FORMULATING THE PLASTIC DEFORMATION

EXPRESSION ON COATED SUBSTRATE AS A COATING

SELECTION TOOL

Nagentrau s/o Muniandy

A thesis submitted infulfillment of the requirement for the award of the

Degree of Masters

FACULTY OF MECHANICAL AND MANUFACTURING ENGINEERINGUNIVERSITI TUN HUSSEIN ONN MALAYSIA

MAY 2017

“To my parents, teachers and almighty God....”

To my beloved mother and father,

Mr. Mrs. Muniandy & Nookkaretnam

For being the backbone of my life by supporting me from the very beginning

To my supervisor and mentors,

A/Prof. Dr. Waluyo Adi Siswanto

Dr. Abdul Latif Mohd. Tobi

Prof. Dr. Mohd. Nasir Tamin

For their consistent encouragement, guidance and support throughout theresearch

To my family and friends

For their trust, cooperation and motivation during this project

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Acknowledgement

First and above all, I praise the Almighty God for blessings me the strength andcapability to complete the research successfully. This thesis owes its presence tothe motivation, support and assistance of several good souls. Therefore, I wouldlike to express my sincere gratitude to all of them. My greatest appreciation goesto my parents, my siblings and relatives for their unconditional love, patience andbeen together through my thick and thin. With their genuine love and respect Iwholeheartedly dedicate this thesis to my family.

I would like to express my deepest thanks to my supervisor Assoc. Prof.Dr. Waluyo Adi Siswanto, who has been a constant source of guidance andenthusiasm during my research. I greatly appreciate for his intellectual supportand persistent encouragement throughout this research project and thesis writing.I am thankful for his flexible discussion time, precious suggestions and mostimportantly his trust on me which really boosted my effort towards completingthe research.

A special thanks goes to my co-supervisor Dr. Abdul Latif Mohd. Tobibecause without him, I would never been interested in this research. I amgrateful for his financial assistance from setting up the research proposal untilthis research completed. I am also indebted to him because he moulded me intoa good researcher. Particular gratitude goes to Prof. Dr. Mohd. Nasir Taminfrom Universiti Teknologi Malaysia (UTM) for insightful ideas with his leadingknowledge contribution. My endless thanks to him for his inspiration in orderto accomplish this research and guiding me in journal writing.

I warmly thank all of my colleagues for their unflagging support and careduring the research. Last but not least, I would never forget all the good heartedpersonalties that have been an indispensable impetus who contributed directlynor indirectly to the progress of my research and the thesis writing.

Nagentrau s/o MuniandyBatu Pahat, Johor

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Abstract

This study presents the investigation of elastic coating performance responseson elastic-plastic substrate of advanced alloys using Finite Element (FE)method with an explicit numerical algorithm under quasi-static condition.Cylinder-on-flat contact configuration subjected to a normal and tangentialloading is examined. Two aeroengine materials, namely Ti-6Al-4V and SuperCMV (Cr-Mo-V) alloys are employed. Coating mechanical properties which areapplied load (500 N − 1000 N), sliding displacement amplitude(0.005 mm − 0.12 mm), friction coefficient (0.3 − 0.9), coating elastic modulus(100 GPa − 400 GPa) and coating thickness (0.01 mm − 0.1 mm) areinvestigated. The effect of coating parameters on stress-strain distributionsalong with plastic deformation of the coated substrate are evaluated. The FEmodel is validated by comparing with theoretical Hertzian contact solution forhomogeneous substrate, meanwhile coated substrate is validated by comparingwith reported literature. The evolutions of contact pressure, von Mises stress,equivalent plastic strain, tangential stress and surface profile are examined forvarious coating parameters. There is a clear increasing trend of development instress-strain distributions along with plastic deformation, maximum pile-up anddepth-in values with the increase in all coating parameters for both coatedsubstrates (except for coating thickness effect on Super CMV material, wherethe contradict trend is noted). Friction coefficient acts as the significant coatingparameter that leads to plastic deformation failure of coated substrate. Therelatively higher stiffness and yield strength of the coated Super CMV alloyregistered limited plastic deformation compared to the coated Ti-6Al-4V alloy.The coated substrate plastic deformation mathematical expressions areformulated according to the Lagrange multivariate interpolation which can beused as an alternative method for FE approach. The weighted scoring methodis practised in coating selection approach based on plastic deformation failure ofcoated substrate. The verified coating selection tool can contribute toknowledge in suitable material selection for coating based on its performance.

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Abstrak

Kajian ini membentangkan pengaruh pelbagai paramater lapisan (coating)elastik pada substrat elastik-plastik di bawah keadaan kuasi-statik denganmenggunakan kaedah unsur terhingga (FEM). Konfigurasi silinder atas platrata tertakluk kepada bebanan normal dan tangen dikaji. Dua jenis bahanaeroengine, iaitu aloi-aloi Ti-6Al-4V dan Super CMV (Cr-Mo-V) dikaji dalamanalisis ini. Antara sifat mekanikal lapisan yang dikaji ialah beban digunakan(500 N − 1000 N), amplitud anjakan (0.005 mm − 0.12 mm), pekali geseran(COF) (0.3 − 0.9), modulus elastik (100 GPa − 400 GPa) dan ketebalan lapisan0.01 mm − 0.1 mm). Kesan parameter lapisan pada tegasan-terikan bersamadengan deformasi plastik substrat dinilai. Model FE tanpa lapisan disahkandengan teori Hertzian contact manakala model FE dengan lapisan disahkandengan penyelidikan sebelum ini. Evolusi tekanan contact, tekanan von Mises,ketegangan plastik, tegasan tangen dan profil permukaan dikaji mengikutpelbagai parameter lapisan dan bahan substrat. Didapati satu trend yangmeningkat pada tegasan-terikan bersama dengan deformasi plastik, pile-up dandepth-in maksima dengan peningkatan kesemua parameter pada kedua-duasubstrat (kecuali untuk paramater ketebalan lapisan pada Super CMV). Pekaligeseran bertindak sebagai parameter yang jelas menyebabkan deformasi plastik.Hasil kajian juga menunjukkan bahawa kekuatan yield yang lebih tinggi padaaloi Super CMV berbanding dengan aloi Ti-6Al-4V menyebabkan deformasiplastik yang minima. Seterusnya, ungkapan matematik (mathematicalexpression) diperolehi dengan menggunakan interpolasi multivariat Lagrangeberdasarkan deformasi plastik pada substrat melalui simulasi. Ungkapanmatematik yang dihasilkan boleh digunakan sebagai pendekatan alternatifuntuk menggantikan teknik analisis kaedah unsur terhingga. Selain itu, kaedahpemarkahan wajaran (weighted scoring method) digunapakai dalam pendekatanpemilihan lapisan berdasarkan deformasi plastik. Pendekatan pemilihan iniboleh menyumbang kepada pengetahuan dalam pemilihan bahan yang sesuaiuntuk lapisan berdasarkan prestasinya.

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Contents

Declaration ii

Dedication iii

Acknowledgement iv

Abstract v

Abstrak vi

List of Figures x

List of Tables xvii

List of Appendices xix

List of Symbols xix

Chapter 1 Introduction 11.1 Research Background 21.2 Problem Statement 41.3 Research Objectives 41.4 Research Scopes and Limitations 51.5 Research Significance 61.6 Thesis Organisation 7

Chapter 2 Literature Review 82.1 Contact mechanics 8

2.1.1 Tribology 92.1.2 Friction 92.1.3 Hertzian elastic contact 10

2.2 Stress-strain relations 11

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2.3 Plasticity 162.3.1 Plastic deformation mechanism 172.3.2 Plasticity work hardening 20

2.4 Coating and Substrate 242.4.1 Tribological coating 252.4.2 Coating selection 26

2.5 Normal loading and tangential loading 272.6 Coating failure 29

2.6.1 Macromechanical friction and wearmechanisms on coating surface 29

2.7 Finite element method (FEM) 302.7.1 Previous Finite element analysis of coated

substrate 302.8 Coated substrate plastic deformation 352.9 Multivariate Lagrange Interpolation 372.10 Weighted scoring method 402.11 Summary of literature review 41

Chapter 3 Methodology 423.1 Introduction 423.2 Research flow 423.3 Basic Assumptions 473.4 Finite element modelling 48

3.4.1 Finite element simulation software 483.4.2 Physical model 493.4.3 Model Geometry 503.4.4 Assigning the material 533.4.5 Finite element meshing 543.4.6 Contact interaction 563.4.7 Boundary condition and step configuration 57

3.5 Assessment parameters and method 613.5.1 Assessment parameters 613.5.2 Assessment methods 63

3.6 Analysis techniques 633.6.1 Output Data Base (ODB) file 643.6.2 Area under the curve for energy terms 673.6.3 Surface profile analysis 68

3.7 Development of mathematical expression 693.8 Coating selection approach 70

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Chapter 4 Results and discussion 744.1 Introduction 744.2 Finite element model validation 75

4.2.1 Homogeneous substrate 754.2.2 Coated substrate 80

4.3 Evolution of internal energy terms 824.4 Coating Parameter study 84

4.4.1 Effect of contact Loading 844.4.2 Effect of sliding displacement amplitude 974.4.3 Effect of friction coefficient 1094.4.4 Effect of coating elastic modulus 1224.4.5 Effect of coating thickness 136

4.5 Surface profile 1494.6 Plastic deformation mathematical expressions of

coated substrate 1554.7 Coating selection 170

Chapter 5 Conclusion and recommendation 1745.1 Conclusion 1745.2 Knowledge contribution 1765.3 Recommendation for future works 178

References 179

Vitae 200

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List of Figures

Figure No Title Page

2.1 A frictional force,F is needed to cause motion rollingand sliding (Ludema, 1996) 10

2.2 Hertzian non-conforming contact (Johnson &Johnson, 1987) 11

2.3 Different material stress-strain response (Roesleret al., 2007) 12

2.4 Elastic deformation stress-strain relationship(Hibbeler, 2011) 13

2.5 Elastic-plastic deformation stress-strain relationship(Hibbeler, 2011) 14

2.6 Stress-strain diagram for ductile material (steel)(Hibbeler, 2011) 15

2.7 Relationship between shakedown behaviour and wearrate under repeated sliding condition (Fouvry et al.,2001) 17

2.8 Deformation process by slip; (a) undeformed, (b)elastically deformed (c) elastically and plasticallydeformed, (d) plastically deformed (Lal & Reddy,2009) 18

2.9 Most closely packed atoms in (a) ace-centred cubic,(b) hexagonal close packed and (c) body centred cubiccell structures (Lal & Reddy, 2009) 19

2.10 Twinning process (Lal & Reddy, 2009) 192.11 Isotropic hardening yield surface (Rees, 2012) 212.12 Kinematic hardening yield surface (Rees, 2012) 222.13 Process of delamination wear mechanism (Tobi et al.,

2009) 232.14 Coated substrate illustration 24

x

2.15 Tribological process of coated surface (Koutsomichaliset al., 2009) 26

2.16 Typical coating applications in the mechanicalengineering field (Grzesik, 2003) 27

2.17 Normal pressure and tangential traction on contacthalf-plane (Johnson & Johnson, 1987) 28

2.18 Macromechanical contact conditions for differentmechanisms which influence friction when a hardspherical slider moves on a coated flat surface(Holmberg et al., 2006) 30

2.19 Schematic illustration of the scratch tester stylusdrawn along the coated sample. The materialloading and response can be divided into threephases: ploughing, interface sliding and pulling thecoating (Holmberg et al., 2006) 31

2.20 Schematic illustration of the three-dimensional finiteelement mesh (Holmberg et al., 2006) 32

2.21 (a) Full model mesh, (b) contact region mesh detailand (c) schematic view of the cylinder on flat coatedsubstrate model (Mohd Tobi et al., 2011) 33

2.22 Comparison of FE-predicted and experimental wearprofiles for coated SCMV pair for (a) 100,000 cycles,(b) 300,000 cycles, and (c) 600,000 cycles (Mohd Tobiet al., 2011) 34

2.23 Model configuration (a) overall model layout and (b)mesh around the indentation contact in multilayercoatings (Zhao et al., 2011) 35

2.24 Effect of deformation, yield and the cracks formationon the stress-strain curve obtained from sphericalindentation of coated specimen. (1) elastic–plasticdeformation; (2) plastic deformation; (3) fracture ofcoating (Kot et al., 2013) 36

3.1 Overall research flowchart 433.2 Detailed flowchart of stage 1 : Coated substrate

parametric study 443.3 Detailed flowchart of stage 2 : Formulating coated

substrate plastic deformation mathematical expression 46

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3.4 Detailed flowchart of stage 3 : Verifying coatingselection tool 47

3.5 Schematic of crossed cylinder-on-flat geometry usedfor fretting tests 50

3.6 Schematic view of the half cylinder-on-flat coatedsubstrate contact configuration 52

3.7 Assembly of half cylinder and flat plate 523.8 Plastic stress-strain curves for Ti-6Al-4V and SCMV

(Benedetti & Fontanari, 2004; Leen et al., 2001) 543.9 Variations of the predicted maximum contact pressure

with element size 553.10 Mesh module of the 2D contact model 563.11 Master and slave surfaces interaction 573.12 Contact property of the coated substrate 573.13 Initial step - applying boundary condition 593.14 Step 1 - Applying the load 593.15 Step 2 - Applying the displacement 603.16 Step 3 - Unloading 603.17 Abaqus coated substrate simulation ODB file 653.18 Analysis techniques to extract FE predicted results 653.19 FE result extraction from specific element from ODB

field output 663.20 FE result extraction using path line technique 663.21 Initial and final contact point position 673.22 Diagram illustrating Simpson’s rule 683.23 Schematic diagram of surface deformation occurred on

elastically coated elastic-plastic substrate 693.24 Coated substrates material weightage breakdown 713.25 Coated substrate parameter weightage breakdown 72

4.1 Contact pressure and contact half-width lengthcomparison between FEM analysis and Hertziancontact theoretical solution for Ti-6Al-4V 77

4.2 Contact pressure and contact half-width lengthcomparison between FEM analysis and Hertziancontact theoretical solution for Super CMV 78

4.3 Sub-surface stress along Y-axis comparison betweenFEM analysis and Hertzian contact theoreticalsolution for Ti-6Al-4V material 79

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4.4 Sub-surface stress along Y-axis comparison betweenFEM analysis and Hertzian contact theoreticalsolution for Super CMV material 79

4.5 FE predicted tangential stress distributionscomparison between FEM analysis and previousstudy for coated Ti-6Al-4V substrate 81

4.6 FE predicted tangential stress distributionscomparison between FEM analysis and previousstudy for coated Super CMV substrate 82

4.7 Evolution of internal and kinetic energy of the coatedsubstrate system 84

4.8 Contact pressure distribution of different contactloading condition 86

4.9 Von Mises stress history of different contact loading condition 894.10 Equivalent plastic strain history of different contact

loading condition 914.11 Equivalent plastic strain after complete unloading for

500 N contact loading 934.12 Equivalent plastic strain after complete unloading for

750 N contact loading 944.13 Equivalent plastic strain after complete unloading for

1000 N contact loading 954.14 Tangential stress distribution of different contact loading 964.15 Contact pressure distribution of different sliding

displacement amplitude 984.16 Von Mises stress history of different sliding

displacement amplitude 1014.17 Equivalent plastic strain history of different sliding

displacement amplitude 1034.18 Equivalent plastic strain after complete unloading for

0.005 mm sliding displacement amplitude 1054.19 Equivalent plastic strain after complete unloading for

0.05 mm sliding displacement amplitude 1064.20 Equivalent plastic strain after complete unloading for

0.12 mm sliding displacement amplitude 1074.21 Tangential stress distribution of different sliding

displacement amplitude 1094.22 Contact pressure distribution of different friction coefficient 1114.23 Von Mises stress history of different friction coefficient 114

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4.24 Equivalent plastic strain history of different friction coefficient 1164.25 Equivalent plastic strain after complete unloading for

friction coefficient of 0.3 1184.26 Equivalent plastic strain after complete unloading for

friction coefficient of 0.6 1194.27 Equivalent plastic strain after complete unloading for

friction coefficient of 0.9 1204.28 Tangential stress distribution of different friction coefficient 1224.29 Contact pressure distribution of different coating

elastic modulus 1254.30 Von Mises stress history of different coating elastic modulus 1284.31 Equivalent plastic strain history of different coating

elastic modulus 1304.32 Equivalent plastic strain after complete unloading for

coating elastic modulus of 100 MPa 1324.33 Equivalent plastic strain after complete unloading for

coating elastic modulus of 200 MPa 1334.34 Equivalent plastic strain after complete unloading for

coating elastic modulus of 400 MPa 1344.35 Tangential stress distribution of different coating

elastic modulus 1364.36 Contact pressure distribution of different coating thickness 1384.37 Von Mises stress history of different coating thickness 1414.38 Equivalent plastic strain history of different coating thickness 1434.39 Equivalent plastic strain after complete unloading for

coating thickness of 0.01 mm 1454.40 Equivalent plastic strain after complete unloading for

coating thickness of 0.05 mm 1464.41 Equivalent plastic strain after complete unloading for

coating thickness of 0.1 mm 1474.42 Tangential stress distribution of different coating thickness 1494.43 Surface profiles of the coated substrates upon

complete unloading under different load 1504.44 Surface profiles of the coated substrates upon

complete unloading under different slidingdisplacement amplitude 151

4.45 Surface profiles of the coated substrates uponcomplete unloading under different friction coefficient 151

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4.46 Surface profiles of the coated substrates uponcomplete unloading under different coating elastic modulus. 152

4.47 Surface profiles of the coated substrates uponcomplete unloading under different coating thickness 152

4.48 Decimal digits truncation effect on mathematicalexpression accuracy 157

4.49 Equivalent plastic strain curve of Ti-6Al-4V as afunction of time and loading 160

4.50 Equivalent plastic strain curve of SCMV as a functionof time and loading 161

4.51 Equivalent plastic strain curve of Ti-6Al-4V as afunction of time and sliding displacement 162

4.52 Equivalent plastic strain curve of SCMV as a functionof time and sliding displacement 163

4.53 Equivalent plastic strain curve of Ti-6Al-4V as afunction of time and friction coefficient 164

4.54 Equivalent plastic strain curve of SCMV as a functionof time and friction coefficient 165

4.55 Equivalent plastic strain curve of Ti-6Al-4V as afunction of time and coating elastic modulus 166

4.56 Equivalent plastic strain curve of SCMV as a functionof time and coating elastic modulus 167

4.57 Equivalent plastic strain curve of Ti-6Al-4V as afunction of time and coating thickness 168

4.58 Equivalent plastic strain curve of SCMV as a functionof time and coating thickness 169

A.1 Tangential stress contour during loading 191A.2 Normal stress contour during loading 192A.3 Shear stress contour during loading 192A.4 Tangential stress contour during sliding 193A.5 Normal stress contour during sliding 193A.6 Shear stress contour during sliding 194A.7 Tangential stress contour during unloading 194A.8 Normal stress contour during unloading 195A.9 Shear stress contour during unloading 195B.1 Surface spatial displacement at elemental nodes

during loading (Step-1), sliding (Step-2) andunloading (Step-3) for Ti-6Al-4V 196

xv

B.2 Surface spatial displacement at elemental nodesduring loading (Step-1), sliding (Step-2) andunloading (Step-3) for SCMV 197

xvi

List of Tables

Table No Title Page

3.1 Materials properties of coated substrate 543.2 Smooth step amplitude for analysis 613.3 Different coated substrate configuration models 623.4 Coated substrates material weightage 713.5 Coated substrate parameter weightage 72

4.1 Validation of homogenous substrate FE model 784.2 Validation of coated substrate FE model 814.3 Internal and kinetic energy of the coated substrate system 844.4 Maximum contact pressure of different contact

loading condition 874.5 Contact half-width length of different contact loading

condition 874.6 Maximum von Mises stress of different contact loading

condition 894.7 Maximum equivalent plastic strain of different contact

loading condition 914.8 Maximum contact pressure of different sliding

displacement amplitude 994.9 Contact half-width length of different sliding

displacement amplitude 994.10 Maximum von Mises stress of different sliding

displacement amplitude 1014.11 Maximum equivalent plastic strain of different sliding

displacement amplitude 1044.12 Maximum contact pressure of different friction coefficient 1124.13 Contact half-width length of different friction coefficient 1124.14 Maximum von Mises stress of different friction coefficient 114

xvii

4.15 Maximum equivalent plastic strain of different frictioncoefficient 116

4.16 Maximum contact pressure of different coating elasticmodulus 125

4.17 Contact half-width length of different coating elastic modulus 1264.18 Maximum von Mises stress of different coating elastic

modulus 1284.19 Maximum equivalent plastic strain of different coating

elastic modulus 1314.20 Maximum contact pressure of different coating thickness 1394.21 Contact half-width length of different coating thickness 1394.22 Maximum von Mises stress of different coating thickness 1414.23 Maximum equivalent plastic strain of different coating

thickness 1434.24 Maximum pile-up and depth-in of the coated

substrates according to different parameters 1544.25 Mathematical expression truncation with different

decimal digits 1574.26 Coated substrate plastic deformation mathematical

expressions for different coating parametersaccording to the substrate material. 159

4.27 Coating selection table considering plasticdeformation of coated substrate 171

4.28 Coating selection options 172

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List of Appendices

Appendix Title Page

A Stress components comparison of coated substrate 191B Surface spatial displacement at elemental nodes

during simulation 196C Matlab script file for mathematical expression 198

xix

List of Symbols

α Dundurs parameters on elastic mismatch

β Dundurs parameters on Poisson’s ratio difference

δ Sliding amplitude

µ Friction coefficient

ρ Density

σf Fracture stress

σpl Proportional limit

σu Ultimate stress

σy Yield stress

εe Elastic strain

εp Plastic strain

εx Total deformation

A Distance along the first line

a Contact half-width length

B Distance along the middle line

C Distance along the last line

c Coating

E Elastic modulus

h Distance betwen each line

P Normal load

p(x) Contact pressure distribution

po Maximum contact pressure

q(x) Tangential traction

xx

R Radius

s Substrate

t Time

v Poisson ratio

2D Two-dimensional

3D Three-dimensional

AS Aggregate score

COF Coefficient of friction

CPRESS Contact pressure

FE Finite Element

FEM Finite Element Method

GPa Giga Pascal

LSMB Lower Specimen Mounting Block

LVDT Linear Variable Displacement Transducer

MISES Von Mises stress

mm Milli Meter

MPa Mega Pascal

N Newton

ODB Output Data Base

PEEQ Equivalent plastic strain

PW Parameter weightage

S Score

S11 Tangential stress

SCMV Super Chromium Molybdenum Vanadium alloy

SW Substrate weightage

Ti-6Al-4V Titanium alloy

U Spatial displacement at nodes

USMB Upper Specimen Mounting Block

xxi

Chapter 1

Introduction

Contact mechanics is concerned as the study of solid deformation when twobodies (surfaces) are touching and interacting with each other at one or morepoints. Mechanics of contact is deliberated as an essential subject because mostof the engineering applications deal with it consequently. Contact mechanicsoffers an interesting, yet challenging research subject as it can lead to failureeffect on contacting materials such as plastic deformation, fracture, fatigue,wear and others. Obviously, the contact failure can be significant and able todevelop under corrosive, erosive, high temperature and heavy contactenvironment.

In the recent decades, minimizing contact failure on contacting bodies isconsidered to be a major concern, especially in safety engineering as contactmechanics failure can result in costly flaws and fatality. So, the knowledge inprotecting contacting bodies from failure is very crucial, thus advances inreducing contact mechanics failure need to be studied extensively. One of themost competent approach to protect contact engineering components is bycoating application. A coating can be defined as a covering that is applied onthe surface of a body which referred as the substrate. Coating widely used fordecorative, functional or both purposes.

The coating purpose broadly noticeable in engineering applications suchas manufacturing, automotive and aeroengine industries in order to minimizecontact damage which will extend the operational life of components.Interestingly, as coating offers the first line of defence to the underlyingsubstrate, it is vital to study the coating architecture along withcoating-substrate failure mechanism to prolong the contacting componentslifetime. So, the application of appropriate coatings will contribute in reducing

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the major economic loss by protecting contacting components from any failure.

1.1 Research Background

Recent developments of coating application in contact mechanics have led to arenewed interest in coating selection. In addition, coatings are generally used toprovide protection for surfaces that subjected to contact loading, i.e. pistonrings, bearings, machine tools, cams and followers (Oliveira & Bower, 1996).Coating approach is practised in order to alter the surface properties of asubstrate such as wettability, wear resistance, adhesion or corrosion resistance.For instance, thermal barrier coating is applied in gas turbine engines to guardcomponents from thermal shock; hard and corrosion resistant coatings areapplied on machining and drilling tools to protect the components from friction,wear and fretting damage; in medical devices or implants coating offers wearresistance, chemical durability and biocompatibility (Demidova et al., 2012).

Failure of the coating or substrate is a major concern in coatedengineering applications. Obviously, the deterioration of coated substrate canbe associated with the failure of coating itself or the substrate deformation(Mohd Tobi et al., 2013). It appears from the aforementioned studies that mostattention has been paid to contact mechanics of different coating/substratecomposites (Komvopoulos, 1989; Gupta & Walowit, 1974; Weppelmann &Swain, 1996). Hard elastic coatings are suitable solutions to enhance surfaceperformance and improving endurance life of contact components. Most hardcoatings are made up of ceramic compounds such as nitrides, carbides, cermets,ceramic alloys and metastable materials like cubic boron nitride and diamond.

According prior evidence, hard elastic coating are considered as a greatchoice in providing protection for substrate that are subjected to contact loadingas this type of coating capable to reduce friction due to contact stresses. Inaddition, various failure mechanism of hard coated surface such as delamination,thickness cracking, coating buckling, spallation and substrate plastic deformationare observed and studied by previous researchers (Demidova et al., 2012; Bull &G-Berasetegui, 2006). A number of studies investigating coated contact subjectedto normal and tangential loading by FE simulations proved that the stresseswithin the layer varied with the coating thickness and elastic mismatch level(Holmberg et al., 2006; Mohd Tobi et al., 2011; Zhao et al., 2011). Holmberget al. (2006) carried out a numerical study of 3D scratch test on TiN coatedsteel substrate under elastic-plastic deformation and compared with experimentalstudies where the first crack observed is associated with tensile and bendingstresses at coating surface as scratched by the indenter.

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Michler & Blank (2001) studied the coating fracture and substrateplasticity induced by the spherical indenter; their analysis revealed that theinitial damage of coated substrate system is noticed to occur by plasticdeformation of substrate or below the interface. This is consistent with theexperimental and numerical study on spherical indentations of coating-substratesystem demonstrated by Kot et al. (2013), which showed that coated substratedamage initiated as substrate elastic-plastic deformation followed by plasticdeformation and finally fracture of coating.

Titanium alloy (Ti-6Al-4V) and high strength steel Super CMV(Cr-Mo-V) are the materials which show high interest on coating utilization dueto their wide application in various field. Interestingly, Ti-6Al-4V and SuperCMV alloys had been investigated by several researchers because of their highstrength, ductility, excellent corrosion and fatigue resistance characteristicsalong with their extensive application in aeroengines and gas turbines (Ford,1997; McColl et al., 2004; Tobi et al., 2009; Hyde, 2002). Besides that, coatingutilization on Ti-6Al-4V and Super CMV alloys is necessary as both areaeroengine specific materials where potentially can experience contact damagessuch as in the spline coupling (Leen et al., 2002) and the dovetail fan blade joint(Ciavarella & Demelio, 2001; Rajasekaran & Nowell, 2006).

So, appropriate surface coating selection will be able to contribute onimproving reliability and lifetime of these contact components. Previous studieshave been primarily concentrated on coating failures such as coating brittlefracture, through thickness cracking, spallation, delamination coating bucklingand others (Bull & G-Berasetegui, 2006; Bull, 1991; Mohd Tobi et al., 2013).However, studies related to substrate failure are relatively scanty and there hasbeen limited investigation on substrate plastic deformation. In addition,substrate plastic deformation occurs before any other failure triggers in coatedsubstrate system (Zheng & Sridhar, 2006).

Obviously, substrate plastic deformation can develop without the coatingfailure but this phenomena has not yet received much attention as only limitedstudy focused on this particular type of failure. In the present study, theinfluence of coating mechanical properties on substrate plastic deformationunder quasi-static condition is focused. Comprehensive numerical analysis isconducted to examine the stress-strain distributions along with plasticdeformation of coated substrate subjected to normal and tangential loading. Inaddition, plastic deformation mathematical expression is formulated byconsidering FE equivalent plastic strain data. Last but not least, coatingselection tool considering coated substrate plastic deformation is developed.

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1.2 Problem Statement

Generally, the awareness on the contacting components safety design and itsprotection is a concern in minimizing costly flaws and human safety in variousengineering field. It may however be noted that rigorous study can be doneto reduce potential danger of coated contact components failure due to plasticdeformation of the substrate. In the future, coating need to be designed in orderto withstand substrate plastic deformation. It should be noted that there is stillshortage of systematic studies on the effect of mechanical properties of coatingon substrate plasticity.

Substrate plastic deformation failure should get an insight as the initialcoated substrate system failure maps occur by substrate yield (Michler & Blank,2001). Sooner or later, the substrate plastic deformation will cause failure on thecoating surface which acts as first line of defence for a coated substrate system(Kot et al., 2013). In addition, coating selection based on substrate yield forspecific applications is a complicated task due to many factors such as appliedexternal load, sliding displacement amplitude, contact friction coefficient coatingmaterial and thickness should be taken into consideration.

For time being, most of the coating selection with the aim of minimisingsubstrate plasticity is done by trial and error basis which might lead to costlyflaws and degrade the sustainability of the industry. In particular, FiniteElement approach should be practised in predicting substrate plasticdeformation which will aid in coating selection as this method is an effectivetool in determining stress-strain field in coated substrate system. Besides that,the understanding on coating selection and purpose should be polished as lackof fundamental understanding in term of coated substrate mechanics analysis.Furthermore, a comprehensive guide for coating design based on the plasticdeformation of Ti-6Al-4V and Super CMV alloys has not yet been established.

Hence, protecting these materials from plasticity by proper coatingselection often necessary because of their wide application in engineering fields,i.e. aeroengine specific, armour plating, spacecraft, naval ships, missiles, highperformance automotive parts, medical devices and industrial machines.

1.3 Research Objectives

The purpose of this research are as follows :

(a) To evaluate the influence of coating mechanical properties, i.e. normalloading, sliding displacement amplitude, friction coefficient, elastic

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modulus of coating and coating thickness on stress-strain distributionsalong with plastic deformation response of coated substrate subjected tocontact loading.

(b) To formulate mathematical expressions based on numerical plastic strainresults of coated substrate system verified in numerical problem cases.

(c) To verify an effective coating selection tool based on the substrate plasticdeformation.

1.4 Research Scopes and Limitations

The scopes and limitations of the study are designed to achieve the goals of theresearch. Hence, the scopes and limitations for this research are as follows :

(a) Modelling two dimensional (2D) cylinder-on-flat contact configurationsusing coated aeroengine specific materials made of Titanium alloy(Ti-6Al-4V) and high strength steel Super CMV (Cr-Mo-V).

(b) Studying concerned variables of different coating mechanical properties,i.e. applied external load, sliding displacement amplitude, contact frictioncoefficient, coating material and thickness and their influence onstress-strain behaviour along with plastic deformation of the coatedsubstrate..

(c) Modelling perfectly bonded elastic coating on elastic-plastic substratesubjected to contact loading with three steps, i.e loading, sliding andunloading without any slippage and delamination occur at the interfaceregion.

(d) Fracture mechanics concept is not implemented as the coating assumed towithstand higher compressive and tensile forces.

(e) Coating debonding due to unloading, thermal stress and strain rate effectsare not considered in this particular study.

(f) Formulating plastic deformation mathematical expressions based onnumerical plastic strain results by using multivariate variable technique.

(g) Verifying an effective coating selection tool using weighted scoring approach.

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1.5 Research Significance

The contribution of this study is obvious as the resulting outcomes can becapitalized as guidelines to evaluate contacting coated substrate mechanics andthe failure associated by substrate plastic deformation thorough theimplementation of numerical analysis. The uniqueness of this research will bean advancement in the fundamental understanding of coating selection based onsubstrate yielding in engineering applications. In addition, the long termimplications of this study will benefit multi-billion industries which deals withhard elastic coating usage on ductile substrate by proper coating selection toprotect materials from plastic deformation, thus sustainability of the industry isguaranteed.

Since there has been a significant rise on aerospace industry all over theworld, this study will lend a hand in the growth of technology in aerospaceindustry. Besides that, Malaysian government has planned to invest RM177million by year 2020 in local aerospace company named Strand AerospaceMalaysia Sdn. Bhd. under the Economic Transformation Plan in order toimprove human capital for High Value Engineering Services. So, the findings ofthis study will improve performances and reducing maintenance cost ofaeroengine components via proper coating selection as aeroengine specificmaterials i.e. Ti-6Al-4V and Super CMV alloys are focused in this particularresearch. In addition, the benefaction of this study to our knowledge is not onlyin addressing the view point in academics but also assist in nation economicdevelopment especially, Human Capital Development in infrastructure.

The current research will contribute to our knowledge by addressing threeimportant issues as follows :

(a) The influence of coating mechanical properties on stress-straindistributions along with plastic deformation response of coated substratecan be studied in detail. Thus, any failure associated by coated substrateplastic deformation can be prevented and life time of coated contactingcomponents prolonged.

(b) Verified mathematical expressions according to Multivariate lagrangeinterpolation concerning coating parameters and substrate materials.These mathematical expressions can be used to predict equivalent plasticstrain of coated substrates with high accuracy results without requiringany expensive computational time.

(c) An effective coating selection tool based on the substrate plasticdeformation will be provided. The coating selection tool will act as a

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guideline for proper coating selection by comparing and identifyingsuitable coating according to substrate material and working conditions.

1.6 Thesis Organisation

The outline of thesis is given below :

Chapter 1 contains the research background, problem statement, objectives,scopes and significance of the research. In addition, a brief introduction isgiven by addressing the motivation and importance of the study.

Chapter 2 reviews pertinent literatures on the understanding of stress-strain,plastic deformation mechanism, finite element analysis, coating-substratefailure and coating selection in the study of coated substrate plasticdeformation prediction for coating selection tool.

Chapter 3 outlines the numerical methodology using explicit finite elementapproach pursued with different mechanical coating properties. Theapproach of formulating mathematical expressions and verifying coatingselection tool is focussed.

Chapter 4 provides verification and validation of numerical model withanalytical solution and previous research results. The numerical result ofstress-strain distributions along with plastic deformation of coatedsubstrate is studied in detail. The mathematical expressions based onnumerical plastic strain results of coated substrate system are formulatedbased on Lagrange multivariate interpolation and an effective coatingselection tool is verified using weighted scoring method.

Chapter 5 offers the conclusions for this particular work. The recommendationfor coating selection based on substrate plastic deformation is discussed.

Chapter 2

Literature Review

2.1 Contact mechanics

The study of solid deformation where it touches each other at one or more thanone points is referred as contact mechanics. A contact is said to be occurred if:

• Mechanical interaction of bodies along surfaces

• Surfaces should “touch”

• Occurrence of force pressing two bodies

• Contact area rely on materials, forces, temperature and geometry.

Generally, contact concern both elastic and plastic strain ranges beneathasperities (Ludema, 1996). Contact mechanics also distinguish betweenconforming and non-conforming contacts. When the surfaces of two bodies fitclosely without deformation, then the contact is said to be conforming, whilebodies which have contradictory profiles are considered to be non-conforming(Johnson & Johnson, 1987). Virtually, each and every movement in this planetrelated with contact and friction such as running, walking, driving cars orstreaming of trains (Wriggers & Laursen, 2006). The base aspects that takeninto account in the contact mechanics topic are normal direction, frictionalstress acting tangentially between surfaces and the adhesion or pressure whichacting perpendicularly to the contacting body surfaces.

The surface contact points which can be seen widely in engineeringpractice often execute complex motions which addressed to transmit both forcesand moments (Johnson & Johnson, 1987). In addition, contact mechanics isfoundational to the mechanical engineering field as it provides proper

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information for the safe and energy efficient design of technical system fortribology study. The area of interest in contact mechanics are normal contact ofelastic and inelastic solids, tangential loading and sliding contact, rollingcontact of elastic and inelastic bodies, calendering and lubrication, dynamiceffects and impact, thermoelastic contact and rough surface contact.

2.1.1 Tribology

Tribology is defined as science and engineering of contacting surfaces in relativemotion. It comprises the study and application of the fundamental of friction,lubrication and wear. Tribology is the ’Ology’ or science of ’Triben’ where theword is originated from the Greek root known as ’Tribulation’. A directtranslation defines Tribology as the study of rubbing and sliding. The modernand expanded meaning of tribology known as the study of friction, lubricationand wear (Ludema, 1996). In addition, tribology is a crucial aspect as themovement of one solid surface on another is fundamentally essential to theoperation of many mechanisms such as natural and artificial (Hutchings, 1992).Tribology research and development benefit mankind in many ways such asquality, raw material savings, economical savings, energy savings, environmentaland health factors.

For instance, in automotive industries the minimization in themechanical losses because of effective tribology around 10 % could reduce thefuel consumption as 1.5 % equivalent to 340 litre of petrol during the lifetime ofthe car (Tzanakis et al., 2012). Obviously, tribology plays a pivotal role inmanufacturing to analyse and solve the costly flaws due to friction and wear inindustries.

2.1.2 Friction

Friction is a force which known as the resistance encountered by one movingbody on another body (Hutchings, 1992). In addition, friction also can bedefined as a force which resists sliding where it described in term of coefficientand mostly assumed as constant and specific to each material (Ludema, 1996).This wide definition embraces two major classes of relative motion such assliding and rolling. Figure 2.1 presents in both ideal sliding and rolling thepresence of tangential force is required to translate the upper body over a staticcounter face. The ratio of frictional force and normal force is established as thefriction coefficient µ. The law of friction are stated as follows:

• the friction force is proportional to the normal load

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• friction force is independent of the apparent contact area

• friction force is independent of the sliding velocity

Amontons equation states that frictional force described as F = µW wherestatic friction or kinetic friction coefficient is stated as µ while the normal loadas W (Bhushan & Gupta, 1991). Frictional contacts can be classified into fewclasses such as force transmitting components, energy absorption-controllingcomponents, quality control components and low friction components (Ludema,1996).

Figure 2.1: A frictional force,F is needed to cause motion rolling and sliding(Ludema, 1996)

2.1.3 Hertzian elastic contact

There are two types of Hertzian elastic contact, i.e. conforming contact andnon-conforming contact. Conforming contact is considered as a contact wheretwo mating bodies touch exactly each other without any deformation (Johnson& Johnson, 1987). Journal bearing and a flat slider bearing are the examples ofconforming contacts. Meanwhile, two bodies with various profiles are said to benon-conforming. When two non-conforming solids are in contact, the occurrenceof deformation around their initial point of contact under the existence of loadis takes place (Johnson & Johnson, 1987). A theory of contact is necessary toforecast the shape of contacting area and how it will transform if the applied loadis rising. There are some assumptions in the Hertzian theory to formulate thecalculation is valid. This particular type of contact is well displayed in Figure 2.2.The assumptions are as follows:

• The surfaces are continuous and non-conforming: a ≤ R

• The strains are minimum.

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• The surfaces are friction free: qx = qy = 0

• Each solid can be considered as an elastic half-space: a ≤ R1,2, a ≤ l

Where the major dimension of the contact area, R is the relative radius ofcurvature and l is the significant of the bodies both literally and in depth.

Figure 2.2: Hertzian non-conforming contact (Johnson & Johnson, 1987)

2.2 Stress-strain relations

Components which are widely used in engineering applications made up ofdifferent dimensions and complicated geometry which results in load that varystrongly all over the component. Plasticity theory considered as a branch ofmechanics that involves with the calculation of plastic strain and stressdistribution of a body, commonly a ductile material which deformedpermanently when force is applied (Chakrabarty, 2006). A Stress-strain curvesnormally describes the elastic-plastic behaviour of a material. The stress-straincurve of different material will vary according to the material classes as shownin Figure 2.3 (Roesler et al., 2007).

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Figure 2.3: Different material stress-strain response (Roesler et al., 2007)

Studies have found that a material can exhibit two types of ofdeformation such as elastic and plastic deformation respectively. Elasticdeformation can be defined as the recovering of a loaded material to its originalstate upon complete unloading and the stress and strain are directlyproportional to each other (Gurtin, 1973; Hibbeler, 2011). Figure 2.4 illustratesthe stress-strain relation during elastic deformation. The stress-strain growthshows a linear trend which known as linear elastic deformation. Eq. (2.1) is usedto calculate the elastic modulus, E of a material based on the stress-straincurve. The elastic modulus, E corresponds to the particular proportionalityconstant between stress and strain. The elastic modulus, E is obtained from theslope of the elastic deformation linear curve.

E = σ

ε= σb − σaεb − εa

(2.1)

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Figure 2.4: Elastic deformation stress-strain relationship (Hibbeler, 2011)

Meanwhile, the plastic deformation is different from the elasticdeformation where the loaded material does not recover to the initial positionupon complete unloading (Prevost, 1985; Hibbeler, 2011). This leads to thedeformation is permanent. The initiation of plastic strain can lead to largedeformation when small amount of stress is induced and this will cause furtheryielding until the material failure. The point where permanent deformationbegins to occur known as Yield stress, σy. Figure 2.5 demonstrates the responseof stress-strain during elastic-plastic deformation. It is noted that plastic strain,εp is significant than the elastic strain, εe. The total deformation of a material,εx is the resultant of the plastic strain, εp and elastic strain, εe.

Eq. (2.2) presents the stress-strain curve in terms of the Ramberg-Osgoodexpression. Eq. (2.2) is originally developed for non-linear metals which encounteryielding and plastic strain. It involves the initial Young’s modulus (E0), the proofstress (σp) corresponding to the plastic strain (p) and (n) which determines thestrain hardening of the stress-strain curve.

ε = σ

E0+ p

(σ

σp

)n(2.2)

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Figure 2.5: Elastic-plastic deformation stress-strain relationship (Hibbeler, 2011)

Numerous studies have attempted to explain the stress-strain behaviourof ductile materials (Dieter & Bacon, 1986; Hibbeler, 2011). Figure 2.6 presentsthe stress-strain diagram for ductile material especially steel. Based on theFigure 2.6, it is noted that there are 4 types of region in a stress-strain diagramfor ductile material, namely elastic region, yielding region, strain hardeningregion and necking region. The material behaves differently depending on theinduced strain amount. In addition, the composition of the material, loadingrate, temperature, microscopic imperfection and test time affect the stress-strainbehaviour of a material (Hibbeler, 2011; Kienzler & Herrmann, 2012).

Elastic behaviour of the material is occur at the elastic region as shownin Figure 2.6. The behaviour of this particular region is consistent with theFigure 2.4 as discussed earlier. In this region, the stress is proportional to thestrain and the material is said to be linear elastic. The proportional limit, σpl isdefined as the upper stress limit of the linear elastic straight line. Once the stressexceeds the proportional limit, σpl it reaches the elastic limit. Upon achieving this

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particular point, the material tend to return to its initial shape upon completeunloading. In fact, elastic limit rarely determined for steel since it is very close tothe proportional limit, σpl and very difficult to detect (Hibbeler, 2011; Kienzler& Herrmann, 2012).

A slight advancement in stress which exceeds the elastic limit will lead tomaterial breakdown and permanent deformation; such a behaviour is called asmaterial yielding which lead to plastic deformation. Yield stress or yield point, σyis the critical stress that causes material yielding. The material will continuouslyelongate (strain) without any increment in load as exhibited in Figure 2.6 and thisstate often referred as perfectly plastic (Hibbeler, 2011; Kienzler & Herrmann,2012).

Once the material achieved complete yielding, then an increment in loadcan be withstand by that particular material. This is mainly due to curve thatincreases continuously until reach the the maximum stress known as the ultimatestress, σu. The increment in such a manner is referred as strain hardening asdepicted in Figure 2.6 (Hibbeler, 2011; Kienzler & Herrmann, 2012).

Last but not least, the necking region is where the material elongates as thestress exceeds the ultimate stress, σu. In deed, such a case will result in decreasingof cross-sectional area in a localized region of the material. Thus, constrictionor ’neck’ is formed as the material further elongates and finally break at fracturestress, σf as shown in Figure 2.6 (Hibbeler, 2011; Kienzler & Herrmann, 2012).

Figure 2.6: Stress-strain diagram for ductile material (steel) (Hibbeler, 2011)

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2.3 Plasticity

Plasticity can be defined as deformation of a material which undergoingpermanent change or non-reversible response due to applied force. Most of thereal materials will experience some permanent deformation, whichnon-reversible after elimination of the load or force. Normally, major permanentdeformations happen when stress achieves some critical value, known as yieldstress which is a material property. Classical theory of plasticity states thatinitially materials deform elastically and deform plastically when reaching theyield stress. Perfect plasticity can occur as material undergo an irreversibledeformation without any increment in stress due to load.

It is noted that the accumulation of the plasticity by ratcheting cancause exhaustion of ductility which induced to cracks and delamination wear(Mohd Tobi et al., 2013). As shown in Figure 2.7, the occurrence of globalsurface shear plasticity shows higher wear regime meanwhile, local asperityplasticity shows lower wear regime (Fouvry et al., 2001). Discontinues ofvelocity and large deformation makes plasticity problems difficult to resolveanalytically than elastic problems (Yin & Komvopoulos, 2012).

17

Figure 2.7: Relationship between shakedown behaviour and wear rate underrepeated sliding condition (Fouvry et al., 2001)

2.3.1 Plastic deformation mechanism

There are two noticeable mechanism of plastic deformation such as slip andtwinning. Slip considered as prominent mechanism of plastic deformation inmetal. The blocks of crystal slide over each other along the definitecrystallographic planes which named as slip planes. It is compulsory that largescale of slipping of adjacent atomic plane (more than half atomic spacing) mustoccur to cause plastic deformation. It is equivalent to a deck of cards pushedfrom one end as slip happens when applied shear stress greater than a criticalvalue.

The crystal lattice structure remains same during slipping process,meanwhile there is modifications in atoms relative position as shown inFigure 2.8 (Lal & Reddy, 2009). The structure of each lattice has definite slipplanes and direction laterally where slipping can take place. By the way,slipping occur more freely along the planes at closely packed atoms. Figure 2.9shows face centred cubic, hexagonal close pack and body centred cubicstructure.

18

Twinning can be termed as a portion of crystal follows an orientationwhich is related to the orientation of remaining untwinned lattice in a definite,symmetrical way. The twinned part of the crystal can be considered as reflective(mirror) image of parent crystal. The symmetry plane is known as twinning plane.The chief role of twinning in plastic deformation is it acts as a root in changing theplane orientation to cause advancement in slip. Figure 2.10 illustrates twinningprocess results in atoms dislocation (Lal & Reddy, 2009).

Figure 2.8: Deformation process by slip; (a) undeformed, (b) elastically deformed(c) elastically and plastically deformed, (d) plastically deformed (Lal& Reddy, 2009)

19

Figure 2.9: Most closely packed atoms in (a) ace-centred cubic, (b) hexagonalclose packed and (c) body centred cubic cell structures (Lal & Reddy,2009)

Figure 2.10: Twinning process (Lal & Reddy, 2009)

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2.3.2 Plasticity work hardening

Work hardening can be defined as material strengthening due to plasticdeformation. Work hardening also can prevent the nucleation of more newdislocations. The material strengthening can act as a resistance to thedislocation formation, hence plastic deformation can be minimised. In metalliccrystals, permanent deformation is typically carried out on a microscopic scaleby imperfections named dislocations, which usually formed by local stress fieldfluctuations in the particular material resulting in rearrangement of lattice dueto propagation of dislocations over the lattice. Annealing process annihilatesthe dislocations at room temperature.

In addition, the interaction of dislocations with one another leads toaccumulation and act as obstacles which significantly obstruct their motion.Thus, the yield strength of the material is improved and successive decreases inductility. The yield surface for hardening materials can develop in space bythree ways such as isotropic hardening, kinematic hardening and combinedisotropic-kinematic hardening.

Isotropic hardening

The first type of plasticity hardening is known as isotropic hardening. For thisparticular hardening the yield surface grows bigger in size meanwhile the centreremains at fixed position in the stress space as shown in Figure 2.11. For isotropichardening, if plastically deform a solid, then unload it, then try to reload it again,its yield stress (or elastic limit) would have increased compared to what it wasin the first cycle (Rees, 2012). Again, when the solid is unloaded and reloaded,yield stress (or elastic limit) further increases. as long as it is reloaded past itspreviously reached maximum stress. This advances until a stage (or a cycle) isreached that the solid deforms elastically throughout this isotropic hardening.

Besides that, if the yield stress in tension increases due to hardening thecompression yield stress grows the same amount even though you might nothave been loading the specimen in compression. This type of hardening widelypractised in mathematical models for finite element analysis to describe plasticityalthough it is not exhibit real material characteristics.

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Figure 2.11: Isotropic hardening yield surface (Rees, 2012)

Kinematic hardening

Kinematic hardening is the second type of plasticity hardening. Unlike isotropichardening, the centre of the yield surface translates in stress space, while theyield surface size remains fixed in this hardening as illustrated in Figure 2.12. Inorder to model the Bauschinger effect and similar responses, where a hardeningin tension will lead to a softening in a subsequent compression, one can use thekinematic hardening rule (Rees, 2012; Chaboche, 1991). The Bauschinger effectrefers to a property of materials where the material’s stress/strain characteristicschange as a result of the microscopic stress distribution of the material.

For example, an increase in tensile yield strength occurs at the expenseof compressive yield strength. This is where the yield surface remains the sameshape and size but merely translates in stress space. The law of evolution for thismodel comprises of kinematic hardening component which describes the yieldsurface translation in stress space via the back stress, α.

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Figure 2.12: Kinematic hardening yield surface (Rees, 2012)

Combined isotropic-kinematic hardening

The third type of yield surface evolution is known as combinedisotropic-kinematic hardening where both isotropic and kinematic hardeningproperties are present (Axelsson & Samuelsson, 1979; Bathe & Montáns, 2004).In the combined isotropic-kinematic hardening, yield surface orientation mightchange as well. Although isotropic hardening is the most common form of yieldsurface evolution utilized in FE models for plastic deformation simulation, it isnot necessarily the most accurate (Eterovic & Bathe, 1990). The combinedisotropic-kinematic hardening model can considered as the most accurate fromthe three types of hardening models.

This hardening model consists of two components which are kinematichardening component and isotropic hardening component. The non-linearkinematic hardening component explains the yield surface translation in stressspace via the back stress. The isotropic hardening component explains thetransformations of the equivalent stress defining the size of the yield surface as afunction of plastic deformation. Combined isotropic-kinematic hardening willdemonstrate the real material characteristics as it displaying both isotropic andkinematic hardening behaviour (Kang, 2010).

Wear

Wear can be defined as the process of material removal from one or both solidsurfaces which are in contact due to sliding or rolling on each other (Bhushan &Gupta, 1991). The removal rate in wear is slow but continuous and steady. In

23

addition, wear mechanism can be classified as mechanical, chemical and thermalwear while wear modes classified as abrasive, adhesive, flow and fatigue wear(Stachowiak, 2006). The process of deformation and fracturing is considered asmechanical wear, where the deformation process takes place in ductile materialand fracture occur in brittle materials.

Wear occurs almost in every components and devices including teeth andbone joints, piston rings, roads, tires, dirt seals, brakes, liquid seals, belts, floor,fabrics, shoes, electrical contacts, CD reader heads, cannon barrels, dies, rollingmills, forgings, conveyors, ore crushers, home appliances, door hinges, zippers,saws, drills, pump impellers, razor blades, pipe bends, valve seats, erasers,plastic moulding screw and others as well (Ludema, 1996). Basically, yieldingand plastic deformation will lead to wear of material as shown in Figure 2.13(Tobi et al., 2009). An attention should be given in minimising plasticdeformation of material in order to protect the material from wear, thus canprolong the lifetime of engineering components.

Figure 2.13: Process of delamination wear mechanism (Tobi et al., 2009)

Generally, plastic deformation and wear should be eliminated or reducedto ensure efficiency of materials or machinery in order to enhance its performance.There are some plastic deformation and wear reduction steps such as:

• Retain low contact pressure

• Retain low sliding speed

• Retain smooth bearing surface

• Avoid high temperature

24

• Usage of hard materials

• Ensure low friction coefficient

• Apply a lubricant/coating

2.4 Coating and Substrate

Coating can be referred as a covering that is applied on the surface of an objector substrate as shown in Figure 2.14. Coating can be used for decorativepurposes, functional purposes or sometimes both. Coating which covering thesubstrate completely is known as all-over coating while some coatings only covercertain part of the substrate. The major purpose of coating is to alter thecomponent’s surface properties to ensure substrate material bulk characteristicscan be exploited when unsupportable situation occurs (Strawbridge & Evans,1995). Obviously, coatings are widely used in engineering applicationsespecially, manufacturing, automotive and aeroengine industries in order toreduce friction, increase resistance of plastic deformation and wear on contactsurfaces (Mohd Tobi et al., 2013). There are few type of coatings that widelyused such as surface welding coatings, thermal spraying coatings, electrodeposition coatings, plastic coatings, physical and chemical vapour depositioncoatings.

A substrate is known as a primary or underlying substance or layer asshown in Figure 2.14. Normally, substrate is surface of an object where othermaterials for instance, coating, ink, paint or treatment are enforced. Moreover,a substrate also can be defined as medium or solid substance where anothersubstance is applied where the adherence of second substance occurs. Generally,the purpose of the coating is to protect the surface of the substrate to maximiselife time of the substrate.

Figure 2.14: Coated substrate illustration

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