A Thesis Entitled Ultraviolet Bonding of Diamond Abrasive Tools for Lap

Home , Lapping

A Thesis

entitled

Ultraviolet Bonding of Diamond Abrasive Tools for Lap-Grinding Process

Lei Guo

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

Master of Science Degree in Mechanical Engineering

______Dr. Ioan D. Marinescu, Committee Chair

______Dr. Hongyan Zhang, Committee Member

______Dr. Sarit Bhaduri, Committee Member

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

The University of Toledo

December 2012

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

Ultraviolet Bonding of Diamond Abrasive Tools for Lap-Grinding Process

Lei Guo

Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science Degree in Mechanical Engineering

The University of Toledo

December 2012

Different from grinding, the lapping process is always taken place on higher precision machining process at low speed and low pressure. Conventional lapping process is based on a slurry process, abrasive particles mixed with lapping fluid supplied during the process and the abrasives keep rolling and scratching in the working zone. By these actions, the material is removed from the work piece. As known, the lapping efficiency is limited due to the abrasion mechanism that is illustrated as three-body abrasion. In order to improve the lapping efficiency and surface finishing, a novel manufacturing method of lapping plate fabrication is proposed in this research.

Since the ultraviolet-curable resin first introduced to industry, the rapid prototyping technology developed significantly and become one of the most important techniques. Recently, researchers studied on the manufacturing of abrasive tools by ultraviolet-curable resin and some promising achievements have been made. Therefore, our idea is typically base on the rapid prototyping technique. By curing the mixture of ultraviolet-curable resin and abrasive particle to fabricate the lapping plate, the grains bonded within the mixture. Thus, the abrasion type during lapping change from three-

iii

body mechanism to two-body mechanism and the lapping efficiency can be improved then.

In this research, the material properties of the ultraviolet-curable resin and mixture are studied. The optimum combination of resin and diamond abrasive is selected. Moreover, in order to see the practical lapping performance, the lapping plate is developed. Based on rigid brittle material lapping experiments, the lapping efficiency and surface finishing is discussed and concluded. At last, two ways to improve the performance of lapping plate is carried out. One of them is based on the influence of abrasive type, and the other one focused on the polymerization of resin and bonding strength of the mixture.

This thesis is dedicated to my parents, Yaohui Guo and Aimin Zhang. Thank you for bringing me to this world and give me unconditional love and support. XieXie, Wo Ai

NiMen.

Acknowledgements

I wish to convey my deepest appreciation to Dr.Marinescu, my advisor, for his enthusiasm, dedication, and expertise. Thanks for his continued concern and support through my study.

I would like to thank the group member of PMMC and Mr.Todd.Gearig. I really appreciate their help in my study and experiments.

I would also like to show my gratitudes to the MIME machine shop staff members

Mr. John Jaegly, Mr. Tim GrIvanos and Mr. Randall Reihing. This research could not have gone so far without their help.

Finally, I want to say thanks to my girlfriend, Ting Wen. Thanks for your love and support. I love you.

Table of Contents

Abstract ...... iii

Acknowledgements ...... vi

Table of Contents ...... vii

List of Tables ...... x

List of Figures ...... xi

List of Abbreviations ...... xiv

1 Introduction ...... 1

1.1 Overview ...... 1

1.2 Research Objective...... 3

2 Lapping Processes...... 5

2.1 Introduction ...... 5

2.2 Two-Body and Three-Body Abrasion (Mechanisms) ...... 6

2.3 Classification of Lapping Process ...... 11

2.3.1 Single Side Lapping...... 13

2.3.2 Double Side Lapping ...... 14 vii

2.3.3 Cylindrical Lapping ...... 15

2.4 Lapping Component ...... 16

2.4.1 Lapping Plate ...... 16

2.4.2 Lapping slurry and fluid ...... 21

2.4.3 Lapping Abrasive...... 22

3 Ultraviolet-Curable Resin ...... 24

3.1 Ultraviolet-Curable Resin Research Development ...... 24

3.2 UV-Curable Resin Introduction ...... 28

3.3 Key Factors in the UV-Curing Process ...... 30

3.4 Bonding Mechanism of Abrasives ...... 31

3.4.1 Mechanical Bonding ...... 31

3.4.2 Adsorption bonding ...... 33

4 Experiments Design and Set up ...... 34

4.1 Material and Equipment Preparation...... 34

4.1.1 Ultraviolet-Curable Resins ...... 34

4.1.2 Abrasive Particles ...... 35

4.1.3 Ultraviolet Curing System ...... 37

4.1.4 Lapping Machine ...... 41

4.2 Experimental Procedure ...... 41

viii

4.2.1 Tensile Strength ...... 41

4.2.2 Porosity ...... 44

4.2.3 Hardness and Wear Resistance ...... 46

4.3 Manufacturing of the Lapping Plate...... 50

4.3.1 Spin-Curing Method ...... 50

4.3.2 Slices Curing Method ...... 51

4.4 Silicon Wafer Lapping Experimental Program ...... 53

5 Optimization and Improvements in Lapping Performances ...... 56

5.1 MA Abrasive and Surface Treated MA Abrasive ...... 56

5.1.1 Lapping Experimental on Aluminum Oxide ...... 57

5.1.2 Experiments Results and Discussion ...... 59

5.2 Influence of Nano-Particle Additives...... 63

5.2.1 Nano-Particle Additives Introduction ...... 63

5.2.2 Nano-Particle mixed Resin Material Experiment Details ...... 64

5.2.3 Lapping Performance Discussion ...... 72

6 Summary and Conclusion ...... 73

References ...... 75

List of Tables

2.1 Possible Situation-based Classification of Wear Abrasion [17] ...... 11

4.1 Three Ultraviolet-cured Resins ...... 34

4.2 Dymax 5000 UV Curing Flood Lamp Specification ...... 40

4.3 Porosity of Resins ...... 46

4.4 Hardness and Abrasion of Resin-Abrasive Mixtures...... 49

4.5 Experimental conditions ...... 54

5.1 Experiment Condition of MA and ST-MA Lapping...... 59

5.2 Hardness and Abrasion test results ...... 69

List of Figures

2-1 Model of the Lapping Plate ...... 7

2-2 Schematic of Two-body and Three-body Abrasion ...... 8

2-3 Two-body and Three-body abrasion Comparison ...... 9

2-4 Single Side Lapping Process ...... 12

2-5 Double Side Lapping Machine ...... 13

2-6 Single Side Lapping Mechanism Illustration ...... 14

2-7 Position of the Work Piece in Double Side Lapping [19] ...... 14

2-8 Schematic Illustration of Cylindrical Lapping Method ...... 15

2-9 SEM Image of Silicon Surface after Rough Grinding [20] ...... 17

2-10 Cast Iron (Fe) Lapping Plate [21] ...... 错误!未定义书签。

2-11 Tin Lapping Plate [22]...... 19

2-12 8inch Ceramic Lapping Plate [23] ...... 20

2-13 Cooper Composite Lapping Plate [24] ...... 21

3-1 Reaction of UV Curing Process...... 27

3-2 Adhesion Mechanisms...... 32

3-3 Nickel Coated Diamond Abrasive ...... 32

4-1 SEM Image of Normal MA Abrasive (11288) ...... 36

4-2 SEM Image of Surface Treated MA Abrasive (R0206) ...... 36

4-3 Particle Size Distributions of the Abrasive Particles ...... 37 xi

4-4 Dymax 5000 Flood Ultraviolet Curing System ...... 38

4-5 Wavelength Distribution of Dymax 5000 Curing System...... 38

4-6 Innovative Machine Co. UV-100 Curing System ...... 39

4-7 Ultraviolet Lamp of UV-100 Curing System ...... 39

4-8 Lapmaster 12 Lapping Machine ...... 41

4-9 Curing Pattern Designed for Tensile Test ...... 42

4-10 Tensile Stress vs. Tensile Strain of Abrasive-Resin Mixture ...... 44

4-11 Microscopy of 921vt Resin...... 45

4-12 Microscopy of 425 Resin...... 45

4-13 Microscopy of HTU363 Resin ...... 46

4-14 Curing Patterns for Different Test ...... 47

4-15 Samples Prepared for Wear Resistance Test ...... 47

4-16 Photos of Samples for Hardness Test ...... 48

4-17 Wear Resistance Experiment ...... 48

4-18 Spin-coating Method Introduction...... 50

4-19 Curing Pattern Designed for Slice of Lapping Plate ...... 51

4-20 Whole Plate Consists of 12 Slices ...... 52

4-21 Dressing Process of Lapping Plate ...... 53

4-22 Surface Roughness of UV Lapping and Conventional Lapping ...... 54

5-1 Comparison of MA and Surface Treated MA ...... 57

5-2 Aluminum Oxide Rings ...... 58

5-3 Surface AFM Image of Unfinished Ceramic Ring ...... 58

5-4 Roughness of Surface Treated MA Lapping Plate ...... 60

xii

5-5 Roughness of MA Lapping Plate...... 60

5-6 Roughness Comparisons of MA and ST-MA Plate...... 62

5-7 Cure Depths vs. Cure Times ...... 65

5-8 Stresses Curve of Nano-particle Mixed Resin...... 67

5-9 Fixed Abrasive Lapping Process ...... 68

5-10 Microscopy of the Nano-particle Mixture ...... 70

5-11 Microscopy of the Mixture without Nano-particle ...... 70

5-12 Roughness by MA Resin Plate with Nano Al2O3 Particle...... 71

5-13 Roughness by MA Resin Plate without Nano Al2O3 Particle ...... 71

5-14 Roughness Comparisons between Plates with and without Nano Al2O3 ...... 72

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List of Abbreviations

AFM ...... Atomic Force Microscope

CBN ...... Cubic Boron Nitride

FAP ...... Fixed Abrasive Pad

MA ...... Metal Bond Diamond Powder

PVA ...... Polyvinyl Acetate

RA ...... Resin Bond Diamond Powder

SEM...... Scanning Electron Microscope ST-MA ...... Surface Treated Metal Bond Diamond Powder

UV ...... Ultraviolet

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Chapter 1

Introduction

1.1 Overview

Abrasive processes have been employed in industry manufacture for more than a century and the origin could be traced back to Neolithic times. Grinding, lapping and polishing processes researches significant developed in twentieth century and the application of machining technology to achieve high quality and efficiency increased rapidly. Compare with grinding, lapping and polishing is similar process by which fine surface finish, high dimensional accuracy, flatness and minimal sub-surface damage obtained. These techniques have been used in optical lens, semiconductor and electronics industry on a widely range of material from silicon, glass, ceramic to metal and their alloys. Lapping, as one of the most important techniques in surface finishing, has become more and more important in ceramic industry [1].

The traditional lapping procedure is based on a slurry process and the abrasive grains are freely moved in the process. The major problem for conventional lapping process is to get a uniform slurry distribution between the lapping pad and the work piece.

During the lapping process, the lapping pad become smoother and the pores would fill with chips and pad material. The transportation of the slurry to and debris from the work piece surface will be weakened, which is always called glazing phenomenon. As a result, the material removal rate and machining efficiency will drop. On the other aspect, slurry handling and disposal is another problem that should be considered for conventional lapping [2].

With huge development and remarkable progress of material science, the modern machining technology needs to achieve high quality and efficacy. Therefore, a new lapping process named fixed-abrasive lapping appeared in recent year [3]. Fixed-abrasive lapping, sometimes called Nano-grinding or Lap-grinding, is first proposed by Gatzen[4].

Subsequently, other researchers did some further research. A thermo-curable fixed abrasive pad was carried out by Choi et al to get a submicron-level surface roughness [5].

Thermo-curable fixed abrasive pad has loading and glazing phenomena, especially when applied an ultra-fine abrasive grain. Then, they developed a hydrophilic FAP and self- conditioning mechanism by water swelling of the polymer. The experiments results showed nanometer level roughness on work piece surface. Tomita et al have studied on development of new bonding materials for fixed abrasives of grinding stone and proposed a new process by using a double-side lapping machine with a grinding stone [6]. The conclusion shows that the mixture of PVA resin and some other thermo-curable resin or polyurethane is the best bonding material.

Recently, researchers investigated ultraviolet-curable resin can be used in fabrication of abrasive tools. Since 1968 when a German made the ultraviolet-curable

resin commercialized for the first time [7], ultraviolet-cured rapid prototyping technology has been developing quickly and become to one of the most advanced techniques in manufacturing industry. Tanaka et al proposed a method to develop a grinding wheel with cured resin and Peng et al studied on different abrasive tools using ultraviolet- curable resin [8-11].

1.2 Research Objective

Based on the previous study on manufacturing process of ultraviolet-curable resin abrasive tools, this research aims to develop an abrasive lapping plate which can be used in Nano-grinding process to improve material removal rate and finishing performance.

Moreover, different experiments method was applied to find ways to optimize the ultraviolet-curable resin bonded lapping plate.

A brief introduction of each chapter is given below

In Chapter2, the basic concept of lapping process and tribology are stated, and the material removal mechanism in different modes is explained to give a fundamental understanding of the whole research.

In Chapter 3, the ultraviolet-curable resin is introduced, the curing theory and curing mechanism will basically illustrated.

In Chapter 4, initial experiments are designed to see if the resin material properties satisfy the requirement to be a bonding material. Moreover, the combination of

resin and diamond used in fabrication will select from various types. The manufacturing method of lapping plate will be introduced.

In Chapter 5, lapping plate manufacturing and lapping experiments will be focused. Compare experiment are finished between conventional lapping and ultraviolet resin bonded lapping. In addition, the lapping performance differences by abrasive types are discussed. In addition, we proposed a method on which we put Nano-size Aluminum

Oxide particle into the mixture to improve the material properties and the plate performance.

In Chapter 6, the very last chapter, the summary and conclusion has been made.

Chapter 2

Lapping Processes

2.1 Introduction

A range of loose abrasive processes used for precision machining, and loosed abrasive processes are mainly employed in high-requirement surface finishing process to generate a desired quality and characteristics. The finishing abrasive processes using loose abrasives can be approximately classified into the following categories: lapping, polishing, and field-assisted processes. These types of operations are capable of producing fine finishes on both ductile and brittle materials [1].

Lapping is a fine machining method, on which material removed to produce a smooth, flat surface and to obtain a high surface quality and dimensional accuracy. The lapping plate will rotate at a low speed (<80 rpm) and a mid-range abrasive particle (5-

20μm) is typically used. Furthermore, conventional lapping process is a loose abrasive machining process that combines abrasive grains with an oil or aqueous medium, which always named slurry. The choosing of the slurry is depending on the material being finished. Abrasive particles applied with the slurry to working surface continuously or at

specific intervals, then an abrasive grain film is formed with the lapping plate and work piece. Each of the abrasive particles is hard enough and has sharp irregular shapes and when a pressure is loaded and related motion is introduced, these sharp angles on each grain are forced into the work piece and cut material away.

Like Fig.2.1 shown, each of the abrasive grain acts as a small cutting tool to make indentation or actions like rolling, plowing and scratching. Even though all these abrasive particles are irregular in size and shape, they applied in huge quantities and micron meter level and thus the continuous actions taken place on the entire contact surface to make sure the finishing quality [12].

2.2 Two-Body and Three-Body Abrasion (Mechanisms)

In lapping process, one of the most important mechanisms in wear is abrasive wear. The detachment of the unwanted material from the work piece within the lapping process is caused by breakout and the abrasive particles between the opposite surfaces.

As we know, the objective of abrasive machining is to reduce the friction and wear of the abrasive while increase the abrasive wear of the work piece. In other words, that means to improve the tool life and material removal rate at the same time.

Figure 2-1 Model of the Lapping Plate

The common classification of abrasive wear into two-body abrasion and three- body abrasion is widely accepted by researchers in this field. The basic meanings of the two-body/three-body in the terms are illustrated in the following figure. The primary meaning of the two-body/three-body concept (Fig.2.3) is to describe whether the abrasive particles are bound (two-body) or free to roll or slide (three-body).

Figure 2-2 Schematic of Two-body and Three-body Abrasion

In Two-body abrasion, the abrasive particles assumed to be constrained by the tool. The relative motion between the abrasive and work piece is usually considered to pure sliding. Meanwhile, two-body abrasion is considered as the result of the displacement of material from a solid surface due to hard particles sliding along the surface or when rigidly held grits pass over the surface like a cutting tool. In practical applications, many components, such as conveyor belt, shuttle, tillage tools and wind blades are subjected to two-body abrasive wear. In three- body abrasive wear, the grits are free to roll as well as slide over the surface. Two-body abrasive wear is a complex process often involving high strain and plastic deformation and fracture of micro volumes of the material, which might be described as the removal of discrete surface by a harder substance which tends to gauge, score or scratch. Four different mechanisms in abrasive wear process are micro ploughing, micro cutting, micro fatigue and micro cracking [15].

Figure 2-3 Two-body and Three-body abrasion Comparison

For the three-body abrasion, as mentioned above. The abrasive grains are free to rotate and slide, experiencing collision both with the work piece and with the pad and other abrasive grains. From an energy viewpoint, this is obviously a less efficient process since each collision leads to energy dissipation. However, the advantage of three-body abrasion is that as the grains rotate, new cutting edges can be brought into action.

In practice, a two-body abrasive process involves an element of three-body abrasion, since abraded material from the work piece and fractured abrasive particles from the grains can form a three-body action in grinding and honing. In general, three- body action in two-body processes is an effect that causes quality problems, since the loose material can be embedded in the work piece surface. Embedded particles detract from the surface texture and create an abrasive finished surface on the work piece that can damage other surfaces with which the part comes into contact. [12]

In 1998, Gates illustrated that the classification of two-body and three-body abrasion is indistinct since there exists condition when those two concepts could create 9

misunderstanding. In his viewpoint, the two-body and three-body abrasion classification can be used in the only situation when describing whether abrasive grains are rigidly held or free to toll. From a tribology viewpoint, one should take into account the severity of wear behavior: mild, severe, and extreme [16]. Consideration should be given to the specific situation: gouging abrasion, high-stress abrasion and low-stress abrasion. The difference of the high-stress abrasion and low-stress abrasion is whether the abrasive particles are broken during abrasion. This is important since fracturing can create sharp cutting edges and give higher wear rates. Gates proposed this new classification as an improvement that shown in table below.

On the other hand, Trezona in 1999 proposed that it is difficult to determine how the wear was occurring due to a range of condition near the transition between wear processes. They indicated two new terms to replace the two-body and three-body abrasion, the “grooving abrasive wear” and “rolling abrasive wear” [18]. The grooving abrasive wear means an abrasive process in which the same region of the abrasive particle or asperity is in contact effectively with the wearing surface throughout the process. Besides, the rolling abrasive wear describes a process that the region of the abrasive grain in contact with the wearing surface is continually changing. Wear surfaces produced by grooving abrasion are characterized by grooves parallel to the direction of sliding while the ones produced by rolling abrasion are characterized by a heavily deformed, multiply indented appearance and little or no directionality [18].

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 they can exercise higher pressures.

Another difference is the effect of particle (or protuberance) size on wear rate.

Table 2.1 Possible Situation-based Classification of Wear Abrasion [17]

Contact stress Abrasive Particles Low (Particles do High (Particles Extreme (Gross not fracture) Fracture) deformation)

Low-stress High-stress Free Free-abrasive Free-abrasive

Low-stress High-stress Extreme-stress Constrained Fixed-abrasive Fixed-abrasive Fixed-abrsive

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 work piece 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 [12].

2.3 Classification of Lapping Process

Lapping process can be classified in different ways. Firstly, it can be divided into single side lapping, double side lapping and cylindrical lapping due to the lapping machine difference. Figure below is a common single side lapping process of lapping

plate, and sometime doubles sides lapping process also used for various requirements.

Like the machine showed in Fig.2.4, double side lapping machine has two lapping plate.

One of the plates is placed down side to work with the bottom surface of the work piece, also it used as a foundation bed to hold the work piece. On the up side, another lapping plate works with the top surface of the work piece. During the process, those two plates rotate in opposite direction to do the lapping at the same time. For the cylindrical lapping, it is mainly used to machine cylindrical parts or cylindrical portions of a particular component.

Figure 2-4 Single Side Lapping Process

Figure 2-5 Double Side Lapping Machine

2.3.1 Single Side Lapping

Single side lapping is most widely used method to achieve desirable flatness and surface roughness. A large quantity of work pieces can be machined at the same time and consistent cut rates and close accuracy can be obtained with the simple set-up. During the lapping process, the work piece machined under a certain load and abrasive slurry supplied so that a thin abrasive film generated to remove material. The significant factors involved to affect the results include the flatness of the lapping plate, the uniform distribution of the abrasives, and the interval of the continuously supplied slurry and so on [12].

Figure 2-6 Single Side Lapping Mechanism Illustration

2.3.2 Double Side Lapping

Double side lapping process is the most accurate method in terms of parallelism and uniformity. Within double side process there is less chance that foreign particles introduced into the process to settle between the work piece and the load.

Figure 2-7 Position of the Work Piece in Double Side Lapping [19]

The double side lapping can machine the work piece into optically flatness and micrometer surface roughness. Moreover, double side lapping process increase lapping efficiency by machining two surfaces simultaneously.

2.3.3 Cylindrical Lapping

Cylindrical lapping process provides method to finish the cylindrical part to a high degree of geometric accuracy, with low surface roughness. As Fig.2.8 shown, the machines for cylindrical lapping utilize two annular lapping plates, each of them mounted on a vertical spindle. One or both of the lapping plates rotate during the process. The work pieces are placed within a holder that guides them between the lap faces to produce an abrading action. The work holder is guided in the center by a rotating pin that can be adjusted to move eccentricly to the center of the lower lap. The cylindrical parts are placed in slots, the centerline of which are tangent to a circle in the center of the work holder. The rolling action of the parts causes the work holder to rotate. Controlled lapping occurs as the parts slide and slip during rolling, caused by the nonradial position of the work holder.

Figure 2-8 Schematic Illustration of Cylindrical Lapping Method

2.4 Lapping Component

2.4.1 Lapping Plate

The component of lapping process includes lapping plate/wheel, slurry, lapping medium and lapping abrasives. All of these are factors that influence the lapping characteristics.

The lapping plates are characterized by their macro-geometry, their surface topography as well as by their material, their mechanical properties and the structure. The

Lapping plate plays a significant role in the production of high quality specimens. The material and the type of the lapping plate can be various depend on different requirement.

The material removal rate, the surface flatness, the surface roughness and the hardness of the specimen could be the factor to determine the selection of lapping plate.

As the most important characteristic of lapping plate, the material types differs from cast iron, ceramic, steel to glass. Different types of material determine different mechanical properties, structure, macro-geometry and surface topography. Mechanical properties and structure affect the plate resistance and wear abrasion, which would influence the tool life and production efficiency. The macro-geometry and surface topography takes effects on the distribution of the slurry during the lapping process as well as the finished surface quality of work piece.

The hardness of the lapping plate influences the movement type of the abrasives; a softer lapping plate keeps the abrasive firmly in its position and as a result, the abrasive particle scratches the work piece to remove material from the work piece. Thus, a surface of very low roughness is formed and the machining mark consists of fine ridges like that

Fig.2.9 illustrated. As long as a hard material lapping plate is applied, more rolling action than sliding action will take place during the process. Moreover, the abrasive particles would be embedded into the work piece and then gliding marks are generated on the lapping plate. This phenomena will damage the surface quality of work piece been machined. Multi-metal lapping plate consists of two or more materials, which are combined in spiral inlays, mosaic, or annular shape. This kind of lapping plate is much more expensive than the conventional ones. However, they are extremely suitable for the use of diamond media [1]. Examples of existing lapping plates used in industry are given in below figures.

Figure 2-9 SEM Image of Silicon Surface after Rough Grinding [20]

Figure 2-10 Cast Iron (Fe) Lapping Plate [21]

Metal Plate

Cast iron lapping plate is the most widely used plate in modern industry, it is commonly applied for general surface machining as well as high material removal rate.

Cooper plates are similar to cast iron plate in several aspects. However, cooper plate designed mainly for softer material where fine lapping and polishing are the primary requirements. Specimens around 8-10 on Mohs Hardness scal can be lapped on cast iron plate and for cooper plate, the value is around 5-9. Tin plate is another metal plate which is way softer than cast iron and cooper and it works with a wide range of material, such as metal, ceramic and glass. They are typically used with very fine diamonds as the abrasives to minimize fracturing and chiopping tendencies when lapping crystal components. As we can see in Fig.2.11, Tin plate can produce a mirror surface on fiber

optic terminals like the surface itself. Tin plate involved lapping process has a low material removal rate and it is suitable for charging extra-fine particles [22].

Figure 2-11 Tin Lapping Plate [22]

Ceramic Plate

As one of the hardest plate, ceramic lapping plate generally applied in lapping or polishing ceramic parts and some other stain-sensitive hard material where a clean process is necessary or where there metallic type contamination cannot be accepted. The ceramic mentioned here is particularly means the composite ceramic rather than natural ceramic. Ceramic plates provide fine surface roughness with a medium material removal

rate.

Figure 2-12 8inch Ceramic Lapping Plate [23]

Composite Lapping Plate

Composite lapping plate consists of metallic and non-metallic components that held together in a resin system. Composite plate can be used to machine hard material around 7-10 Mohs Hardness. Compare with the pure metal lapping plate, the advantages of composite plate can be concluded in two mainly aspects;(1) Composite lapping plates normally takes a more uniform charge of diamond particles compared to pure metal plates.(2) Composite plate is extremely suitable in the condition where lapping and polishing combined in one step. Cooper composite plate and steel composite plate is the most widely used in industry. The composite plate can take a uniform charge of diamond when using diamond particles. Therefore, composite plates have excellent performance on high quality surface finish and stock removal with superabrasive.

Lapping process is a surface copy process and due to this reason, the design of the lapping plate is very important to the lapping performance. In order to avoid slurry stasis

and to improve the chip removal, lapping plate can be grooved sometime. This is particularly advantageous in the case of machining work pieces of large surfaces.

Figure 2-13 Cooper Composite Lapping Plate [24]

2.4.2 Lapping slurry and fluid

The conventional lapping process is a slurry-based process, and the slurry consists of lapping abrasive and lapping medium that can be fluid or paste. The characteristic of the slurry is determined by a certain volumetric concentration that always named abrasive concentration in slurry. The abrasive concentration varies by different material removal and surface quality requirement, also, the supply amount of slurry affect the performance of the process; if the slurry supply is too high, the parts may float (aquaplaning effect) and a thick lapping film will formed between the plate and work piece. Thus, the material removal rate will be reduced. A break off the lapping film may lead to cold welding through the direct contact of the lapping wheel and the work piece. The limits of the lapping film thickness, which vary in the case of different materials, are reached if no 21

typical lapping pattern can be seen on the surface of the work piece. The surface then resembles the grinding pattern with directional machining marks [1].

During lapping process, to get an even and continuous distribution of the abrasive particle on the plate, an oil or aqueous medium is always selected. The medium is named as the lapping fluid, which also include lubricant and coolant. Compare with the other abrasive process such as grinding and honing, lapping is not a huge heat generated process. Therefore, the lapping fluid typically focus on the lubrication function rather than cooling the temperature down. Besides controlling the wear of the friction during the process, the lapping fluid also provide the ability to suspend and uniformly disperse the abrasive particles. Moreover, the debris from the work piece is taken away from the lapping zone by the lapping fluid.

2.4.3 Lapping Abrasive

The lapping abrasive is different in material type, shape of grain and grain size distribution, and the relationship between each factor is very close. The shape of the abrasive is typically determined by the material properties like hardness and toughness;

Even with equal average grain size, every type of lapping abrasive has a characteristic grain size distribution also determined by the grain shape. The abrasives are considered as the cutting tools with unspecified and irregular cutting edges and the abrasive selecting of lapping is depend on the properties of the work piece to be machined. Moreover, Surface finish requirement, desired material removal rate and economic influence could be also considered [1].

In industry, material used as abrasives consists of natural minerals and synthetic products. There are several types of abrasives that are considered as the conventional abrasives. These abrasive are dramatic difference in properties and cost. For example, the hardness of each abrasive is listed below:

Diamond 7000 kgmm-2

Garnet 1400 kgmm-2

SiC (Silicon Carbide) 2700 kgmm-2

-2 Al2O3 (Aluminum Oxide) 2500 kgmm

CBN (Cubic Boron Nitride) 4500 kgmm-2

In our research, diamond particle is mainly used and in order to improve the lapping performance, a type of surface treated diamond is introduced to the manufacturing of the lapping plate. It is illustrated in details in chapters after.

Chapter 3

Ultraviolet-Curable Resin

3.1 Ultraviolet-Curable Resin Research Development

Today’s mainstream abrasive tools are made of metal bonding agents or resin bonding agents. The manufacturing processes consist mainly the pressing and roasting the agent in oven. For instance, in the case of phenolic resin agent, the process is achieved by using a curing furnace with a temperature of hundreds of degrees for about two hours. This process is comparatively high in cost and energy use and low in product efficiency. Furthermore, industrial exhaust gas is produced during the sintering process.

Some typical methods like the electroplating process, are also applied, for instance, in wire saw manufacturing. But the long time required and the treatment of waste liquid for the electroplate process are very costly. The main challenge facing the abrasive tool industry is the growing demand for cleaner and more cost effective manufacturing processes. With the development of the semiconductor industry, more attention is required to innovate abrasive tools and the manufacturing technologies in this field.

The UV curing technology used in manufacturing can be traced back to the beginning of the 1950s. American engineers applied the technology of light solidifying to make typography board with UV-curing resin. In the past 20 years, researchers have developed photo-manufacturing technology in various applications. Research by

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).

With the application of UV-curing resin in the fields of rapid prototyping, molding material, medical treatment and micro manufacturing, various kinds of UV-curing resin with different features have been developed in the past years. This progress has made possible some kinds of UV-curing resin that appropriate the physical properties of the thermoset resins, like phenolic resin, which is used as a bonding agent for conventional abrasive tools.

In the UV-curing resin, there are two components. One is the basic oligomer and the other is called a photoinitiator. The key characteristics of thee photoinitiator is that it will not react with the resin by itself; the photoinitiator must absorb ultraviolet light before anything can happen (i.e. change of physical properties). When the UV light is delivered, the photoinitiator will undergo a chemical reaction and produce some byproducts that cause the adhesive to harden. Figure below shows the principle of the reaction. For the photoinitiator to react correctly, it must be exposed to light of the correct wavelength and of sufficient intensity. Otherwise, the chemical reaction will not happen, 25

or it may not happen completely, resulting in poor or inconsistent adhesive performance.

Compared with the thermosetting resin, this chemical reaction converts the resin from a liquid to a solid as a result of an increase in molecular weight, without the volatilizing.

From this principle, the use of UV-curing resin brings many benefits to product manufacturers.

(1) Short process of solidifying: It needs only a few seconds or minutes to finish the process of solidifying.

(2) Process consistency and flexibility without large field and equipments

(3) Reduced environmental considerations: There are almost no organic dissolvent in the use of UV-curing resin.

(4) Less energy consumed: In comparison to the heat-solidifying process, this process could save about 90% of energy.

These advantages suggest that in the near future UV-curing resin will replace thermosetting resin and become the new bonding agent in the manufacture of Thinner

Oligomer abrasive tools.

Figure 3-14 Reaction of UV Curing Process

Researchers have been investigating the use of UV-curing resin as the bonding agent of grinding wheels. In 1997, Tanaka, etc presented a grinding wheel made by the process of rapid prototyping [8]. Peng, etc presented a new dicing blade made of UV- curable resin in 1999. Takehara, etc presented a diamond wire saw with UV-curable resin in 1999 [25]. However, these creative ideas are still in the laboratory research stage, mainly due to wear resistance in use. In fact, there is a lack of fundamental understanding about several important issues from the UV-curing resin to the new abrasive agent. This lack of fundamental understanding has hindered further advancement of the new technology and its implementation in the industry.

3.2 UV-Curable Resin Introduction

There is a wide variety of UV-curable materials available for a broad range of applications. In this research, two types of UV-curable resins and their mixtures will be evaluated according to the mechanical properties of the abrasive tool. The first type of adhesive to become familiar with is an epoxy-based material. While some people use the term epoxy as a generic reference to all high performance, engineering resins, epoxy has a quite specific meaning within the adhesive world. The second is an acrylic-based, UV- curable resin that we will exam because it differs from epoxy-based resins. As a potential bonding agent for manufacturing of abrasive tools, whose basic properties will be studied.

The first are the mechanical characteristics, such as tensile strength, hardness and wearing resistance. The second are the processing abilities, such as the curing depth and shrinkage during the UV-curing process. The final are the adhering abilities to abrasives and other additives, such as a variety of fibers and micro particles used to reinforce the material or modify the special properties.

Epoxy resin will be studied in this project because it is one of the stiffest plastic materials. In the UV-curing process, epoxy resins use a catalytic curing mechanism. The catalyst is a by-product from the reaction of the photoinitiator to UV light. By definition, a catalyst is something that promotes a chemical reaction but is not consumed in the reaction. One consequence of this is that UV-curing epoxy resins exhibit a special capability: material that is not directly exposed to UV light will cure eventually. This may be propitious to the uncompleted curing problem concerned with the abrasive’s resistance to UV light. The other advantage using epoxy-based resins is easy their 28

modification capability, achieved by mixing them with different additives. This advantage may create great potential to modify the resin to meet the special needs of abrasive tool making.

Acrylic resin, the most wildly used UV-curable resin, is the second one we will study in this project. They result from an entirely different chemistry and a different type of photoiniator than epoxies. Curing of acrylic resins is a free radical mechanism. The free radicals are produced by the photoinitiator when it is exposed to ultraviolet light.

However, the free radicals are consumed in the adhesive curing process, so acrylic resins can only cure where UV light is delivered. At least one of the components being bonded must be UV-transparent to some degree. Modification of properties in acrylic resins is more often conducted at the chemical level, through changes in formulation or combination with other base resins. Wide range of properties can be utilized: impact resistance, surface insensitivity, environmental resistance, and others.

In fact, there are few kinds of pure UV-curable resin to be used as a bonding agent because high composite characteristics are required. However, the theory of composite material indicated that development of such materials is possible. Like cement should be used in the form of concrete filled with sand and reinforced with steel bars, resin should also be modified by filling it with additives according to the needs of bonding agents for abrasive tools.

The early stage of rapid prototyping technology is mostly used to manufacture prototypes for the quick verification of designs 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 a part’s properties similar to those of its serial version cannot be built with the UV-curing process because of the limited material properties of UV-curable resins. However, many opportunities to reinforce resins exist, one of which is filling the resin with powders such as ceramics, which shows great promise. Research has shown that different powder-filled, UV-curable resins can theoretically be used to manufacture highly loadable parts and tools. In general, the stiffness, wearing resistance, and the thermal and chemical resistance of the composite are higher than those of the pure resin. Based on the fact that several micro-scale powders have been successfully synthesized with UV-curable resin in the field of rapid prototyping, in this proposed research, ceramics powder like nano-size Al2O3 particle, will be studied to reinforce the bonding agent material.

3.3 Key Factors in the UV-Curing Process

Two factors affect the curing process most in the UV curing process. One is the rheological property of the paste, which will affect the flowing in the mold for final products. The other is the filler, which affects the depth and polymerization of UV-curing

In general, the high amount of fillers in the available resin is supposed to offer improved material properties, as mentioned before. However, this leads to an increased viscosity. The rheological properties of the paste affect the forming process before UV- curing. In most cases, the paste is formed in a mold or the adding the thin layer method according to different shapes of abrasive tools. Some of the abrasive tools are very thin in size. For instance, the dicing blade used in the silicon wafer dicing process requires a depth of 0.02 - 0.2 mm. If the viscosity is too high, the forming process will be 30

complicated or even impossible. On the other hand, the rheological properties of the paste will also be connected with abrasive deposition, which is extremely difficult in the precision abrasive tools. Therefore, the rheological properties of the paste should be well understood, and a satisfactory shear viscosity coefficient of the paste should be acquired.

3.4 Bonding Mechanism of Abrasives

3.4.1 Mechanical Bonding

The bonding strength of abrasives is related to the efficiency of cutting tools and life of abrasive tools. The boding mechanism by the UV-curing process is different from the conventional thermoset process and many theories have been developed to explain the process of bonding in adhesive structures. Individually, each of these theories is inadequate to describe the complete process of bonding in most situations. However, each theory contributes an understanding of the overall process of bonding and, therefore, is important. Figure below illustrates the five predominant mechanisms of adhesion.

According to the mechanical bonding theory, in order to work well, an adhesive must fill the surface valleys of abrasives to be bonded, as well as displace trapped air.

Adhesion is the mechanical interlocking of the adhesive and the abrasives, and the overall strength of the bond is dependent upon the quality of this interlocking interface. To this end, a rough surfaced abrasive is highly recommended for optimum bonding. If the surface of the abrasive, in most cases diamond, is too slippery for better bonding, a pre- process will be taken into consideration. Coating is one pre-process to be used in this research, not only for its rough surface but also for forming a chemically reactive surface.

The significant effect of the coating was reported. Figure below shows the diamond 31

coated with nickel suited to epoxy adhesive Thus this is one key point to study for bonding strength.

Figure 3-15 Adhesion Mechanisms

Figure 3-16 Nickel Coated Diamond Abrasive

3.4.2 Adsorption bonding

The adsorption mechanism theory suggests that bonding is the process of intermolecular attraction (van der Waals bonding or permanent dipole, for example) between the adhesive and the diamond at the interface. According to this theory, an important factor in the strength of the bond is the soakage of the diamond by the adhesive.

Soakage is the process in which a liquid spreads onto a solid surface and is controlled by the surface energy of the liquid-solid interface versus the liquid-vapor and the solid-vapor interfaces. In the proposed research, in a practical sense, to soak the diamond surface, the adhesive should have a lower surface tension than the diamond.

Chapter 4

Experiments Design and Set up

4.1 Material and Equipment Preparation

4.1.1 Ultraviolet-Curable Resins

Three different Ultraviolet-cured resins selected in this research, they 921-V and

425 resin was purchased from DYMAX corporation and the HTU-363 was purchased from HUITIAN co., and all of the technical specifications are listed in Table 4.1 The

Ultraviolet-resin can be cured by Ultraviolet light and has certain hardness, strength and other basic mechanical or chemical properties. For the bonding material in lapping plate, it must meet some requirements to be used, so the hardness, porosity and tensile strength tests were carried out to select the appropriate one.

Table 4.1 Three Ultraviolet-cured Resins

Resin 425 921-VT 363 Durameter Hardness D80 D75 D80 Water Absorption 0.7% 1.1% 1.8% Viscosity (mPa·s) 4000 11000 1300-1700 Thermal Limit -40o-150oc -43o-177oc -45o-160oc Tensile at break 6200 psi 5200 psi 5000 psi Elongation at break 7.3% 35% 12% 34

4.1.2 Abrasive Particles

As the hardest abrasive in industry, diamond is preferred in lapping process. By which, a fine surface finish and higher material removal rate can be achieved. In modern industrial manufacturing, there are two kinds of diamond abrasive grains used mainly: the

Metal Bond Diamond Powder and the Resin Bond Diamond Powder. The Metal Bond

Diamond Powder (MA) is mono-crystalline micron and nanometer size diamond that exhibits 3-D blocky particle shape, it provides an aggressive stock removal and superior surface finish properties; and the Resin Bond Diamond Powder (RA) is mono-crystalline diamond that exhibits typical sharp friable-resin bond crystals, and micronized from selected industrial grade powders. The particle size distribution of MA and RA grains are around 10µm to 20µm. Both of the RA and MA abrasive powder is purchased from Engis

Company, and for the initial experiment in our research, RA and MA abrasive particle will mixed with different types of UV resin to find out the best combination.

In addition, surface treated MA abrasive is introduced later for the manufacturing of the lapping plate. As same as the normal MA abrasive, surface treated MA has a size distribution around 12-22 micrometer and the comparison figures can be seen below.

Fig.4.1 and Fig.4.2 gives a SEM of those two particles.

Figure 4-17 SEM Image of Normal MA Abrasive (11288)

Figure 4-18 SEM Image of Surface Treated MA Abrasive (R0206)

Figure 4-19 Particle Size Distributions of the Abrasive Particles

4.1.3 Ultraviolet Curing System

Ultraviolet curing system mainly consists of an ultraviolet lamp and a power supply and it is used to process the transform of the resin from liquid to solid. Our curing system using alters on different samples. In Fig.4.4, the system shown that is used for small samples designed for tensile test, hardness and wear abrasion test. UV light is an electromagnetic wave of 100 to 380nm wavelength, longer than that of X-rays but shorter than that of visible rays. For different, the required wavelength for cure is specific to the resin chemistry. The wavelength distribution of Dymax is around 300nm to 450 nm as the figure shown below.

Figure 4-20 Dymax 5000 Flood Ultraviolet Curing System

Figure 4-21 Wavelength Distribution of Dymax 5000 Curing System

Figure 4-22 Innovative Machine Co. UV-100 Curing System

Figure 4-23 Ultraviolet Lamp of UV-100 Curing System 39

Table 4.2 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 from the

bottom edge of the reflector 225 mW/cm2

housing

Dimensions of Illuminated Area 5" x 5" (12.7 cm x 12.7 cm)

EC Input Power Requirements Universal 90-265 VAC (grounded), 47-63 Hz

Power Supply Weight 12.25 lbs. (5.6 kg)

Power Supply 12" L x 16" W x 4.25" H (30.5 cm x 40.6 cm x

Dimensions (approximate) 10.8 cm)

Reflector Housing 6.75" L x 6.75" W x 8" H (17.2 cm x 17.2 cm

Dimensions (approximate) x 20.3 cm)

Reflector Housing Weight 2.7 lbs (1.2 kg)

UV-100 curing system comes with a large lamp that can be seen in Fig.4.7, and which give a large area cover of UV lights. Thus, it is more suitable for curing large samples since a more uniform energy distribution. The working zone is based on a rotating belt whose speed can be changed in 10 levels, and the system power can be controlled by 3 levels (125watts/inch, 200watts/inch, 300watts/inch).

4.1.4 Lapping Machine

The lapping experiment of the ultraviolet-curable resin plate is taken place on the

Lapmaster 12 lapping machine that is made by Lapmaster Company. The lapping plate can be easily mounted or dismounted, and it comes with three condition rings. The lapping speed can be switched from 0 to 60 rpm.

Figure 4-24 Lapmaster 12 Lapping Machine

4.2 Experimental Procedure

Before we build the lapping plate, basic properties of the mixture by UV resin and diamond abrasive particle include tensile strength, hardness and wear resistance should be studied.

4.2.1 Tensile Strength

Tensile test, which as known as tension test, is a basic material science test in which a sample is applied to uniaxial tension until failure. The results from the tensile test

are mainly consists of two curves: the tensile stress versus tensile strain and the maximum load versus maximum elongation. Moreover, the Young’s modulus, the

Poisson’ ratio and yield strength can be obtained by the test. All of these parameters determine the mechanical properties of the material and these data is always used to select material for different purpose. In our research, three types of UV resin and two types of diamond abrasive combined together to make the samples used in tensile test.

The abrasive concentration selected for the mixture is 25%. The abrasive concentration for abrasive tools is a key factor that affects the tool’s performance. For the diamond concentration of a diamond tool, it means that the weight of the diamond in each cubic centimeter of the diamond tool’s working layer, and the international standard has prescribed that if each cubic centimeter contains 0.88g super-abrasives, its concentration is 100%. The each increase of decrease of 0.22g will cause the concentration by 25% correspondently [26]. In lapping and such precision machining process, a lower abrasive concentration is used frequently.

Figure 4-25 Curing Pattern Designed for Tensile Test

As Fig.4.9 shown, steel pattern is designed to make the samples used in tensile test. Both of the pure resin samples and abrasive-resin mixture samples are prepared for the experiment. The UV resin and diamond abrasive particle is firstly mixed together, and then the mixture will stirred for couple of hours to make sure the uniform distribution of the particles. The mixture will be left over night to improve the polymerization and remove the internal bubble [27].

The tensile stress and tensile strain curve of the six combination is shown in

Fig.4.10. No matter what kind of abrasive diamond is added to 425 resins, the mixture shows an outstanding performance on maximum tensile stress, which means they can afford more load than other to break does. 921t UV resin mixture breaks at 20 and 30

Mpa and it shows the weakness of the mechanical properties. For the HTU363 resin mixture, the maximum elongation is much larger than the others. The elongation or tensile strain can be viewed as the representation of material elasticity, which shows the deformation is easy to taken place when force loaded. As a bonding material in lapping plate, the deformation of the plate will cause the abrasive particle embedded into the material and transform three-body abrasion to two-body abrasion. Sometime, this phenomenon helps lapping process in material removal rate and surface finish. However, if the material is too soft, the lapping plate will be hard to hold the particles and then huge amount of edges on the abrasives would be submerged. Thus the machining efficiency could be influenced.

Figure 4-26 Tensile Stress vs. Tensile Strain of Abrasive-Resin Mixture

4.2.2 Porosity

The lapping plate made by resin keeps porous not only in UV-curable resin bonding material but also in thermo-curable resin, since it is a material property of resin itself.

Previous studies form researchers investigated that the lapping efficiency is significantly affected by porosity. The porosity can change the hydrodynamic performance between the lapping plate and the work piece, as well as provide space for grains, chipping and lapping fluid during lapping process.

Microscopies of each resin are carried out in figures below. Different size and density can be found in different resin, the desultorily distribution of these pores help fluid permeating on the plate.

Figure 4-27 Microscopy of 921vt Resin

Figure 4-28 Microscopy of 425 Resin

Figure 4-29 Microscopy of HTU363 Resin

4.2.3 Hardness and Wear Resistance

The hardness of lapping plate is one of the key factors because the lapping plate undergoes load and produce deformation during the lapping process. The certain hardness of the lapping plate may keep the grain in the situation of scraping or rolling, thereby affect the material removal rate and surface roughness of the woke piece. The hardness of

Table 4.3 Porosity of Resins

Pore size 20-100 10-110 10-100 [m]

Porosity More Medium Less

three kinds of resin filled with two different diamond powders was tested.

Furthermore their wearing properties were compared by lapping those samples on the 46

same condition and measuring the loss of thickness of the sample. Samples are made with the pattern shown in Fig.4.14. With the blue plastic patter, 18mm samples made of different ultraviolet-cured resin were prepared.

Figure 4-30 Curing Patterns for Different Test

Figure 4-31 Samples Prepared for Wear Resistance Test

The Vickers Hardness of the cured samples was tested by Clark Micro hardness

Tester CM400-AT, and the results are shown in Table4.4.

Figure 4-32 Photos of Samples for Hardness Test

Figure 4-33 Wear Resistance Experiment

Table 4.4 Hardness and Abrasion of Resin-Abrasive Mixtures

Resin UV-resin(921) UV-resin(425) UV-resin(363)

Diamond MA4 RA MA4 RA MA4 RA powder 12.5%vol 12.5%vol 12.5%vol 12.5%vol 12.5%vol 12.5%vol (15μm)

Hardness 11.1 12.3 15.4 13.6 8.3 7.7 (kgf/mm2)

Abrasion 0.074 0.082 0.078 0.077 0.038 0.052 (mm)

In order to determine the wear resistance of each combination, the samples are lapped under the same condition to see the loss in thickness. The results are listed in the above table.

4.3 Manufacturing of the Lapping Plate

4.3.1 Spin-Curing Method

Small sample pieces of resin-abrasive mixture is easy to make since the small area is easy to control and also the ultraviolet energy distribution can be uniform in a small area. For a large area of resin-abrasive mixture that needs to be cured, the curing process is much more complicate. Spin-coating method is proposed in previous studies and the idea is illustrated in figure below.

Figure 4-34 Spin-coating Method Introduction

In spin-coating process, the resin-mixture is spread on the cast iron plate and curing at one time. It seems like this method will reduce the curing time and generate a uniform distribution of the mixture. However, two mainly problems are exist during the process; First, a lot of heat will generated within the curing process on the surface of the mixture film, the uneven distribution of the heat absorption will cause deformation of the surface and then surface waveness occurred. The waveness of the whole place is difficult to deal with before lapping experiment taken place. Second, it is impossible for the ultraviolet lamp to produce uniform energy distribution due to the large surface area of

the plate. The distance from UV lamp to the center of the plate is different from the distance that from UV lamp to the periphery area of the plate, therefore, the cure depth can be various in one plate. Besides these two problems, the contact area of the resin- abrasive mixture and the cast iron plate is cured or not is still uncertain. Even if it is cured, whether the adhesive force is strong enough to handle the lapping process is still a problem.

4.3.2 Slices Curing Method

In our experiment, another curing method to build a lapping plate with resin- abrasive mixture is introduced. The plate is divided into 12 slice evenly, and each slice has a suitable size to be mounted in curing system in order to get a uniform energy distribution from the UV light as much as possible. Fig.4.19 shows the steel pattern used for curing slices of lapping plate

Figure 4-35 Curing Pattern Designed for Slice of Lapping Plate

The cured pieces are then agglutinated together on the cast iron plate to form the plate, as Fig.4.20 shown below, the finished lapping plate is consists of 12 slices. Since the agglutinating process in hand working process and it is hard to keep the adhesive film thickness exact the same, an dressing process is necessary before the plate used for lapping experiment. A grind wheel is applied to take a face from the top of the plate to ensure the flatness of the lapping plate, this process also can be done with conditioning ring and slurry and the idea is based on conventional lapping process. However, the efficiency of grinding is much more preferable to lapping.

Figure 4-36 Whole Plate Consists of 12 Slices

Figure 4-37 Dressing Process of Lapping Plate

4.4 Silicon Wafer Lapping Experimental Program

For the estimation of lapping performance of the developed UV-plate, some experimental program is designed. The lapping finish experiments are carried out in the

Lapmaster 12 lapping machine. Typical rigid brittle materials silicon wafer is selected as experimental work-piece. Tap water is used as coolant during the lapping. Traditional cast iron lapping plate is also used in the experiments as a reference experiment. During process of traditional lapping, the slurry concentration of the green SiC particles is 20% by weight. The experimental conditions are listed in Table 4.5. In this basic experiments, we used the silicon wafer samples with initial surface roughness of Ra 0.15 µm . Silicon wafers are lapped for 30, 60, 90,120 and 150 minutes by UV-plate and traditional cast iron plate respectively.

Figure 4-38 Surface Roughness of UV Lapping and Conventional Lapping

Table 4.5 Experimental conditions

Parameter Value

Lapping machine Lapmaster 12

Work piece 4 " silicon wafer

Min steady rotate speed [rpm] 1

Maximum rotate speed [rpm] 50

Round count precision[˚] ±0.5˚

Lapping load[ kgf/cm² ] 0.018

Lapping period[min] 30, 60, 90,120,150

Roughness tester Mahr, P12THOM1T12 S II

Silicon wafer is selected first to be machined on the comparison because of the easily machinable properties in manufacturing. Both in conventional lapping and in some new lapping technology, silicon is an important type of material that should be considered. Before going deep in our research, a basic comparison is needed to see the advantage of UV plate lapping technology to conventional lapping process.

The results in Fig.4.22 show that the surface roughness lapped by cast iron plate is not rapidly changed in the first 30 minutes. The wafer surface machined by an UV-plate produces remarkable improvement, the roughness reduced rapidly within the first 60 minutes and it keeps stable after that. It is because cast iron plate is based on conventional loose abrasive lapping method in which the loose abrasive grain was rolled between the work-piece and the lapping plate. Each abrasive grain in the ways of rolling action removes the material. This is the reason why the conventional loose abrasive lapping process has low machining efficiency, which also leads an improvement in material removal rate of the process. In conclusion, the new UV-plate carries with outstanding machining characteristics.

Chapter 5

Optimization and Improvements in Lapping Performances

5.1 MA Abrasive and Surface Treated MA Abrasive

Abrasives play the most significant role in lapping process, even in UV resin plate lapping. The abrasive embedded into the cure resin and act as a micro cutting tool.

Therefore, the type of the abrasive eventually affect the lapping performance and surface finish quality. Diamond, as the hardest and most efficient abrasive, is widely used in lapping process. As we mentioned in previous chapter, Metal bond diamond and Resin bond diamond mixture and compare in different experiment and the MA obviously leads the competition.

For more lapping experiments on plate, we choose MA as the abrasive particle.

As a comparative experiment, Surface Treated MA (ST-MA) is introduced. The material that coated on the surface of the particle determines surface treatment of abrasives, the coating material can be chosen from types of metals, such as cooper, nickel, titanium.

The main purposes of the surface treatment process are focused in two aspects. First, the coating material can change the weight ration and increase the brittleness. The machining efficiency is then affected since the fracture of the abrasive is defined by the brittleness of 56

abrasive particle. On the other hand, the surface treatment changes the shape of the particle by forming more wrinkles on the surface. It can be clearly seen in the compare figure below.

Figure 5-39 Comparison of MA and Surface Treated MA

5.1.1 Lapping Experimental on Aluminum Oxide

In order to see the machining capacity of UV resin lapping plate on harder material such as ceramic, aluminum oxide rings will used as the work piece in this group of experiments. With an initial roughness of 0.45 µm, the Al2O3 rings are lapped for different period, the experiment condition is given in Table 5.1.The roughness test will be done in Hommel Tester 1000, and the comparison results are shown below.

To get an intuitive understanding, AFM is used in the experiment. The AFM is a very high-resolution type of scanning probe microscopy. It is one of the foremost tools for imaging, measuring and manipulating matter at the nanoscale. The information of

surface morphology and roughness is gathered with a mechanical probe. An AFM image of the unfinished ceramic ring surface is given in Fig.5.2.

Figure 5-40 Aluminum Oxide Rings

Figure 5-41 Surface AFM Image of Unfinished Ceramic Ring

Table 5.1 Experiment Condition of MA and ST-MA Lapping

Lapping Machine Lap Master M10

Roughness Test Machine Hommel Tester 1000

Lapping Speed (rpm) 40

Lapping Coolant Engis MISC.1040 Grinding Lubricant

Lapping Time (minute) 15, 30, 45, 60, 90, 120

Lapping Load (N/cm2) Low(2.38), High (6.35)

Work Piece ø20mm Aluminum Oxide Rings

5.1.2 Experiments Results and Discussion

The roughness of the finished work piece has been compared in Fig.5.4 and

Fig.5.5. For both of these two figures, the roughness value drops down rapidly within the first 15 minutes, that is because lap-grinding is different from the conventional lapping process, which is based on a slurry process and the loose abrasive particle rolling between plate and work piece, the material is removed by the rolling action of the abrasive grain. For UV resin plate lapping process, which is considered as a fixed abrasive lapping process, the abrasive grain is softly bonded in the resin and it is more like a grinding process in the starting period, and that is the reason why UV-plate lap- grinding has an excellent machining efficiency when compare to the loose abrasive lapping. In addition, it has been proved that UV resin plate lapping has a better performance in material removal rate. All of these results are just the same as what we done before on the comparison between UV resin plate lapping and conventional lapping.

Figure 5-42 Roughness of Surface Treated MA Lapping Plate

Figure 5-43 Roughness of MA Lapping Plate

Lapping load is another factor that we considered in this experiment, from the results given in figures, we can see the roughness of the higher load work pieces drop down faster than low pressure ones. The lapping pressure is always related with the material removal rate and finish efficiency, since the larger the load applied, the more free particles would be embedded into the plate. Therefore, more two-body abrasion taken place and the machining efficiency improves. In conclusion, material removal rate is linear related with lapping pressure. For the roughness, a better result is always obtained under low speed and low pressure. The roughness of the work piece under low pressure would be smaller if we increase the lapping time.

When contrast Fig.5.4 and Fig.5.5, the roughness of surface treated plate drop down from 0.45 to 0.15 and the normal diamond plate is just down to 0.2, there is a 0.05 difference and it is not a small value in this level. For another part, in surface treated diamond plate experiments, the roughness drops down rapidly and especially in the starting 15 minutes. However, for normal diamond experiments, the roughness drops down in a smooth curve. When we look at the SEM images of the two diamonds, surface treated diamond have more rumples and edges and they are very helpful to increase the bonding force between the abrasive and resin. The more wrinkles on the diamond abrasive surface means the larger surface area, therefore the contact area of resin and abrasive increased when compare to the normal MA. In other words, the increased surface roughness and increased surface area of surface treated MA help to increase the bonding strength in the curing process. The stronger bonding strength of the surface treated MA plate help to keep the abrasives fixed in the plate for a longer time during

lapping process. That is the reason why surface treated diamond abrasive plate experiment has a rapidly drop down in roughness test along with time.

Figure 5-44 Roughness Comparisons of MA and ST-MA Plate

When the abrasive grain is fixed on the plate, those small edges one the particle surface play like a micro cutting tool on the surface of the abrasive grain and an aggressive stock removal is obtained. When the grain fall off from the bonding material, the abrasive particles will roll, slide and scratch within the lapping plate and work piece to remove material. In the comparison figure, both curves keep stable after 75 minutes and this is because the main action of the process is instead by three-body abrasion and the abrasive particles became blunt and dull.

5.2 Influence of Nano-Particle Additives

5.2.1 Nano-Particle Additives Introduction

The curing process of the resin is another part should be considered besides abrasive particle. Nano-scaled materials are now being considered as filler material to produce high-performance composite structures with further enhanced properties.

Improvements in mechanical, electrical, and chemical properties have resulted in major interest in nano composite materials in numerous automotive, aerospace, electronics and biotechnology applications. Nano-fibers such as carbon nano fibers, due to their high tensile strength, modulus, and relatively low cost, are drawing significant attention for their potential applications in polymer reinforcement. Various kinds of nano particles, such as nano-SiC, nano-ZrO2, nano-ZnO, nano-TiO2, nano-CaCO3, nano-diamond particles, are widely used as fillers in poly composites to improve mechanical characteristic especially wear-resistance. Based on previous research of nano composites, both nano-fibers and nano-particles In particular, significant increase of wear-resistance is expected, mostly by nano-particles, which is closely concerned with the lifetime of abrasive tools.

In our experiment, the Al2O3 nano-particle is selected as additives filler in the resin mixture, and they were purchased from LECO Corporation, the size of the nanoparticles is 50 nm according from the manufacturer. In order to see the improvement of the material properties of resin with nano-particle, a compare experiment was carried out, one group of the samples is diamond abrasive mixed resins and the other group of

samples is made of diamond abrasive resin mixed with nano-particles. A 12.5% diamond concentration is selected as a same standard in our experiments.

The concentration of the nano-particles is another parameter need to be concerned and from the former researches we knew that the material properties of the polymer can be changed with a very low concentration, which is always less than 1% in mass. In this research, the nano-particle concentration is set at 0.75% in mass content as a standard.

The mixture of resin, abrasive diamond and nano-particle is stirred for half an hour to make a uniform distribution of the nanoparticles and then the stirred mixture was left overnight in order to obtain a better aggregation and to remove the internal bubbles.

To get the cured samples can be used in various properties tests, the nanoparticle mixed resin was placed into different curing molds which illustrated in figures given before.

5.2.2 Nano-Particle mixed Resin Material Experiment Details

The cure depth is a key parameter in photo-polymerization, which is complicate and affected by a serious of factors. Previous researchers studied that photo-initiator concentration, additive suspensions, penetration depth of light are all related with cure depth. However, resins used in our research are purchase directly from manufacturer and the chemical compound is already optimized and fixed, so the photo-initiator and chemical compound effect will be ignored. Because of the Al2O3 nanoparticle involved in our resin mixture and surely the ultraviolet light penetration will change by different concentration. At first, a comparison in cure depth of diamond abrasive mixtures needs to be finished to find out an optimal curing condition, so that the further experiments can be done at the same standard. The measurement method used in cure depth is based on ISO

4049. A group of different curing time from 25s, 50s, 75s, and 100s to 125s was chosen 64

to see the trend. Finished samples were cleaned and then measured. Results are shown in figure below.

Figure 5-45 Cure Depths vs. Cure Times

It is clearly to see that the cure depth increased rapidly in the beginning and the increasing drop down after 50 seconds, and then the depth keeps in a stable value around

2mm and 1.5mm. In our experiment, the top layer cured rapidly within the first 50 seconds and that is because the surface resin absorbed the ultraviolet light energy from the UV bulb immediately, and then the photo-polymerization process occurred, the top layer resin transferred from liquid to solid state. After the chemical reaction, the cured resin generated a solid film which covered on the uncured resin. For light penetration in turbid media such as composite resin, the intensity of light is decreased down due to absorption and scattering [28]. In our experiment, the cured resin film, the diamond abrasive powder, the nanoparticle filler and the resin itself are all factors that affect the light penetration. Therefore, with the cure time growing in experiment, less and less

ultraviolet light penetrated through the resin mixture, and much less energy can be absorbed by the uncured resin to process the photo-polymerization. The depth difference between mixture with nanoparticle and mixture without nanoparticle is due to the UV energy absorption and refraction by the Al2O3 nanoparticles.

From former researchers’ studies, the thickness of cured resin is not directly proportional to exposure time due the decrease of UV energy penetration in the composites. If 70% of the UV energy is absorbed in the top 0.01" of coating, then 70% of the remainder or 7% of the initial amount will be absorbed in the second. Thus, a two- fold increase in the cure depth requires a ten-fold increase in UV intensity [29]. If we just increase the cure time in curing process, the top layer would absorb too much energy and the surface temperature would be much higher than the thermal limit.

Micromachining is a precision process and the thickness loss of the machining tools such as lapping plate and polishing pad could be very small even to Nano scale, therefore, a millimeter scale thickness is acceptable in manufacturing of those machining tools. In addition, the curing process can be done on both sides of the work piece to increase the cure depth if needed.

Nano scale fillers increase the polymer mixture in both elasticity and strength, and the stress curve comparison shows the improvement in Fig.5.8. The tensile strain increased almost two times compares to the non-filler resin, and it means that the material elongation in tensile test increased significantly. When the abrasive particles added into the UV resin, the polymer structure defect taken place due to the size of these micro- particles, therefore the pure resin have a better mechanical properties than the abrasives mixture. In our experiment, Al2O3 nanoparticles added as filler into the mixture and a

much smaller size particle could not affect too much on polymer structure. Moreover, the

Nano-particles have high specific surface energy while monomer molecules have dipole moments and nanoparticles direct monomers during the polymerization process. A polymer crystal is formed because of the ordered structure, by which the material strength and elasticity improves. . On the other hand, the uniformly distribution nanoparticles in polymer acts as a large amount of crystallization centers and increases the resin strength

[29].

Figure 5-46 Stresses Curve of Nano-particle Mixed Resin

Fig.5.9 gives an illustration of the resin bonded lapping process mechanism. The particle 1 and 4 is the freely move particles which remove the material by scraping and rolling, and this is basically the conventional lapping process which have a lower material removal rate and caused work piece surface damage. The embedded particle 2 acts as small cutting tools within the process and obviously, it is more efficient compare to the former one. 67

Figure 5-47 Fixed Abrasive Lapping Process

With the material strength improvement, the amount of abrasive particles that rolling within the gap between machining tool and work piece reduced. The abrasive particle is hard to remove from the polymer, and that means they can have a longer lifetime work as an embedded particle. By this reason, the machining efficiency of the abrasive tools can be improved. Even if the particles drop off from the polymer, nanoparticle mixture polymer receives less damage due to the stronger bending strength.

Meanwhile, some of the free abrasive particles can be embedded in the resin by lapping load because of the soft and elastic material features

Figure 5.10 and Fig.5.11 is the microscopy of these two mixtures after lapping in same condition. Much more surface damage can be seen in the non-nanoparticle mixture and that is because the weak material properties. When the embedded particles drop off from the polymer, some of the materials around the particle were removed at the same time, and then the rough surface would cause more damage on the work piece.

Table 5.2 Hardness and Abrasion test results

Material With Nano Filler Without Nano Filler Value

12.8 10.6 Random Point 13.4 10.9 Hardness (kgf/mm2) 13.1 10.0

Average Hardness 13.1 10.5 (kgf/mm2)

Abrasion (mm) 0.070 0.078

For the nanoparticle mixture, the particle removes and very little material around drop off due to the strong resin polymerization caused by those nanoparticles, and then only a small pit left on the surface. These small pits provide rooms for grain, chips, and fluid during the process and it affect the hydrodynamic performance between the woke- piece and lapping plate in good way.

Figure 5-48 Microscopy of the Nano-particle Mixture

Figure 5-49 Microscopy of the Mixture without Nano-particle

Figure 5-50 Roughness by MA Resin Plate with Nano Al2O3 Particle

Figure 5-51 Roughness by MA Resin Plate without Nano Al2O3 Particle

5.2.3 Lapping Performance Discussion

Lapping experiments are finished on ceramic rings as the same, and the results shown in Fig.5.12 and Fig.5.13. When take account to the lapping pressure, as it supposed to be, the roughness does not differ a lot as the lapping time increase after a certain period. This results indicate that the abrasive type affect the lapping performance much more than the resins which have similar characteristics.

The improvement of the nanoparticle mixed resin show the benefits in mechanical properties and manufacturing of the plate. The more important aspect is that when we look at the detail of the figures, the roughness by MA resin plate with nano-particle stays in lowest since 30 minutes. On the other hand, the roughness by MA resin plate without nano-particle decrease gently and keep stable after 60 minutes or even later. The comparison can be clearly seen in Fig.5.14.

Figure 5-52 Roughness Comparisons between Plates with and without Nano Al2O3

Chapter 6

Summary and Conclusion

This research focuses on the development of a lapping plate with ultraviolet- curable resin and diamond abrasive. Based on different types of material properties test, the possibility of UV-curable resin’s using as a bonding material in lapping plate manufacturing is proved. The fabrication of the lapping plate is then carried out in order to see the lapping performance. According from the results obtained in final experiment, the following conclusion can be drawn.

i. It is practicable that the fixed abrasive lapping pad is made by new

bonding materials ultraviolet curing resin.

ii. In comparison of conventional lapping plate and lapping process in rigid

brittle material machining, ultraviolet-curable resin bonded plate improve lapping

performance and machining efficiency significantly.

iii. UV-curable resin curing process can be done within seconds to minutes

and this characteristic improves the manufacturing efficiency in industry. In addition,

UV-cured resin plate manufacturing save much energy cost than thermo-curable

resin and has less environmental issues.

iv. In lapping process in rigid brittle material machining such as silicon wafer

and ceramic ring in this project, the surface finish quality affected by lapping

pressure and lapping time.

v. The surface treatment of the diamond abrasive particles increases the

bonding strength and then benefits the lapping performance both in material removal

rate and in surface roughness of work piece.

vi. The nanometer size Al2O3 additives help polymerization of resin and

improve the material properties of the abrasive-resin mixture. Therefore, the plate

made by nano-particles has a better result in experiment.

While the research on the ultraviolet-cured resin plate lapping is at primary experimental stage, there still exist some insufficiencies in lapping process, such as scratches of work-piece surface, self-sharpening properties. With further work a deeper understanding of the lapping process could be gained, and after long term tests the stability lapping process for using UV-curable resin lapping plate can be realized.

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