Entropy Alloys [6]. HEAs are new type of alloys with potential use in structural applications. In the last 10 years around 600 papers were published on HEAs, however, mostly studies were related to mechanical properties. The present paper will be focussed on the background of HEAs and their corrosion resistance properties. From the knowledge of physical metallurgy the binary and ternary phase diagrams may predict a large number of phases (ordered phases, complex intermetallics, Lave phases etc.) and simple ordered bcc, fcc, hcp phases for the multiple element alloy systems leading to a complex system with brittle nature. This would probably be difficult for the analysis of the structure and also would have very limited applications. However, the contribution of high Sconf. value in the solid solution formation of these multiple element alloy system will reduce the number of phases. Because of the alloy formed from the multiple principal elements properties of these HEAs are special. These alloys possess exceptionally high resistance to wear, corrosion and oxidation and also the high temperature strength and stability. Some non-conventional properties exhibited by them draw the attention of the scientists, engineers and industries. So far around thirty elements have been used to produce more than three hundred alloy systems thus exciting for the material scientists.

4.2 Background of HEAs Yeh et al. in their effort to find the multi principal alloys found less than the expected disordered solid solutions and the formed compounds [3]. It was suggested that this could be due to the contribution of high entropy of mixing. Based on the lowest Gibbs free energy the equilibrium phase formation for some alloys system is known to follow the relation as: Gmi x = Hmi x. −TSmi x

(4.1)

(where H is the enthalpy, T is the temperature and S is the entropy). It is obvious from Eq. (4.1) that the phase formation is affected by the entropy. The disordered phases with high configurational entropy are more dominating than the ordered phases with

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lower entropy. Thus, negative enthalpies of mixing and also at the higher temperatures some of the ordered phases do not appear in the phase formation. This Scon f. can be given by Eq. (4.2) as: Scon f. = −R

n 

Xi ln(Xi)

(4.2)

i

(where R: gas constant; Xi: fraction of atom of the ith element and n: total number of constituting elements). Configurational entropy: S = R ln(n) which is equal to 1.61 R where n = 5 (for equimolar alloy with same concentration of elements). Miracle et al. [6] suggested the configurational entropy of HEAs can be given by Sconf. ≥ 1.5 R which does not include possibilities of the phase formation otherwise for 5 elements system it would have been 1.36 R. Phase Formation in HEAs From Hume–Rothery rules for low-entropy alloys the parameters controlling the solid solution formation; atomic size difference, electronegativity, electron concentration, several parameters like enthalpy of mixing and configurationally entropy along with the atomic size differences δ have been identified [12–14]. From Eq. (4.2) entropy can be calculated and the enthalpy of mixing from Eq. (4.3) given as: Hmix = −R

n 

ωijXiXj ,

(4.3)

i=1, i=j

where, ωij = 4Hmix; I and j represents melt-interaction parameter and enthalpy of mixing of binary alloys HmixAB . The difference of atomic size can be obtain by following Eq. (4.3) given as below   N   ri 2 δ = 100 Xi 1 − ¯r i=1

(4.4)

(where, 100: factor for numerical clarity, ri : atomic radius of element I; and ¯r weighted average atomic radius of the elements in the alloy system). Rein et al. [15] drawn a plot between Hmix and δ for some alloy system as represented in Fig. 4.1. It should be follow the trend −22 ≤ Hmix ≤ 7 kJ/mol, δ ≤ 8.5 and 11 ≤ Sconf. ≤ 19.5 J (mol K)−1 for solid solution formation [13, 16]. This condition was suggested to be quite reasonable by Tsai and Yeh [17]. When enthalpy is too large negative the intermetallic phases can be formed and too large positive value results in the phase separation. If the difference of size of the atoms is large it will increase the stain energy thus the structures can be destabilized. The large configurational entropy is finally playing the role and in these conditions ordered phases of simple bcc, fcc and hcp solid solutions may be produced with and no complex intermetallics.

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Fig. 4.1 δ versus Hmix in some high entropy alloys (alloy with only solid solution represented by S; and with intermetallics by C; S1-S18 [15]; 9–14 [8], 9: CrFeCoNiAlCu0.25 , 10: VCuFeCoNi, 11: Al0.5 CrFeCoNi, 12: Ti2 CrCuFeCoNi, 13: AlTiVYZr, 14: ZrTiVCuNiBe [15]

The trend −15 ≤ Hmix ≤ 5 kJ/mol, δ ≤ 4.3 and 12 ≤ Sconf. ≤ 17.5 J (mol K)−1 is followed by the disordered phases [13, 17]. Yang and Zhang [18] presented the criterion of formation of simple phase in HEAs based on the competition between enthalpy and entropy. As suggested by them:  = Tm Smix/Hmix

(4.5)

(where, Tm : melting temperature (weighted average) of the elements in the alloy). Larger value of  indicates the formation of simple disordered phase. If,  = 1.1 and less than 6.6 the difference in the size of the atoms there is the possibility of formation of simple disordered phases. However, Tsai and Yeh [17] yet to propose a modified version with a new parameter of the formation of simple disordered phases. Guo et al. [16] had shown the possibility of formation of bcc or fcc and the phase stability directly dependent on the Valence Electron Concentration (VEC). It was shown that, the stable bcc phase formed if VEC is less than 6.87 and stable fcc phase formed for VEC greater than 8 was. Both the phases can be formed if it lies between this range. These can be represented as shown in Fig. 4.2.

4.3 Corrosion Resistance of HEAs Corrosion of materials has been a subject of serious concern and the researchers have been attentive towards identifying the corrosion problems and its improvement. In industrial sectors, materials degrade due to various reasons during the service life of products and equipment. The corrosion of materials causes a huge economic loss in failure and replacement of materials in service. In 2013, US alone born the corrosion related loss of $1 trillion which in terms of gross domestic product i.e. loss incurred around 6.2% of the GDP. Estimated loss at the Global level costs around

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Fig. 4.2 VEC versus fcc, bcc phase stability for HEA systems (fully closed symbols: fcc phase, fully open symbols for bcc phase; top-half closed symbols for mixes fcc and bcc phases [15]

3% of the global GDP [19]. Improvement of corrosion resistance of materials would greatly reduce the cost of corrosion related materials loss and hence the economical loss for which researchers always look for the development of corrosion resistance materials and surface modifications by alloying or deposition of layers on the surface of the materials to extend the life span of the materials. A host of literature exists on the corrosion resistance of materials largely based on stainless steel, Ni-, Tibased alloy. In these alloys low concentrations of elements like Cr, Ni, W, N, Mo etc. have been added for the improvement of corrosion resistance. The enhancement of corrosion resistance of these alloys is largely due to the formation of protective oxide layer/adherent oxide layer which minimizes the corrosion of the materials. Various industries food and chemical, nuclear and waste containment, automobile and aerospace have been benefitted by using these corrosion resistance alloys and have been in a great demand. Stainless steel and some other alloys have been studied extensively and formulated certain rules for the development of corrosion resistant alloys. For example, PREN (pitting resistance number) which gives the idea for the selection of stainless steel for the application where resistance to pitting corrosion is required. PREN is related to the alloying elements [20–23] which can be represented as: P R E N = wt.% + 1.6 wt.% + 3.3 wt.% + 16 wt.% N

(4.6)

However, the studies on corrosion resistance of all possible alloying elements are yet to be studied to understand the corrosion resistance of HEAs. The alloying elements which form the protective layer like W, Cr, Ti etc. may significantly minimize the corrosion of HEAs. But yet the large compositional variations of corrosion resisting elements make the HEAs different from conventional alloys. The properties of HEAs are also based on the interactions of these compositional elements in HEAs [17].

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Chen et al. [24] worked on HEA CuNiAlCoCrFeSi and compared with AISI 304 stainless steel. It was reported that the HEA shows more noble behavior than the individual elements. HEAs show unique corrosion resistance properties which make them use for corrosion resistant coatings. The corrosion resistant elements like Ni, Cr, etc. render these alloys resistant to corrosion which depends on the amount and distribution of these elements and also on the presence of the galvanic cells. The random arrangement of multiple elements in the solid solution in HEAs causes a chemical environment which is locally disordered which may be attributed to the corrosionresistance properties [25]. Earlier studies mostly in the last decade were made on the corrosion behavior of HEAs in various aqueous environments e.g., acid, salt water, high-temperature and high-pressure water [24, 26–30]. HEAs containing chomium, nickel, molybdenum etc. had shown equivalent or superior resistance to corrosion in comparison to conventional alloys [24, 26]. Apart from superior corrosion resistance properties the other properties of HEAs like combined noble strength-ductility [31, 32], superior resistance to fatigue [33–35], improved fracture toughness [36, 37] and high thermal stability [38] make these alloys highly demanding in various industrial sectors as superior structural alloys suitable to extreme service conditions like nuclear, turbine and aerospace industries. It was realized that the synthesis of HEAs by arc melting, casting is more expensive than the deposition of HEAs on the materials surface. Thus, the low cost of HEAs coating on material surface to obtain corrosion resistant materials is very advantageous. It has already been reported that the HEAs layer deposited on various steels, Al alloys and Si by following laser cladding [39–42], electro-spark deposition [43], magnetron sputtering [44–46]. The deposited layer of HEAs acts as the stable barrier layer by forming the passive films which prevents the ingress of corrosives and thus minimizes the electrochemical reactions underneath the deposited layer. On the other hand, superior mechanical properties with novel corrosion resistance of the deposited layer of HEAs confer a significantly high resistance to cavitation-erosion in salt water [47]. Similarly, HEA layer with combined irradiation and corrosion resistance properties finds the application in nuclear fuel and also for the application in the high pressure vessels [47]. Zhang et al. [47] had already proved that the HEA layer with the composition similar to the bulk material shows even better corrosion resistance. It was suggested that the more homogeneous microstructure of the layer might be responsible for a better corrosion resistance.

4.3.1 Corrosion Resistance of HEAs in Chloride Environment The corrosion property of the materials is commonly assessed by following potentiodynamic polarization tests in a given electrolyte. First the corrosion parameters are known and then assessed the corrosion behavior of the materials. The corrosion

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Fig. 4.3 Represents a typical potentiodynamic polarization curve which shows the corrosion parameters (icorr , E corr , ipass , E pit ), passivation area

parameters Icorr (corrosion current density), Ecorr (corrosion potential), ipass (passive current density), Epit (pitting potential), Eb (breakdown potential) can be calculated from the potentiodynamic polarization tests of the materials. Figure 4.3 is the typical polarization curve obtained from the corrosion test performed in the 3.5% NaCl solution [48]. From the icorr value the average corrosion rate can be calculated by following equation Corrosion rate(mm/year) = 3.27 × 10−3 × i corr /ρ × EW

(4.7)

(where; ρ: the density of the alloy (in g/cm3 ), icorr (in A/cm2 ): corrosion current density and EW: the equivalent weight of the alloy. EW can be calculated by using the equation EW = (Σn i fi /Wi )−1 ;

(4.8)

(where; ni : valence of the ith element, fi : mass fraction of the ith element and Wi : atomic wt. of the ith element in the alloy). To understand corrosion resistance behavior, HEAs were subjected to corrosion tests and compared with the conventional alloys of similar compositions. The studies were performed in chloride containing electrolytes. Chou et al. worked on Co1.5 CrFeNi1.5 Ti0.5 Mo0.1 HEA to understand the effect of sulfate ion in various NaCl concentrations and also in 1 M NaCl + 0.1 M NaNO3 chloride solutions and determined the Epit and critical pitting temperature (CPT) [49]. The CPT was found to be 70, 60 and 60 °C in 0.1, 0.5, and 1 mol/L NaCl respectively. Epit was found to linearly related to the concentration of chloride at logarithmic scale at temperatures

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70 and 80 °C. Effect of sulphate ions added in the chloride solutions was found to be positive for both these parameters where the SO4 2− /Cl− was greater than 0.5. Lee et al. [50] worked on Alx CrFe1.5 MnNi0.5 HEAs and shown the effect of Al addition on corrosion resistance by following the laboratory test in NaCl solution (1 M) by following potentiodynamic polarization. It was shown that the Al addition lowered the Epit values significantly when compared to the CrFe1.5 MnNi0.5 alloy with no Al. The cyclic polarization test had shown that the Al free CrFe1.5 MnNi0.5 alloy has no tendency to undergo pitting; in contrast to that the alloy with Al was found to be susceptible to pitting in an electrolyte containing Cl− ion. The tendency to pitting increased with the increase in the Al content. It was suggested that the increase in Al and hence decrease in Cr content leads to the formation of porous oxide film which causes severe corrosion by allowing the Cl− ion to pass through the oxide film to the substrate surface. What happens when the material is in contact with the Cl− containing electrolyte is that the Cl− adsorbs on the passive layer. The adsorbed chloride anions then pass through the oxide films. If the passivating elements are present in the sufficient concentration a protective adherent oxide film forms which acts as the barrier layer and prevent the corrosives to penetrate through the layer. Hence, the passive layer formation and its protective property depend on the alloying element. Whether the alloying element forms a good adherent oxide layer and forms the protective layer or forms a porous oxide layer to allow the corrosives to pass through the layer to reach to the surface of the material whereupon causing corrosive degradation. It has been found that the Al addition in the Alx CrFe1.5 MnNi0.5 alloy alters the FCC and FeCr (CrFe1.5 MnNi0.5 ) microstructure to FCC along with the BCC microstructures of (Al0.3 CrFe1.5 MnNi0.5 ) [31, 51]. The BCC phase is Al–Ni enriched with the depletion of Cr resulting in elemental segregation. The dominating BCC phase with depleted Cr forms less protective layer and hence reduces the corrosion resistance when compared with the FCC phase. Similar is the case with the Alx CoCrFeNi HEAs which on addition of Al looses the corrosion resistance property [52]. With the increase in Al concentration the porous oxide layer forms in the sulfuric acid. When the Cl− anions come in contact with the surface of the materials it adsorb on the oxide layer which then dissolve following the formation of metal complex in sulfuric acid as:  − [Al (SO4 )] + Al + SO2− 4 = [Al (SO4 )] + 3e ;

(4.9)

and/or  − − Al (OH) SO4 Al + SO2− 4 + OH = Al(OH)SO4 + 3e

(4.10)

With the application of external potential, the inner ion complexes move toward the passive layer/electrolyte interface where Cl− cannot form the passive layer rather the complex formation and hence dissolution continues. As the addition of Al forms the porous oxide layer, it is required to be improved by forming a protective oxide layer. This may be achieved by anodic treatment such as immersion of sample in

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the 15 wt.% H2 SO4 and keeping in the passive region to form a stable passive film which lowers the current density. Thus, a stable passive oxide layer so formed may have better protectiveness. Lee et al. [53] reported the enhancement of corrosion resistance property by following anodic treatment and thus improvement in the property of passive layer. Hsu et al. studied on the effects on corrosion behaviour after Cu addition to FeCoNiCux alloy in 3.5 wt.% NaCl solution [54]. In this study FeCoNiCrCu0.5 and FeCoNiCrCu was immeresed in the electrolyte for 30 days which had shown the occurence of the localized corrosion (Fig. 4.4). The attack in these HEAs was found alongside the interdendrites due to segregation of Cu which was caused by the weaker binding force. This might have caused by the enthalpy of mixing with other elements which are Fe, Co, Ni and Cr alloying elements [55]. Hence, the Cu enriched and Cr depleted interdendrites couple as anode and cathode respectively and thus form a galvanic coupling leading to galvanic corrosion. The Mo addition to Co1.5 CrFeNi1.5 MoTi0.5 Mox HEAs and its effects on corrosion response in 1 N NaCl electrolyte was presented by Chou et al. [56]. For the enhancement of corrosion resistance of stainless steel Mo addition is already proved to be an

Fig. 4.4 Surface morphology of HEAs a Fe Co Ni Cr, b Fe Co NiCrCu0.5 ,c FeCoNiCrCu and d 304L stainless steel after immersion in 3.5 wt% NaCl electrolyte for 30 days [54]. Arrows indicate localized attack at interdendritic sites

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important alloying element. Mo forms its oxide MoO4 2− and adsorb on the surface layer which acts as the barrier layer for the corrosive attack. Cyclic polarization test had shown that the Mo free alloy is prone to pitting corrosion than the Mo added alloys. But the presence of Cr in these alloys forms Cr enriched σ phase hence Cr depleted sites become corrosion prone and which in turn reduces the passivation property. Chou et al. [57] also suggested that corrosion resistance of HEAs can be improved by adding sufficient amount of suitable inhibitors to the electrolyte. From classical theory, the inhibitors adsorb on the surface competitively with the adsorption of Cl− ions. Adsorption of these ions reduces the susceptibility of pitting corrosion, some inhibits the propagation of the pit and the other nucleation of pits.

4.3.2 Corrosion Resistance of HEAs in Acidic Environments and the Role of Alloying Elements Earlier the corrosion behavior of HEAs in acidic environment has been studies. Unlike corrosion in chloride environment where the possibility of pitting is more, the corrosion in acidic environment in absence of chloride ions the pitting is not common. However, the high concentration of hydrogen ions may affect the oxide films which in turn affect the resistance to corrosion especially general corrosion.

4.3.2.1

Al Addition to HEA and the Effect on Corrosion Resistance

In previous section already been discussed about the findings of Lee et al. [50] that the increasing concentration of Al in the high entropy Alx CrFe1.5 MnNi0.5 alloy decreased the Ecorr and increased the icorr and ipass in the solution of H2 SO4 (0.5 M) thus indicating reduced general corrosion resistance of the alloy. Figure 4.5 shows the variation of content of Al and the effects on corrosion resistance. From Fig. 4.5b, it is evident that the Epit value of Alx CrFe1.5 MnNi0.5 decreased in 1 M NaCl solution when compared to AlxCrFe1.5 MnNi0.5 (where x = 0). On the other hand Lee et al. [50] from the cyclic polarization tests found that the Al free alloy shown negative hysteresis loop indicating the possibility of repassivation whereas the alloy containing Al shown the positive hysteresis that is the degradation of passivation of the alloy. It was found that the alloys without Al i.e. CrFe1.5 MnNi0.5 , had shown the lowest icorr (6.86 × 10−4 A/cm2 ) value possessing the FCC and α-FeCr features. Moreover, the alloy Al0.3 CrFe1.5 MnNi0.5 contained both the FCC and BCC phases. As explained in the previous section that the increase in Al contents the FCC phase disappears and only the BCC phase appeared in the microstructure. Based on these findings it was concluded that the alloy free from Al was more corrosion resistant alloy because of the presence of FCC phase and also it was suggested that it may be because of the similarity with the austenitic phase in the stainless steel.

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Fig. 4.5 a Potentiodynamic polarisation curves of Alx CrFe1.5 MnNi0.5 system (x = 0, 0.3, 0.5) in 0.5 M H2 SO4 ; b Alx CrFe1.5 MnNi0.5 (x = 0, 0.3, 0.5) in 1 M NaCl [50]

BCC phase as the only phase present in the microstructure might have acquired the property of ferritic stainless steel with less resistance to corrosion. However, the understanding of the effects of Al addition to these alloys on corrosion resistance is yet to be cleared. The electrochemical impedance studies (EIS) revealed the decreased corrosion resistance as the charge transfer resistance decreased. From the second capacitive loop of the Nyquist plot it was concluded that the alloys surface favored the hydrogen adsorption on the alloy surface [50]. The hydrogen adsorption resulting in the complex formation in acid is evidenced by the release of Al as: Al + H2 O = Al (H2 O)ad + H+ + e−

(4.11)

Al (H2 O)ad + 5H2 O + H+ = Al3+ + 6e−

(4.12)

The release of Al is caused by the dissolution of the Al complex which leaves the surface with the porous corrosion product. The thickness of the aluminum adsorbed layer increases with the increasing Al content and the Al enriched layer with Cr depleted BCC phase forms. This layer loses its initial resistance to corrosion/passivity (Fig. 4.6). Similar reduction in corrosion resistance was found in the case of Alx CoCrFeNi in H2 SO4 (0.5 M) [52]. Li et al. [58] worked on the corrosion behavior of HEA FeCoNiCrCu0.5Alx (x = 0.5, 1 and 1.5) in both 0.5 M NaCl and 0.5 M H2 SO4 solutions. It was suggested that the Al content increased from 0.5 to 1.0 the initial FCC phase evolves to the BCC phase. In the case of Al1.5 the microstructure containing both the FCC and BCC phases. So, in 0.5 M NaCl electrolyte the icorr was shown by alloy containing Al contents 1.0 and 0.5 to be higher than that of alloy containing Al1.5. However, in 0.5 H2 SO4

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Fig. 4.6 Representation of SEM micrographs of Alx CrFe1.5 MnNi0.5 alloys with increasing amount of Al a x = 0, b x = 0.3, c x = 0.5 mol; d represents the micrograph after anodic polarization exceeded the breakdown potential (>1.25 VSHE ) in H2 SO4 in 0.5 H2 SO4 [50]

the corrosion resistance was found to follow the order as Al1.0 > Al0.5 > Al1.5 with the icorr was found to be lower for Al1.0 than the Al0.5 alloy. BCC phase had shown to have better resistance to corrosion than the FCC phase in solutions containing chloride ions and the sulfuric acid. It was also suggested that the duplex structure containing both the FCC and BCC phases in Al1.5 alloy.

4.3.2.2

Ti Addition to HEA and the Effect on Corrosion Resistance

It is already known that pure Ti is exhibits the active-passive behaviour in most aqueous environments. It forms a thin, impervious and adherent layer of TiO2 on the surface which renders the metal passive and thus enhancing the corrosion resistance [58]. As the Ti is an active metal, it can form several intermetalic compounds the elements in HEA [7, 59–65] e.g. Fe2 Ti-type, Co2Ti-type Lave phases in the HEAs AlCoCrFeNiTi1.532 and CoCrCuFeNiTi respectively [7, 60, 61]. Thus, heterogeneous microstructures developed in most of the HEA containing Ti containing HEAs. It has also been reported that the Ti acts as the catalyst for the Ni2.67 Ti1.33 -type R phase, the FeCr-type sigma phase and the Co2 Ti-type Laves phase formation in the HEA CoCrFeNiTi0.5 . These heterogeneous structures make the Ti containing HEAs prone to corrosion.

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Fig. 4.7 Representation of corrosion behaviour of Al2 CrFeCo-CuNiTix alloy and Q235 steel after potentiodynamic polarisation tests in 0.5 mol/l HNO3 solution [66, 67]

Ti containing HEA Al2 CrFeCoCuNiTix had shown the lower values of icorr when compared to Ti free HEA in 0.5 HNO3 as shown in Fig. 4.7 [66, 67]. On the other hand, with increasing amount of Ti the values of icorr was found to be decreased. The nobler Ti dispersed in the HEAs matrix may also be responsible for the localized corrosion. The presence of trace amount say x = 0.2 of Ti or Si degraded the corrosion resistance of both the HEAs Al0.3 CrFe1.5 MnNi0.5 Tix and Al0.3 CrFe1.5 MnNi0.5 in 0.6 M NaCl [66, 67]. This degaradation was caused by the formation of intermetallic compounds. Microcapillary electrochemical cell method would be suitable for the clarity of Ti containing phases on the corrosion of HEAs [68–71].

4.3.2.3

Cr Addition to HEA and the Effect on Corrosion Resistance

Chromium (Cr) is known to be one of the important alloying components in stainless steels and corrosion resistant alloys like Ni–Cr alloys. Cr renders these alloys stainless by covering its surface with an impervious, thin and adherent oxide layer. The oxide layer makes it passive and thus reducing the corrosion of these alloys. It was reported by Shiobara et al. that the potential greater than 400 mV versus SCE applied to Fe– Cr alloys, pure Fe and Cr makes it passive in 1 M H2 SO4 solution. At the lower applied potential the concentration of Cr required for the passivation of Fe and low chromium Fe alloys in 1 M H2 SO4 is around 12–13 at.% [72–75]. It was observed that the surface of the low Cr containing Fe alloys covers up with iron oxide/hydroxide layer [68, 69] whereas the at.% of Cr > 13% makes it to cover up with the hydrated Cr oxide/hydroxide passive layer. This shows a passivation region in potentiodynamic polarization curves and the Cr2 O3 oxide layer shown to be stable within a wide pH range between 2 and 12 [72, 76]. It was concluded that the Cr amount should be greater than 12 at.% in Ni-Cr, Fe–Cr, Fe–Cr–Ni alloys to exhibit the stainless behavior [72]. Based on this idea the HEA alloys should also contain the amount of Cr greater than 12 at.%.

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HEA Co1.5 CrFeNi1.5 Ti0.5 Mo0.1 shows a significantly high general corrosion and pitting resistance [49]. But the mechanism as to how the resistance to corrosion enhanced, by surface films or the intersolubility of Cr in rest of the alloying elements is yet to be clear. Amongst other elements (Fe, Co, Ni) Al and Cu shown to be detrimental to corrosion resistance of Cr containing HEAs e.g. in Cu0.5 NiAlCoCrFeSi alloy the Epit was found to be less than 0 VSHE . Same is the case with the Alx CrFe1.5 MnNi0.5 alloy [54, 77]. In the previous section it was already discussed that the Cr depleted dendritic structures (acts as the anodic site) and Cr enriched interdendritic structure (acts as the cathodic site) in case of Cr containing HEAs AlCoCrFeNi constituting the galvanic coupling which is responsible for the corrosion of the alloy [52]. This indicates that the Cr enriched surface layer or the Cr2 O3 precipitate is not sufficient for the alloy to render stainless behavior. The other elements should also be considered for their passive/corrosion resistance behavior. The passive layer formed by the noblest element may contain the other elements in their dispersed oxidized or unoxidized states. Hence, to understand the corrosion behavior the oxide layer needs to be further studied.

4.3.2.4

Mo Addition to HEA and the Effect on Corrosion Resistance

Molybdenum is one of the important alloying elements of austenitic stainless steel. Mo addition improves the passive behavior of the steel and also it improves the resistance to pitting corrosion [78]. Corrosion resistance improves due to the formation of MoO2 passive film. The ion MoO24 – formed on the surface may also be converted to MoO2 layer at the pit sites and thus passivate the surface thus reduces the corrosion. On the other hand elemental Mo pit repassivate the surface [79–81]. It was found that the Mo containing HEAs form Mo and Cr enriched sigma (σ) thus depleting these elements from the matrix of the alloy system. Hence, the corrosion resistance reduces in these HEAs [82, 83]. The corrosion behavior studied in different electrolytes 0.5 M H2 SO4 , 1 M NaOH and 1 M NaCl by Chou et al. [83] had shown in Fig. 4.8. For the alloy with and without Mo the corrosion resistance in 0.5 H2 SO4 was found to be almost similar but for the Mo containing alloy the tests in NaOH solution had shown an increase in icorr and a decrease in Eb breakdown potential.

4.3.2.5

Ni Addition to HEA and the Effect on Corrosion Resistance

Nickel (Ni) addition in stainless steel stabilizes the austenite phase containing high amount of Cr [78]. For the improvement of corrosion resistance Ni can be added with Mo, Cr and Cu also keeping the ductile FCC structure [84]. It was reported that the Ni content (x = 1) in the HEA Al2 CrFeCoCuTiNix had shown lower icorr value in neutral and alkaline solutions but this value was increased with the increase in Ni content. It was reported that Ni and Al form Al-Ni enriched B2 phase which could be detrimental to the resistance to corrosion in the Ni containing HEAs Al2 CrFeCoCuTiNix [52]. It was suggested that the Ni forms FCC and BCC phases with the other alloying

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Fig. 4.8 Representation of corrosion behavior after potentiodynamic polarization of the HEA. a Co1.5 CrFeNi1.5 Ti0.5 Mox (x = 0, 0.1, 0.5 and 0.8 mol) in deaerated 0.5 M H2 SO4 NaCl [83]; b Al2 CrFeCoCuTiNix in 3.5% NaCl [67]; c as-cast FeCoNiCrCux along with 304L in 3.5% NaCl at 25 °C [54]

elements and exhibits poor corrosion resistance may be because of the smaller size of Ni atom as compared to the other alloying elements. This decrease in atomic radius may create lattice distortion in high Ni containing HEAs which makes it prone to corrosion. However, further studies are required to clear the ambiguity why the increase in the Ni content decreases the resistance to corrosion.

4.3.2.6

Cu Addition to HEA and the Effect on Corrosion Resistance

It has been observed by several workers that with the addition of Cu to FeCoNiCrCux HEAs (with x = 0, 0.5 and 1) the values of icorr increases and Ecorr and Epit decreases in 0.6 M NaCl [85–87]. Previously, it was found that in FCC matrix of Cu containing FeCoNiCrCux HEAs the Cu segregation forms Cu-enriched interdendrites [88–90]. Hence, the microstructure consists of Cu enriched and Cu depleted but with the increase in Cr content phases. The interdendritic phase was found to be prone to corrosion due to the microgalvanic cells developed as a result of Cu segregation and also interaction of Cu with other alloying elements and hence positive binary mixing. Ren et al. studied the corrosion of Cu containing HEAs CuCrFeNiMn by following the laboratory corrosion tests like immersion and potentiodynamic polarization tests in 1 M H2 SO4 [85]. It was found that the CuCr2 Fe2 Ni2 Mn2 alloy with the lower amount of Cu has less segregation and hence shown the significantly high resistance to corrosion. On the other hand, the alloy Cu2 CrFe2 Ni2 Mn2 with the higher Cu content had shown significant segregation of Cu resulting in the lowest resistance to corrosion.

4.3.2.7

B Addition to HEA and the Effect on Corrosion Resistance

Boron is added to the HEAs to form borides for the enhancement of hardness and resistance to wear [91]. The corrosion behavior of B containing HEAs

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Fig. 4.9 Corrosion parameters of some selected HEAs in quiescent 0.6 NaCl at 25 °C [28]

Al0.5 CoCrCuFeNiBx (x varied between 0 and 15.4 at.%) in 1 N H2 SO4 was studied by Lee et al. [91]. From Fig. 4.9 it is observed that these alloys form negative hysteresis indicating these are not prone to localized corrosion. The icorr values of these alloys shown to be increased with increasing content of B along with a decrease in the repassivation potential Erp . Zhang et al. [92] studied the corrosion behaviour of B containing HEAs FeCrNiCoBx (0.5 ≤ x ≤ 1.0) in 0.6 M NaCl and suggested that the addition of B initially improves the corrosion resistance but later with the increase of B content the corrosion resistance degrades. It may be because of the presence of (Cr, Fe)2 B orthorhombic phase more resistant to corrosion than the FCC matrix in the microstructure. It was suggested that with the increased amount of B the phase transformation of (Cr, Fe)2 B orthorhombic to (Fe, Cr)2 B tetragonal structure degraded the corrosion resistance. However, for better clarity of the mechanism of effect of B on corrosion resistance requires more experimental evidences.

4.3.2.8

Sn Addition to HEA and the Effect on Corrosion Resistance

The addition of Sn in HEAs CoCuFeNi and AlCoCrFeNi was found to enhance the tensile strength [93–96]. After polarization in 3.5% NaCl the icorr and Epit values were found to be lower in Sn containing HEAs FeCoNiCu than that of 304 SS. In 5% NaOH the Icorr value was found to be higher and Epit value was lower than that of 304 SS. The mechanism of the corrosion behaviour of Sn containing alloys is still remained to be unfolded.

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4.3.3 Electrochemical Response of High Entropy Alloys and Pitting Potential Qui et al. [28] presented the corrosion parameters of some HEAS obtained after potentiodynamic polarization in 0.6 NaCl taken at 25 °C and compared with the published galvanic series (Fig. 4.9). It has been shown that the HEAs have Ecorr value within the range ~−498 to −180 mVSCE . Some HEAs had shown more noble behaviour than mild steel and aluminium, some in between ferritic and austentic stainless steel (ASS) and some of them even more nobler than austenitic ASS. Some HEAs had shown higher Epit than ferritic and some higher than that of ASS e.g. Ti0.3 (CoCrFeNi)0.7 shown significantly higher noble potential 1040 mVSCE . It is found that the HEAs have corrosion potential different than its constituent elements. This indicates that the HEAs have heterogeneous microstructures consisting of different phases and show their own electrochemical behaviour. For example in the case of HEA AlCoCrCuFe the most noble elements Cr and Cu show the OCP values below −0.35 to −0.4 VSCE respectively in chloride solutions. But the alloy has its own corrosion potential not of the constituent elements. Considering the case of CoFeNi in which Cr addition makes the alloy CoCrFeNi which is nobler and hence exhibit nobler Ecorr and improved resistance to pitting (higher Epit , Fig. 4.1). Likewise Ti addition forms Ti0.3 (CoCrFeNi)0.7 and with Al forms Al0.3 CoCrFeNi (AM (arc melting), Al0.6 CoCrFeNi (AM) or Al0.9 CoCrFeNi (AM) further increases the value of Epit . Now considering the potentiodynamic polarisation tests in 0.6 M NaCl and evaluating the corrosion parameters from the resulting curves of as cast AlCoCrCuFe, (TiAl)0.7 V0.15 Fe0.1 Ni0.05 and AlTiVCrSi HEAs alloys. From the electrochemical data it is found that these HEA alloys spontaneously undergo passivity but less so in the latter one. The latter one AlCoCrCuFe had shown more cathodic kinetics and undergo more corrosion. The Cu segregation is another reason which causes the deterioration in corrosion resistance. The Cu segregate into interdendritic structures where Cr enriched FCC phase forms between Cu depleted dendrites. The binary enthalpy of mixing for Cu with other elements like Al, Co, Cr, Fe is very near to zero or positive which is responsible for weak bonding force between Cu and other elements and thus causes segregation into interdendritic regions. This situation causes the development of microgalvanic cells with Cu depleted region as the cathodic sites and the Cu enriched interdendritic region as the anodic sites. The latter one will have the tendency to corrode in the corrosive environment. Similar is the case with spark sintered CoCrFeNi alloy which consists of multiphase microstructure. Praveen et al. [95] observed that the spark plasma sintering at 1000 °C of the alloy CoCrFeNi produced the major FCC and minor Cr enriched σ phase mixture in the microstructure. σ phase may be CrFe/CrCo/CrFeCo Cr enriched phase which acts as the cathode and the FCC with Cr depleted matrix acts as the anode develop microgalvanic cell. Thus the galvanic coupling causes the FCC matrix to dissolve in the corrosive electrolyte.

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4.3.4 High Temperature Corrosion Resistance of HEAs The HEAs with their excellent corrosion resistance property not only in aqueous environment but also in the high temperature service conditions e.g. in the high temperature and pressure environments of water. High temperature corrosion resistance of the Alx CoCrFeNi (x = 0.15 and 0.4) HEAs had been studied by Liu et al. [29] in supercritical water at 500–600 °C. The surface was found to be covered with dense and very thin spinel type oxides of (Fe, Cr)3 O4 layer. The oxide layer formed on the surface of the HR3C steel (25Cr–20Ni–Nb–N) compared with the layer formed on the HEAs. It was found that the latter was much thinner consisting of more homogeneous and smaller oxide particles. The corrosion resistance of HEAs was due to the dense, homogeneous spinel oxide (Fe, Cr)3 O4 layer. Xiang et al. [97] found the order of rate of repassivation and the resistance to stress corrosion in high pressurized water of four HEAs as TaNbHfZrTi > Co1.5 CrFeNi1.5 Ti0.5 Mo0.1 > 690TT > AlCoCrFeNiSi0.1 . This suggests that first two alloys with the higher repassivation rate may be used for the components or coatings in the nuclear power plants from the safety point of view.

4.3.5 Surface Coatings of High Entropy Alloys for the Enhancement of Corrosion Resistance Surface engineering has attracted very much to the industrial sectors for prolonging the service life of tools, components etc. In this method a variety of techniques are utilized to deposit the layer of desirable thickness with improved, wear and corrosion resistance properties and thermal shock or stress protecting applications. As for example, yttria stabilized zirconia (YSZ) coatings on Ni/Co based superalloys as high temperature coatings deposited by thermal spray coatings. HEAs possess very good corrosion resistance properties and thermal stability as discussed in the above sections. High corrosion resisting properties of HEAs have been utilized for the corrosion resisting coatings to safeguard the material and prolong their service life. A host of literature exists on the HEA coatings of various thicknesses deposited by following chemical vapour deposition, laser cladding, magnetron sputtering, electro spark deposition, plasma spray coatings, high velocity oxy-fuel coatings. The following sections present the review of the corrosion resistance coatings of HEA by following Thermal/plasma and laser surface processing.

4.3.5.1

Thermal Spray Coatings of HEAs

The thermal spray coatings have been recognized as the versatile and efficient technology. The thermal spray technology having a very few limitations and choice for the deposition of large number of layers with various properties like corrosion

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resistance, wear resistance, oxidation resistance etc. and cost effectiveness made it a versatile and popular in industrial sectors. The thermal spray consists of feedstock materials/mixture as powder/wire/rod form taken in the spray torch where the materials are heated to molten state completely or partially then sprayed on to the surface of the base material. The molten material on the base material is then rapidly quenched/cooled and solidifies as the deposited layer [98–100]. There are several process parameters like gas and flow rates of the carrier gas, stand-off distance, powder feed rate which can be controlled to achieve the desirable coatings. Thermal spray technology is divided into the three different categories: (a) high velocity oxy fuel spray (HVOF), (b) atmospheric plasma spray (APS) and (c) cold spray (CS). The thermal spray processes are represented as below (from Ref. [101]). HVOF involves ignition of hydrocarbon fuel Cx Hy (kerosene, methylene, acetylene etc.) with air or oxygen as oxidizer. Thus, temperature and pressure generated and heated and under molten condition of the feedstock materials accelerated through the Laval nozzles may be up to the speed of 2000 m/s onto the surface of the material (Fig. 4.9). The higher particle speed results in the good coatings with less porosity and in-flight oxidation. Atmospheric plasma spray coating (APS) is a proven and popular technology with a few limitations only has attracted to the researchers and industrial sectors [101]. The process involves introducing the feedstock material into the plasma stream (Fig. 4.10). The feedstock material undergoes complete or partial melting and propelled towards the surface of the material to be deposited and finally deposited as lamellar splats (Fig. 4.12).

Fig. 4.10 Representation of thermal spray processes [101]

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The process parameters power input, gas flow rate, carrier gas flow rate, powder feed rate, the standoff distance of the substrate control the density, uniformity and ultimately to the desired level quality of the coatings. Since the process is performed under ambient conditions it is economical, however suffers from the limitation that the feedstock particles undergo in-flight oxidation. Alternately, the process may be optimized for minimizing the oxidation by processing in the inert gas environment or in soft vacuum or in the reducing atmosphere. This process already proved to deposit large varieties of ceramic/cermet coatings for oxidation resistance/corrosion resistance (Fig. 4.11). Atmospheric pressure plasma spray coating (APS) is a proven and popular technology with a few limitations only has attracted to the researchers and industrial Fig. 4.11 HVOF coating gun. a Photograph of the gun, b schematic representation of the HVOF process and c photograph showing the HVOF process (Sulzer, Germany, housed in the Nanotechnology Laboratory, EBRS-KISR) [102]

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sectors [103]. In this process the feedstock material in the form of wire or powder is introduced into the plasma stream. The feedstock material undergoes complete or partial melting and propelled towards the surface of the material to be deposited and finally deposited as lamellar splats. The process parameters power input, gas flow rate, carrier gas flow rate, powder feed rate, the standoff distance of the substrate control the density, uniformity and ultimately to the desired level quality of the coatings. Since the process is performed under ambient conditions it is economical, however suffers from the limitation that the feedstock particles undergo inflight oxidation. Alternately, the process may be optimized for minimizing the oxidation. The process may be performed in the inert gas shroud or in a soft vacuum or reducing environment. The process already proved to deposit large varieties of ceramic/cermet coatings for oxidation resistance/corrosion resistance. Recently, solution precursor plasma spraying (SPPS) process as an emerging technology for coating with desirable properties has gained much attention. The nanostructured coatings by following SPPS has became very attractive for the surface engineering of the materials. The process involves the use of homogeneously mixed solution precursors as a feedstock and the starting solution precursors e.g. chemistry, concentration and solvents are the controlling factors to develop the desirable microstructure. Figure 4.12 (A → C) shows the schematic of the conventional and solution precursor plasma spray processes. Cold spray also called as kinetic spray is another category of thermal spray coating which involves the high pressure gas acceleration of feedstock particles through the Laval nozzles (Fig. 4.13). The advantage of this technology is the low temperature process which does not heat the feedstock particle to melt and avoid the risk of in-flight oxidation and phase transformation. Thermal spray technology already proved to develop coatings with significantly improved properties by depositing the composites, ceramics, cermets, alloys etc. At the industrial scale thermal spray technology is popular and demanding as it is capable to generate reproducible coatings for the aerospace and power generation applications. It has also been identified that the HEAs coatings are better alternatives to conventional materials coatings to produce novel protective coatings for the industrial application. The feedstock material for the thermal spray is generally the gas atomized (GA) powder. In the HEA coating the use of GA powder is very popular. The liquid alloy is propelled at high pressure through the nozzle in the inert gas environment. During its journey the liquid fragments into spherical droplets that undergo rapid solidification [107]. Gas atomization generates the spherical particles possessing good flowability, homogeneous alloy formation and with wider range of size. The desired size of the particles can be sieved for the feedstock materials. Mechanical alloying with the post treatment to improve the feedstock property may also be the alternative way to produce feedstock powder for the thermal spray [108]. Yin et al. presented the various shapes of the high entropy alloy particles following gas atomization (Fig. 4.14) [109]. Huang et al. first presented the work on HEA coating in the year 2004. In this study, atmospheric pressure plasma spray coatings of HEAs AlSiTiCrFeCoNiMo0.5 and AlSiTiCrFeNiMo0.5 , were followed for the enhancement of oxidation and wear

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Fig. 4.12 Schematic of plasma spray processing. a Plasma spray gun, b deposition process, c solution precursor plasma spraying process [104, 105]

Fig. 4.13 Schematic representation of cold spray coating gun [106]

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Fig. 4.14 Represents the high entropy alloy particles after gas atomization. a Shows spherical and satellite particles and; b dendritic growth [109]

resistance [110]. After 2016, a rapid rise in the HEA coatings by following thermal spray has been studied by several workers. Wang et al. [111] reported the HEA coating of Nix Co0.6 Fe0.2 CrySizAlTi0.2 and explained the microstructure and strengthening mechanism. Ang et al. [112] plasma sprayed the two HEAs AlCoCrFeNi and MnCoCrFeNi and correlated the microstructure and properties. These two articles explaining the structure to property correlation had been the attraction and cited by several workers. In the previous sections it has been presented that mostly the CrFeCoNi [98] HEA had been taken as the baseline and explored the properties by alloying with the additions Al, Si, Ti, Cu, Nb, Mn and Mo. The desirable microstructure and properties can be tailored by selecting the suitable composition of the feedstock materials and the process parameters. Addition of Al, Ti, Si, Cr required to improve oxidation resistance whereas Cr for corrosion resistance and Co–Ni combination to enhance plasticity. Yu et al. [113] coated the alloy AlCrFeCoNiCu by following low velocity plasma spray coating. This alloy has widely been studied in its bulk form. Ang et al. [114] followed atmospheric plasma spray for coating AlCrFeCoNi and found a significant in-flight oxidation whereupon the two phase mechanically alloyed powder into composite of alloy-oxide multiphase structure. This is caused by the reactivity of nanocrystalline feedstock with the atmosphere. The homogenous lamellar microstructure coating had shown good anisotropic mechanical properties. Cheng et al. [115] in the year 2019 reported the coating of HEA AlCrFeCoNi by following APS and studied the effect of particle size and spray parameters like argon as primary gas flow rate, plasma current etc. It was suggested that the phase formation may be controlled by adjustment of the spray parameters. In this study, the starting phases were B2 Ni–Al-rich dendritic and BCC Fe–Cr interdendritic microstructures. The phase transformation from BCC to FCC phases at the temperature of 600 °C followed by r phase at 800 °C occurred. The latter one dissolved on further annealing at 1000 °C and again transformed to FCC phase.

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Mu et al. in 2019 [116] further studied the plasma sprayed HEA coatings and suggested that during spraying IFO influenced the nano-oxide formation dispersed in the alloy matrix. It was formed as a result of HEA, plasma and air interaction. Recently, in 2020 Anupam et al. [117] revealed that the plasma sprayed coated layer as studied by Ang et al. [114] consisting of multiple phases formed as a result of HEA, plasma and air interaction. It was shown that a medium sized (5–15 μm) ball milled AlCrFeCoNi particle at the highest temperature zone (1200–2300 °C) in the plasma plume. It melts completely then spheroidize and oxidize partially in the flight then splat on impact as presented in Fig. 4.15. Anupam et al. [118] also cold sprayed to produce the HEA mechanically alloyed AlCrFeCoNi layer. Feedstock powder had not been not undergone IFO because of the low temperature (99% purity, leaving no need to apply any purifying treatment, and was subsequently stored at a low temperature before use. The MMA has a density of 0.94 g/cm3 , with a molecular weight of 100.12 g/mol and it is slightly soluble in water. Ammonium persulfate (APS) was used as the initiator for the reaction. The APS was purchased from Sigma–Aldrich, its density is 1.98 g/cm3 , has a molecular weight of 228.2 g/mol and is completely soluble in distilled water. The copper sulfate (CuSO4 ) and ethylenediaminetetraacetic acid (EDTA) used in this process were acquired from Sigma-Aldrich. TheSiO2 nanoparticles that possess an average diameter of 15 nm were also used here. All materials used here were analytical grade and were used without further purification. Chloroform obtained from Archos was used during the processing of the PMMA thin films; its density was 1.492 g/cm3 .

5.3 Materials Processing Techniques 5.3.1 PMMA/SiO2 Synthesis The nanoparticle infused PMMA was synthesized via in situ polymerization using ultrasound irradiation. The process began with 320 mL of distilled water being heated in a sonication bath to 70 °C. Next a predetermined wt% of aerosil silica nanoparticle was weighed using the scale (e.g. 1 wt% = 1350 mg), added and sonicated using

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the high intensity ultrasonic horn (Ti-horn, 20 kHz, 100 W/cm2 ) for 3 min at 50% amplitude. The temperature of the mixture equilibrated at 70 °C, after which 491.2 mg of CuSO4 and 986.4 mg of EDTA (weight ratio of 1:2) were added and the mixture was subsequently sonicated for a further 2 min. Once the CuSO4 and EDTA were completely dissolved, 120 mL of MMA was added drop wise so as to maintain the required temperature over a period of 5 min while sonicating. Once all of the MMA was added to the solution, 1600 mg of APS was added and the mixture was sonicated over 50 min. The mixture was removed from the sonication bath where 30 ml of distilled water was added and was then allowed to cool over a period of 2 h. This was done by utilizing a temperature controlled magnetic stirrer. The product was then centrifuged for 10 min at 10 × 103 RPM at room temperature. Distilled water was added to rinse the sample and this process was repeated 3 times in order to remove any unreacted materials and other bi-products. Thus obtained PMMA (pellet) was then collected and left to dry overnight in the oven at 80 °C. The sample yield was approximately 22.3 mg. The aerosil silica nanoparticle weight percentage was considered within the range of 0–5 wt%.

5.3.2 Fabrication of Thin Films The as prepared PMMA/SiO2 powder was used to synthesize thin films using the Laurell Spin Coater System. In preparation for the spin coating process, 12 g of PMMA/SiO2 was dissolved in 42 g of the solvent chloroform using the magnetic stirrer, with an established weight ratio of 1:3.5 for PMMA/SiO2 to chloroform. This solution was obtained using the temperature controlled magnetic stirrer over a time frame of 2 h at approximately 480 rpm or until the entire polymer content was no longer visible. Approximately 23 mL of this solution was dispensed onto the center of a stationary silicon wafer (surface area = 123 cm2 ), that was held under vacuum pressure. Oncethe solution was deposited, the wafer was spun at 80 rpm for a period of 10 min to set the solution, followed by a cycle of 20 min at 100 rpm to give the solvent sufficient time to evaporate. The wafer was removed from the Spin Coater and left to set and dry thoroughly overnight at room temperature. Once dried, the thin film was removed by allowing the silicon wafer with the deposited thin film to sit in a sonication bath of distilled water for 5 min at room temperature. The samples thus obtained were shown in Fig. 5.4 and they were later used for thermal, mechanical and morphological characterizations.

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Fig. 5.4 Thin films of a commercial PMMA (Lucite); b lab synthesized neat PMMA; PMMA with c 1 wt%; d 2 wt%; e 3 wt%; f 4 wt%; and g 5 wt% SiO2

5.4 Experimentation 5.4.1 Scanning Electron Microscopy (SEM) Scanning Electron Microscopy was used to characterize the microstructure and unit cells of the samples. The samples were analyzed using JEOL-JSM 5800 SEM. Prior to the characterization, the samples were coated with a thin film of gold-palladium using the Hummer 6.2 Sputtering System to make them electrically conductive whilst preventing charging of the sample.

5.4.2 Transmission Electron Microscopy (TEM) Transmission Electron Microscopy was conducted using JEOL JEM-2010. The sample was prepared in a room temperature sonication bath with ethanol. A very small amount of the sample was then transferred to a copper grid. This was then transferred to the sample holder for characterization.

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5.4.3 Gel Permeation Chromatography (GPC) Gel Permeation Chromatography is used to determine the relative molecular weights as well as the distribution weight of polymer samples. GPC setup is very similar to liquid chromatography and experimental results are usually accurate within ±5% range. This test was performed at Lucite International, Inc. in Cordova, TN. The samples weighing 0.03 g each were dissolved in 10 mL of THF and toluene was used as the flow marker to confirm peak validity on the instrument. The samples were allowed to dissolve overnight to ensure complete dissolution in the THF, after which approximately 1 mL of the solution was injected into each column. As a reference, the unit ran 3-PL gel 10 µm mixed B and 1-PL gel 10 µm 50 A column. The software used for analysis was CIRRUS by Polymer Labs, Inc.

5.4.4 Optical Microscopy (OM) The Optical Microscope was used to produce micrographs of the PMMA thin film. The system used was the Olympus SZX16 Microscope. The samples were prepared by cutting thin strips, no larger than 5 mm × 3 mm (l × w), perpendicular to the direction in which the quasi-static tensile testing samples were cut. The samples were then attached to carbon tape on a glass slide and loaded upright with the crosssectional area exposed for analysis.

5.4.5 Thermogravimetric Analysis (TGA) Thermogravimetric Analysis is a thermal characterization technique designed to determine the changes in weight with respect to temperature changes, therefore the results describe the thermal stability of the sample. The nitrogen sample purge gas was used to control the atmosphere in the furnace. The PMMA thin film samples were prepared by shaping the test samples into miniature disc like shapes so that they would sit in the bottom of the pan to avoid vibrations when the sample purge gas is flowing. The weight of the samples was 12 mg ± 5%. Once the sample and sample pan were loaded onto the TGA balance, there was a predetermined testing program that was initiated. For the first phase of the testing the sample was heated at a rate of 10 °C/min from room temperature (typically ≈25 °C) to 500 °C, during which time data was collected. Once the maximum temperature was attained, the second phase of the program started; the cooling phase was executed at a rate 40 °C/min.

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5.4.6 Tensile Analysis For tensile characterization a specimen was subjected to tensile load and the elongation of the specimen over some distance was taken into consideration. Tensile tests were used to determine the tensile properties such as modulus of elasticity, elastic limit, elongation, proportional limit, area reduction, tensile strength, yield point and yield strength. Tensile tests were performed by using the Zwick/Roell Materials Testing Machine and by applying the 20 N load cell with grips that provided stable and adequate clamping for the thin films. For this analysis, the specimens possessed the following dimensions 30 mm × 5.5 mm × 0.1 mm (l × w × t) and were tested according to ASTM D882—10. A cross-head speed of 1 mm/min was used during testing after acquiring a pre-load tension of 0.05 N using the same cross-head speed. Stress-strain curves were produced using the Testware-SX software to obtain tensile properties from the strain versus strain graph. Four samples were tested from each thin film for reproducibility of the curves.

5.5 Results and Discussion 5.5.1 Scanning Electron Microscopy The particle morphology of the lab synthesized PMMA and PMMA/SiO2 were studied using SEM micrographs. Figure 5.5 shows the micrographs for neat PMMA, 1, 2, 3, 4 and 5 wt% of SiO2 in PMMA. The micrographs were taken with the same accelerating voltage of 8 kV with similar magnification for comparison of the samples. The polymerization of the monomer at 70 °C produced PMMA particles that were spherical in shape with noticeably smooth surfaces. Although the particle shape was uniform, the particle sizes were not uniform. The particle sizes for the neat PMMA were observed to be ≤7.5 µm. However, with the addition of the 1 wt% SiO2 to the PMMA during synthesis, the particle size for the PMMA microspheres decreased to ≤2.5 µm. The particle sizes for the higher weight percentages were noticeably larger than the neat PMMA; 75% increase in diameter was observed for 5 wt% sample when compared the neat sample. The commercial PMMA (Lucite) particles have diameters ≤100 µm and werealso spherical with smooth surfaces. The particles also exhibited more uniformity in size when compared to the lab synthesized PMMA. For the PMMA/SiO2 , it was observed that the nanoparticles were uniformly dispersedand there was increased nanoparticle presence with the PMMA when the 1 wt% was compared to the 5 wt%.

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Fig. 5.5 SEM micrographs a commercial PMMA (Lucite); b lab synthesized neat PMMA; PMMA with c 1 wt%; d 2 wt%; e 3 wt%; f 4 wt%; and g 5 wt% SiO2

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Fig. 5.6 TEM micrographs of PMMA/SiO2 5 wt%

5.5.2 Transmission Electron Microscopy TEM analysis was performed on the PMMA/SiO2 5 wt% (Fig. 5.4) and the results are in agreement with the SEM analysis; the nanoparticles are indeed fused onto the surface of the PMMA microspheres. TEM analysis shows that the PMMA microsphere has a 0.5 µm diameter and the aerosil silica diameter was confirmed to be 15 nm as stated in the manufacturer’s specifications. Powders of any particle size tend to form agglomerates when being mixed traditionally, thus sonication was shown to be an effective means of deagglomerating and dispersing the aerosil silica as seen in Figs. 5.4 and 5.5, therefore overcoming the bonding forces ofaerosil silica nanopowder. TEM micrographs in Fig. 5.6 show a unique dispersion of the nanoparticle with few indications of agglomeration.

5.5.3 Gel Permeation Chromatography GPC characterization was used in this work to determine the molecular weight, average molecular weight and molecular weight distribution for PMMA. The commercial film produced by Lucite International ranges from 30 to 600 k in Mw due to the wide variety of applications they are required and the sample received from Lucite International, as previously mentioned, has a molecular weight between 300 and 400 k. The average viscosity molecular weight (Mv) was also determined to be 156,924. The analysis of the PMMA sample synthesized in these lab facilities resulted in a polymer with average molecular weight of 193,803. Typically, the higher the Mw the stronger and harder the material, therefore it is usually more stiff and has better chemical resistance. These results dictate that the lab synthesized PMMA has a medium Mw range, thus it should have a good balance of flexibility and hardness.

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Fig. 5.7 OM Micrographs for cross sectional area of a Lucite; b PMMA

5.5.4 Optical Microscopy Analysis The optical microscopy characterization was used to determine the surface morphology of a material. The OM was used to determine the thickness of the Lucite and PMMA thin films prior to tensile testing, however the analysis also highlighted the transparency of the thin films. Three measurements were taken across the cross-sectional areas seen in Fig. 5.7 for a more accurate assessment of the thin film thickness. The Lucite thin films had an average thickness of 120 µm and lab synthesized PMMA thin films had an average thickness of 97 µm. This 30% difference in thickness was attributed to different weight ratios used in solution preparation of the thin films. The varying ratios meant that the Lucite and PMMA solutions had different viscosities; therefore each solution wet the silicon wafer differently during spin coating, thus producing films with varying thickness.

5.5.5 Thermogravimetric Analysis of PMMA/SiO2 Thermogravimetric analysis was used to collect information on the thermal stability of the PMMA thin films. The TGA results shown in Fig. 5.8 represent the weight loss percentage versus temperature change of the neat PMMA and Lucite thin films in comparison to the PMMA/SiO2 thin films with 1–5 wt%. The analysis of these results for the thermal decomposition temperature can be seen in Table 5.2 where the benchmarks are taken at 10 and 50% weight loss, followed by 3 step decomposition temperature analysis. The Lucite, neat PMMA, and all the PMMA/SiO2 thin films exhibit 3 distinct phases of decomposition, with the exception of the PMMA/SiO2 thin films with 3wt% of aerosil silica that only exhibited one phase of decomposition. The first phase of decomposition for the thin films is noted by the degradation temperature below 200 °C. The Lucite thin films exhibit the greatest weight loss of 13.8% at this point, while the PMMA/SiO2 thin films with 1wt% of aerosil silica showed the least weight loss at 153 °C. It can also be seen that with increased

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Fig. 5.8 TGA curves for Lucite, neat PMMA and PMMA/SiO2

Table 5.2 TGA results for Lucite, neat PMMA and PMMA/SiO2 Sample

T10 (°C)

T50 (°C)

First step weight loss (%/°C)

Second step weight loss (%/°C)

Third step weight loss (%/°C)

Lucite

164.18

330.28

13.8/184

43/316.5

84.3/404.2

Neat

201.6

388.46

9/196.2

29.5/318.9

71.2/409.6

1%

196.9

395.88

3.6/153.3

22.9/319.6

70.8415.2

2%

171

386.9

5/154.8

32/321.9

75.4/416

3%

189.7

389.1

0/150

4/320.1

70/417.7

4%

160

397

4.8/145

26.1/319.69

73.1/421.2

5%

160

398

4.6/139.9

24/317.8

73.4/423.1

nanoparticle weight percentage, the first step decomposition temperature decreased by approximately 30% from the neat PMMA to the 5 wt% aerosil silica. This first phase of decomposition was attributed to the weak linkages arising from stress on the polymer chains which are resulted from the interaction between PMMA chains and the silica surface [33]. Greater percentage of weight loss occurs just above 300 °C; this occurs during the second phase of decomposition. However, there is not a distinct variation (±1.6 °C) in decomposition temperatures at this point amongst the thin films and the highest temperature at this point was seen in the 2 wt% aerosil silica thin film at 321.9 °C. The second phase of weight loss is due to the decomposition of organic compounds, i.e. loss of carbon, hydrogen and oxygen in the thin films. The third phase seen on the curve is the area where rapid weight loss was observed, with difference seen between Lucite and neat PMMA as well as increasing decomposition temperature between neat PMMA and nano infused PMMA. In this phase the Lucite thin film exhibits a decomposition temperature that was 2% less than the PMMA. The third phase shows the random scission of PMMA main chains (this

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occurs with the residual polymer. Improvement in the final decomposition temperature was seen with the addition of the aerosil silica nanoparticle. The degradation temperature increased by 3% from the neat PMMA with 409.6 °C to the PMMA/SiO2 5 wt% thin film with 423.1 °C. The increase in the final degradation temperature shows that the aerosil silica content affected the property of the PMMA therefore improving its thermal stability from as a result of more interfacial bonding due to effective dispersion of the aerosil silica in the polymer matrix. The PMMA/SiO2 3 wt% exhibited a difference in degradation in that there was only one distinct phase for decomposition. The degradation was delayed all the way from 100% to approximately 30% where rapid weight loss occurs at 417.7 °C. This also suggested an increase in the thermal stability of the thin film that was credited to nucleation of the polymer matrix.

5.5.6 Tensile Test Analysis The tensile properties of Lucite, PMMA and PMMA/SiO2 (1–5 wt%) thin filmswere evaluated by tensile testing and were summarized in Table 5.3. This polymer is typically brittle and this is one of its main disadvantages; hence tensile testing was performed to determine the effects on the polymer when aerosil silica was added and the stress-strain curves were shown in Fig. 5.9. Compared to the neat PMMA, the nanoparticle infused PMMA thin films exhibitedsignificantimprovement in tensile strength. The PMMA/SiO2 5 wt% thin film was 1.4 times higher than the neat PMMA. However, there was an initial 10% decrease between the neat and the PMMA/SiO2 1 wt% that can be attributed to insufficient effects of interfacial reaction between the polymer and the nanoparticle. From the stress-strain curves it was observed that the PMMA thin films with 1 and 3 wt% aerosil silica showed brittle failure under tension because their stress-strain relationship was strictly linear although there were significant differences in the strength. The other thin films, with the exception of the Lucite thin film, showed a small deviation curve after the linear stress-strain relationships, therefore entering the ductile region. Table 5.3 Mechanical properties of Lucite, PMMA and PMMA/SiO2 from quasi-static tensile testing Modulus, E (GPa)

Strength (MPa)

Elongation (%)

Lucite

0.23 ± 0.04

22.43 ± 1.12

69.24 ± 11.70

Neat PMMA

0.38 ± 0.20

25.44 ± 3.44

6.11 ± 2.59

PMMA/SiO2 1%

0.44 ± 0.16

22.97 ± 2.89

3.86 ± 0.41

PMMA/SiO2 2%

0.56 ± 0.13

34.16 ± 2.31

4.28 ± 0.36

PMMA/SiO2 3%

0.62 ± 0.09

38.38 ± 2.26

4.54 ± 0.84

PMMA/SiO2 4%

0.54 ± 0.04

44.46 ± 4.31

8.26 ± 2.48

PMMA/SiO2 5%

0.66 ± 0.08

62.03 ± 1.52

8.90 ± 1.81

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Fig. 5.9 Stress-strain curves for Lucite, neat PMMA and PMMA/SiO2

In addition to this, the tensile modulus also showed improvement in a similar manner with a 42% increase from neat PMMA to PMMA/SiO2 3%, however there was an approximately 13% decrease from 3 to 4 wt% leading to a 22% increase from 4 to 5 wt%. The commercial thin film, Lucite, exhibited a modulus that was 40% less than the neat PMMA modulus. The elongation however did not show a steady improvement; there was a similar trend as shown with the strength in that there was a decrease from neat to the 1 wt% thin film. But with the addition of the nanoparticle from 2 to 5 wt% there was a steady increase (showing a 45% increase), thus showing increased ductility for the typically brittle polymer. The Lucite thin film performed differently under the tensile loading exhibiting bottle neck before failure. This type of behavior is indicative of plastic deformation, hence the dramatic difference in elongation compared to the other thin films. The improvement in elongation might be due to the increase in interfacial bonds between the aerosil silica nanoparticle and PMMA thermoplastic polymeric matrix and a less interfacial defect [34].

5.6 Conclusions In the work presented here, PMMA/SiO2 nanocomposites were explored by use of the polymer which included manufacturing of thin films. This application was used in an effort to study their thermal, mechanical, and thermophysical properties. The aerosil silica weight percentage was varied to investigate the influence of nanoparticles on the polymer nanocomposite. The results showed overall improvement of the properties with the addition of the nanoparticle aerosil silica. The polymer nanocomposite, PMMA/SiO2 , was synthesized in the lab, with the aerosil silica wt% varied between 0 and 5 wt%. This nanocomposite was first manufactured as a thin film along with the commercially obtained PMMA (Lucite). The thermal characterizations revealed that the PMMA/SiO2 with 5 wt% exhibited the highest thermal stability and improvement

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over the neat PMMA and the Lucite. The tensile analysis also showed that the PMMA with 5 wt% had the best mechanical performance with ductile behavior however, the Lucite exhibited elastic behavior. The difference in mechanical behavior was a result of the commercial PMMA and the lab synthesized PMMA having different molecular weights. This work also shows that the addition of the SiO2 nanoparticle enhances the properties of the nanocomposites, allowing improved performance of the applications so that they may be used more efficiently in their applications such as automotive, protective coatings and biomedical applications. The as-prepared PMMA/SiO2 composite particles can be further studied for their shear fluid thickening (SFT) applications by increasing the SiO2 percentage of loading. Using this method STF can be designed for a specific medical application such as a surgical hand gloves, where softer polymer for flexibility/comfortability and harder nanoparticles for the puncture resistance are needed. This work also can be extended to other nanoparticles such as CNTs, Graphene, Cu, Ag infusion using in situ PMMA polymer synthesis techniques for various composites and medical applications. Thermal conductivity of these thin films can be studies for a specific application. These thin films can be consolidated to fabricate a uniformly dispersed nanoparticles thick composite panels. Acknowledgements The authors would acknowledge the financial support of NSF-RISE #1459007, NSF-CREST#, 1735971 and NSF-MRI-1531934.

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Chapter 6

Characterization of Coatings Through Indentation Technique B. Sridhar Babu and Kaushik Kumar

Abstract Instrumented indentation tests are most promising, reliable, easy nondestructive testing procedures in the materials research and these procedures are extended to characterize the coatings developed on the surface of the substrates. Indentation tests are conducted at different length scales i.e. Macro to Nano levels. The indentation tests data is used to determine the different mechanical properties of the coatings. This chapter gives the different numerical procedures or analytical models used to evaluate the Elasto-plastic deformation behaviour of coatings by using indentation data. Keywords Indentation · Elasto-plastic deformation · Indentation load · Mechanical behaviour · Slope · Young’s modulus · Indentation depth · Yield stress · Coatings · Indenter · Hardness

6.1 Relevance of Instrumented Indentation in Materials Research Instrumented indentation test is a non destructive testing procedure used in metal industry and it is most commonly used to find out the elasto-plastic behaviour of various materials like bulk metals, coatings, ceramics and bio-logical materials [1– 10]. The instrumented indentation has so many advantages over conventional hardness testers, in this testing procedure the indentation load, P, and the displacement of the indenter, h, are measured and controlled over the complete indentation process, when the indentation process is taking place. Instrumented indentation tests are also called as depth sensing instrumented indentation. A single instrument can be used to characterize nearly all types of materials and the loads can be varied from μN to B. Sridhar Babu (B) CMR Institute of Technology, Hyderabad, India e-mail: [email protected] K. Kumar Birla Institute of Technology, Ranchi, India e-mail: [email protected] © Springer Nature Switzerland AG 2021 K. Kumar et al. (eds.), Coatings, Materials Forming, Machining and Tribology, https://doi.org/10.1007/978-3-030-62163-6_6

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hundreds of mN. The depth of the indentation also can be varied from 1000 nm to 0.2 nm, respectively [11–14]. Because of the numerous advantages and the control level, sensitivity and data acquisition features of instrumented indentation testers, this process has become most predominant in the field of materials research particularly to study the mechanical behaviour of materials at micro to nano scale range. The instrumented indentation testers are used to study the fracture behaviour of ceramics, mechanical behaviour of coatings, bone, residual stresses, time-dependent behaviour in polymers, mechanical behaviour of bulk materials, scratch resistance of coatings and wear resistance of metals and also tribological behaviour of surfaces with additional feature i.e. lateral probe motion [15–18].

6.2 Nanoindentation Test The Nanoindentation experiments are mostly commonly used to investigate the mechanical behaviour of Coatings with spherical and Berkovich tip. A nanoindenter commonly operated in CSM (Constant stiffness mode),. It enables the stiffness (S) to be determined at regular interval of indentation load and depth curve (p–h). The loading follows the exponential function. Depth control mode and load control mode can be enabled while indentations were made on samples at a constant nominal strain rate. Specimens are mounted on to the nanoindentation specimen holder using adhesive (Glue). It is important for the nanoindentation test to keep the specimen surface in the same level and specimen should be adhered to the holder strongly enough so that there is no movement along x or y direction during nanoindentation test.

6.3 Working Principle of Nanoindenter The instrumented indentation system is represented in the schematic illustration in Fig. 6.1. It consists of mainly 3 parts i.e. hard material indenter, indentation load applied on the indenter and sensor to determine the indenter penetration into the material. By using an electromagnetic coil load is applied on the indenter and it is mounted on the vertical shaft of berkovich indenter by leaf springs connected in series. Load applied on the indenter is measured by the deflection of the leaf springs. The indenter penetration depth into the coating is determined continuously with the help of a capacitive sensor; the instrumented indentation tests need a person who is having good experimental skills and theoretical knowledge about the indentation. Such instrumented indentation systems are very sensitive to mechanical vibrations and thermal drifts. Therefore, for accurate results the experiments should be conducted in a free noise and vibration environment conditions. Figure 6.2 shows the Berkovich indenter residual impressions in the nanoindentation tests.

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Fig. 6.1 Schematic illustration of instrumented indentation tester

Fig. 6.2 Series of Berkovich indentations

6.4 Interpretation of Load-Displacement Curve (P–h) Instrumented indentation testing procedure completes with loading and unloading process. The indentation depth ‘h’ is taken as the penetration depth of indenter tip in the coating material. During the loading cycle of indentation elastic and plastic deformation takes place in the coating material and while unloading process only a recovery of the elastic displacements takes place in the material. Because of this the unloading process is studies by modelling it as an elastic contact problem, where the indenter is considered as rigid for elastic material.

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Fig. 6.3 Dimensions of the spherical indenter residual impression read from the profile on the optical profilometer

In the nineteenth century Hertz [19] considered the indentation problem as elastic contact problem. The problem was considered between two elastic solids with the indenter displacements and stresses. Hertz gave a mathematical relation for load (P) and the indentation displacement considering the indenter as a rigid sphere. √ 8 R E 3 h2 P= 2 3 1−ν where R was the sphere radius, Experimental findings and theoretical predictions confirm that the cycle of loading and unloading processes are dictated by the geometry of the indenter. Kick’s Law clearly defines the loading during indentation, if sharp indenter s are used in the indentation process, P = Ch 2 where C is material constant and termed as loading curvature.

6.5 Indentation Crater Profile The dimensions (diameter, depth and lip height) of the residual impressions formed (see Fig. 6.3) during Nanoindentation tests can be measured using Bruker optical profile meters. Indentation crater profile is shown in Fig. 6.3.

6.6 Contact Stiffness Contact stiffness is an important parameter to consider in the instrumented indentation data analysis. Consider the case of the elastic constant for flat specimen and a

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Fig. 6.4 Schematic diagram of conical indentation

cone, shown in Fig. 6.4. In this case sneddon’s equation gives the load and indenter displacement values. P=

2 ∗ E tan αh 2 π

Differentiating the above Eq. with respect to displacement ‘h’   2 ∗ dP =2 E tan α h dh π Substitute above equation in the ‘P’ equation. P=

1 dP h 2 dh

The conical indenter penetration depth ‘h’ can be given as h=

π a cot θ 2

and since A = pa2 dP = 2E ∗ a dh These last 2 equations refers to the axis-symmetric indenter elastic contact (such as a cone, sphere, cylindrical punch) to be calculate from the contact stiffness and the contact radius.

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6.7 Berkovich Indenter Berkovich indenter is most commonly used to study the behaviour of bulk specimens and coatings. The ratio between the indentation load and the indenter contact area is used to find out the mean pressure which is equal to the hardness value. The indenter contact area is calculated by means of indentation depth. Approximately the strain rate value is 8% in the indentation. Figure 6.5 shows the berkovich indenter geometry. The Berkovich indenter contact area formed on the material in the indentation process can be derived as follows. l tan 60◦ = a/2 √ 3 l= a 2 √ 3 2 al = a A pr oj = 2 a h cos 65.27◦ = b √ a = 2 3h tan 65.3◦ √ A pr oj = 3 3h 2 tan2 65.3◦ = 24.26h 2

Fig. 6.5 Berkovich indenter geometry

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6.8 Oliver-Pharr Procedure to Determine the Coatings Mechanical Properties from the Instrumented Indentation Response Doerner and Nix [17] has given a step by step procedure to find out the material mechanical properties i.e. Hardness and the Young’s modulus from the instrumented indentation data. Indenter contact area in the indentation process is used to measure the hardness of the coating and initial slope of P–h unloading curve is used to determine the Young’s modulus (E) of the coatings. The Power laws gives the nature and behaviour of the Unloading loading curve. However, In this procedure effect of pile-up and sink-in behaviour of the coating was not included to determine the mechanical properties of coatings. As per the Oliver-Pharr procedure the hardness ‘H’ of the coating is expressed as ratio of maximum indentation load ‘Pmax ’ applied on the indenter and the projected contact Area ‘Aproj ’. H=

Pmax A pr oj

The projected area for the Berkovich indenter is A pr oj = 24.56 h 2c Pmax hc = h − γ S ‘γ ’ is the indenter geometric constant depends on indenter geometry and its value is 0.75 for Berkovich indenter. ‘E’ of the coatings can be evaluated by finding the unloading curve initial slope (S) is equal to S=

dp = α · m(h − h f )m−1 dh

m·α, and hf are the variables which are called as the best-fit constants. ‘hc ’ is the indentation depth and the ‘E’ of coating is determined by using the below equation.   1 − νi2 1 1 − νs2 = + E∗ Ei E √ S π E∗ = √ 2 A pr oj E * is the elastic contact modulus, νi = 0.07 and Ei = 1141 GPa. The accuracy in the E values from this procedure depends on the accuracy in the measurement of area of contact and these measurements include the piling-up and sinking-in. Because of the piling-up or sinking-in surface effects the Young

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Fig. 6.6 Berkovich indenter residual indentation impression

modulus value may vary from real value. With the advent of newer technologies in indentation equipment, assessing material properties has become easier and not only that the inputs and output measurements can be maintained at smaller scales (Fig. 6.6).

6.9 Dao’s Reverse Analysis Dao (2001) [20, 21] proposed reverse analysis schemes to derive the elastic-plastic properties of coating with the help instrumented indentation data. Using dimensional analysis, mechanical material properties were derived from dimensionless functions and the indentation data. Dao’s dimensional analysis gives the C expression as C = σ0.033

 E ∗  σ0.033 1

where σ0.033 is the representative stress at 0.033 strain and dimensionless function 1 . When εr = 0.033 1 is equal to   ∗ 3   ∗ 2  E ∗  E E = −1.1321 ln + 13.635 ln σ0.033 σ0.033 σ0.033 1   ∗  E + 29.267 − 30.194 ln σ0.033

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The work hardening exponent ‘n’ is equal to    E∗ 1 d pu = σ0.033 ,n 2 σ0.033 E ∗ h m dh m dp is the slope of unloading curve, and dimensionless function where dh m defined as

 2

can be

   E∗ 1 d pu ,n = ∗ = (−1.0557n 3 + 0.77526n 2 + 0.15830n − 0.06831) 2 σ0.033 E h m dh m 3  E∗ ln + (17.93006n 3 − 9.2209n 2 + 2.37733n − 0.86295) σ0.033 2  E∗ ln + (−79.99715n 3 + 40.55620n 2 + 9.00157n − 2.54543) σ0.033   E∗ ln + (122.65069n 3 − 63.88418n 2 − 9.589362n + 6.20045) σ0.033

The below equation gives  the correlation n between (σ0.033 ) representative stress E∗ and yield stress σ0.033 = σ y 1 + σ y 0.033 Magnitudes of σ y and n can be determined by substituting C and E* , S and hm in the above equations

6.10 Indentation Size Effects Model Relation between H and h i.e. indentation hardness and depth of indentation, established by Nix and Gao. Nix and Gao [21] arrived at these equations based on GNDs model and Taylor dislocation model, This equation rationalizes the ISE because the conventional plasticity theories are not sufficient model ISE because they does not incorporate length scales of the materials. As per the Nix and Gao the indentation depth dependent relation of indentation hardness is expressed as h∗ H2 =1+ H0 h In the above equation H0, and h* are hardness values for a large indenter penetrations and characteristic length in mm that mostly rely on the indenter material. H0 and h* can be calculated from the below equations √ Ho = 3 3αμbρs

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 2 81 1 μ 2 2 h = bα tan θ 2 2 f Ho ∗

Here α varies between 0.3 and 0.5, and angle between the surfaces of indenter and specimen is θ, f is a scaling factor ranging from 0 to 3.5 and, b is the Burgers vector the indentation size effected with the help of above relations with a variable known as ‘intrinsic material length scale’ which is derived from strain gradient plastic theory. By using the Tabor relation with Misses flow rule the strain gradient is determined as follows. 

σ σo

2

9 = 1 + l ∧χ ≈ 1 + l ∧χ 8

σ and σo are the stresses developed in the material due to the presence of a gradient and absence of strain gradient. intrinsic material length scale l∧ is calculated by l∧ = b



μ σo

2

6.11 Strain Rate Sensitivity of Materials Strain rate is termed as the rate of deformation during indentation. It is usually related with the loading rate applied or displacement rate on the indenter over the surface of the material. The strain rate is considered to act perpendicular to the materials ) divided by displacement surfaces and is expressed as the displacement rate (h = dh dt (h). Strain rate ‘ε’ is expresses as ε=

dh 1 dl h(t)

Strain rate sensitivity m is equal to m=

d ln H d ln ε

In engineering applications the components are most often subjected to accidental shock loadings, strain rates is very essential to study the material behaviour.

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6.12 Conclusions 1. Instrumented indentation procedures are very effective non destructive procedures to characterize the coatings based on the indentation load and indentation depth response of coatings. 2. Because of the phenomena of pile-up or sink-in, true projected contact area of the residual impression and the apparent projected contact area of the indentation area are not possible to be measure accurately and influence the end results. 3. Because of the indentation size effects, the hardness values for the same material measured under different conditions and scales may vary very much, which makes it difficult to correlate these hardness values. 4. There were experimental evidences that materials do exhibit strain rate sensitive mechanical properties of materials.

References 1. C.P. Sharma, A.K. Bhargava, in Mechanical Behaviour and Testing of Materials (PHI, 2011) 2. T. Kondo, Y. Takigawa, T. Sakuma, High-temperature tensile ductility in TZP and TiO2 –doped TZP. Mater. Sci. Eng. 231, 163–169 (1997) 3. D. Tabor, Hardness of Metals (Clarendon Press, Oxford, 1951) 4. K.L. Johnson, in Contact Mechanics (Cambridge University press, 1985) 5. X. Hernot et al., Influence of penetration depth and mechanical properties on contact radius determination for spherical indentation. Int. J. Solids Struct. 43, 4136–4150 (2006) 6. J.S. Field, M.V. Swain, A simple productive model for spherical indentation. J. Mater. Res. 8, 297–306 (1993) 7. W.C. Oliver, G.M. Pharr, An improved technique for determining the hardness and elastic modulus using load and displacement sensing indentation experiments. J. Mater. Res. 7, 1564– 1583 (1992) 8. M.F. Doener, W.D. Nix, A method for interpreting the data from depth-sensing indentation instruments. J. Mater. Res. 1, 601–609 (1986) 9. N. Huber, Tsagrakis, C. Tsakmakis, Determination of constitutive properties of thin metallic films on substrates by spherical indentation using neural networks. Int. J. Solids Struct. 37, 6499–6516 (2000) 10. J.M. Antunes et al., A study on the determination of plastic properties of metals by instrumented indentation using two sharp indenters. Int. J. Solids Struct. 44, 5803–5817 (2007) 11. B. Sridhar Babu, A. Kumaraswamy, B. Anjaneya Prasad, in Investigation of Elasto-Plastic Deformation Behavior of Haynes242 Alloy Subjected to Nanoscale Loads through Indentation Experiments. Transactions of Indian institute of metals (Springer, Berlin, 2015) 12. B. Sridhar Babu, A. Kumaraswamy, B. Anjaneya Prasad, Effect of indentation size and strain rate on nanomechanical behavior of Ti-6Al-4V alloy. Trans. of Indian Inst. Met. 67(5) (2014) 13. J.L. Bucaille et al., Determination of plastic properties of metals by instrumented indentation using different sharp indenters. Acta Mater. 51, 1663–1678 (2003) 14. N. Chollacoop, M. Dao, S. Suresh, Depth sensing instrumented indentation with dual sharp indenters. Acta Mater. 51, 3713–3729 (2003) 15. J.G. Swadener et al., Determination of elasto plastic properties by instrumented sharp indentation. J. Mech. Phys. Solids 50, 681–694 (2002) 16. S. Suresh et al., A new method for estimating residual stresses by instrumented sharp indentation. Acta Mater. 46, 5755–5767 (1998)

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17. W.H. Poisl, W.C. Oliver, B.D. Fabes, The relation between indentation and uniaxial creep in amorphous selenium. J. Mater. Res. 10, 2024–2032 (1995) 18. Y. Liu, A.H.W. Ngan, Depth dependence of hardness in copper single crystals measured by nanoindentation. Scrpta Mater. 44, 237–247 (2001) 19. M.J. Mayo, W.D. Nix, A micro indentation study of super plasticity in Pb Sn and Sn-38 wt-percent-Pb. Acta Metall. 36(8), 2183–2192 (1998) 20. M. Dao et al., Computational modelling of the forward and reverse problems in instrumented sharp indentation. Acta Mater. 49, 3899–3918 (2001) 21. W.D. Nix, H. Gao, Indentation size effects in crystalline materials: a law for strain gradient plasticity. J. Mech. Phys. Solids 46, 411–425 (1998)

Part IV

Simulation and Optimization

Chapter 7

FE-RSM Modeling of Wire Drawing of Brass-Plated Steel Wire Anup Kr. Pathak, Aditya Singh, Gitanshu Raj, Milind, and Bappa Acherjee

Abstract Wire drawing is a material deformation process in which the cross-section of the wire or rod is reduced by pulling the wire or rod through a single or a series of converging dies. A three-dimensional elasto-plastic FE (finite element) model of wire drawing process is developed using ANSYS® . The brass-plated steel wire is used as the wire material, while the polycrystalline diamond is used as the die material. The von Mises yield criteria, associative flow rule, and isotropic work hardening is implemented in the plasticity model. The FE model predicts the drawing stress on the wire and die, in response to the selected wire drawing process parameters. The results of the FE model are used to feed the experimental matrix designed to develop the empiric models using the RSM (response surface method). Models are validated using test results and found to be consistent within the range of the parameters studied. The effects of the wire drawing parameters on drawing stress are also investigated and discussed. Keywords Wire drawing · Finite element model · Response surface model · Drawing stress · Parameter effect · Brass-plated steel wire

7.1 Introduction Wire drawing is a process of metal-working in which the cross-sectional area of the wire is reduced by passing the wire through a single die or often a series of drawing dies as per the requirement [1]. Among the numerous applications of wire drawing, the primary application areas are electric wires, tensioning structural elements, cables, wire stock for fences, springs, stringed musical instruments, and wheel spokes. The extrusion process is quite similar in practice except the wire is pulled through the die in the wire drawing while the wire is forced through the die in the extrusion [2]. Wire drawing process is usually performed at room temperature, and is thus categorized under the cold working process. Nevertheless, it is often A. Kr. Pathak · A. Singh · G. Raj · Milind · B. Acherjee (B) Department of Production Engineering, Birla Institute of Technology, Mesra, Ranchi 835215, India e-mail: [email protected] © Springer Nature Switzerland AG 2021 K. Kumar et al. (eds.), Coatings, Materials Forming, Machining and Tribology, https://doi.org/10.1007/978-3-030-62163-6_7

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done at elevated temperatures to reduce the forces when drawing large wires. Before drawing, physical methods such as filing, hammering, rolling, or swaging are used to form the leading part of the wire to fit it into the die to pull it through the die afterward. As the volume of metal stock remains constant throughout the process, the reduction in the diameter is compensated for by the increase in the length of the wire. The procedure involves more than one drawing through progressively smaller dies to achieve the required cross-section for a sizable reduction in diameter [3]. The material properties of the wire substrate are affected because of cold working during wire drawing. The effective reduction the cross-section region in smaller wires is observed to be about 15–25%, while in larger wires it is 20–45%. Annealing is required before the redrawing phase where the requirement of reduction in crosssection area is more than 50%. Figure 7.1 draws a schematic representation of the wire drawing process and die geometry, where, D is the initial diameter of the wire and D0 is the final diameter of the wire after passing through the wire drawing die. The arrow mark associated with pulling force P shows the direction of pulling of the wire. Die geometry is specified by several parameters, such as entrance angle (ω), reduction angle or die angle (γ ), reduction length (β), bearing length (L), and back relief angle (δ). An entrance angle is given in the die to lead the wire through the die. This also guides lubricant flow on the wire surface during the drawing phase. A reduction angle is given in the die to carry the wire to the die nib, which reduces the cross-section of the wire. The die length aligned with the reduction angle is called the reduction length. The purpose of bearing length in drawing die is to keep the wire from recuperating its original size by radial elongation, and to ensure that the final wire is round, straight and surface finished. The back relief angle is used to strengthen the die exit and to make it possible for the wire to emerge easily from the bearing, preventing interaction with any hard edge that could chip off the wire. During the wire drawing process, a certain amount of plastic deformation is induced into the substrate based on the drawability of the material. Because of the

Fig. 7.1 Schematic representation of the wire drawing process and die geometry

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cold processing during wire drawing the yield stress of the drawn wire is increased as a result of strain hardening of wire material. The drawing process depends on several factors including die geometry (especially the die angle and bearing length), drawing velocity, and coefficient of friction at the die-wire interface, which regulates the drawing stress for the necessary plastic deformation. Drawing velocity is the speed at which the wire is drawn through the die. The coefficient of friction at the die-wire interface is dependent upon lubrication between the die and wire [3–7]. A high plastic deformation value in the material may result in extreme damage within the wire. The damage sustained depends primarily on the die angle and, to a smaller degree, on the proportion of cross-section reduction [8]. Several numerical and experimental studies [1–17] are performed to explore the various facets of the wire drawing mechanism and to refine the process parameters to optimize the process. Numerical simulations are used to research the fundamental dynamics and parametric effects of wire drawing. Numerical methods, especially FE (finite element) method, proved to have more precise results than analytical models [5, 10], and are thus preferred in modeling of wire drawing process [4, 5, 8–15]. This chapter presents a study on the wire drawing process through numerical and empirical modeling of the process. Wire drawing process is simulated using FEM in ANSYS® and the results obtained from the simulation model are used to develop the empirical model using response surface method (RSM). Models are validated using test results and found to be consistent within the range of parameters investigated. The effects of wire drawing process parameters on the response of interest are also investigated and presented.

7.2 FE Simulation of Wire Drawing Process Three-dimensional mechanical FE analysis of the wire drawing process is performed on ANSYS® . An elasto-plastic FE model is therefore developed to simulate the wire drawing process. Figure 7.2 presents a flowchart showing the steps involved in the development of the FE model.

Fig. 7.2 Flowchart of steps involved in FE modeling

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7.2.1 Formulation of the Model The simulation model is developed using the ANSYS® FE software based on the elasto-plasticity principle of material. The application of forces to material causes stress within it and the material undergoes deformation. Deformation or strain is proportional to the stress to a level known as the proportional limit. Thereafter, stress and strain have a nonlinear interaction but do not actually become inelastic. The relation between stress and strain can be expressed as [18]: {σ } = D{ε}

(1)

 T {σ } = σx σ y σz σx y σ yz σx z

(2)

 T {ε} = εx ε y εz εx y ε yz εx z

(3)

where, {σ} is the stress vector,

{ε} is total strain vector,

[D] is the stiffness matrix, specified by the mechanical properties viz. Young’s modulus, Poisson’s ratio and shear modulus of materials. The total strain vector, {ε}, comprises of elastic, plastic, thermal, creep and swelling strain vectors. The total strain vector for wire drawing simulation is considered as:     {ε} = εel + ε pl

(4)

where, suffixes el and pl denote the elastic and plastics strain vectors, respectively. Once induced stresses in the substrate reach the yield point of the material, plastic deformation occurs. The plasticity model consists of three central elements of plastic analysis, including the yield criteria, the hardening rule, and the flow rule. The yield criteria define the degree of stress under which yielding starts. In the case of multi-component stresses, the yield criterion may be interpreted as a function of the individual components, F({σ }), as: σe = F({σ })

(5)

where, σ e is equivalent stress. Plastic strains tend to develop in a substance where the corresponding stress is equivalent to the substrate yield stress. F({σ }) = σ y

(6)

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The yield criterion in the present simulation is assumed to be based on the von Mises yield criterion. von Mises stress is used to estimate the yielding of materials under multiaxial loading conditions using data from basic uniaxial tensile experiments.

7.2.2 Material Geometry and Meshing Solar wafer cutting wire is chosen as wire material. It is a brass-plated high carbon steel wire, used to cut solar wafer in space applications. The brass-plated steel wire manufacturing involves an intermediate step of brass plating of the steel wire and a final step of brass-plated steel wire drawing to the finished product. The coating weight is in the range of 3–12 g/kg of steel wire, so the brass coating has negligible influence on the physico-mechanical properties of the wire material; however, the coating contributes to the drawing process by serving as a solid lubricant. The brass coating also improves the fracture life of the wire and prevents the base metal from corrosion. Polycrystalline diamond (PCD) is used as a die material. The physicomechanical properties of wire and die materials, such as density (7.86 g/cm3 for the wire, 3.8 g/cm3 for the die), elastic modulus (206 GPa for the wire, 840 GPa for die), Poission’s ratio (0.3 for the wire, 0.075 for the die), yield stress (929 MPa for the wire), failure stress (3713 MPa for the wire, 7460 MPa for the die), ultimate strain (0.0307 for the wire) are used for material modeling during FE simulation [19]. The diameter of the wire is estimated to be 0.13 mm at the entry and 0.12 mm at exit, in a single stage. Figure 7.3 presents the cross-sectional view of the drawing die, modeled using ANSYS® . Different sections of the die are modeled, such as entrance cone (70° angle, 0.45 mm length), transition cone (30° angle, 0.003 mm length), reduction area (8°

Fig. 7.3 The cross-sectional view of the die with different die sections

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Fig. 7.4 a Isometric view and b sectional view of die and wire mesh model

angle, 0.18 mm length), bearing (0° angle, 0.06 mm length), back relief (16° angle, 0.12 mm length), and exit cone (60° angle, 0.16 mm length) [19]. To reduce the total number of nodes needed to reduce the solution time, a rough meshing is done to the die, except for the area where the wire and die come into contact. Selective meshing for denser nodes is performed in areas of interest, i.e. surfaces in contact between the wire and die. Figure 7.4 presents the mesh model for the wire-die geometric model.

7.2.3 Simulation and Results The FE simulation model is governed by Eq. (4). The boundary condition of the die is assumed to be zero displacements and rotations in all directions. The boundary conditions applied to the wire is considered as zero velocity in the x and y directions and 20 mm/s on the z-direction. von Mises yield criteria, associative flow rule, and isotropic work hardening is implemented in the plasticity model. The developed elasto-plastic FE model is validated by using the published data from Ref. [19]. The results presented in Fig. 7.5 shows a reasonable agreement between the simulated and the published results. Figure 7.6 shows the shaping of the wire during the reduction of the cross-section of the wire when passing through the nib and bearing of the die. The maximum drawing stress of 3632.7 MPa is developed in wire during the drawing process at the wire-die contact region. It is seen that the deformed wire does not recover its original diameter after passing through the bearing zone, rather elongates in the longitudinal direction to compensate for the cross-sectional reduction. The developed model can be used to perform parametric analysis, as the model inputs can be varied. The results of the FE model are used to feed the experimental matrix designed to develop the empirical model by RSM (response surface method) to perform the parametric investigations of the wire drawing process.

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Fig. 7.5 Validation of the developed mode

Fig. 7.6 Contours of stress distribution during wire drawing

7.3 Empirical Modeling of Wire Drawing Process RSM is a combination of mathematical and statistical methods, used to draw the relationship between the input variables with the output responses through the development of an empirical equation [20]. The experiment is designed based on the central composite design of RSM. Bearing length, die angle, coefficient of friction, and drawing velocity are selected as wire drawing parameters. The response of interest is the drawing stress, S (MPa).

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Table 7.1 Wire drawing process parameters and the selected parameter ranges Parameters

Units

−α

−1

0

+1

+α

A: Bearing length

mm

0.04

0.05

0.06

0.07

0.08

B: Die angle

degree

C: Coefficient of friction D: Drawing velocity

mm/s

6

7

8

9

10

0.040

0.045

0.050

0.055

0.060

18

19

20

21

22

7.3.1 Design of Experiments A four-factor five-level design of experiments is planned as shown in Table 7.1. Design Expert® software is used to design the experiments and also to derive the empirical relationships. Table 7.2 presents the experimental plan composed of 30 experiments. For each of the experimental run, the response value is computed from the FE model by using the values of the input parameters.

7.3.2 Development of the Empirical Models Design Expert® software is used for the analysis of the response and the determination of the best fit model. The same software is used to perform f -test, lack-of-fit test, and analysis of variance (ANOVA) to check the adequacy of the developed empirical model. The f -test and lack-of-fit test suggest a quadratic model with significant model terms, thus a quadratic empirical model is selected to map the interrelation between parameters and the response. The R2 value of 0.9566, which is close to unity also confirms the fitness of the model. It is observed from the ANOVA table furnished in Table 7.3, that the associated p-value of the model is less than 0.05, thus the model is significant at a 95% confidence level. The model terms with an associated p-value of less than 0.05 indicate that they are statistically significant [21]. The insignificant model terms are eliminated from the equation, except the model term C to maintain the hierarchy. The developed empirical models for drawing stress are given as follows: In terms of coded-factors: S = 2623.40 + 49.33A + 130.02B + 15.26C − 177.04D + 223.13B 2

(7)

In terms of actual factors: S = 23,726.06 − 55,244.16A − 3223.17B − 83,870C − 342.98D + 223.12B 2 (8) It can be observed from the ANOVA tables that the drawing velocity has the strongest effect on drawing stress, followed by die angle and bearing length. These

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Table 7.2 Experimental plan and the computed values of response S. No.

A

B

C

D

S

1

0.05

7

0.045

19

2914.6

2

0.07

7

0.045

19

2964.5

3

0.05

9

0.045

19

3256.2

4

0.07

9

0.045

19

3274.7

5

0.05

7

0.055

19

2942.3

6

0.07

7

0.055

19

2994.6

7

0.05

9

0.055

19

3345.6

8

0.07

9

0.055

19

3296.4

9

0.05

7

0.045

21

2514.4

10

0.07

7

0.045

21

2599.7

11

0.05

9

0.045

21

2795.4

12

0.07

9

0.045

21

2876.9

13

0.05

7

0.055

21

2531.8

14

0.07

7

0.055

21

2612.3

15

0.05

9

0.055

21

2814.2

16

0.07

9

0.055

21

2934.4

17

0.04

8

0.05

20

2503.7

18

0.08

8

0.05

20

2876.2

19

0.06

6

0.05

20

3332.3

20

0.06

10

0.05

20

3632.7

21

0.06

8

0.04

20

2662.2

22

0.06

8

0.06

20

2707.7

23

0.06

8

0.05

18

2843.9

24

0.06

8

0.05

22

2374.3

25

0.06

8

0.05

20

2623.4

26

0.06

8

0.05

20

2623.4

27

0.06

8

0.05

20

2623.4

28

0.06

8

0.05

20

2623.4

29

0.06

8

0.05

20

2623.4

30

0.06

8

0.05

20

2623.4

parameters should be tuned carefully because the drawing stress is highly sensitive to these process parameters.

162

A. K. Pathak et al.

Table 7.3 Results of analysis of variance Source

Sum of squares

Degrees of freedom

Mean square

F-value

p-value

Model

2.62E+06

14

1.87E+05

23.63