A Dissertation Entitled Study of the Influence of Nanosized Filler on The

Home , Abrasive machining

A Dissertation

entitled

Study of the influence of nanosized filler on the UV light-curable resin bonded

abrasive tool

Lei Guo

Submitted to the Graduate Faculty as partial fulfillment of the requirements for the

Doctor of Philosophy Degree in

Mechanical Engineering

______Dr. Ioan Marinescu, Committee Chair

______Dr. Abdollah Afjeh, Committee Member

______Dr. Sorin Cioc, Committee Member

______Dr. Liangbo Hu, Committee Member

______Dr. Chunhua Sheng, Committee Member

______Dr. Patricia R. Komuniecki, Dean College of Graduate Studies

The University of Toledo

May 2016

This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author. An Abstract of

Study of the Influence of Nanosized Filler on the UV-Curable Resin Bonded Diamond Abrasive Tools

Lei Guo

Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Mechanical Engineering

The University of Toledo

May 2016

As one of the most broadly employed machining process, abrasive machining is a complex material removal process in which a number of abrasive grains randomly cut material off from the workpiece. Common examples including grinding, lapping, honing and polishing can be all categorized to abrasive machining process. In the manufacturing of abrasive tools like grinding wheel and lapping plate, two of the most significant components are primarily considered: the abrasive grain and the bonding agent. Abrasive grains are usually selected from Aluminum oxide, Silicon carbide, Diamond and CBN in modern industry, while metal bonding agent and resin bonding agent is the most widely used matrix material.

Thanks to a remarkable development on rapid prototyping, the ultraviolet (UV) curing technology has been applied at a wide range in manufacturing. In this research, the

UV curing techniques was introduced into the fabrication of resin bond abrasive tool. In order to optimize the time consuming, reduce the energy cost of the traditional

iii manufacturing process and improve the machining performance of the tool, the thermosetting resin agent was replaced with UV light-curable resin.

To verify the manufacturing feasibility and examine the machining performance of the tool, an UV light-curable resin bond grinding wheel was developed with diamond as the abrasive grain. A group of comparative experiments on ceramic workpiece was carried out to study the machining capacity of the tool, in terms of surface finishing and material stock removal. Moreover, nanosized alumina particle was selected as a filler additive in the manufacturing process in order to improve the material properties of the resin matrix, and strengthen the bonding between the matrix and the diamond grain. Interestingly, the result showed that the utilization of nanosized alumina filler influenced not just the manufacturing of the tool, but also the performance of the tool in a very positive way.

In order to determine the influence of filler loading on the bonding mechanism of each single diamond grain, a theoretical force model was established based on elastic mechanics to quantitively describe the retention capability of the resin matrix, in terms of compressive stress generated from the shrinkage of resin. Because of the micron scale of diamond grains in resin matrix, it is hard to experimentally verify this force model.

Therefore, a numerical model was also developed based on FEM simulation to validate the reliability of above-mentioned force model. A series of calculation and simulation were carried out to comparatively study the retention force on each diamond grain in pure resin matrix and filler loaded resin matrix. The results obtained are consistent with reasonable errors that could be explained by the difference in application conditions of each model.

From a perspective of process prediction, these models could provide not just a direct output including matrix volume shrinkage and compressive force generated on each

iv diamond grit, but also an indirect output as the tool’s machining capacity that largely determined by diamond retention.

Acknowledgements

I would like to express my greatest gratitude to my advisor, Dr. Ioan Marinescu. I would never have been able to finish this work without the help and guidance from him.

I would like to thank the entire dissertation committee: Dr. Abdollah Afjeh, Dr.

Sorin Cioc, Dr. Liangbo Hu and Dr. Chunhua Sheng, for providing me all sorts of help and suggestion.

I would like to thank to all the members from PMMC for creating a friendly research group to work with.

I would like to thank all my friends, my family, especially my parents AiMin and

Yaohui, to whom this work is dedicated.

Table of Contents

Abstract ...... iii

Acknowledgements ...... vi

Table of Contents ...... vii

List of Tables ...... xi

List of Figures ...... xii

List of Abbreviations ...... xvii

List of Symbols ...... xviii

Chapter 1 Introduction ...... 1

1.1 Rapid Prototyping and UV Light Curing Technology ...... 1

1.2 UV-curable Resin in Manufacturing of Abrasive Tools ...... 4

1.3 Introduction of Abrasive Machining Process ...... 6

1.3.1 Fixed Abrasive Machining Process ...... 8

1.3.2 Loose Abrasive Machining Process ...... 11

1.4 Mechanism of Resin Bond Abrasive Machining ...... 15

1.5 Filler Additive in Manufacturing of Resin Tool ...... 18

1.6 Introduction Summary and Research Motivation ...... 23

1.6.1 Introduction and Background Summary ...... 23

1.6.2 Research Motivation ...... 24

vii

Chapter 2 Research Objectives and Dissertation Structure ...... 27

2.1 Research Objectives ...... 27

2.2 Research Framework and Dissertation Structure ...... 29

Chapter 3 UV Light-Curable Resin Bond Diamond Lap Grinding Wheel ...... 32

3.1 Experimental Procedure ...... 32

3.1.1 UV Light-curable Resin Prepare ...... 32

3.1.2 Abrasive Prepare ...... 33

3.1.3 UV Light Curing System Setup ...... 36

3.1.4 Manufacturing of the UV-Resin Bond Diamond Grinding Wheel .. 38

3.2 Experiment of Lap-Grinding on Ceramic Workpiece ...... 43

3.3 Results and Discussion ...... 45

3.4 Conclusion ...... 52

Chapter 4 Influence of Nanosized Aluminum Dioxide Filler Additive ...... 54

4.1 Experimental Procedure ...... 54

4.1.1 Materials Prepare ...... 54

4.1.2 Methods...... 56

4.2 Results and Discussion ...... 59

4.2.1 Cure Depth of Resin-Diamond Composite ...... 59

4.2.2 Hardness of Cured Resin-Diamond Composite ...... 62

4.2.3 Ultimate Tensile Strength ...... 64

4.2.4 Worn Surface Microscopy Observation ...... 65

4.3 Lap Grinding Experiment on Ceramic Workpiece ...... 69

4.3.1 Surface Roughness of Workpiece ...... 69 viii

4.3.2 Material Removal Rate ...... 72

4.4 Conclusion ...... 75

Chapter 5 Study on the Surface-Textured Diamond Grain and UV-Resin Matrix ...... 77

5.1 Diamond Abrasive ...... 77

5.2 Diamond Etching Mechanism ...... 78

5.3 Comparative Study on Surface Textured Diamond Matrix ...... 81

5.3.1 Light Transmittance of Resin-Diamond Composite ...... 81

5.3.2 Wear Performance of Surface Textured Diamond Tool ...... 83

5.4 Conclusion ...... 86

Chapter 6 Model Development on Single Diamond Grit Retention ...... 88

6.1 Introduction and Background of Diamond Retention in Matrix of Abrasive

Tools ...... 88

6.2 Theoretical Force Model of Mechanical Retention ...... 91

6.2.1 Literature Review of Modeling on Diamond Retention Capability of

Matrix in Abrasive Tool ...... 91

6.2.2 Development of the Theoretical Mechanical Retention Force Model

...... 93

6.2.3 Determination of the Model Parameters ...... 100

6.3 Numerical Modeling of the Mechanical Retention on Single Diamond Grit in

UV resin matrix...... 103

6.3.1 Development of the FEM Simulation Model in Abaqus 6.14 ...... 103

6.3.2 Determination of the Input Parameters in Abaqus Simulation ...... 105

6.3.3 Results of the Simulation on Pure Resin Diamond Matrix and

Alumina Additive Filled Resin Diamond Matrix ...... 113

Chapter 7 Sapphire Lap-Grinding with UV-Resin Bond Diamond Tool ...... 116

7.1 Introduction and Background of Sapphire Machining...... 116

7.2 Fabrication Method of UV Resin Diamond Tool for Sapphire Abrasive

Machining ...... 118

7.3 Discussion ...... 121

7.3.1 Surface Profile of Sapphire Workpiece ...... 121

7.3.2 Surface Roughness of Sapphire Workpiece ...... 123

7.4 Conclusion ...... 127

Chapter 8 Conclusion and Future Work ...... 129

8.1 Research Conclusion and Original Contribution ...... 129

8.2 Limitation, Future Work and Research Direction ...... 132

References ...... 135

List of Tables

Table 1.1 Possible situation-based classification for abrasive wear ...... 13

Table 1.2 Proposed severity-based classification for abrasive wear ...... 14

Table 3.1 Material technical specifications of UV light-curable resin ...... 33

Table 3.2 Porosity of the cured UV light-curable resin ...... 34

Table 3.3 Dymax 5000 UV curing flood lamp specification ...... 38

Table 3.4 Experiment conditions ...... 44

Table 4.1 Material specification of the UV-curable resin ...... 54

Table 4.2 Specifications of the resin-bonded diamond composites ...... 55

Table 5.1 Resin diamond composite specifications ...... 80

Table 6.1 Mechanical and thermal properties of resin matrix and diamond ...... 106

Table 7.1 Measurements of sapphire surface roughness and PV value by different types of lapping process ...... 125

List of Figures

Figure 1-1 Curing process of the UV light-curable resin ...... 2

Figure 1-2 Stereolithography (SLA) ...... 3

Figure 1-3 Manufacturing of resin-bond grinding wheel ...... 4

Figure 1-4 Manufacturing process of UV light-cured diamond abrasive plate ...... 6

Figure 1-5 Microscopic interaction modes in grinding...... 10

Figure 1-6 Structure of a typical grinding wheel ...... 11

Figure 1-7 Lapping model for hard, brittle material ...... 13

Figure 1-8 Two body and three body abrasion mechanism ...... 16

Figure 1-9 SEM images of silicon nitride (A) and silicon carbide whisker (B) ...... 20

Figure 3-1 SEM micro observation of resin bond diamond powder by Engis ...... 35

Figure 3-2 Abrasive particle size distribution of Resin bond diamond powder by Engis .35

Figure 3-3 Dymax 5000 flood ultraviolet curing system ...... 36

Figure 3-4 Innovative Machine Co. UV-100 Curing System ...... 37

Figure 3-5 Wavelength Distribution of Dymax 5000 Curing System ...... 37

Figure 3-6 Different types of curing mold ...... 39

Figure 3-7 Spin coating process of resin bonded abrasive plate ...... 40

Figure 3-8 Assembly curing of lap grinding plate ...... 41

Figure 3-9 Cast iron lapping plate ...... 41

xii

Figure 3-10 Effect of grooves in lapping plate ...... 42

Figure 3-11 Surface roughness (avg.) of ceramic workpiece machined by iron plate lapping and UV curable light-curable resin bonded plate machining ...... 45

Figure 3-12 Schematic representation of an indentation event ...... 47

Figure 3-13 Lapping mechanism contrast ...... 50

Figure 3-14 AFM micro observation on ceramic workpiece machined by iron plate lapping…...... 51

Figure 3-15 AFM micro observation on ceramic workpiece machined by UV light- curable resin bonded abrasive plate lapping ...... 51

Figure 4-1 DYMAX 5000-EC UV Curing Flood Lamp ...... 56

Figure 4-2 Manufacture process of UV light-curable resin bonded diamond plate ...... 57

Figure 4-3 Visible light transmittance of the uncured composites with different ratios of

Al2O3 filler ...... 61

Figure 4-4 Thickness of the cured resin diamond composites with different loading of

Al2O3 filler ...... 61

Figure 4-5 Hardness of the UV resin-bonded diamond composites with different loading of Al2O3 filler ...... 63

Figure 4-6 Tensile strength of the UV resin-bonded diamond composites with different loading of Al2O3 ...... 64

Figure 4-7 Diamond bond conditions in cured resin composites ...... 65

Figure 4-8 Diamond abrasive conditions in cured resin-bond composite (a, b) ...... 68

Figure 4-9 Diamond abrasive conditions in cured resin-bond composite (c, d, e, and f) ..68

Figure 4-10 Surface roughness of workpiece after machining ...... 70 xiii

Figure 4-11 Ceramic workpiece after lap grinding on resin-bond diamond plate ...... 71

Figure 4-12 Ceramic workpiece after lap grinding on Al2O3 filled resin-bond diamond plate...... 72

Figure 4-13 Mass loss of ceramic workpiece during machining ...... 73

Figure 4-14 Worn surface of filler loaded plate ...... 74

Figure 4-15 Worn surface of non-filler loaded plate ...... 75

Figure 5-1 SEM micrograph of unetched abrasive diamond particles (12-22µm) ...... 79

Figure 5-2 SEM micrograph of etched abrasive diamond particles (12-22µm) ...... 79

Figure 5-3 A manufactured resin bond diamond grinding wheel ...... 80

Figure 5-4 Ultimate tensile strength and Vickers hardness of cured composites with unetched diamond and etched diamond ...... 82

Figure 5-5 Surface roughness varying with machining time ...... 85

Figure 5-6 Definition of major, minor and minimum diameter on abrasive grain ...... 85

Figure 6-1 Forces on working diamond cutting point in a drill or saw blade ...... 92

Figure 6-2 Schematic illustration of embedded diamond grit in matrix ...... 94

Figure 6-3 Spherical coordinates ...... 95

Figure 6-4 Schematic diagram of the uniform distribution of diamond grit within resin matrix… ...... 100

Figure 6-5 3D model of a diamond grit and meshing ...... 104

Figure 6-6 3D model of the resin matrix around diamond grit ...... 104

Figure 6-7 Model of a diamond grit embedded in UV-resin matrix ...... 107

Figure 6-8 Stress distribution around an embedded diamond grit in 425 UV-resin matrix

(Y-plane cross section) ...... 108 xiv

Figure 6-9 Stress distribution of diamond-resin contact area in 425 UV-resin (Y-plane cross section) ...... 108

Figure 6-10 Stress distribution of diamond-resin contact area in 425 UV-resin (Z-plane cross section) ...... 109

Figure 6-11 Stress distribution on the diamond grit that embedded in 425 UV-resin matrix...... 109

Figure 6-12 Stress distribution around an embedded diamond grit in alumina filled 425

UV-resin (Y-plane cross section) ...... 110

Figure 6-13 Stress distribution of the diamond-resin contact area in alumina filled 425

UV-resin (Y-plane cross section) ...... 110

Figure 6-14 Stress distribution of the diamond-resin contact area in alumina filled 425

UV-resin (Z-plane cross section) ...... 111

Figure 6-15 Stress distribution on the diamond grit that embedded in alumina filled 425

UV-resin matrix ...... 111

Figure 6-16 Comparison between stress distribution in pure 425 UV-resin matrix (a) and alumina filled 425 UV-resin matrix (b) ...... 112

Figure 6-17 Comparison between compressive stress predictions from FEM numerical model and theoretical force model ...... 113

Figure 7-1 Sapphire wafer manufacturing process flow ...... 117

Figure 7-2 Manufacturing process flow of UV-curable resin bond diamond grinding wheel… ...... 119

Figure 7-3 SEM observation of diamond grains used in manufacturing of UV-curable resin bond grinding wheel ...... 120 xv

Figure 7-4 Experiment set-up for lapping tests on sapphire samples ...... 120

Figure 7-5 Optical surface profiler topography of sapphire sample (a) lapped under fixed diamond grinding wheel (b), slurry-steel lapping wheel (c) and UV-resin bond diamond grinding wheel (d) ...... 122

Figure 7-6 Schematic diagram of UV-resin bond diamond wheel lap grinding process .122

Figure 7-7 Average surface roughness and Peak-to-Valley height of original sapphire substrate (a) and samples lapped under fixed diamond lapping wheel (b), diamond slurry lapping wheel (c) and UV-resin bond diamond lapping wheel (d) ...... 126

Figure 7-8 Material removal rate of sapphire substrate lapping in fixed diamond lapping

(b), diamond slurry lapping (c) and UV-resin bond diamond lapping (d) ...... 127

Figure 8-1 Framework of UV light-curable resin bond abrasive machining modeling ...134

xvi

List of Abbreviations

ANOVA ...... Analysis of variance ASTM ...... American Society for Testing and Materials

CBN ...... Cubic Boron Nitride

RPM ...... Revolutions per Minute

SEM ...... Scanning Electron Microscope

UV ...... Ultraviolet UVA ...... Ultraviolet A

xvii

List of Symbols

I ...... Moment of inertia of the whisker ρ ...... Density of resin v ...... Stickiness of resin 휔...... Mean value of angular velocity of the whisker γ ...... Euler constant L ...... Length of the whisker d...... Diameter of the whisker θ ...... Angle between transverse direction and whisker’s axial direction E ...... Strength of the electric field pointed to transverse direction 휀푝 ...... Electrical conductivity α ...... Angle of incidence β ...... Angle of distortion c ...... Equilibrium length of the lateral crack 훼0 ...... Material independent constant 퐾푤 ...... Fracture toughness of the workpiece 퐸푤 ...... Young’s modulus of the workpiece 퐻푤 ...... Hardness of the workpiece 푃푖 ...... Normal load per load-bearing particle h...... lateral cracks from the bottom of the plastic zone α1 ...... Constant depending on particle shape 푉푖 ...... Volume of workpiece material removed per indentation α2 ...... Constant depending on particle shape α3 ...... Constant depending on particle shape 푃푡표푡 ...... Total load on workpiece R ...... Mean radius of the particles 퐴푤푝 ...... Area of the workpiece Z ...... Material removal rate

HV ...... Vickers hardness P ...... Load d...... arithmetic mean of the two diagonals

xviii

σ ...... Retention force 휀 ...... Matrix shrinkage rate on diamond 퐸1...... Elastic modulus of diamond 퐸2 ...... Elastic modulus of matrix 푆1 ...... Radius of diamond grit 푆2 ...... Radius of metal-wrapped diamond unit R ...... Retention ratio A ...... Contact area between diamond and matrix 푝1 ...... Matrix compressive stress 푝2 ...... Contact force 휇 ...... Diamond-matrix friction a...... Inner radius of resin-wrapped diamond unit b...... Outer radius of resin-wrapped diamond unit 휃 ...... Degree measured from the positive z-axis to a radius ∅ ...... Degree measured round the z-axis in a right handed sense u...... Displacements component in r direction v...... Displacements component in 휃 direction w ...... Displacements component in ∅ direction 휎푟 ...... Radial component of the stress vector 휎휃 ...... Tangential components of the stress vector 푓푟 ...... Body force 휖푟, ...... Radial component of strain 휖휏 ...... Tangential component of strain E ...... Elastic modulus of the matrix 휇 ...... Poison’s ratio 푝1 ...... Inner compressive load 푝2 ...... Outer compressive load 푢1 ...... Radial displacement of diamond grit 푢2 ...... Radial displacement of the resin layer 훼1 ...... Coefficient of linear thermal expansion of diamond grit 훼2 ...... Coefficient of linear thermal expansion ∆푅1 ...... Total radial displacement of diamond grit ∆푅2 ...... Total radial displacement of resin layer M ...... Total weight of diamond grits in each cubic centimeter matrix m ...... Weight of each diamond grit v ...... Volume of each diamond grit 휌 ...... Density of diamond grit ∆푇 ...... Temperature decrease Q ...... Total amount of heat energy absorbed by diamond-resin matrix I ...... UV light intensity t ...... UV light curing time 휑 ...... Energy absorption efficiency C ...... Heat capacity of the diamond-resin matrix

xix

Chapter 1

Introduction

1.1 Rapid Prototyping and UV Light Curing Technology

Since ultraviolet (UV) light-curable resin has been introduced into industry from the 1950s, the rapid prototyping technology has been developed significantly and became one of the most important techniques in manufacturing. UV light-curable resin is a polymer composite that changes its properties when exposed to ultraviolet region of the electromagnetic spectrum [1]. These changes are often manifested structurally, for example hardening of the material occurs as a result of cross-linking when exposed to light. Fig. 1-

1 below shows the curing process that a liquid photopolymer transforms from liquid phase to solid structure through the exposure under UV light.

Generally, UV light-curable resin consists of a mixture of multifunctional monomer, oligomer and photoinitiator. The photoinitiators are compounds that upon radiation of light decompose into reactive species that activate polymerization of monomers and oligomers in order to achieve the desired physical properties. The UV light- curable resin must be exposed to light of appropriate wavelength and intensity. Otherwise,

the reaction will not be initiated or react incompletely as to affect the performance of the solid matrix.

Figure 1-1 Curing process of the UV light-curable resin [2]

In the past two decades, researchers have utilized photo-manufacturing technology in various applications in the fields of prototype molding, medical treatment and precision micro manufacturing. Related research has been explored by scholars including Borden, et al. in radiation curable compositions (March 1976); Roskott in processing for UV curable resin (July, 1976); Guarino, et al. (January, 1978) and Due, et al. (February, 1978) in UV curable coating; Nate, et al. in mounting parts on circuit boards (June 17, 1980), Marutani in photo-prototyping (May, 1984 and Aug, 1988); Kumatani in mold manufacturing

(January, 1995), and Yishigawa in fiber reinforced resin (October, 2002).

Recently, the UV light-curable resin has began to play an important role in additive manufacturing as one of the feed stock materials. A set-up illustration in Fig. 1-2 shows the process of stereo lithography, in which the liquid-curable resin transforms from liquid phase to solid phase when it is exposed to the laser light. The laser beam traces a cross- 2

section of the part pattern on the surface of the liquid resin, then the build platform descends by a distance after the pattern layer has been traced. Then, a sweeper sweeps across the cross section of the part, re-coating it with fresh material. The cross-section pattern of this new layer is traced on this new liquid surface and then jointed to the previous layer. A complete 3D part is formed through this repeated process.

Figure 1-2 Stereo lithography (SLA) [3]

1.2 UV Light-Curable Resin in Manufacturing of Abrasive Tools

Resin composites is also broadly applied as one of the bonding materials for manufacturing of abrasive tools, such as resin bond grinding wheel, abrasive disk and lap- grinding plate. The manufacturing process of these abrasive articles usually includes ingredients mixing, shape molding, firing and finishing. In some cases, the molding and firing process can be completed at the same time. As shown in Fig. 1-3 below, the resin bond abrasive grinding wheel was fabricated through thermal compressing technology.

The ingredients which consist of abrasive grains, resin and additives are uniformly mixed together and molded into the desired shape. After mixing and molding, the wheel is fired in an oven or furnace at a temperature of 300 to 400 degree Fahrenheit (150 to 200 degree

Celsius). Firing melts the resin around the abrasive grains to form a solid structure of the grinding wheel. The firing process is always followed by a finishing step which helps reaming the center hole of the wheel, correcting the size and balancing the wheel as well.

Figure 1-3 Manufacturing of resin-bond grinding wheel [4]

Conventionally, thermosetting resin is primarily selected as the bonding agent to manufacture abrasive articles through thermal pressing technique. Recently, researchers have developed a UV curing process which fabricates tools by transferring UV light- curable resin (or UV-resin) from liquid phase to solid phase under the exposure of UV light

[5, 6]. Compared to the conventional thermal pressing manufacturing, the UV curing technology has brought a series of advantage to the production of abrasive tools:

 Short Process of solidifying (Curing time varies from seconds to minutes).

 Process consistency and flexibility without large field and equipment.

 Less energy cost and environmental friendly.

In 2001, Tanaka, et al. developed a new grinding wheel with resin cured by ultraviolet light. His research has verified the possibility of manufacturing grinding wheels with UV light-curable resin, and proposed the large capacity for grinding based on the sufficient bending strength and elastic modulus [7]. A year later, Enomoto, et al. presented a diamond wire saw with UV-resin [8]. In his study, ultraviolet-curable resin was used as the bonding material of a resinoid diamond wire tool to replace thermosetting resin, and the experiment has revealed that the newly developed wire tool can be manufactured at low cost and has a decent mechanical property and machining performance. Recently, Guo, et al. have developed a lap grinding plate with UV-curable resin, in which the grains were initially embedded in the bonded resin matrix and worked same as the grains in grinding process. As the machining process continued, some of the grains started to become dull and fall off from the abrasive plate due to the weak bond. These grains existed in the machining zone between the workpiece and abrasive plate begun to act as loose abrasive grains as lapping process. His study proposed a method in which the workpiece could be 5

machined by both grinding and lapping process, in order to get a better surface finish at an efficient material removal rate.

Figure 1-4 Manufacturing process of UV light-cured diamond abrasive plate

1.3 Introduction of Abrasive Machining Process

Abrasive machining processes have been employed in industry manufacture for more than a century and the origin could be traced back to Neolithic times. Abrasive machining uses a large number of multipoint or random cutting edges for effective material removal at smaller chip sizes than in the machining methods that using cutting tools with a defined edges [9]. As the most widely applied manufacturing process, grinding, lapping and polishing researches were significantly developed in the twentieth century and the applications of machining technology to achieve higher quality and efficiency was increased rapidly as well. In abrasive machining, the main objectives are usually to minimize friction and wear of the abrasive while maximizing abrasive wear of the workpiece. Moreover, another objective is concerned with the quality of the workpiece,

including the achievement of a specified surface texture and roughness, and avoidance of thermal damage as well.

As one of the most important abrasive machining process, grinding can be classified as a subset of cutting. It is a material removal and surface generation process used to shape and finish components made of metals, ceramic and other material. The surface finishing quality and dimensional accuracy machined by grinding could be ten times better than milling or turning process. Grinding has a relatively higher material removal rate than other abrasive machining process, and it always applied as an initial machining in precision manufacturing industry.

Compared to grinding, lapping and polishing are similar processes by which extra fine surface finish, high dimensional accuracy, flatness and minimal sub-surface damage are obtained. These techniques are usually used in optical lens, semiconductor and electronics industry on a widely range of material from silicon, glass, ceramic to metal and their alloys [10].

In all classes of abrasive machining process, abrasive grain is the most important component. Material used as abrasives include both natural minerals and synthetic products. A primary requirement of a good abrasive is that it should be very hard, and sharp edges and good cutting ability would be concerned as important requirements as well. The abrasive industry is largely based on five types of abrasive material. Aluminum oxide

(alumina, Al2O3), silicon carbide (SiC) and garnet are considered to be conventional abrasives. Diamond and cubic boron nitride (CBN) are termed superabrasives.

The properties, cost and application range between conventional abrasives and superabrasives have significant difference. As an example, the hardness, which is considered as the primary property of abrasive, is shown below (Knoop scale hardness):

 Diamond: 7000 kg mm-2

 CBN: 4500 kg mm-2

 SiC: 2700 kg mm-2

 Alumina: 2500 kg mm-2

 Garnet: 1400 kg mm-2

Most abrasives used in industry are synthetic due to the economic cost. Aluminum oxide is used in about 75% of all grinding operations, and it is primarily used to grind ferrous metals. The second largest used abrasive is silicon carbide for grinding on softer, no-ferrous metals and high density materials, such as cemented carbide or ceramics. As superabrasives, CBN and diamond is used for harder material.

Based on how the grains work on the workpiece, abrasive machining process can be divided into two main categories: fixed (bonded) abrasive process and loose abrasive process.

1.3.1 Fixed Abrasive Machining Process

Machining process including grinding, honing, wire cutting and sawing, sanding and abrasive belt machining is all fixed abrasive machining. In fixed abrasive machining process, the abrasive grains are held together within a matrix which can be metal, vitrified or resin. The combined shape of these particles determines the geometry of the machined

workpiece. The grain and the bond have very different physical-mechanical properties. The hardness is about one order of magnitude different. The main contact between the abrasive tool and workpiece takes place on the hard and sharp edges of the grains, the hardness of these grains are sufficient to plastically deform the workpiece material, as required in ploughing and cutting the workpiece [11]. As the machining process continues, the abrasive grains would be deformed and became dull and worn. However, the deformations of the abrasives are negligible in comparison with deflections of the bond [12]. On the other hand, the elastic properties of the tool are also favorable for finishing and super- finishing actions, and can be achieved by an appropriate choice of the type, hardness, and structure of the bond.

For example, in a typical grinding process, the abrasive grains are bonded together in a wheel, as the grinding wheel is fed to the workpiece, each of the grain start to work as a micro cutting tool, therefore the material of the workpiece can be removed. Fig. 1-5 shows the different types of microscopic action mode of abrasive grain in grinding, and it indicates the machining mechanisms as well. Based on the various contact conditions between the abrasive grain, bond and workpiece, the microscopic interactions can be divided into three categories: cutting, plowing and sliding. The material is mainly removed by cutting process and deformed by plowing and sliding. As a result, the material removal rate of grinding process is primarily affected by the number of grains that work as cutting grain, and the significant influence on surface roughness of machined workpiece is from the plowing and sliding grains.

Regarding the efficient material removal rate and productive flexibility, grinding is always selected for machining brittle material including ceramic, silicon, silicon carbide, 9

glass and so on. Most of the grinding tools consist of abrasive grain and bonding material,

Fig. 1-6 below indicates the typical structure of a grinding wheel. The abrasive grain (or named abrasive particle) is the key element that actually performs machining actions during the machining process. Diamond and CBN are the most widely used abrasive grains in grinding due to the high hardness and great wear resistance.

Figure 1-5 Microscopic interaction modes in grinding

Figure 1-6 Structure of a typical grinding wheel [12]

1.3.2 Loose Abrasive Machining Process

Processes such as lapping or polishing employs loose abrasive technology and belong to a family of fine-finishing or super-finishing process, which is mainly applied in high-precision process to obtain a high dimensional accuracy and fine surface quality. The loose abrasive machining process consists of a workpiece, a supporting element (i.e. a lap plate or polish pad) and abrasive grain. The supporting abrasive element could be supplied by means of a fluid suspension to transport the abrasive grains. The fluid suspension, also named slurry, lubricates and cools the machining process. Since there is not structure connecting the grains and the abrasive grains can move independently, they are always forced into the workpiece by a load.

Lapping and polishing process are the most common loose abrasive machining, which are precision finishing processes involve different mechanical arrangements.

Lapping is a slow material removal operation. Although the lapping process tends to decrease the original surface roughness and improve the surface quality, the main purpose

is to remove material and modify the shape. Lapping is primarily used to improve dimensional accuracy rather than to reduce surface roughness. Form accuracy includes, for example, flatness of flat surface and sphericity of a ball. In contrast, the term polishing implies better finish with little attention to form accuracy.

Lapping and polishing are used for many work materials including glass, ceramic, plastic, metals and their alloys, sintered materials, stellite, ferrite, copper, cast iron, steel, etc. Typical examples of the diverse range of processed components are pump parts (seal faces, body castings, rotating valves, impellers), transmission equipment (spacers, gears, shims, clutch plates), cutting tools (tool tips, slitter blades), hydraulic and pneumatics

(valves plates, seals, cylinder bodies, castings, slipper plates), aerospace parts (lock plates, gyro components, seals), inspection equipment (test blocks, micrometer anvils, optical flats, surface plates), and stamping and forging equipment (spacers, type hammers, bosses, and a variety of other tools with complex shapes). The mechanism of lapping process on brittle material is shown in Fig. 1-7 below.

The relative removal speeds in lapping and polishing are much lower than in grinding. Consequently, the concentration of energy in the contact area is much lower. The benefit is that average temperature increases tend to be lower than in grinding and may be negligible; the disadvantage is that specific heat is higher, although, since the volumes of material to be removed are small, this is not usually important.

Generally, loose abrasive machining and fixed abrasive machining is the mostly accepted classification to study abrasive wear. However, this process could be classified from a different prospective, in terms of the contact stress between the abrasive particle

and work piece. Table 1.1 and Table 1.2 below shows the classification of abrasive wear based on the contact stress.

Figure 1-7 Lapping model for hard, brittle material [13]

Table 1.1 Possible situation-based classification for abrasive wear [17]

Contact stress Abrasive Low High Extreme particles (Particles do not facture) (Particle facture) (Gross deformation) High stress, Free Low stress, free abrasive free abrasive Low stress, High stress, Extreme stress, Constrained fixed abrasive fixed abrasive fixed abrasive

Table 1.2 Severity-based classification for abrasive wear [17]

Typical a Abrasive wear mode Situations Mild Severe Extreme Particle size Small Moderate Large

Constraint Unconstrained Partially Strongly constrained by constrained counter face

Particle shape Rounded Sharp Sharp

Contact stress Low, insufficient to Moderate, Very high, may fracture particles sufficient to cause macroscopic fracture particles deformation or brittle fracture of material being worn

Dominant b Micro-ploughing Micro-cutting Micro-cutting mechanism and/or micro- facture Equivalent Low stress abrasion High stress Gouging abrasion terms c abrasion

Scratching Grinding abrasion d High stress, two abrasion d body e

Low stress three High stress, three body e body e

Low stress, two body e a – Not all aspects of the “typical” situation necessarily apply simultaneously. b – Debris-removal mechanism is highly material-dependent. c – It has already been demonstrated at length that these alternative terminologies are ambiguous, therefore, no fully reliable correspondence with the proposed new terms can be expected. d – Term not favored even within the alternative classification scheme. e – Dominant sense of two body/three body distinction.

1.4 Mechanism of Resin Bond Abrasive Machining

Wear is the term that represent gradual material removal from a surface due to a mechanical movement and/or chemical process [14]. One of the most important mechanism, due to its frequency, is abrasive wear, which means detachment of material from surfaces in relative motion, caused by protrusion and/or hard particles between the opposing surfaces or fixed in one of them. The mostly widely used classification of abrasive wear into two body abrasion and three body abrasion is categorized by the mechanism of the abrasive grain action, shown in Fig. The primary meaning of the two body and three body concept is to describe whether the abrasive grains are bonded (two body) or free to roll or slide (three body) [15]. The term two body abrasion refers to wear caused by abrasive grains or asperities which are rigidly attached (embedded) in a second body (bond), while three body abrasion represents the wear caused by free and loose abrasive grains which existing as interfacial elements between a solid body and a counter body [16].

In two body abrasion, the abrasive grains assumed to be constrained by the tool, and the relative motion between the abrasive grain and workpiece is usually considered to pure sliding. Also, two body abrasion is considered as a results of the displacement of material from a solid surface due to hard grains sliding along the surface or when rigidly held grains pass over the surface like a cutting tool. In practical applications, many components such as a conveyor belt, tillage tools and wind blades are subjected to two body abrasive wear.

Since the abrasive particles in two body mechanism are bonded rigidly, they are able to cut deeply into the workpiece material, while in the case of three body abrasion, the abrasive grains may spend only part of their time cutting into the material. Therefore, two body abrasion is considered to produce wear rates three times bigger than the three body mechanism under the same loading condition [17].

Grinding is considered as a typical two body abrasive process because the grain is fixed in the grinding wheel. Lapping is primarily considered a three body abrasive mechanism due to the fact that it employs free abrasive grains that can roll or slide between the workpiece surface and lapping plate. Mostly, the material of the lapping plate has a relative good hardness to keep the abrasive grains from being embedded into the plate.

However, some grains still occasionally become embedded in the lap leading to two body abrasion. In this case, the process involves both two body abrasion and three body abrasion at the same time.

Figure 1-8 Two body and three body abrasion mechanism

In 1998, Gates illustrated that this categorizing of two-body and three-body abrasion is ambiguous because there are situations when these two concepts could create misinterpretations. In his opinion, this classification can be used only to describe whether abrasive grains are rigidly held or free to roll. From a tribological point of view, one should take into account the severity of wear behavior: mild, severe, and extreme. Consideration should be given to the specific situation: gouging abrasion, high-stress (or grinding) abrasion, and low-stress (or scratching) abrasion. The difference between high-stress and low-stress abrasion is whether or not the abrasive grains are broken during abrasion. This is important since fracturing can create sharp cutting edges and give higher wear rates.

Gates proposes this new classification as an improvement (without being considered precise).

Even if mechanisms in two-body and three-body abrasion are the same, there are some obvious differences between the two methods. In two body abrasion, the abrasive grains are constrained against the abraded surface and higher pressures can be exercised by them. Another difference is the effect of particle (or protuberance) size on wear rate.

In three-body abrasion, the distribution of grains in the contact area is subject to greater uncertainty. With an ample supply of abrasive, the average pressure on the grains is likely to be lower than in a two-body process. The pressures exerted by an abrasive particle also tend to depend on the grain size. The pressures are likely to be higher with large grain sizes. This affects the scratch depths on the workpiece surface. With low pressure and fine particle sizes, the scratches will be very small. For this reason, polishing, which is a predominantly two-body process, can produce very low surface roughness.

Resin bonded abrasive tools have a relatively soft bond between the abrasive grain and bonding material, while the bond force in vitrified or metal bonded abrasive tools are much more stronger. As a result, the abrasive grains in resin bonded tools are easier to be pulled out from the resin matrix leading a three body abrasion within the machining zone.

Therefore, a pure grinding or two body abrasion process could be transferred to a complicate process which consists both fixed abrasive process (grinding, two body abrasion) and loose abrasive process (lapping, three body abrasion). As mentioned above, in lapping machining, the material of the lapping plate has a relative good hardness to keep the abrasive grains from being embedded into the plate. However, some grains still occasionally become embedded in the lap leading to two body abrasion. In this case, the process involves both two body abrasion and three body abrasion at the same time. The resin bonded grinding process therefore could be considered as a reverse transformation of

“three body abrasion to two body abrasion”.

1.5 Filler Additive in the Manufacturing of Resin Tool

UV light-curable resin, as a bonding agent, the requirements of the compositive characteristics are always high. In fact, it is unusual to use pure resins individually in industrial manufacturing due to their weakness in material characteristics. However, the theory of composites material indicated the development of a suitable resin composites with desirable characteristics is accessible by introducing filler additives into the composites. Like cement should be used in the form of concrete filler with sand and reinforced with steel bars, resin should also be modified and improved by filling it with appropriate additives or fibers according to the needs of bonding agents for abrasive tools. 18

The early stage of UV light-curing technology is primarily used to fabricate prototypes for quick verification of designs or feasibility study, or prototypes with a low range of functionality. These prototypes give a first impression of a part’s properties. Fully functional prototypes with the full range of properties cannot be built with the UV-curing process because of the limited material properties of UV light-curable resin. However, many opportunities to reinforce resins exist, one of which is filling the resin with powders such as ceramics and it shows a great promise. Nearly most researches about effect of filler in light-curable resin has been done on dental resin composites and has shown that the stiffness, wear resistance, thermal and chemical resistance can be modified. Also, based on the fact that several micro-scale powders have been successfully synthesized with UV light-curable resin in the field of rapid prototyping, it can be proposed that the influence of filler additive on material characteristics of UV light-curable resin exists and affect the properties of composites in a positive way.

Adding a micro powder to resin could enhance the material properties of the composites though, such as the expected improvement in material hardness, wear resistance and thermal chemical resistance. However, the influence on tensile strength is usually unexpected. In resin bonded abrasive tools, tensile strength of the bonding agent is one of the key material properties. For instance, in order to save material and improve production efficiency, a thinner cut groove is desired in semiconductor dicing process.

Therefore, the thickness of the abrasive dicing blade is desirable in current industry and the strength of the bonding material is the key factor that determines how thin the dicing blade could achieve.

Figure 1-9 SEM images of silicon nitride (A) and silicon carbide whisker (B) [18]

In reality, tensile strength of the bonding agent is not only concerned with the strength of the tools like a thinner dicing blade or even super-thin abrasive tool, but also with the wear resistance in use. These needs consequentially raise up the requirement of the strength of abrasive tools. In order to fill the gap, researchers developed various methods to improve the composites material. Whisker filling and Nano-material filling are the two mainly applied approaches.

Xu, etc. studied the reinforcement of different whiskers in dental resin and revealed that whisker type and whisker/silica ratio are key microstructural parameters that determine the composites properties and reinforcement with silica-fused whiskers results in novel dental composites that possess substantially higher strength and fracture toughness [18].

Aluminum borate whisker, zinc oxide whisker, carborundum whisker and potassium titanate whisker are also used to reinforce different composites [19-20]. Some of the researches also shows that the efficiency of whisker filling could be increased by controlling the direction of whisker in resin. The enhancing efficiency of whisker in length

direction is much greater than in diameter direction. To make whiskers more efficient, the electrical field could be used to align the whisker in the direction of the electrical field as shown in Fig. 1-10 below. The moving equation can be expressed as:

푑2휃 푑휃 퐼 ∙ + 퐶 ∙ + K ∙ sin 휃 ∙ cos 휃 = 0 푑푡2 푑푡

(1.1)

16π 휌 ∙ 푣 ∙ 퐿3 퐶 = ∙ 3 2 ∙ 푙푛[8푣/(푑 ∙ 퐿 ∙ 휔)] − 2훾 + 1

(1.2)

π 퐾 = ∙ 퐿2 ∙ 푑 ∙ 휀 ∙ 퐸2 4 푝

(1.3)

Figure 1-10 Whisker filler resin in electric field

Here I is the moment of inertia of the whisker; ρ is the density of resin; v is the stickiness of resin; 휔 is the mean value of angular velocity of the whisker; γ is the Euler 21

constant. L is the length and d is the diameter of the whisker; θ is the angle between transverse direction and whisker axial direction; E is the strength of the electric field pointed to transverse direction, 휀푝 is the electrical conductivity.

Besides the whisker filling, nano materials are now being considered as a filler material, too. The nano-material filled resin composites can be used to produce high performance structures with further enhanced properties. Improvements in mechanical electrical and chemical aspects have attracted major interest in nano material filled composites from automotive, aerospace industry to consumer electronics and biotechnology applications.

Nano fibers such as carbon nano fibers are drawing great attention for their potential applications in polymer reinforcement because of their high tensile strength, modulus and relatively low cost. Moreover, various types of nano particles including nano silicon carbide, nano zirconium dioxide, nano zinc oxide, nano diamond particles etc. are broadly used as filling material in composites to improve mechanical characteristics [21-22]. Based on the previous researches, a conclusion can be drawn that both nano fibers and nano particles could be applied as a reinforcement filler in resin composites to improve the mechanical properties. Particularly, a significant increase in wear resistance is expected at the same time, which is closely concerned with the life time of resin bonded abrasive tools.

1.6 Introduction Summary and Research Motivation

1.6.1 Introduction and Background Summary

As one of the base materials in rapid prototyping, UV light-curable resin has the capability to be cured and shaped by ultraviolet light to build 3D models, and this technique of fabrication stimulated researchers to develop resin bond abrasive tools. Because of the limitation of UV resin’s material properties, the bond between abrasive grits and resin matrix is relatively weak than that in metal bond abrasive tool. As a result, both of the Two- body abrasion and Three-body abrasion exist at the same time during machining.

Therefore, this unique abrasive machining process can be considered as a dynamic combination of fixed abrasive machining and loose abrasive machining.

In the case of UV resin bond abrasive tools, excluding the characteristics of the abrasive grains and bonding agent, the wear performance of resin bond abrasive tools are primarily affected by the bond strength between the grains and bonding material. A strong bond could help to prevent premature loss of the abrasive grains before they performed the working life completely, and thereby to improve the machining efficiency of the tool.

Yokogawa et al. indicated that the coarser grain and stronger bond in CBN grinding wheels tend to have a higher abrasive grain retention and better grinding performance. [23]

A variety of treatments focused on abrasive grain and resin matrix was applied by previous researches that intended to improve the grain retention. S.Y. Luo et al. investigated that the diamond retention of resin-bonded abrasive tool could be improved by adding various types of fillers into the composites. [24] B. Strzemiecka et al. also studied the effect of different fillers on resin abrasive composites and compared the mechanical-thermal

characteristics. [25] These previous studies were mainly based on thermosetting resin composites though, nearly none research emphasized on the UV-curable resin composites that used in the manufacturing of abrasive articles.

Filler filled UV-curable resin composites was studied widely in dental industry, it was the primary option as direct filling material in dental filling because of its quick response curing mechanism. Filler loaded UV-curable resin composites is partly translucent and scatter light. Light penetration decreases with increased material thickness due to the absorption and scattering of light by fillers and other additives. Nazanin Emami et al. revealed how filler properties, filler fraction, sample thickness and light source affect light attenuation in particulate filled resin composites. [26] However, the researches on filler loaded UV-curable resin composites in dental industry was concentrated on the applications as a filling material. The filler loading influence on this type of composites as a bonding material in the manufacturing of abrasive tool has never been studied.

1.6.2 Research Motivation

Through the literature review and feasibility analysis. The following summary can be made:

1) The industrial needs for a practical and accessible manufacturing method to develop

UV light-curable resin bond abrasive tools:

 A general process of manufacturing method of UV light-curable resin bond abrasive

tool, which can be widely utilized on the fabrication of a various types of abrasive

tools, such as grinding wheel, lapping plate, polishing pad, wire saw, abrasive blade,

etc.

 A comparative study between the UV light-curable resin bond tools’ machining and

conventional abrasive machining (metal bond, vitrified bond, etc.), and a

comparison research between UV light-curable resin bonded abrasive tools and

thermosetting resin bonded tools.

 Practical methods to overcome the limitation of UV resin’s material properties, to

optimize the diamond grit retention in resin matrix, and thereby to improve the work

performance of UV light-curable resin bond abrasive tool.

 A comprehensive understanding of the process parameters including UV light

exposure time, the depth of cure, the abrasive concentration, the additive filler

loading, etc., and how these factors affect the characteristics of the tool.

 A model that can be used to predict the machining capability of the UV resin bond

abrasive tools as an output, with the input from manufacturing process conditions.

2) Based on previous research and related work, the gaps between the researches and

industrial need can be concluded.

 A usable UV light-curable tool should be developed to show performance over

conventional abrasive tools and thermosetting composite bonded tools.

 The UV light originated process parameters should be studied to improve the

manufacturing of UV light-curable resin bonded abrasive tools.

 The bond between abrasive grain and UV light-curable resin in abrasive tools should

be studied as potential direction to improve the work performance of UV light-

curable resin bonded tool, since most of the study on resin bond was concentrated

on thermosetting resin and polymer.

 A prediction model should be established to describe the relationship between the

work performance of UV light-curable resin bond abrasive tool and manufacturing

process parameters

Chapter 2

Research Objectives and Dissertation Structure

2.1 Research Objectives

The first goal of this research is to determine a manufacturing method of UV light- curable resin bond abrasive tools, and then to develop a grinding wheel that could be used to experimentally study the work performance difference between UV resin bond abrasive tool and conventional abrasive tool. The influence of abrasives, resin and UV light curing condition on abrasive machining will be also studied.

Based on the literature review, it can be concluded that the UV light-curable resin bond abrasive tool is still at experimental stage, and lack of in-depth study on the process optimization and practical application. Hence, the second goal of this research is to investigate potential methods to improve the performance of UV light-curable resin bonded abrasive tool. Since the UV resin and diamond abrasives were purchased from suppliers and ready to use, the investigation was mainly focused on the modification and reinforcement of the resin-diamond composite. Therefore, this objective can be sub- grouped as following.

 To improve the material properties of the resin-diamond matrix by

introducing nanosized aluminum oxide filler as a reinforcement additive.

 To strengthen the retention capability of matrix to retain diamond grit by

introducing surface textured diamond.

As of another primary goal of this dissertation, a theoretical model will be developed to describe the relationship of the UV light curing process and the work performance of the UV resin bond tool. As the input parameters of this model, the manufacturing process conditions are complicate to specify, however, the mechanical locking offered by the resin matrix, or the mechanical stress on each single diamond grit after the tool was manufactured is the dominated factor to the tool’s performance, when the material characteristics of abrasives and bonding agent are fixed. Therefore, a theoretical force model to predict the mechanical stress on single diamond grit is necessary for evaluating the machining capability of a resin bond abrasive tool.

Furthermore, to verify the theoretical force model, a numerical model based on

FEM simulation was carried out to comparative study the results, in terms of stress and strain distribution on each single diamond grit within resin matrix.

In addition, the last goal of this study is to utilize this UV resin bond abrasive tool to a broader usage on sapphire workpiece, to verify the potential and feasibility of

UV resin abrasive tool for participating in the production chain of sapphire surface finishing.

2.2 Research Framework and Dissertation Structure

Based on the research scope and structure, this proposed research is organized into

8 chapters as following. A brief summary of each chapter is introduced.

Chapter 1: Background introduction, literature review, analysis of the gaps between industry needs and academic research.

 The background of rapid prototyping and UV light curing technology,

introduction of UV light-curable resin and its applications in manufacturing

of abrasive tools.

 The literature review on the manufacturing of various UV light-curable

resin bond abrasive tools are conducted.

 The research and method on improving the performance of thermal setting

resin bonded abrasive tools are also reviewed as reference, and the industrial

need for improvement on UV light-curable resin bonded abrasive tools are

identified as the motivation of this research.

Chapter 2: Research objectives description and dissertation structure.

 The research objectives to develop an UV resin bond abrasive tool, to

improve the performance of the tool, and to establish models to describe the

manufacturing process and work performance of the tool is described in this

chapter.

 The research scope of this study and the structure of this dissertation is listed

in this chapter.

Chapter 3: Development of an UV light-curable resin bond diamond lap grinding wheel.

 The material properties of UV light-curable resin and diamond composites

are studied and a lap grinding wheel is fabricated.

 The work performance of the tool on ceramic workpiece is examined and

the results are comparatively studied with that from conventional lapping

process.

Chapter 4: Study on the influence of nanosized aluminum dioxide filler in the manufacturing process of UV resin bond tool.

 Nanosized aluminum dioxide filler is introduced into the manufacturing of

UV light-curable resin bonded diamond abrasive tool. The influence on lap

grinding performance of the tool is studied and the results are analyzed

based on the material removal rate and surface finish of the machined

workpiece.

Chapter 5: Study on the influence of shape and surface characteristics of abrasive diamond grain in the manufacturing of UV resin bond tool.

 A comparative experiment is conducted to compare the influence of surface

characteristics of diamond abrasive grain on the work performance.

 An investigation on the relationship between diamond grain retention and

machining capability of the tool.

Chapter 6: Development of the models to describe UV-curing manufacturing process and work performance of the tool.

 A theoretical force model is established based on UV curing mechanism

and elastic mechanics in resin matrix, to describe the mechanical stress on

single diamond grit.

 A numerical FEM model is developed as well based on FEM method, in

order to verify the theoretical force model.

Chapter 7: The utilization of UV resin bond abrasive tool in sapphire workpiece.

 The machining capability of UV resin bond abrasive tool is examined on

sapphire workpiece, and the results in terms of material removal rate and

surface roughness of workpiece is compared to conventional grinding and

lapping.

Chapter 8: Research limitation, conclusion of this dissertation and prediction of future work.

 The limitation of this research is summarized from the experimental and

theoretical perspective.

 The conclusion is drawn based on the all of the experiments, data analysis

and modeling results.

 The potential research direction of future work that derived from this study

is also discussed in this chapter.

Chapter 3

UV Light-Curable Resin Bond Diamond Lap Grinding Wheel

3.1 Experimental Procedure

3.1.1 UV Light-Curable Resin Prepare

In manufacture of UV resin bond abrasive tool, the UV light-curable resin is used as the bonding material and it composes of epoxy or acrylate resin, performed polymer, diluents, photo initiator and other additives. In this research, three types of UV light- curable resin were supplied by Dymax Corporation. These durable resins are environmental safe and exhibit exceptional tensile strength as well as vibration and impact resistance. Some of the technical specifications of these three types of resin is listed in the

Table 3.1 below.

As one of the important material properties, the porosity in cured resin structure is generated due to the shrinkage of the resin after phase transition from liquid to solid.

Previous related studies have investigated that the abrasive machining efficiency is significantly affected by porosity, which could change the hydrodynamic performance between the abrasive tool and workpiece. The porosity of the bond connected with each

other, forming a perfect network of pores in the wheel, which can efficiently arrest crack propagation, accelerate the removal of the chippings, and aid cooling fluid flow on the abrasive surface. Furthermore, the impact strength, grinding efficiency, self-dressing, and service life of the abrasive wheel could be also improved [27]. In this study, all of the three types of UV light-curable resin was cured separately and the microstructures were examined under microscope respectively.

Table 3.1 Material technical specifications of UV light-curable resin

Resin 921-VT 425 430 Durameter Hardness D75 D80 D80 Water Absorption 1.1% 0.7% 1.8% Viscosity (mPa·s) 11000 4000 1300-1700 Thermal Limit -43o-177oc -40o-150oc -45o-160oc Tensile at break 5200 psi 6200 psi 5000 psi Elongation at break 35% 7.3% 12%

3.1.2 Abrasive Prepare

As the hardest abrasive in industry, diamond is preferred in precision abrasive machining process including lapping, polishing and fine grinding. An ultras fine surface finish and relatively higher material removal rate can be achieved by using diamond as the abrasive grain. In industry, two primary types of diamond grain is commonly selected as the abrasive for manufacturing of diamond abrasive tools: metal bond diamond grain and resin bond diamond grain. The former one, metal bond diamond grain is the powder with high concentrate particles, accurate size and irregular shape. It is always used for high

performance metal bond tools to ensure excellent abrasive action and tight particle size distribution. Abrasive tools with metal bond diamond could benefit from its aggressive stock removal and good surface finish properties.

Table 3.2 Porosity of the cured UV light-curable resin

921-vt 425 430

Micrograph

Pore size [m] 20-100 10-110 10-100 Porosity More Medium Less

On the other hand, resin bond diamond powder with irregular shapes and rough surface area of crystals is the result of synthesis of soft diamond powders with fractured and slice-like structure. The specific feature of these types is the permanent renewal of cutting edges of crystal thanks to breaking away of small fragments. Thus, resin bond diamond is recommended for cutting hard, brittle and delicate material such as ceramics, carbides, and exotic metals where low heat generation or improved surface is desired.

In this study, resin bond diamond powder was selected as the abrasive for manufacturing of ultra-fine grinding wheel. The abrasive powder was supplied by Engis

Corporation and the grain size distribution is around 12 to 22 micrometer. An SEM micro observation can be seen in Fig. 3-1 below, and the abrasive particle distribution is also shown in Fig 3-2.

Figure 3-1 SEM micro observation of resin bond diamond powder by Engis

Figure 3-2 Abrasive particle size distribution of Resin bond diamond powder by Engis

3.1.3 UV Light Curing System Setup

Ultraviolet (UV) light is an electromagnetic radiation with wavelength from 400nm to 100nm, shorter than that of visible light but longer than X-ray. An ultraviolet curing system for laboratory use mainly consists of an ultraviolet lamp and a power supply, through which the composite paste of UV light-curable resin and diamond grains could be cured to form a solid structure. In this study, two sets of ultraviolet curing system were employed to meet the need for curing parts with different dimensions. The required wavelength for curing UV light-curable resin is specific to the resin chemistry, and the ideal wavelength in this case is around 365 nm which was given out by the resin manufacturer. The wavelength distribution of Dymax curing system around 300 nm to 450 nm as Fig. 3-5 shown.

Figure 3-3 Dymax 5000 flood ultraviolet curing system 36

Figure 3-4 Innovative Machine Co. UV-100 Curing System

Figure 3-5 Wavelength Distribution of Dymax 5000 Curing System

Table 3.3 Dymax 5000 UV curing flood lamp specification

Output Power 400 Watts UV Light Source Unfiltered UVA flood Typical Intensity in the UVA range (320-390 nm1) Measured three inches 225 mW/cm2 from the bottom edge of the reflector housing Dimensions of 5" x 5" (12.7 cm x 12.7 cm) Illuminated Area EC Input Power Universal 90-265 VAC Requirements (grounded), 47-63 Hz Power Supply Weight 12.25 lbs. (5.6 kg)

Power Supply 12" L x 16" W x 4.25" H Dimensions (approximate) (30.5 cm x 40.6 cm x 10.8 cm) Reflector Housing 6.75" L x 6.75" W x 8" H Dimensions (approximate) (17.2 cm x 17.2 cm x 20.3 cm)

Reflector Housing Weight 2.7 lbs (1.2 kg)

3.1.4 Manufacturing of the UV-Resin Bond Diamond Grinding Wheel

3.1.4.1 Mixing of Resin and Abrasive Diamond Powder

A relatively lower abrasive concentration is preferred in lapping and polishing or high precision grinding, a 25% abrasive concentration was chosen in this study. The diamond abrasive concentration is the weight of diamond in a unit volume of the diamond tools’ working layer, and it is defined that when each cubic centimeter contains 0.88g diamonds, the concentration is 100%. The layer mentioned above should be the resin in this research. A 25% diamond concentration stands for 0.22g diamond in each cubic

centimeter of resin. The desired weight of diamond grain was mixed with certain volume of resin and stirred for one hour under vacuum condition to get uniformly distributed diamond composites and remove the air bubble which could be created during the stirring.

The resin diamond composites was then injected into different shapes of mold for curing.

Figure 3-6 Different types of curing mold

3.1.4.2 Spin Coating Curing Method

Small pieces of resin cured part can be achieved through the various types of mold, but this method might not be suitable for a larger area curing. Spin coating method was proposed in previous studies and could be employed in this research, the process idea is illustrated in Fig 3-7 below. In spin coating process, the composites of UV light-curable resin and diamond grain is spread on a base plate to generate a thin film. The resin diamond composites coated plate was then processed to the UV light exposure to complete the curing.

Figure 3-7 Spin coating process of resin bonded abrasive plate

The advantages of spin coating method includes the short process time and completeness of the tool. However, there are still some issues existing during the process.

Firstly, since the curing area is relatively much larger than small pieces, it is hard to control the UV light distribution. For instance, the distance from the center of the plate to the lamp is much shorter than that from the peripheral area to the lamp. As a result, an uneven energy absorbed plate in which some of the resin was cured and some of them was not might be fabricated. Even if all area of the film was cured after curing process, the cure depth could be various. Also, the uneven distribution of energy could cause deformation of the film and generate a waveness on the surface. Hence, it can be concluded that spin coating method could be used for small scale curing but not suitable for a large area curing.

3.1.4.3 Slices Assembly Curing Method

To cure a large scale surface, another curing method named assembly curing can be used in for this study. Like Fig. 3-8 shown below, an abrasive lap grinding plate or vertical grinding wheel can be divided into pieces evenly, therefore each slice has a suitable dimension to get a uniform energy distribution during curing process. 40

Figure 3-8 Assembly curing of lap grinding plate

Figure 3-9 Cast iron lapping plate

After curing, the cured pieces were then adhered on top of the base plate to form a whole abrasive machining surface. Since the alignment of each piece is hand working

process and it is also impossible to keep the adhesive film thickness the same exactly, a finishing process is needed to take a surface off form the top of the cure resin film. In can be also considered as a dressing process in which the flatness of the abrasive plate was adjusted and a new layer of diamond was revealed.

Figure 3-10 Effect of grooves in lapping plate

As an abrasive tool design for lapping or lap grinding, the grooves between each of the slice was also desired. Generally, the surface of the lapping plate is grooved and these grooves serve to hold slurry, remove cutting chips, and evenly supply slurries to the workpieces. More importantly, grooves unifromize the distribution of the pressure under the workpiece, which works to improve the flatness accuracy of the workpiece as indicated in Fig 3-10. When even distribution load is applied to the workpieces that placed on the non-grooved lapping plate, the stress distribution under the workpieces becomes the biggest in the periphery of the workpiece, although it is not as notable as in the elastic body that is pressed against the rigid body as often argued in the elasticity theory. 42

When the lapping plate is grooved, the pressure is distributed between the neighboring grooves. The pressure distribution is interrupted by the grooves, making the pressure distribution under workpieces small and uniform. As both workpieces and lapping plate work relatively, the workpieces can be lapped to a uniform and high accuracy flatness.

3.2 Experiment of Lap-Grinding on Ceramic Workpiece

In order to estimate the abrasive machining performance of the UV light-curable resin bonded diamond plate, a group of comparative experiments on ceramic lapping was conducted. A conventional lapping process performed by cast iron lapping plate was selected as the contrast, a standard water based diamond (same size as the diamond in resin bonded plate) slurry was used as abrasive. For the lapping of resin bonded plate, tap water was used as the fluid. The experimental conditions are listed in Table 3.4 below and the experimental set up is also illustrated in Fig. 3-11.

The ceramic workpieces with an initial surface roughness of 0.45 µm was cleaned through ultrasonic washing and then left till dry, the weight of each workpiece was examined by electronic balance before lapping. For each group of experiment, six workpieces were machined together in one holder at the same time, and the surface roughness and weight loss was examined and recorded at 10min., 20min., 30min., 40min.,

50min., and 60min. The surface roughness of machined workpiece was measured randomly on a small area of the surface from each of them, then a mathematical average was taken for comparison in Fig. 3-12.

Table 3.4 Experiment conditions

Parameter Value Lapping machine Lap Master 12 Workpiece ø20 mm silicon wafer Minimum steady rotate speed [rpm] 1 Maximum rotate speed [rpm] 50 Round count precision[˚] ±0.5˚ Lapping load[ kgf/cm² ] 0.018 Lapping period[min] 10, 20, 30, 40, 50, 60

Figure 3-11 Experimental set-up for lapping experiment

roughness and weight loss was examined and recorded at 10min., 20min., 30min., 40min.,

Figure 3-12 Surface roughness (avg.) of ceramic workpiece machined by iron plate

lapping and UV curable light-curable resin bonded plate machining

3.3 Results and Discussion

Fig. 3-12 shows the surface roughness obtained from a conventional slurry based lapping process and a UV light-curable resin bond plate involved machining process. The

workpieces were started to be machined with a 0.45µm initial roughness on both of the two groups, thus the surface roughness value is coincident at 0.45 when the machining time equals to 0. It can be seen from the figure that roughness value significantly drops down within the first 20 min. and then floats in a small range. Moreover, the decreasing rate of roughness value is relatively higher in iron plate lapping. In the first 10 minutes, the roughness value in iron plate dropped down from 0.45µm to 0.29µm, the difference is

0.16µm. Meanwhile, the difference in UV resin bond machining is 0.19µm. This phenomenon could be explained by the material removal mechanism of each process.

In conventional slurry based lapping, the diamond grains were delivered into the work zone between iron plate and the ceramic workpiece. The diamond grains started to work as loose abrasives immediately and perform indentation on the brittle surface of ceramic workpiece to remove material, the mechanism of indentation is illustrated in Fig.

3-13 From previous study, the indentation of a sharp particle will deform the workpiece surface elastoplastically, producing a plastic zone with residual tensile stress which generate and propagate the lateral crack. According to Marshall et al. [29], the equilibrium length, c, of the lateral crack is given by

퐸 3/8 푐 = 훼 푤 푃5/8 0 1/2 1/2 푖 퐾푤 퐻푤

(3.1)

Where 훼0 is a material independent constant which depends on particle shape; 퐸푤 refers to the Young’s modulus of the workpiece, 퐾푤 is the fracture toughness of the

workpiece, 퐻푤 is the hardness of workpiece and 푃푖 is the normal load per load-bearing particle. The lateral cracks h originate from the bottom of the plastic zone and is given by

1/2 퐸푤 1/2 ℎ = α1 푃푖 퐻푤

(3.2)

where α1 is another constant depending on particle shape.

Figure 3-13 Schematic representation of an indentation event [28]

The volume of workpiece material removed per indentation above the fracture threshold is given by [30]

2 푉푖 = π푐 ℎ

(3.3)

Substituting equations 1 and 2 into 3, it becomes to

5/4 퐸푤 7/4 푉푖 = α2 2 푃푖 퐾푤퐻푤

(3.4)

It is assumed that under steady state abrasion conditions a constant number of abrasive particles is at all times in simultaneous contact with lapping plate and workpiece, and every indentation event of the rolling particles removes a chip from the workpiece the size of a lateral crack [31]. With this assumption, the material removal rate can be considered as the volume removed from each indentation times the number of abrasive particles within a certain time period. It was found to be given by

3/4 5/4 푣푃푖 푃푡표푡 퐸푤 푍 = α3 2 푅퐴푤푝 퐾푤퐻푤

(3.5)

where 훼3 is constant depending on particle shape, 푃푡표푡 is the total load, R is the mean radius of the particles and 퐴푤푝 is the area of the workpiece.

From Fig. 3-12, it appears that the lowest roughness value iron plate lapping could achieve is around 0.2 µm . From the previous analysis, it can be concluded that in brittle material lapping the surface roughness R can be associated with the depth of lateral fracture h. Based on equation (3.1), the parameters that could influent roughness including the abrasive particle shape, modulus and hardness of the particle and the load per load-bearing particle. In our two groups of experiment, the abrasive particles were from one same batch of powders and the difference in particle shape and material properties can be neglected.

Thus, it is clearly to summarize that the difference in final roughness was generated by the

different load per particle in two groups. The hardness of cured resin plate is much smaller than that of cured resin plate, and a resin plate is relatively easier to deform when a load was applied on. In our experiment, once the test started and the load was applied on, a number of abrasive grains would be pressed into the resin plate like the Fig. 3-14 shows below. As a result, the number of abrasive grains that contacted the workpiece increased and the contact area of each grain that contacted with plate increased as well. Since the total load applied on each workpiece was the same, these increase caused a decrease on load per load-bearing grain. From equation (3.1), the roughness is directly proportional to the load per grain. Therefore, the workpiece machined on UV light-curable resin bond plate could achieve a better surface finish with a roughness lower than 0.18µm. Moreover, a number of abrasive grains were pressed into the resin matrix also reducing the effective average abrasive grain size. In case of iron plate lapping, the load-bearing grains are apparently larger in size than those are not bearing load, so the effective average grain size is above the total average grain size. As a dominant factor, grain size primarily influents the surface finish in abrasive machining. Compared to abrasive grains in small size, a larger grain size will directly increase the surface roughness when other variable is fixed and decrease the material removal rate of machining as well.

Figure 3-14 Lapping mechanism comparison between UV resin bond lapping and

conventional lapping

In equation (3.5), the load per load-bearing particle, 푃푖, is also proportional affected the material removal rate. By considering that, it can be expected that the machining process with higher material removal rate should firstly reach the ultimate surface roughness it could achieve. It is illustrated in Figure 3.12 that the roughness in each group both rapidly drop down within the first 10 min. and the decreasing slope from iron plate lapping is even bigger. The sharply decrease in roughness indicated the higher material removal rate and it matches the analysis based on equation (3.5).

The surface profile of the machined ceramic workpieces were examined under atomic force microscope, two 3D views selected is shown in Fig. 3-15 and Fig. 3-16. Based on comparison from these figures, it can be clearly seen that a rough surface with more

“hills” and “valleys” was finished by iron plate, while a better surface was obtained from

UV light-curable resin bonded abrasive plate lapping.

Figure 3-15 AFM micro observation on ceramic workpiece machined by iron plate

lapping

Figure 3-16 AFM micro observation on ceramic workpiece machined by UV light-

curable resin bonded abrasive plate lapping 51

3.4 Conclusion

A novel manufacturing method was experimentally developed for abrasive machining, in which UV light-curable resin was selected as the bonding material. In order to study the work performance, a group of comparative experiments were carried out on ceramic workpiece. According to the results and analysis based on the specific abrasive machining mechanism, the following conclusion can be drawn.

 It is practicable to use UV light-curable resin as the bonding material for

manufacturing of abrasive tool like the abrasive machining plate in our

research. Compared to the conventional manufacturing of resin bond

abrasive article, this method significantly reduces the time consuming and

energy cost. Also, UV light-curable resin bond abrasive tools’

manufacturing is environmentally friendly in which nearly none by-

products generate.

 In machining of brittle material like ceramic workpiece in our study, with

the abrasive diamond grains in same size, the machined surface roughness

under UV light-curable resin bond abrasive machining is improved by more

than 10% comparing with slurry-based iron plate lapping. It indicates that a

better surface quality could achieved by former application when the

average abrasive grain size in both machining process is the same.

 Compared to conventional lapping in this research, the abrasive machining

on UV light-curable resin bond plate has a relatively lower material removal

rate. It can be explained by the disadvantages in hardness and strength. In

future works, the improvement on material properties of the resin/grain composites could be a potential direction.

Chapter 4

Influence of Nanosized Aluminum Dioxide Filler Additive

4.1 Experimental Procedure

4.1.1 Materials Prepare

The resin used for this study was provided by DYMAX (Light Weld® 425 optically clear structural adhesive, 431 Ultra-Light Weld® Versatile Glass-to-Metal Bonding

Adhesive), both of the uncured and cured properties from supplier are shown below in

Table 4.1.

Table 4.1 Material specification of the UV-curable resin

Properties Dymax 431 Dymax 425

Density (g/ml) 1.04 1.07 Viscosity cP (20 rpm) 500 4000 Hardness (Durometer) D70 D80 Tensile (psi) 7000 6200 Elongation (%) 61 7.3 Modulus of Elasticity (psi) 82000 500000

From previous studies of the filler-loaded resin matrix, the mechanical properties of filler-resin composites basically depend on the filler content, size and morphology [32,

33]. Moreover, researches in dental resin composites (DRC) revealed that increasing the filler content and reducing the average filler size is one of the methods to produce DRC that require adequate strength and wear resistance to endure mastication forces [34]. The average size of alumina (α-Al2O3) fillers used in this study was 0.5 µm, and the diamond abrasive grain used was supplied by Engis Corporation and sized at 12-22 µm. Table 4.2 showed the specifications of the diamond resin composites filled with alumina filler in this study. Diamond concentration of a diamond tool means the weight of the diamonds in each cubic centimeter of the diamond tool, and it is an important feature of diamond tools that greatly influencing their abrasive machining efficiency. In high precision abrasive machining, low concentration resin bond diamond tools are frequently used. The diamond abrasive concentration in our study was selected at 12.5% vol. which indicates that each cubic centimeter resin composites contain 0.11 gram diamond grains.

Table 4.2 Specifications of the resin-bonded diamond composites

Specimen Filler Diamond Size Concentration

A None 12-22 µm 12.5% vol.

B Al2O3 2.5% wt. 12-22 µm 12.5% vol.

C Al2O3 5.0% wt. 12-22 µm 12.5% vol.

D Al2O3 7.5% wt. 12-22 µm 12.5% vol.

E Al2O3 10 % wt. 12-22 µm 12.5% vol.

Figure 4-1 DYMAX 5000-EC UV Curing Flood Lamp

4.1.2 Methods

The composites of UV-curable resin, abrasive diamond, and alumina filler was uniformly mixed together in vacuum condition to remove the air bubbles which could be created during the stirring process. The prepared diamond composites in liquid form were injected into different shapes of mold and then processed to the exposure of light primarily in the UV range (320-390 nm wavelength). Fig. 4-1 shows the UV curing system used in this research, and the process of UV curing could be seen in Fig. 4-2 below.

4.1.2.1 Determination of the Transmittance of Visible Light

The uncured resin composites paste with different ratio of alumina filler was prepared in a vacuum condition, and then injected into a transparent plastic mold with a depth of 10 mm. The transmittance of visible light passing through the composites were observed at room temperature using spectrophotometer. 56

Figure 4-2 Manufacture process of UV light-curable resin bonded diamond plate

4.1.2.2 Curing Depth of the Resin Composites

The mixed composite was injected into a nontransparent “cup” mold with 10 mm inner diameter and a depth of 20 mm. All of the samples were cured under UV light for 60 seconds and then left for 24 hours. The samples were cleaned as to remove the uncured portion of the composites and then left at room temperature till dry. The thickness of the cure depth of the samples were measured with caliper.

4.1.2.3 Vicker’s Hardness

The samples of each cured composites were fabricated in a stainless steel mold

(10mm diameter*2mm depth) and kept for 24 hours at room temperature prior to hardness test. The test was carried out with a micro hardness tester Clark CM-400AT. Vickers hardness was determined with the application of 200g for 10s. After the load P was applied,

the value of diagonal indent lengths d1 and d2 were measured. Three specimens prepared for each composites and the mean value was used for the analysis. The Vickers Hardness was calculated by the following equation according to the ASTM E384-11e1:

136° 2푃푠𝑖푛 푃 퐻푉 = 2 ≈ 1854.4 푑2 푑ퟐ

(4.1)

Where HV is Vickers hardness, P is Load in kgf, d is arithmetic mean of the two diagonals, d1 and d2 in mm

4.1.2.4 Tensile Test

Three rectangular shaped sample (10mm width*20 mm length) of each composites were prepared for tensile test, all of the specimens were left at room temperature for 24 hours prior to test. The tensile test was performed on Instron 5569 Testing System, at a speed of 2mm/min. A strain gage was used to determine the elongation and tensile modulus, yield strength and ultimate strength.

4.1.2.5 Wearing Test and Microscopy

Three samples of composites A and E was prepared for the wearing test and microscopy observation, all of the specimens were cured in same conditions and left at room temperature for 24 hours prior to test. The worn test was performed on a lapping machine equipped with 1000-grit lap grinding plate. The samples were lapped for 30 seconds at a speed of 50 rpm and Trim C270 (Master Chemical) was used as the coolant.

After wearing test, the samples were cleaned and dried, then the worn surface of the samples were examined by microscopy.

4.1.2.6 Lap Grinding Performance

Composites A and E was selected to compare the influence of alumina filler loading on the machining performance of cured diamond lap grinding plate. The composites paste was evenly spin-coated on the top surface of an aluminum base plate, and then the composites coated plate was cured by UV light. The cured plates were left at room temperature for 24 hours before they had been dressed at same condition. The manufacturing process of UV resin bond plate is illustrated in Fig. 4-2.

The lap grinding experiment was carried out on a double side lapping machine and ceramic ring with initial roughness of 0.4 µm was selected as the workpiece, the lap grinding speed was selected at 50 rpm and the load was 7 kPa. Trim C270 supplied by

Master Chemical was used as the coolant. Three workpieces machined each time for 10 minutes, the surface roughness and material mass loss of the workpiece was recorded for every 2 minutes.

4.2 Results and Discussion

4.2.1 Cure Depth of Resin-Diamond Composite

Cure depth is a major issue for light-cured resin diamond composites since the resin is partly translucent and scatter light, light penetration decreases as the resin thickness increased. This is due to the absorption and scattering of light by the diamond particle, the

additives and porosity of the composites. Since the resin in this study is UV-reactive, the state and structure of the resin would be affected around the range of UV light wavelength due to the polymerization of the monomers. Meanwhile, the interfacial bond between the diamond and resin, the Al2O3 filler and resin would also be affected because of the volume shrinkage of the composites. All of these factors mentioned above would influent the light transmittance of the uncured composites and lead to an erratic fluctuation. Hence, the visible light wavelength range from 450 nm to 800 nm was selected for light transmittance test.

Figure 4-3 shows the transmittance of uncured resin-diamond composites with different ratios of Al2O3 filler within the range of visible light wavelength. The figure indicates that more than 95% (1 -T %) of the light was absorbed and scattered by the resin- diamond composites, regardless of Al2O3 filler loading. When there is no filler introduced, the transmittance value of the composites float between 3.0% and 3.5%, and it approaches to 4.0% in 450 nm wavelength. The transmittance value drops significantly down to 2.5% when the resin-diamond composites is filled with Al2O3 filler, and the continuous decrease of the transmittance is attributed to the increase of the filler loading in the composites. The lowest transmittance value reaches 0.75% when the filler loading was 7.5%wt.

4.0

A 3.5

3.0

2.5 B

2.0

1.5 C

Transmittance (T%) Transmittance D 1.0

E 0.5 400 450 500 550 600 650 700 750 800 Wavelength (nm)

Figure 4-3 Visible light transmittance of the uncured composites with different ratios of

Al2O3 filler

1.6 1.52 Cure depth 1.4

1.2 1.14 1.07 1.0 0.93 0.83

0.8

0.6

Curedepth (mm) 0.4

0.2

0.0 0.0 2.5 5.0 7.5 10.0 Filler loading (wt%)

Figure 4-4 Thickness of the cured resin diamond composites with different loading of

Al2O3 filler 61

Figure 4-4 shows that the cured depth of the composites with different ratios of

Al2O3 filler. It is found that the increase in filler loading leads to the decrease in cure depth.

This can be explained that the increase in filler loading increased the turbidity of the resin- diamond composites, then the light transmittance decreases as a result. Most of the light is absorbed and scattered by the very top layer of the composites to initiate the polymerization. The light penetration decreased as the turbidity of the composites increases, and the lower layer could not absorb enough energy to intimate the reaction. It can be established that the diamond abrasive concentration and filler loading proportion significantly affect the cure depth of the resin-diamond composites, the higher filler loading leads to an increase in composites turbidity and decrease in light transmittance, and then reduce the cure depth as a result.

4.2.2 Hardness of Cured Resin-Diamond Composite

The hardness of the cured composites is a key factor influent work performance of resin bonded abrasive tools. The tool undergoes varieties of forces during the machining process, certain hardness could keep the grain in the situation of scraping or rolling, thereby affect the material removal rate and surface condition of the workpiece.

Figure 4-5 shows that the Vicker’s hardness values of the cured resin-diamond composites with different ratios of Al2O3 fillers, it can be established that the cured resin- diamond composite with Al2O3 filler demonstrate significantly higher HV when compared to the composites without Al2O3 filler (50%-100% higher), and the HV values also increases constantly as filler loading increases. Based on Fig. 4-5, the HV value of the

composites with 2.5% wt., 5.0% wt., 7.5% wt. and 10.0% wt. of Al2O3 filler could reach

15.4, 18.2, 19.7 and 20.2 kgf/mm2 respectively, and the unfilled composite indicates a

2 lower HV value at 10.4 kgf/mm . Al2O3 filler particles are stiffer and more brittle than the resin matrix, therefore, the increased ratio of Al2O3 filler contributes to an increase on HV value of the cured composite.

Hardness 19.7 20.2

) 20 2 18.2

15.4 15

10.4 10

Vickershardness (kgf/mm

0 0.0 2.5 5.0 7.5 10.0 Filler loading (wt%)

Figure 4-5 Hardness of the UV resin-bonded diamond composites with different loading

of Al2O3 filler

4.2.3 Ultimate Tensile Strength

Figure 4-6 shows the ultimate tensile strength of the resin-diamond composites filled with different ratios of Al2O3 filler. The highest strength of the cured composite reaches 45.92 MPa at Al2O3 filler loading of 10 wt. %. In this research, it can be established that the mechanical properties of the resin-diamond composite is related to the filler loading, the tensile strength values of different composites are 34.76, 39.98, 42.14, 44.73 and 45.92 MPa, respectively.

50 Tensile strength 44.73 45.92 42.14 39.98 40 34.76

Tensilestrength (MPa)

0 0.0 2.5 5.0 7.5 10.0 Filler loading (wt%)

Figure 4-6 Tensile strength of the UV resin-bonded diamond composites with different

loading of Al2O3

Statistically, the tensile strength value is increased as filler loading increases within a particular range in our study. The adequate distribution of the nanosized Al2O3 particle in the resin acted as a reinforce material and improved the structure of the composites, and then led to the improvement of the ability to sustain higher stress. Since the continuous increasing loading concentration of filler would also affect the structure of the resin matrix at some point. Further research is need to determine the appropriate range of filler loading concentration.

4.2.4 Worn Surface Microscopy Observation

Based on a number of microscopy observations of the worn surface of UV resin- bonded diamond composites, nearly all of the diamond bond conditions can be summarized as the figure shown below.

Figure 4-7 Diamond bond conditions in cured resin composites

a) The abrasive diamond was bonded in the cured resin and has not started to

perform as an active abrasive particle; b) The sharp tips of the diamond revealed as the composites worn out because

of the different wear resistance and material strength of resin and diamond.

The diamond particle at this situation could not fully perform as working

grains since the limited contact between the tips and the workpiece; c) Active abrasive diamond. Nearly half of the particle embedded in cured

resin, the bond between the resin and diamond helps to hold the grain so the

other half of the particle could work as a fixed micro cutting tool; d) Partial breakage abrasive diamond. As the machining process continues,

some of the diamond might be broken during the machining process since

the particles work under the complicate force involved condition, in terms

of wear compression, shear, etc. Although the particle was broken in this

condition, on the other side, the worn edges and blunt tips had been took

away by fluid and new sharp edges generated due to the break. e) Breakage diamond particle. The larger piece of the broke particle was pulled

out, however, the small piece of the particle still bonded to the composites.

Since the left piece of the diamond is embedded in the pull-out hole and no

edges or tips headed out from the composites, the breakage diamond in this

condition could not be considered as effective working particle. f) Pull-out hole. The diamond particle was pulled out and a hole left in the

composites, these holes provided the space to keep the break particle pieces,

material chips, working fluid, etc. as to change the hydrodynamic 66

performance between the workpiece and abrasive tool, and affect the work

efficiency positively.

The different types of diamond bond conditions in cured resin-bond composites can be seen in Figure 4.8 and Figure 4.9. Based on the comparison of worn surface microscopy between filler loaded and non-filler loaded resin bonded wheel, it is obvious to see that more rough traces and marks were generated on the surface of non-filler load wheel, a large number of diamond particles has been pulled out and the cured resin matrix worn a lot. In contrast, more active abrasive diamond grains in condition (b), (c) and (d) is found on the surface of the filler loaded grinding wheel, and the wear traces and material loss is not as serious as the former one. Since all the working conditions were the same, this difference indicates that the diamond grain embedded in the filler loaded wheel is relatively harder to be pulled out during machining, and a better wear resistance could be achieved through filler loading. It has been revealed that the work efficiency of abrasive tool is significantly affected by the properties of bonding matrix and abrasive grain retention [23]. In this study, a lap grinding experiment was also carried out to examine the influence of filler loading in machining process of diamond abrasive tool.

Figure 4-8 Diamond abrasive conditions in cured resin-bond composite (a, b)

Figure 4-9 Diamond abrasive conditions in cured resin-bond composite (c, d, e, and f)

4.3 Lap Grinding Experiment on Ceramic Workpiece

As shown in Fig. 4-10 and Fig. 4-13, the surface roughness of the machined ceramic workpiece and the material removal rate was compared between the Al2O3 filler loaded resin-bond diamond plate and plate without filler loading.

4.3.1 Surface Roughness of Workpiece

It can be clearly seen in Fig. 4-10 that the workpiece machined on the filler loaded plate obtained an outstanding improvement in surface roughness. The average roughness value of the two tests represented a same trend that they dropped down rapidly in the first

4 minutes from 0.40μm to 0.085μm and 0.124μm respectively, then the drop rate reduced significantly and the roughness approached to 0.080μm and 0.1220μm at 6 minutes. After

6 minutes, the roughness fluctuated around 0.075μm and 0.117μm and the lowest value of each condition was reached at 10 minutes (0.0752μm and 0.1162μm). The near 30% improvement of the surface quality can be firstly explained that the alumina filler also played as an abrasive particle during the lap grinding process. Since the particle size of the alumina filler is smaller than the diamond abrasive particle, the chips taken off by indentation from the filler loaded plate is much smaller than the ones from non-filler loaded plate. Moreover, the free alumina filler particle that pulled out from the resin matrix caused micro-polishing between the abrasive tool and workpiece, which also contributed to a better surface quality. The micro-observation of the machined surface piece was shown in

Fig. 4-11 and Fig. 4-12.

0.45

0.40

0.35 No filler loaded plate Filler loaed plate 0.30

0.25

0.20

0.15

0.10

Roughness(micrometre)

0.05

0 2 4 6 8 10 Time (minute)

Figure 4-10 Surface roughness of workpiece after machining

Secondly, the higher hardness of the filler loaded plate helped to reduce the surface roughness of machined workpiece. Both of lapping and grinding mechanism exists in the lap grinding performed by resin-bonded diamond plate, thus the surface roughness of workpiece determined by two-body abrasion and three-body abrasion collectively. The surface roughness model for description of three-body abrasion can be presented as:

1/2 퐸푤 1/2 푅 = α1 푃푖 퐻푤

(4.2)

퐸푤 is the Young’s modulus of the workpiece, 퐻푤 is the hardness of the workpiece,

α1 is the constant depending on particle shape. 푃푖 is the normal load per load-bearing particle, and it is dependent on the hardness of workpiece and plate. It can be derived from equation (4.2) that, if we fix the other parameter as a constant value, then the increase of

퐻푤 would lead to the decrease in R.

Additionally, the improvement on material ultimate tensile strength also helped to reduce the amount of the broken chips from the composites as to avoid damages to the workpiece and influence to the working zone.

Figure 4-11 Ceramic workpiece after lap grinding on resin-bond diamond plate

Figure 4-12 Ceramic workpiece after lap grinding on Al2O3 filled resin-bond diamond

plate

4.3.2 Material Removal Rate

In order to evaluate the material removal efficiency of the abrasive tool, the mass change of the workpiece was examined every two minutes from beginning of the process.

All of the three ceramic workpieces were unloaded each time and cleaned with an ultrasonic cleaner, the weight loss was measured by electronic balance and then recorded.

A mathematical average was taken to indicate the weight after certain period machining and the results shown in Fig. 4-13.

9441

9440

9439

9438 No filler loaded plate Filler loaded plate Linear fit of no filler loaded plate 9437 Linear fit of filler loaded plate

Massofwork piece (microgram)

0 2 4 6 8 10 Time (minute)

Figure 4-13 Mass loss of ceramic workpiece during machining

It can be seen that the weight dropped down to 9438.90 mg and 9437.00 mg, respectively from 9440.60 mg after 10 minutes lap grinding. The material removal on the filler loaded plate is 3.6mg in average and the value on non-filler loaded plate is 1.7mg on average. It can be also investigated in the figure that the mass loss slope on the filler loaded plate is larger than the slope on non-filler loaded plate, which can be considered to a higher material removal rate can be achieved with alumina filler loaded plate. This improvement can be explained that the filler loading led to an increase in the ultimate tensile strength of the cured composites. In the case of non-filler loaded plate, the diamond particle was easier to be pulled out from the resin matrix and then employed to the three-body abrasion. On 73

the other side, the material properties improvement of filler loaded plate helped to retain the diamond particle and alumina grit to perform the two-body abrasion.

The observations shown in Fig. 4-14 and Fig. 4-15 are the worn surface of the two different plates after machining, the circled part 1 indicated the active diamond and the circled part 2 indicated the pull-out holes. It can be clearly seen that there are more active diamonds in Fig. 4-14 and that implied more two-body abrasion was taken place during the filler loaded plate machining. It was known that three-body abrasion is not as effective as two-body abrasion for material removal in machining. The longer working time as two- body abrasion of each particle contributed to the machining efficiency, and then led the improvement of filler loaded plate on material removal rate.

Figure 4-14 Worn surface of filler loaded plate

Figure 4-15 Worn surface of non-filler loaded plate 4.4 Conclusion

In this research, the nanosized Al2O3 particle was employed in the UV light-reactive resin-diamond composites as filler. The increase in the filler loading within particular range led to an increase in the hardness and ultimate tensile strength. On the other hand, the cure depth of the cured composites decreased as the filler loading proportion increased due to the light absorption and scattering. However, the cure depth of the resin-diamond composites is still suitable for abrasive application such as polishing pad and lapping plate.

In addition, the employment of Al2O3 filler could also improve the wear resistance of the composites. As a reinforce material, the filler strengthened the bond between diamond and resin as well. All of these advantages mentioned above led to an improvement of the UV curable resin-bonded diamond plate on lap grinding, the surface roughness of the machined

ceramic workpiece reduced significantly and the material removal rate of the process was improved as well.

Chapter 5

Study on the Surface-Textured Diamond Grain and UV-Resin Matrix

5.1 Diamond Abrasive

As one of the hardest material in industry, diamond powders are mainly used in abrasive applications, in which diamond particles are bonded on a variety of tools for grinding, sawing, cutting, drilling and slicing applications, or bonded to form a diamond abrasive films or incorporated into slurries and compounds for lapping and polishing process [35]. Based on the investigation that was carried out by L. E. Samuel, H. E. Exner,

R. D. Buchheit, et al., in abrasive machining, the material remove rate is significantly affected by the diamond particle size [36-38]. Moreover, the abrasive particle shape characteristics including minimum and maximum diameter, aspect ratio, convexity and sharpness also majorly influences abrasion efficiency.

In our previous study, the UV light-curable resin was introduced into the manufacturing of an abrasive machining plate in which diamond gain was selected as the abrasive grain. Particularly, the characteristics of the diamond grain played an important role in the absorption and scattering of the UV light as an additive in resin, which leading

an influence on the photosensitive resin polymerization. By considering the studies above, a research about diamond grain features’ influences on UV light-curable resin bond tools’ work performance was proposed.

5.2 Diamond Etching Mechanism

The resin used in this research is supplied by DYMAX (Light Weld® 425 optically clear structural adhesive, 431 Ultra-Light Weld® Versatile Glass-to-Metal Bonding

Adhesive), the specifications of which can be found from Table 7. Two types of diamond abrasive grain was purchased from Engis Corporation, surface textured monocrystalline diamond particles with various degree of surface texture (etching) and standard monocrystalline diamond. Both of the two batches of diamond powder was sized in 12-

22µm in average. Through the diamond etching mechanism given out by Engis [35], the process is a result of water molecules adsorption on the diamond surface, whereas most of them undergo dissociation when colliding with the diamond surface forming chemical bonds: C-H; C=O and C-O-H. Dissociation of water molecules when colliding with one another and with diamond surface, takes place according to chemical reaction:

+ − 퐻2O ↔ 퐻 + 푂퐻

While, as protons, the produced hydrogen ions could be hydrated to hydroxonium ions. As a result, reaction equation can be better expressed as:

+ − 2퐻2O ↔ 퐻3푂 + 푂퐻

By reason of the high acidity and strong basicity, the hydroxonium and hydroxide interact with diamond surface and form chemical bonds: C-H; C=O and C-O-H. Therefore, 78

the diamond surface is etched. A comparison on SEM micrographs of etched and unetched diamond grains are shown in Fig. 5-1 and Fig. 5-2.

Figure 5-1 SEM micrograph of unetched abrasive diamond particles (12-22µm)

Figure 5-2 SEM micrograph of etched abrasive diamond particles (12-22µm) 79

Table 5.1 Resin diamond composite specifications

Resin diamond Resin diamond Specification composites A composites B

Resin Dymax® 431,425 Dymax® 431,425

Engis HYPREZ® Surface textured Engis HYPREZ® 12-22µm MA4 Diamond 12-22µm MA4 diamond powders diamond powders (Prepared according to Patent Appl. No. 12/982, 444.

Abrasive 12.5% vol. 12.5% vol. concentration

Figure 5-3 A manufactured resin bond diamond grinding wheel

Composites A and B was prepared to compare the influence of diamond characteristics on the machining performance of a UV light-curable resin cured abrasive lap grinding wheel. The prepared composites paste was evenly spin-coated on the top

surface of an aluminum base wheel, and then the composites coated wheel was cured by

UV light. Samples for material property study was also prepared. Three rectangular shaped sample (20 mm length*10mm width*2mm thickness) of each composites were prepared for ultimate tensile strength and vickers hardness test. A finished resin bond abrasive lap grinding wheel was is shown in Fig. 5-3. The lap grinding experiment was carried out on a lapping/vertical grinding machine and ceramic wafer with initial roughness of 0.4 µm was selected as the workpiece, the lap grinding speed was selected at 50 rpm and the load was 7 kPa. Trim® C270 supplied by Master Chemical Corporation was used as the coolant.

Three workpieces machined each time for 10 minutes, the surface roughness and material mass loss of the workpiece was recorded for every 2 minutes.

5.3 Comparative Study on Surface Textured Diamond Matrix

5.3.1 Light Transmittance of Resin-Diamond Composite

Fig. 5-4 compared the ultimate tensile strength and hardness of cured samples made from composites A and B. As shown, the composites cured with surface textured diamond has a relatively better performance on material properties in terms of strength and hardness.

For the ultimate tensile strength, the value increased by more than 15% from 33.41 MPa to

38.65 MPa. In the same way, the hardness of cured composites with etched diamond was improved from 10.64 Kgf/mm2 to 12.17 Kgf/mm2. These changes indicates that the surface characteristics of diamond grain effectively influent the curing process, and a better

polymerization and fully reaction can be achieved through the introducing of surface textured diamond particles.

Figure 5-4 Ultimate tensile strength and Vickers hardness of cured composites with

unetched diamond and etched diamond

The abrasive diamond grains in resin could be considered as an additive which affect the light transmittance of the composites. In this case, the energy for resin curing was from the UV light exposure and the composite’ ability of absorbing light significantly determined the material properties of cured tool., In particle dispersed epoxy resin composites, the more the light transmitted indicates the less the light absorbed under the same condition, one method of improving the light absorption is to reduce the light transmittance by reducing the normalized total surface area between the particle and the resin matrix[39]. That is to say, a relatively larger surface area of the particle leads to an improvement of light absorption in curing. 82

Compared to the unetched diamond grain, the etched grain has a number of etching pits on its surface and the surface roughness and surface area increased accordingly. The extra energy absorbed in etched diamond resin composites helped the photoinitiator reacted completely to obtain a better polymerization. Hence, the improvement on hardness and strength of etch diamond resin composites can be explained by the difference in completeness of the UV light-curable resin polymerization.

5.3.2 Wear Performance of Surface Textured Diamond Tool

The surface roughness of machined ceramic wokpiece obtained using the resin bond unetchd diamond wheel and etched diamond wheel is shown in Fig. 5-5. It can be clearly seen that the lowest roughness 0.190µm can be achieved by etched diamond wheel at 6 minutes, the value then floated within a small range from 0.190 to 0.193. Meanwhile, the roughness of workpeice machined from unetched diamond wheel dropped down from

0.450µm to 0.204µm at 8 minute and then varied around 0.205µm, which is higher than that in etched diamond wheel machining. Previous studies have indicated that the under the same machining condition, the roughness obtained from abrasive machining could be determined by the depth of lateral fracture that is given in equation (5.1). The later fracture depth is collectively determined by the effective hardness (Equation 5.2) and the mean size of abrasive particle Lm, c.

1/2 퐸푤 1/2 ℎ = α1 푃푖 퐻푤

(5.1)

훽푤퐻푤 훽푙푝퐻푙푝 1 = 1 훽 퐻 2 훽 퐻 2 [1 + ( 푤 푤 ) ]2 [1 + ( 푙푝 푙푝) ]2 훽푙푝퐻푙푝 훽푤퐻푤

(5.2)

푏 2 2 푃푖 = 퐻푒푓푡(1 − ) 퐿푚,푐 퐿푚,푐

(5.3)

From equation (5.2), the improvement in hardness of the resin bonded tool helped increasing the effective hardness of machining process. As a resulted, the load per particle is risen up. Substituting equation (5.3) to equation (5.1), it can be expected that a larger lateral fracture depth and surface roughness would be achieved from etched diamond bond wheel. However, the results in Fig. 5-5 apparently show that etched diamond bond wheel achieved a relatively lower surface roughness, which indicates a better surface finish. This plausible prediction can be explained by the difference of diamond abrasive grains in shape profile. A definition of major, minor and minimum diameter of abrasive particle is illustrated in Fig. 5-6. The etching process on diamond abrasive grains resulted less angular particle shapes, the major diameter of grains were therefore reduced. Hence, the mean size of the etched diamond grains, Lm, c, was reduced to lower the final surface roughness the tool can achieve. This is in agreement with the results shown in Fig. 5-5.

Figure 5-5 Surface roughness varying with machining time

Figure 5-6 Definition of major, minor and minimum diameter on abrasive grain [40]

Another discovery can be revealed form Fig. 5-5 is the difference in decreasing rate of the surface roughness value. For the unetched diamond bond wheel, the roughness value of the machined workpiece drops down to its stable range (0.205±0.001µm) after 8 minutes while this number in etched diamond bond wheel is 6 minutes. This indicates that a better wear efficiency was performed on later wheel. Thanks to the increased surface area on etch diamond grains, the bonding retention between the diamond and resin matrix was mechanically improved. The strengthening on diamond-resin bond helped retaining abrasive grains in resin to perform two-body abrasion in machining. As a result, the etched particles provided a longer service life as fixed abrasive particles other than unetched ones.

From previous studies, it is generally considered that a fixed abrasive machining is more productive in material removal rate. Under the combined action of a higher tool hardness and stronger bond of abrasive and resin, the higher decreasing rate of roughness on etched diamond tool can be therefore explained.

5.4 Conclusion

The influence of abrasive diamond surface characteristics in UV light-curable resin bond tool was studied in this research. Surface etched diamond and unetched diamond was selected as the abrasive grain for this study. A set of comparative experiments were conducted to examine the impact in material properties and wear performance. From this work, the following conclusion can be drawn:

 The UV light absorption is diamond-resin composite curing is affected by

the surface characteristics of diamond grains, the increased surface area by

etching on etched diamond helped absorbed more energy to obtain a good

polymerization and build a strong bond between the grain and the resin

matrix.

 In resin bond machining, compared to the hardness of abrasive tool, the

grain size and shape profile of the grains play a dominating role that affect

the surface finish

 If the process factors besides abrasive diamond stays the same, a higher

stock removal and better surface finish on workpiece can be achieved

through etched diamond resin bond tool.

Chapter 6

Model Development on Single Diamond Grit Retention in UV-Resin Matrix

6.1 Introduction and Background of Diamond Retention in Matrix of Abrasive Tools

Diamond tools are broadly employed in grinding, polishing and finishing brittle materials include ceramic, glass and stone. A key factor in ensuring superior diamond tool performance and maximum work life is the retention of individual diamond grit in bonding matrix [41]. According to the bond’s properties, the manufacturing process of diamond tools differ from diamond-metal sintering to diamond-resin thermal pressing and curing.

For heavy duty machining as sawing and drilling tools, where dynamic contact load per cutting diamond grit can be more than 1GPa [42]. Metal powders sintering process are selected in this case to create an adequate retention for diamond grit. For lapping and grinding tools, a softer resin or vitrified bonds are employed due to the relatively lower contact loads on each diamond grit.

A well designed diamond tool is expected to hold diamond grits firmly, and guarantee an adequate wear resistance as well. In many cases, a lower wear resistance of

the matrix is desirable because it helps to reveal the fresh diamond grit from the bond. The wear resistance of the metal matrix can be modified in many ways by using soft or hard components according to the mode of cutting and workpiece’s properties [43]. Also, the wear resistance of vitrified bond or resin bond matrix could be changed by adding various filler or additives [44]. However, a much more difficult challenge is to adjust the bonding between the diamond grit and matrix, as to improve the retention capability of the bond to prevent premature loss of diamond.

The retention of diamond grits in matrix occurs as a results of either mechanical or a combination of mechanical and chemical bonding [45]. Because of the superior chemical resistance and high surface energy, the chemical reaction initiated adhesive bonding that taking place during sintering or curing process is negligible. As a result, in most cases, bonding between diamond and matrix relies on mechanical locking [46, 47]. In manufacturing, the diamond grains are tighten by metal or resin matrix during cooling process due to the mismatch of thermal expansion coefficients of diamond and matrix.

Compared to metals or resins, diamond has a much lower coefficient of thermal expansion, which tightens the diamond grit up by shrinking matrix [48]. Therefore, the stress and strain field is generated around each diamond particle to retain it during operation.

In general, the proportion of each diamond particle which performs work is much smaller when compared with that portion which is lost due to the weak retention capability of matrix. As we mentioned above, because the extreme stable chemical resistance of diamond grit, the large majority of metal or resin matrices rely on purely mechanical retention. In practice, it is not very easy to develop a matrix capable of effective utilization of very strong synthetic diamond grit which is perfectly defined blocky shape and smooth 89

faces. The diamond therefore has to be modified to cooperate with the matrix to build a strong bond to reduce its pullout [48]. Diamond manufacturer and tools maker have devoted significant research to improve the retention and have developed a number of options.

In practical manufacturing, there are two main ways where the diamond-matrix bonding strength could be improved. The first method consists in thermal or chemical treating the diamond grit in order to roughen its surface [49-51]. However, an obvious disadvantage that a relatively large quantity of material must be removed from the diamond causes not just a material waste and economic issue, but also a potential risk that the surface etching on diamond surface may affect its the integrity and strength.

Another way to effectively improve diamond-matrix bond strength through coating diamond grits with a thin film of strong carbide former such as titanium [52], chromium and zirconium [53] or silicone [54], tungsten, niobium and alloys [55]. In this case, metal coating helps to modify the surface texture of diamond grit to increase contact area between diamond and matrix, and adjust the interface friction at the same time. Since the compressive stress in vitrify or resin bond is relatively small to sintered metal bond, it is absolutely necessary to keep interface friction between diamond and matrix at an adequate level, to contribute converting compressive stress to retention when bearing contact load

[42]. Researchers have also developed different approaches to address the retention issue, not just by diamond coating technique but also through the modification of the matrix.

Such as sintering at a higher temperature for a longer time, or adding various additives and fillers to the matrix.

According to our previous research, the wear performance of the UV resin-bond diamond grinding wheel largely depends on the material properties and characteristics of the UV resin matrix. In this paper, the UV resin matrix is strengthened by adding aluminum oxide fillers. The effect of filler adding on mechanical properties of the UV resin matrix and the stress distributed around each diamond grit under contact load during operation is presented. Investigates were carried out by finite element modeling using Abaqus 6.14 software.

6.2 Theoretical Force Model of Mechanical Retention

6.2.1 Literature Review of Modeling on Diamond Retention Capability of

Matrix in Abrasive Tool

The mechanical retention, or mechanical locking as we mentioned above, is obtained through the matrix sintering or curing process, in which the matrix shrinks during cooling and generates compressive force to retain the diamond. In order to predict the retention strength and bond quality between diamond grit and matrix, it is needed to develop a model that could describe the correlation between inputs and outputs. Although the related works on resin matrix is very limited, research that concentrates on diamond- metal bond has established models to analyze various involved factors that affect the bond quality and to reveal the bonding mechanism.

The modeling on diamond retention can be categorized to physics modeling and numerical modeling. In numerical modeling, finite element method using software is the

typical approach, in which the material properties of diamond and matrix is used as the input for computer modeling [56] The simulation is usually performed on single diamond grit and it surrounded matrix, the strain and stress distributed in that area is simulated as the output.

Figure 6-1 Forces on working diamond cutting point in a drill or saw blade [42]

The establishment of physical model is based on elastic mechanics, a metal matrix wrapped diamond grit in Fig. 6-1 is usually employed as the analysis object. In some cases, the mechanical retention can be evaluated as the compressive force that the diamond grit undergoes, and the correlation in metal sintering bond is summarized into a simplified format in equation below [57]

휀 σ = 푆 1 ( 2 + ) 푆1퐸1 퐸2

(6.1)

where σ is the retention force, 휀 is matrix shrinkage rate on diamond, 퐸1 is the elastic modulus of diamond, 퐸2 is the elastic modulus of matrix, 푆1 is the radius of 92

diamond grit and 푆2 is the radius of metal-wrapped diamond unit. For this model, the consideration is based on the geometry of single diamond unit and matrix shrinkage rate, which is an empirical parameter.

The retention is also discussed as a ratio R in sintered cobalt bond tool and described as [43]:

퐴푝 휇 푅 = 1 푝2

(6.2)

where A is the contact area between diamond and matrix (Fig. 6-1), 푝1 is the matrix compressive stress, 푝2 is the contact force and 휇 is the diamond-matrix friction. The main analysis object of this modeling is the working diamond grit which partial embedded in the matrix, and a contact force from the workpiece is applied on it. Therefore, the interface area and friction between the diamond and matrix is taken into account. As the output of the previous model, the matrix compressive stress is used as an input in Equation (6.2) and it is directly proportional to the retention ratio. Compare to the previous force model, the establishment of a method to define the retention capacity as a ratio push the diamond retention study a step forward.

6.2.2 Development of the Theoretical Mechanical Retention Force Model

In general, the physical mechanics-based modeling views the compressive force on diamond grit as a representation of retention ability of the matrix. Even though the resin matrix curing is different from other matrix sintering and has its unique characteristics, the

physical mechanics on single diamond grit still has it similarity. Hence, a comprehensive analysis is performed below on resin-bond diamond grit to develop the force model.

Fig. 6-2 below shows a simplified schematic illustration of single diamond grit embedded in UV-resin matrix. In order to simplify the analysis, the diamond grit unit was assumed as a sphere with inner radius a and outer radius b in a spherical coordinates. As shown in Fig. 6-3, 휃 is measured from the positive z-axis to a radius; ∅ is measured round the z-axis in a right handed sense; u, v, w are the displacements components in the r, 휃, ∅ directions, respectively.

Figure 6-2 Schematic illustration of embedded diamond grit in matrix

Figure 6-3 Spherical coordinates

Then the displacement offered by shrinkage during the cooling process is radial.

Hence, the deformation of the matrix is symmetrical with respect to the center of the sphere, that is, the displacement vector is 푢 = 푢푟. Therefore, the equation of equilibrium reduce to single equation [58]:

푑휎 2 푟 + (휎 − 휎 ) + 푓 = 0 푑푟 푟 푟 휃 푟

(6.3)

where the radial component of the stress vector is 휎푟 and the tangential components of the stress vector are equal to 휎휃. 푓푟 represents the body force. The strain-displacement relations are:

푑푢 휖 = 푟 푑푟

(6.4)

푢 휖 = 휏 푟

(6.5)

where the radial component of strain is 휖푟, and the tangential component of strain is 휖휏. The strain-stress relation equations are:

1 휖 = (휎 − 2휇휎 ) 푟 퐸 푟 휏

(6.6)

1 휖 = [(1 − 휇)휎 − 휇휎 )] 휏 퐸 휏 푟

(6.7)

where E is elastic modulus of the matrix and 휇 is the Poisson’s ratio.

In our study, it is assumed that the ideal sphere shape of the diamond grit undergoes both inner compressive load 푝1 and outer compressive load 푝2 , and the body force is neglected. Therefore the boundary condition can be determined as:

휎푟=푎 = −푝1

(6.8)

휎푟=푏 = −푝2

(6.9)

Substitution of equation (6.4 to 6.7) into (6.3) yields the equation with one variable

푢푟:

퐸(1 − 휇) 푑2푢 2 푑푢 2 ( 푟 + 푟 − 푢 ) = 0 (1 + 휇)(1 − 2휇) 푑푟2 푟 푑푥 푟2 푟

(6.10)

The general solution for equation (6.10) is

퐵 푢 = 퐴푟 + 푟 푟2

(6.11)

where A and B is constant to be determined. Then the radial component of stress and tangential component of stress can be represented as

퐸 2퐸 퐵 휎 = 퐴 − 푟 1 − 2휇 1 + 휇 푟3

(6.12)

퐸 퐸 퐵 휎 = 퐴 − 휏 1 − 2휇 1 + 휇 푟3

(6.13)

Substituting the boundary condition (6.8) and (6.9) into equation (6.12) and (6.13), then constant A and B can be written as

푎3푝 − 푏3푝 퐴 = (1 − 2휇) 1 2 퐸(푏3 − 푎3)

(6.14)

푎3푏3(푝 − 푝 ) 퐵 = (1 + 휇) 1 2 2퐸(푏3 − 푎3)

(6.15)

Rewriting equation (6.11) by substituting equation (6.14) and (6.15), we obtained

1 1 + 휇 푢 = [(1 − 2휇)(푎3푝 − 푏3푝 )푟 + 푎3푏3(푝 − 푝 )] 퐸(푏3 − 푎3) 1 2 2푟2 1 2

(6.16)

For a single diamond grit, the retention force, or the compressive force p is generated after the resin matrix is cured. Then the resin layer that surround diamond girt also undergoes a counter force equals p, so we have 97

푝1 = 푝, 푝2 = 0

(6.17)

If we substitute boundary condition (6.17) into equation (6.16), then it can be rewritten as

푝 4 1 + 휇2 3 푢2 = 3 3 [(1 − 2휇2)푎 + 푎푏 ] 퐸2(푏 − 푎 ) 2

(6.18)

where 푢2 is the radial displacement of the resin layer caused by the counter force of diamond grit retention force. Meanwhile, the resin layer temperature reduced to room temperature from a high level after the UV-resin is cured. The radial displacement of shrinkage caused by the decrease of temperature ∆T is

∆푟2 = −훼2 ∙ 푎 ∙ ∆푇

(6.19)

where 훼2 is the coefficient of linear thermal expansion of UV-resin.

Then the total radial displacement of resin layer in our model can be expressed as

푝 4 1 + 휇2 3 ∆푅2 = 푢2 + ∆푟2 = 3 3 [(1 − 2휇2)푎 + 푎푏 ] − 훼2푎∆푇 퐸2(푏 − 푎 ) 2

(6.20)

For the diamond grit, it is assumed that the compressive load, or pressure from the matrix shrinkage was uniformly distributed on its surface. So the radial displacement of diamond grit can represented as

푎푝(1 − 2휇1) 푢1 = − 퐸1

(6.21)

It is also assumed that the temperature decrease in resin matrix is exactly the same as that in diamond, therefor the radial displacement of diamond grit caused by heat loss is

∆푟1 = −훼1 ∙ 푎 ∙ ∆푇

(6.22)

where 훼1is the coefficient of linear thermal expansion of diamond grit.

Then the total radial displacement of diamond grit in our model can be written as

푎푝(1 − 2휇1) ∆푅1 = 푢1 + ∆푟1 = − − 훼1 ∙ 푎 ∙ ∆푇 퐸1

(6.23)

Since the UV-resin surrounded diamond grit was considered as one unit, the displacement in radial direction with respect to the center of the sphere unit must be continues, that is, the radial displacement in resin matrix and that in diamond grit should be equal to each other. So we obtained

∆푅1 = ∆푅2

(6.24)

푎푝(1 − 2휇1) 푝 4 1 + 휇2 3 − − 훼1 ∙ 푎 ∙ ∆푇 = 3 3 [(1 − 2휇2)푎 + 푎푏 ] − 훼2푎∆푇 퐸1 퐸2(푏 − 푎 ) 2

(6.25)

Then the mechanical retention force p can be solved form equation (6.25) and written as

3 3 2(푏 − 푎 )(훼2 − 훼1)퐸1퐸2∆푇 푝 = 3 3 3 3 3 3 [2푎 + 푏 + (푏 − 4푎 )휇2]퐸1 + 2(푏 − 푎 )(1 − 2휇1)퐸2

(6.26)

6.2.3 Determination of the Model Parameters

In order to determine b, the radius of a diamond resin unit that we presumed to derive the force model, it is assumed that all the diamond grits are uniformly distributed within the resin matrix, like Fig. 6-4 shown below.

Figure 6-4 Schematic diagram of the uniform distribution of diamond grit within resin

matrix

It is already known that the abrasive concentration of the grinding tool in our study is 12.5%. Abrasive concentration is defined by a formula based on weight of diamond or

CBN particles in each cubic centimeter of the diamond tool’s working layer. The international standard has prescribed that a 100% concentration represents each cubic

100

centimeter contains 0.88g super-abrasive. The each increase or decrease of 0.22g will cause concentration to change by 25% correspondently. Therefore, in our case, a 12.5% diamond concentration means there are 0.11g diamond in each cubic centimeter of the matrix. Then the number of diamond grit in each cubic centimeter matrix can be expressed as

푀 푀 푀 n = = = 4 푚 푣휌 휋푎3휌 3

(6.27)

where M is the total weight of diamond grits in each cubic centimeter matrix, and it is 0.11g, m is the weight of each diamond grit and it equals to the volume of each diamond grit v multiply its density 휌.

Once we have the number of diamond grit in each cubic centimeter matrix, then the radius of each diamond resin unit b can be written as

3 1 3 4휋푎3휌 3 4휋휌 b = √ = √ = 푎√ 푛 3푀 3푀

(6.28)

Then equation (6.26) can be rewritten as

4휋휌 3 3 2 ( 푎 − 푎 ) (훼2 − 훼1)퐸1퐸2∆푇 푝 = 3푀 4휋휌 4휋휌 4휋휌 [2푎3 + 푎3 + ( 푎3 − 4푎3) 휇 ] 퐸 + 2 ( 푎3 − 푎3) (1 − 2휇 )퐸 3푀 3푀 2 1 3푀 1 2

(6.29)

The common factor 2푎3can be cancelled from right side, so

4휋휌 ( − 1) (훼2 − 훼1)퐸1퐸2∆푇 푝 = 3푀 2휋휌 2휋휌 4휋휌 [1 + + ( − 2) 휇 ] 퐸 + ( − 1) (1 − 2휇 )퐸 3푀 3푀 2 1 3푀 1 2

101

(6.30)

As we know, in UV light curing, the temperature change in diamond-resin matrix is determined by the energy absorbed from the UV light. Therefore the temperature decrease ∆푇 can be represented as a function of UV light intensity and curing time. Then we have

푄 퐼푡휑 ∆푇 = = 퐶 퐶

(6.31)

where Q is the total amount of heat energy absorbed by diamond-resin matrix, I is the UV light intensity, t is the light curing time and 휑 is the energy absorption efficiency.

C is the heat capacity of the diamond-resin matrix.

Substituting equation (6.31) to equation (6.30), we obtain the mechanics model to describe the compressive force on each diamond grit within UV-resin matrix.

4휋휌 퐼푡휑 ( − 1) (훼2 − 훼1)퐸1퐸2 푝 = 3푀 퐶 2휋휌 2휋휌 4휋휌 [1 + + ( − 2) 휇 ] 퐸 + ( − 1) (1 − 2휇 )퐸 3푀 3푀 2 1 3푀 1 2

(6.32)

where

휌 is the density of diamond

푀 is the total weight of diamond grits in each cubic centimeter resin matrix

훼1 is the thermal expansion coefficient of diamond

훼2 is the thermal expansion coefficient of UV-resin

퐸1 is the elastic modulus of diamond

퐸2 is the elastic modulus of cured UV-resin 102

퐼 is the UV light intensity

t is the light curing time

휑 is the energy absorption efficiency

C is the heat capacity of the diamond-resin matrix

휇1 is the Poisson’s ratio of diamond

휇2 is the Poisson’s ratio of cured UV-resin

6.3 Numerical Modeling of the Mechanical Retention on Single Diamond Grit in UV Resin Matrix

6.3.1 Development of the FEM Simulation Model in Abaqus 6.14

For a single diamond grit within resin matrix, the relationship between compressive force and material properties of diamond grit and resin matrix is concluded in the force model above. It is also known from the previous chapters that the introduction of alumina fillers change the material properties of resin matrix, and then strengthen the bond to improve the tool efficiency. Since the data of material properties can be obtained from previous experiment, a computer modeling could be developed by using finite element method to study the compressive strain and stress generated after UV light curing.

103

Figure 6-5 3D model of a diamond grit and meshing

Figure 6-6 3D model of the resin matrix around diamond grit

104

The FEM software used in our study is ABAQUS Version 6.14-2. Firstly, the 3D model of diamond and resin matrix is established as Fig. 6-5 and Fig. 6-6 shows. It is assumed that the diamond grit is in a perfect shape of cubo-octahedral, and the matrix part where the strain and stress distributed is simplified as cylinder shape. In this simulation, the study object is the diamond that completely embedded within resin matrix, as shown in

Fig. 6-6. The diamond is meshed with hexahedral elements of type C3D8R, and the resin matrix is meshed with linear tetrahedral elements of type C3D4.

6.3.2 Determination of the Input Parameters in Abaqus Simulation

During the cooling stage of the UV light curing, a stress field generated in the matrix that surrounding each diamond grit due to a mismatch between the thermal expansion coefficients. As the dependence of mechanical and thermal characteristics of the diamond-resin assembly is unavailable, and the measurement approach is quite limited on examination of the amount of stress and strain due to the micro-scale of diamond. Even though it has been experimentally proved that the application of µ-Raman spectroscopy allows quantification of hydrostatic stress on diamonds partially embedded in cobalt during segment fabrication [59], the analytical evaluation of the stress/strain field and prediction on the magnitude still presents its difficulty. In this simulation, the stress/strain field in resin-diamond unit is reproduced by cooling the system from unstressed condition to room temperature. The model of elastic-plastic material is applied and the experiment data from tensile test stress-strain curves of cured resin is used as input data. All of the known input data is listed in the Table below. It is assumed that the mechanical and thermal

105

characteristics of resin matrix material and diamond are temperature independent within the cooling range.

It is know from the material supplier that the cured UV resin has its thermal limit from -40ºC to 190 ºC. In order to simplify the simulation, it is assumed that the predefined system temperature, as the highest temperature is 190 ºC after the UV-resin is fully cured.

The position constrained assembly is shown in Fig. 6-7. The simulation was firstly applied a predefined filed at 463.15 K, the temperature of the assembly system including diamond and resin matrix was then dropped down to room temperature at 293.15 K. The stress distribution around diamond grit in pure resin matrix and alumina filled resin matrix is compared in figures shown below.

Table 6.1 Mechanical and thermal properties of resin matrix and diamond [60-63]

Alumina filled Matrix properties 425 UV-resin Diamond 425 UV-resin Tensile at yield 7.3 kPa 10.5 kPa - Elastic modulus 1.1 GPa 2.0 GPa 1220 GPa Poisson’s ratio 0.37 0.32 0.2 Thermal expansion 5.5e-5 /K 4.3e-5/K 1.1e-6 /K coefficient Friction coefficient 0.2 0.2 - on diamond

106

Figure 6-7 Model of a diamond grit embedded in UV-resin matrix

Fig. 6-8 showed the cross section area on Y-plane of the diamond and resin assembly after the simulation. Because of the thermal shrinkage of UV-resin matrix, the compressive stress was generated on the surface of the diamond grit. Fig. 6-9 and Fig. 6-

10 showed the stress distribution in the matrix after the diamond grit was take out. As same as Fig. 6-11, it can be clearly seen that the stress was concentrated at the interfacial area of diamond edges and tips.

As a contrast, the shrinkage simulation results of alumina filler loaded matrix was shown in Fig 6-12 to Fig. 6-15. The distribution of compressive stress was similar to that in the pure UV-resin matrix, but with a higher stress value. In Fig.6-15, the color depth on the surface of diamond was much higher than that in Fig. 6-11, it indicated that the compressive stress that the diamond grit bearing in alumina filled matrix is higher than that in pure UV-resin matrix. It can be clearly seen in the stress distribution comparison in Fig. 6-16.

107

Figure 6-8 Stress distribution around an embedded diamond grit in 425 UV-resin matrix

(Y-plane cross section)

Figure 6-9 Stress distribution of diamond-resin contact area in 425 UV-resin (Y-plane

cross section)

108

Figure 6-10 Stress distribution of diamond-resin contact area in 425 UV-resin (Z-plane

cross section)

Figure 6-11 Stress distribution on the diamond grit that embedded in 425 UV-resin

matrix

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Figure 6-12 Stress distribution around an embedded diamond grit in alumina filled 425

UV-resin (Y-plane cross section)

Figure 6-13 Stress distribution of the diamond-resin contact area in alumina filled 425

UV-resin (Y-plane cross section)

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Figure 6-14 Stress distribution of the diamond-resin contact area in alumina filled 425

UV-resin (Z-plane cross section)

Figure 6-15 Stress distribution on the diamond grit that embedded in alumina filled 425

UV-resin matrix

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(a)

(b)

Figure 6-16 Comparison between stress distribution in pure 425 UV-resin matrix (a) and

alumina filled 425 UV-resin matrix (b)

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6.3.3 Results of the Simulation on Pure Resin Diamond Matrix and

Alumina Additive Filled Resin Diamond Matrix

It can be clearly investigated that the compressive stress generated in alumina filled resin matrix is much higher than that in pure resin matrix. The highest stress in alumina filled matrix could achieve 65 MPa at the edge contact area between diamond grit and resin, and it can be explained by the stress concentration caused by the diamond grit geometry.

Figure 6-17 Comparison between compressive stress predictions from FEM numerical

model and theoretical force model

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However, in the case of pure resin bond matrix, the highest stress is below 40 MPa.

It is almost 50% lower than that in alumina filled resin bond matrix. According to the previous research and our material test, both of the elastic modulus and thermal expansion coefficient can be modified by introducing filling additives like nano alumina in our case.

Hence, the volume shrinkage of the resin composite caused by temperature decrease is modified, therefore the compressive stress generated between the diamond grit and matrix is modified as well.

From our previous research, it is known that the diamond grit retention capacity of resin matrix can be studied as the compressive stress generated on the diamond. Based on theoretical force model that we established, the stress can be calculated if we considered the diamond grit as a perfect shape of cubo-octahedral. The result is shown is Fig. 6-17 as

11.4 MPa for pure UV-resin matrix and 23.1 MPa for the alumina filled UV-resin matrix.

The result is also compared to the compressive stress obtained through FEM simulation.

From Fig. 6-17, it is obvious to see that both the maximum and minimum comparative stress in pure resin matrix is lower than those in filled resin matrix. Even though the values are not exactly the same, the trend shows a very good fit to our theoretical force model.

The difference between the stress values that obtained from different models could be firstly explained by the stress distribution. The diamond grit shape in theoretical force model was assumed as a solid sphere for calculation, and the stress calculated is a uniformly distributed stress. However, the diamond grit in FEM simulation was developed as a shape of cubo-octahedral, the compressive stress concentrated on the sharp tips and edges, thereby the maximum stress could reach as high as 40 MPa and 65 MPa in pure resin and filled resin matrix, respectively. It is also noticed that the minimum stress in each matrix is 114

5 MPa and 10 MPa, which mostly occurred on the flat surface of the cubo-octahedral. This fact is also in line with our previous study that etching on diamond grit to texture the surface helped to improve the retention ability of matrix. Moreover, another factor that should be taken into account to explain the results difference between models is the shrinkage mode.

In the case of FEM model, the beginning of the simulation was set as the end of UV curing.

That is to say, the UV-resin matrix polymerization was assumed fully reacted, and the phase of the matrix was completely transformed from liquid to solid before the simulation started. Therefore, the compressive stress obtained from this simulation is a result of thermal shrinkage due to the temperature decrease. However, in fact, the phase transition shrinkage during the curing process also took place to generate compressive stress on the diamond grit. The resin polymerization process is a volume decrease process, in which the shrinkage stress was generated on the diamond grit before the thermal shrinkage occurred.

Hence, the stress obtained through theoretical force model is a combination results of both phase transition shrinkage and thermal shrinkage. In fact, the average stress on diamond grit in pure resin matrix and filled resin matrix is lower than 11.4 MPa and 23.1 MPa, respectively.

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Chapter 7

Sapphire Lap-Grinding with UV-Resin Bond Diamond Tool

7.1 Introduction and Background of Sapphire Machining

Sapphire is the single crystal form of the compound Al2O3 and it is the second hardest natural material after diamond. Single crystal sapphire, as one of the high quality opto-mechatronic materials in industry, has been used broadly in various fields. Because of its superior material properties in physical strength, hardness and high-temperature resistance, sapphire could endure the severe conditions of GaN film deposition, and which make sapphire became the predominate substrate material for LED applications [62-64].

Moreover, sapphire is also a wide gap (∼0.9eV) insulator presenting favorable properties including refractory behavior (Tm=2045°C) and transparency over a wide wave length range, which enable sapphire to be used as laser diodes, optical materials or even insulator in nuclear reactor [65, 66].

Generally, sapphire wafers are most widely used as substrate in industry. The manufacturing process of sapphire wafers usually starts with a sapphire cylinder bar and follow a sequence of machining of slicing, flattening and surface finishing [67-70]. In spite

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of its outstanding physical and chemical properties, machining of sapphire can be a challenge and requires much higher load than conventional abrasive machining and results a rapid tool wear. This particularly makes flattening (lapping, grinding) and surface finishing (mechanical polishing, chemical-mechanical polishing) more challenging since an acceptable material removal rate is desirable as well as the surface quality.

In flattening process, the previous research studied 2-body and 3-body abrasion mechanism on brittle material like sapphire as instance [71, 72]. A relatively higher material removal rate can be obtained through 2-body lapping, while a dense abrasive slurry is needed for 3-body abrasion to achieve the same rate. As a result, both the process waste and cost is increased significantly and the environment pollution could be another potential issue. In order to reduce the slurry consumption, researchers arise their attention toward to 2-body removal mode like fixed abrasive pad and lap grinding wheel [73].

However, in term of surface quality, 2-body abrasion produces a rougher surface than 3- body abrasion. For this reason, some researchers studied the possibility of combining 2- body and 3-body abrasion by delivering slurry to the fixed abrasive pad machining [62].

Figure 7-1 Sapphire wafer manufacturing process flow [62, 67]

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In this chapter, we proposed a new abrasive machining tool fabricated with UV- curable resin and diamond abrasive grains. It helps us to easily generate a fusion process of 2-body and 3-body abrasion during machining of sapphire wafer surface, and saves a lot of process cost at the same time. We compared the machining performance between fixed abrasive lap grinding wheel, slurry-based lapping wheel and this new UV-curable resin lapping wheel. It is hoped that this study could be undertaken to help to explore a new direction of sapphire manufacturing.

7.2 Fabrication Method of UV Resin Diamond Tool for Sapphire Abrasive Machining

13 mm (diameter) x 13 mm (thickness) Kyropoulous grown c-plane sapphire samples were used as the workpiece (99.999% Al2O3, Nanjing Co-Energy Optical Crystal

Co., Ltd.). Three sets of sample were prepared for each abrasive machining process. The sapphire workpiece had been preliminarily machined to achieve a same surface condition

(Ra=0.800 ± 0.020µm).

Fixed abrasive lapping was conducted with mesh #800 (15µm) diamond metal bond lap grinding wheel. Slurry-based conventional lapping was examined on steel lapping wheel with diamond slurry (15µm, Hyprez, Engis Co.). The UV-curable resin bond diamond wheel was fabricated through the process flow shown in Fig. 7-2. The diamond grains (15µm Engis Co.) and filler additives were uniformly mixed together firstly. Then, the prepared composites that in liquid form were evenly spread on the surface of a base

118

plate made of aluminum. The plate coated with resin composites were exposed under UV light curing system (DYMAX® 5000-EC UV Curing System) to trigger the UV-sensitive initiator within resin composites. The photosensitized reaction helped forming a solid matrix of the composites thereby could perform abrasive machining work.

Figure 7-2 Manufacturing process flow of UV-curable resin bond diamond grinding

wheel

The abrasive machining efficiency, in terms of material removal rate was investigated as a function of diamond abrasive size [74]. Therefore, the diamond abrasive grains used in this study was selected at a same average size. The shape and size of diamond grains used for UV-curable resin bond lapping wheel was examined through SEM and the observation is shown in Fig. 7-3. It clearly indicates that the shape of each diamond grain is quite different and the size varies within a particular range, which is 10-20µm in this case. 119

Figure 7-3 SEM observation of diamond grains used in manufacturing of UV-curable

resin bond grinding wheel

Figure 7-4 Experiment set-up for lapping tests on sapphire samples

A picture with schematic diagram shown in Fig. 7-4 illustrates the experimental set-up in this study. The experiment was carried out with the three types of lapping wheel mentioned above. For each wheel, a set of three sapphire samples were lapped under same 120

conditions including pressure, speed and time. After lapping test, the sapphire samples were cleaned and left till dry. The surface roughness of each sample was examined by

Zygo® optical surface profiler and optical microscope. Material removal rate study was based on weight loss of each sample during specific lapping time interval.

7.3 Discussion

7.3.1 Surface Profile of Sapphire Workpiece

The initial surface condition of sapphire sample shown in Fig. 7-5 (a) was very rough and clear to see the grooves and marks generated by previous abrasive grinding process. Sapphire lapping was conducted on each lapping mode for 10 minutes and the surface condition tested with optical interferometry profiler is shown Fig. 7-5 (b), (c), (d).

Fig. 7-5 (b) shows the surface profile of sapphire sample lap-grinded with mesh#800 vertical grinding wheel, which was considered as fixed abrasive lapping and delivered 2-body abrasion. The classic semi-ductile mode removal properties with grooves and scratches can be clearly seen on the surface. The width of these grooves are smaller than that in Fig. 7-5 (a), and this can be explained by the difference in grit size of machining tools. The slurry based 3-body lapping produced a classic brittle removal mode on sapphire surface that shown in Fig. 7-5 (c), the pitted surface was a result of indentation by freely rolling abrasive grain.

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Figure 7-5 Optical surface profiler topography of sapphire sample (a) lapped under fixed diamond grinding wheel (b), slurry-steel lapping wheel (c) and UV-resin bond diamond

grinding wheel (d)

Figure 7-6 Schematic diagram of UV-resin bond diamond wheel lap grinding process

In case of the UV-resin bond diamond lapping, it was assumed that all the diamond grains were embedded in the solid resin matrix and worked as fixed abrasive grain. As the machining process continued, the resin matrix started to wear out due to the relatively lower

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hardness and wear resistance. As a result, some of the initially fixed abrasive grains were released from the bond agent and rolled like a 3-body abrasion grain. Meanwhile, the fresh abrasive grains buried in under layer of resin matrix was revealed and worked as fixed abrasive grain. Therefore, the lapping process of UV-resin bond diamond wheel can be viewed as a fusion of fixed abrasive lapping and loose abrasive lapping. The transition of material removal mode in this case is illustrated in Fig. 7-6.

7.3.2 Surface Roughness of Sapphire Workpiece

Surface roughness of machined sapphire sample was examined under Zygo optical surface profiler, and the measurement was based on a small area of 0.700mm*0.520mm of the sapphire surface. For each sample, three randomly selected areas were measured to get an average roughness value. The maximum peak-to-valley value was also evaluated at the same time. The comparison results obtained from different types of lapping process can be found in Table 1.

The average Ra from each process was 0.328µm, 0.182µm and 0.228µm respectively, and the PV value was 10.640µm, 4.955µm and 6.748µm. The lowest roughness and smallest PV was obtained from diamond slurry lapping process. The diamond fixed lapping delivered the highest roughness at 0.328µm that is nearly 80% higher than the former number, and the PV is 10.640, which is also 50% larger than the value measured from slurry lapping. This difference could be firstly explained by the advantages of 2-body abrasion on material removal than 3-body abrasion. According to previous study by M. Bujis, Rabinowicz and Agarwal [71, 75, 76], most of the abrasive in

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3-body abrasion, which is diamond slurry lapping in our study, has limited effect to material removal and only small amount of abrasive are embedded on the base wheel working as 2-body abrasion. The rolling action of 3-body abrasion in diamond slurry lapping takes off less material than that sliding actions does in fixed diamond lapping, and produces less surface “damage” which could negatively affect the surface quality and increase surface roughness. However, in 3-body lapping, it is assumed that only a small amount of diamond has been pressed into the base plate working as 2-body abrasion abrasive, and the material removal by sliding is negligible. As a result, the material removal rate could be much lower than that in fixed abrasive lapping. This matches our results of material removal rate comparison shown in Fig. 7-8.

In case of UV-resin bond diamond lapping, the roughness achieved was 0.228µm, between that in fixed diamond lapping and slurry lapping. The machined sapphire surface quality is slightly rougher than that machined through slurry lapping, but almost 4 times better that fixed diamond lapping in terms of roughness value. Moreover, UV-resin bond diamond lapping delivered an efficient material removal rate which is 6.19 mg/min in this study. It is about 40% lower than that in fixed lapping but 2 time higher than that by slurry diamond lapping. The reason of this machining performance of UV lapping could be explained by the hybrid of 2-body and 3-body abrasion. As mentioned above, from very beginning to in-process machining, the mainly active abrasion transformed from fixed abrasive machining in 2-body to loose abrasive machining in 3-body, meanwhile, the UV- resin bond diamond lapping wheel keep generating new sharp fixed abrasive layer continuously due to the lower hardness and wear resistance of the resin matrix. This could be understood as a “self-dressing” of fixed abrasive wheel. The integration of these 124

processes help to build a dynamic combination of fixed abrasive and loose abrasive machining.

Table 7.1 Measurements of sapphire surface roughness and PV value by different types of lapping process

Sapphir Mesh#1000 UV-resin bond Preconditioned Diamond e Diamond fixed diamond workpiece slurry lapping sample lapping lapping * ** Ra PV Ra PV Ra PV Ra PV

(µm) (µm) (µm) (µm) (µm) (µm) (µm) (µm) 1 0.812 19.192 0.325 10.805 0.188 5.484 0.220 6.702 2 0.807 18.740 0.330 11.428 0.164 4.712 0.236 6.440 3 0.820 20.024 0.329 9.688 0.193 4.668 0.228 7.102 Averag 0.813 19.319 0.328 10.640 0.182 4.955 0.228 6.748 e

* Ra of a 0.700mm*0.520mm square area ** Maximum Peak-to-Valley height

Generally, compared to the other two lapping process, UV-resin bond diamond lapping not just produced a relatively good surface quality but also remained machining efficiency as well. As we know from Fig. 7-1 above, sapphire substrate manufacturing, especially for the most widely used form of wafer, the process can be summarized into four key steps: saw slicing, profiling, flattening and final surface finishing. If we could apply the UV-resin bond diamond wheel into step 3, the flattening and surface precondition. A more efficient process that offer a relatively better surface quality without taking off too much material could be expected.

From economic and environmental point view, UV-resin bond wheel also has some advantages in manufacturing cost and by-product issue. In case of fixed abrasive lapping,

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the manufacturing of fixed abrasive wheel and dressing during the machining requires a higher cost. For the slurry lapping, the waste of slurry would not just be a cost issue but also a potential environmental problem caused by its chemical solution in waste coolant.

Figure 7-7 Average surface roughness and Peak-to-Valley height of original sapphire substrate (a) and samples lapped under fixed diamond lapping wheel (b), diamond slurry

lapping wheel (c) and UV-resin bond diamond lapping wheel (d)

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Figure 7-8 Material removal rate of sapphire substrate lapping in fixed diamond lapping

(b), diamond slurry lapping (c) and UV-resin bond diamond lapping (d)

7.4 Conclusion

This study proposed a new type of lapping wheel for surface finishing of sapphire substrate. UV-curable resin was selected as a binder in the fabrication of abrasive tool to deliver a faster and cost friendly manufacturing process. The performance of UV-resin bond diamond lapping wheel on sapphire lapping was compared to fixed diamond lapping and slurry-based lapping at a same work condition. The promising results showed that UV- resin wheel could achieve almost the same level roughness as slurry-based lapping does without taking off too much material as fixed abrasive lapping does. This inspires us to

127

find the potential application to replace traditional flattening process in sapphire substrate

(wafer) manufacturing.

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Chapter 8

Conclusion and Future Work

This research involves a comprehensive study on the manufacturing of UV light- curable resin bond abrasive tool, and the influence of nanosized alumina filler additives in resin matrix. A creative approach is firstly developed to fabricate diamond grinding wheel with UV-curable resin, and a series of methods are explored to improve the tool’s performance. Furthermore, the curing mechanism during the manufacturing and the wear mechanism of the resin bond abrasive machining are studied. A theoretical model and a numerical simulation model on single diamond grit retention within UV resin matrix is established for understanding and optimizing the manufacturing process. This chapter summaries this dissertation in details, and discusses the potential research direction for future work.

8.1 Research Conclusion and Original Contribution

A detailed conclusion is summarized as following:

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i) As part of this research, the feasibility of manufacturing resin bond abrasive

tools with UV light-curable resin has been verified. The successful

development of UV light-curable resin bond abrasive tools not just reduces

the economic cost in the manufacturing process, but also eliminated a lot of

environmental hazard which exists in the manufacturing of thermosetting

resin abrasive tool.

ii) The work performance of the tool is analyzed through a comparative study

of UV resin diamond lap grinding wheel and conventional iron plate

lapping, in which ceramic is selected as the workpiece. The results from

experimental data indicates that the resin bond abrasive tool has a better

performance on surface finish, and the surface roughness of the UV resin

tool machined workpiece can be reduced about 10% at same conditions. iii) Nanosized alumina filler additive is introduced in the manufacturing of UV

light-curable resin bond diamond abrasive tool. The increase of the filler

loading leads to an increase in the hardness and tensile strength of the cured

resin-diamond matrix, and the wear resistance is improved as well. As the

reinforcing material in fabrication, the fillers strengthen the mechanical

bond between diamond grit and resin matrix. As fine grinding abrasive

particle in machining, the Al2O3 fillers drop out from resin matrix during

machining and work with diamond grains to form a unique machining

process involving various size abrasives. All of these advantages mentioned

above contribute to an improvement of the UV curable resin-bonded

diamond plate on lap grinding, in terms of a finer surface roughness on the 130

surface of machined workpiece and a higher the material removal rate of

the machining process. iv) Based on a set of comparative experiments between surface-textured

diamond grains and normal diamond grains, the influence of abrasive

diamond surface characteristics in the manufacturing of UV light-curable

resin bond tool is studied. The results revealed the importance of the surface

characteristics of diamond particle in curing and polymerization process of

the resin-diamond composite, and the work performance of the

manufactured tool.

v) The diamond retention capability of the UV resin matrix is analyzed and the

curing mechanism of the resin-diamond composite is studied. Base on the

force analysis of each single diamond grit with resin matrix, the diamond

retention is quantitively determined as the compressive force generated

during the curing process. Therefore, a theoretical force model is developed

to describe the compressive stress obtained through the shrinkage of system.

Furthermore, the theoretical force model is verified with numerical

modeling of FEM simulation. These modeling approach successfully

enhance the understanding of the curing process in fabrication of UV resin

diamond tools, and offer ways to predict the bond quality between diamond

grit and resin matrix to optimize the manufacturing process. vi) As one of the most important brittle material in industry, sapphire can be

machined effectively with UV resin bond diamond tool. The experiment

result shows that the UV resin tool has the potential to take part in the 131

surface finishing of sapphire, and deliver an outstanding surface quality by

removing a relatively less material.

8.2 Limitation, Future Work and Research Direction

In general, the finished work in this dissertation has studied UV resin bond diamond tool from experimental test to theoretical analysis, and also investigated the influence of nanosized filler additives in matrix curing. However, the manufacturing and machining of the UV light-curable resin bond abrasive tool is a complicate interdisciplinary process, which involves optic, chemistry, physics and mechanical engineering. Our study could not cover all aspects and has some limitations as following.

 As a generally considered viscoelastic material, cured resin matrix has a

lower hardness and relatively weak material property. In practice, some of

the free abrasive within machining zone might be pressed into matrix after

they fall off from matrix, this phenomenon is neglected in this research and

we considers all the abrasives that fall off form matrix will work as free

abrasives.

 In our study, it is assumed that all the diamond abrasive grains never breaks

during machining. However, in real machining, the diamond grains break

occasionally as brittle fracture, even the matrix stiffness is much lower.

 In theoretical study, the compressive force modeling on single diamond is

developed by considering that the stress is generated entirely from thermal

shrinkage of the system. The phase transition shrinkage during UV curing 132

process is not covered by the modeling, and this is also one of the reasons

to cause the result error.

The entire research on UV resin bond abrasive tool can be divided into two main categories, the manufacturing of the tool and the machining process on it. Based on the scope in this dissertation, some recommendations are made as potential research direction for future work.

 Although the abrasive machining process involved in this study is at a very

low speed below 100 rpm, therefor the heat generated during machining is

very limited. The machining heat and process temperature could be the next

direction to explore, since resin is a temperature sensitive material and it

decomposes at high temperature.

 For the FEM modeling of compressive stress on single diamond grit, an

interdisciplinary model which simulates from the beginning of curing to the

end of matrix cooling down should be developed. This would enable the

simulation an improved prediction on the retention capability of UV resin

matrix.

 In terms of verification of the theoretical force modeling, advanced

measurement techniques like Raman spectroscopy is necessary to quantify

the strain and stress on each single diamond after curing. Advanced

observation is also encouraged to investigate the diamond conditions.

 For the evaluation of tool’s abrasive machining capability, a comprehensive

study on microscopic abrasive mechanism of the abrasive grain should be

133

considered, and a process model that integrates empirical part and

theoretical part is needed as the Fig. 8-1 shows.

Figure 8-1 Framework of UV light-curable resin bond abrasive machining modeling

134

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