Diamond as a Precision Cutting Tool

Abstract

Ultra precision diamond turning is one of the advanced techniques to generate highly finished optical grade surfaces. The machining at nano scale requires extremely small material to be removed which poses a severe challenge on the cutting tool. At nanoscale, the material strength becomes multi fold and it becomes extremely difficult to remove the material. A machining rule is that the tool must be harder than the workpiece. Diamond is the hardest known natural material. Thus at nano scale, single crystal diamond is commonly used as a cutting tool to remove the unwanted material from the work-piece. There are different orientations of the diamond cutting tool which are utilized depending on the work-piece being machined. Although diamond is the hardest, still there are various challenges in machining different kinds of materials from metals to semiconductors. Diamond machined work-piece surfaces find applications in a range of sectors from energy sector to biomedical and defense to optics industries. The present chapter covers the various aspects of single crystal diamond tool in ultra- precision machining. Tool-workpiece material interaction, material removal mechanisms and tool wear are some of the issues which have been covered in detail.

1. Introduction

Ultraprecision machining techniques have received much attention owing to the increasing demands for precision components with tight dimensional tolerance, high shape accuracies and excellent surface quality. Single point diamond turning (SPDT) or diamond turn machining is an ultraprecision machining technique used to generate the components with exceptional accuracy and surface finish [1]. The SPDT was first explored in 1960s with the need to advance in various fields such as defense, aerospace, computer, electronics and medical. In 1983, Taniguchi produced a plot which showed the evolution of machining accuracy with respect to every passing decade [2] and predicted the accuracy for the year 2000 as shown in Figure 1. After extrapolating, it indicated that the ultraprecision processes at micro/nanoscale would be able to achieve the machining accuracy of 0.01 μm. To achieve this extreme level of accuracy, the processing scale needs to be extremely small of the order of few nanometers, which in turn depends on the accuracies of the machine tool and cutting tool being used. Among the various mechanical micro/nano-scale machining processes, diamond turning technology has brought a revolution in the world of precision machining by achieving submicron level accuracies in the size and shape of the machined component. Other than the integrated technologies like granite bed, air bearing spindle, hydrostatic slide bearings, and optical linear scale feedback system in ultraprecision diamond turning technology, single crystal diamond tool is a key factor to obtain a high quality machined surface. Owing to its exceptional properties suitable for cutting tool, the single crystal diamond tool with edge sharpness of the order of nanometers is extensively used in ultraprecision machining.

Figure 1.

Taniguchi prediction of machining accuracies [3].

Based on the material removal mechanisms, various manufacturing processes can be categorized into mechanical, physical and chemical processes (see Figure 2). The physical and chemical machining processes are limited to certain materials, mechanical machining processes can be applied to a variety of materials and applications. The mechanical based micro/nano machining processes are classified into mechanical tool based cutting and abrasive based machining. Mechanical tool based machining is deterministic in nature as the tool path can be controlled. Whereas abrasive based machining are force controlled and are random in nature. Size and shape control in abrasive machining are difficult to achieve through abrasive based machining processes. The cycle time is much higher in case of abrasive machining. Therefore, precision of the highest level in terms of size and shape control and surface finish is not achieved through abrasive based material removal process [4]. In addition, the cycle time is relatively higher [5].

Figure 2.

Classification of various micro/nano scale manufacturing processes. {MRAFF: Magneto-rheological abrasive flow finishing; MFP: Magnetic float polishing; EEM: Elastic emission machining; EBMM: Electron beam micromachining; LBMM: Laser beam micromachining; MEDM: Micro electric discharge machining; IBMM: Ion beam micromachining; PBM: Plasma beam micromachining; PCMM: Photo chemical micro-machining; ECMM: Electro chemical micro-machining; RIE: Reactive ion etching}.

Currently, the diamond turning is commonly used for various applications, as some examples are listed below:

• cylinders for video tape recorders

• substrates for magnetic discs in computers

• convex mirrors for high output carbon dioxide laser resonators

• infra-red lenses of germanium for thermal imaging systems

• Contact lenses for human vision

• scanners for laser printers and drums for photo copiers

• elliptical mirrors for YAG (yttrium aluminum garnet) laser beam collectors

• X-ray and ultraviolet optics

• molds for Compact Disc lenses

• AIRS for missile guidance system

• Aircraft windscreens

• Computer disk; photocopying application

Single crystal diamond tool is used in ultraprecision diamond turning of nonferrous materials because of its ability to maintain an extremely sharp cutting edge owing to its wear resistance property. Single crystal diamond tool has a round nose with rake angle ranging from 0^o to highly negative and suitable clearance angle (see Figure 3(a)) enough to avoid the contact between clearance face and the machined surface. The tool consists of sharp edge with controlled waviness (< 1 μm) along whole edge. The cutting edge radius is in the order of tens of nanometers. The performance of single crystal diamond tools depends on the quality of the diamond, its crystal orientation, edge radius/sharpness (see Figure 3(b)), and edge irregularities (see Figure 3(c)). Among these, measurement of tool edge radius is a challenge due to the fact that the tool edge is extremely sharp.

Figure 3.

Cutting tool edge geometry.

Asai et al. [6] put forward a measurement technique for diamond tool edge sharpness. They advanced the conventional SEM by employing two secondary electron detectors. The signals from these detectors are processed to produce the fine cutting edge. This method of measurement is efficient for diamond tool with edge radius of 45 nm or less. Yuan et al. [7] performed experimental investigations on different orientations of single crystal diamond tool to find out the optimum crystal plane for ultraprecision machining. They conducted the friction tests on rake and flank face of the tool, noticed the friction coefficients and observed the effect on shear deformation, tool wear and the machined surface quality. Based on the friction coefficients value on different directions of crystal orientation of (100), (110) and (111), it was found that the (100) plane shows highest anisotropy. The literature has contradictions regarding the optimum crystal planes for the rake and flank surface of the diamond tool.

Ultraprecision machining trials including the shear deformation, tool wear and surface quality has indicated that the (100) plane is most suitable for rake as well as flank surfaces. However, Uddin et al. [8] recommended (100) as rake plane and {110} as flank plane for the diamond tool in machining of Si. With theoretical calculations, Zong et al. [9] proposed that it is possible to achieve 1 nm sharp cutting edge radius for the (110){100} crystal orientation.

2. Structure of diamond

In ultra-precision and nanometric cutting operations, the most preferred tool material is single-crystal diamond due to its extraordinary properties, including highest hardness, very strong resistance to wear, perfect chemical stability and satisfactory service life, as well as the least sharpness, when finished. Due to its extreme strength and hardness, recently diamond has become an attractive choice in engineering processes and in a variety of other technological applications. Considering the morphology and structure of diamond, its mechanical and physical properties vary not only with the different crystallographic planes but also with different crystallographic directions on a plane.

Diamond is a crystal allotropic form of Carbon and has diamond cubic crystal structure with a lattice parameter of 0.3567 nm (see Figure 4). Each C atom has four neighboring C atoms in a tetrahedron structure with bond angle between the atoms of 109°28′. The C atoms are covalently bonded together with sharing of one of its outer four electrons with each of four other C atoms resulting in sp³ hybridization. This sharing of electrons results in a strong binding among each pair of C atoms in all three dimensions.

Figure 4.

Crystal structure of single crystal diamond.

Diamond has some outstanding properties which makes it an obvious choice for the cutting tool material.

1. It is the hardest known natural material and hardest among cutting tool materials (see Figure 5).

2. It possesses extreme strength and stiffness.

3. It has highest atom-number density.

4. At room temperature, it has highest thermal conductivity of any solid.

Figure 5.

Hardness of various tooling materials.

Table 1 shows the mechanical properties of diamond. Diamond is categorized into four categories:

1. Natural Diamond

2. Synthetic diamond

3. CVD diamond

4. Diamond like Carbon (DLC)

Property	Values
Density, (kg/m^3)	3520
Elastic Modulus (GPa)	910–1250
Compressive Strength (GPa)	8.68–16.53
Knoop hardness, (Kg/mm^2)	5700–10,400 (Overall); 7500–10,400 (111 plane); 6900–9600 (100 plane)
Poisson’s ratio	0.07–0.16
Diamond-on-diamond Friction coefficient	0.05–0.15 (in air); ~1 (in vacuum)

Table 1.

Mechanical properties of diamond.

Diamond naturally occurs produced in nature at high pressure and temperature in volcanic shafts. With high pressure synthesis, it is able to produce diamond possessing the strength similar to natural diamond. Generally, the natural and synthetic diamond are used for cutting purposes. Diamonds usually exist with other allotropes of carbon and they can be characterized by knowing the structure, atomic vibration and electronic states. The diamond characteristics are usually determined using X-ray diffraction, electron microscopy and Raman spectroscopy. For diamond to be qualified, it must have the following characteristics:

1. A crystalline morphology revealed by electron microscopy

2. Crystalline structure with a single phase identified by X-ray diffraction

3. A sharp peak at 1332 cm⁻¹ detected by Raman spectrum

Diamond occurs predominantly in three different forms in cubic (100), octahedron (111) and dodecahedron (110) and their combinations. These three forms are based on the three planes ((100), (110) and (111)) from a simple cubic crystal structure. Figure 6 shows the schematic of the (100) cubic, the (110) dodecahedral and the (111) octahedral. The octahedral form of diamond crystal is the most common form.

Figure 6.

Schematic diagrams showing the location of {111}, {100} and {110} planes on an octahedron diamond.

Diamond breaks along {111} crystal planes also known as cleavage planes. Bonding is less strong perpendicular to these planes as compared to the other directions due to lower total number of bonds and hence the diamond fractures favorably along these planes. Diamonds are also categorized into four types based on the presence of nitrogen as an impurity in their crystals. These are: Ia, Ib, IIa, IIb. Majority of natural diamonds (~99%) come in type Ia. Type Ia consists of nitrogen in the form of aggregates in the crystal. All synthetic diamonds are of type Ib with an even distribution of nitrogen atoms in the crystal. Types IIa and IIb are very rare in nature but can be synthesized for industrial purposes. In all natural diamonds, there exist many impurity elements such as nitrogen, hydrogen, boron and oxygen. Table 2 shows different classifications of diamond.

Element	Type I		Type II
	a	b	a	b
Nitrogen (ppm)	200–2400	40	8–40	5–40
Boron (ppm)	Nil	Nil	Nil	0.5
Occurrence of atoms in:	clusters	Isolation atoms	Very little	Considerable boron

Table 2.

Diamond classification based on impurity level [10].

3. Fabrication of diamond tools

Predominantly there are two methods used for fabrication of single crystal diamond tools. The diamond tool fabrication is performed in such a way that the tool edge radius is maintained in the order of nanometers and a very smooth rake and flank surface are generated. These methods include mechanical lapping, thermo-chemical polishing, polishing with ion beam, damage-free tribochemical polishing, chemical-assisted mechanical polishing and planarization, chemical polishing with plasma, oxidative etching, and laser ablation.

Mechanical lapping is the most conventional and popular method to mechanically polish the single crystal diamond. Various mechanisms have been proposed in different studies such as: 1) Micro-cleavage or fracture along the (111) plane; 2) thermal wear; 3) Burnout or carbonization will take place at elevated temperature; 4) fracture or micro-chip in hard direction and plastic deformed layer in soft direction; 5) diamond phase transformation; 6) Plastic deformation as a result of brittle-ductile transition. Mechanical lapping is the simplest and most cost effective process method which enables to reach a cutting edge radius of about 70–80 nm [11]. It is very challenging task to manufacture a diamond tool with cutting edge radius smaller than 50 nm. In order to produce very fine cutting edge profile for diamond tool, it is very important to understand the material removal mechanism in lapping of the diamond cutting tool. It will help in optimizing and help controlling the cutting edge sharpness.

Zong et al. [12] put forward the brittle–ductile transition theory to indicate the material removal nature in lapping of diamond surface layer. It explains that plastic deformation is responsible for the dominant removal mode of the lapped surface layer of the single crystal diamond in both the soft and hard directions on the diamond crystal planes. Plastic deformation in diamond crystal takes place when the embedding depth of diamond grit into the lapped surface layer is less than the corresponding critical depth of the cut, which leads to brittle–ductile transition.

In mechanical lapping, control of contact accuracy is extremely important between the high speed rotating scaife and lapped diamond tool. Accurately controlling the contact helps in better cutting edge sharpness. Lapping set-up is shown by schematic in Figure 7. The set up consists of cast iron scaife mounted on air bearing. Diamond tool is placed against the rotating cast iron scaife. Speed of cast iron scaife can be varied from 300 to 3000 rpm. Diamond grits of approximate size of 0.5 μm are used in the scaife for lapping. The relative velocity between scaife and the tool face is approx. 30 m/s. The contact accuracy should be ensured otherwise there will be impacts on the cutting edge and subsequent damages the cutting edge sharpness. Since the variations of the external load will change the number of grits in contact. In fact, only the material removal rate will change, and the mean force of single grit has almost no variations. So the changes of external applied load result in minute influence on the maximal groove depth left on lapped surfaces and are ignored in all experiments. Other variants of lapping are thermo-mechanical lapping, thermo-chemical polishing and chemically assisted mechanical lapping etc. which can also be employed to fabricate diamond tool.

Figure 7.

Schematic representation of the lapping set up [11].

In mechanical lapping, the removal rate takes place in the nanometric level that results in a sharp cutting edge radius ranging from 35 nm to 50 nm. Another common variant of the lapping of diamond tools is thermo-mechanical lapping [13]. Thermo-mechanical lapping is a development over mechanical lapping by using steel scaife in place of cast iron scaife. In thermo-mechanical lapping, material removal mechanisms of diamond are diffusion, graphitization and oxidization. Dominant mechanism is governed by the interface temperature which in turn depends on the lapping compression force and sliding velocity. At low temperature, the dominant mechanism is diffusion of C atoms into the iron matrix. With higher temperature, C atoms from the diamond cubic lattice structure forms dangling bonds and leads to graphitization. On further increased temperatures, oxidization of diamond carbon atoms takes place. The formation of carbon monoxide or dioxide consumes more energy. These deteriorated atomic structures from different mechanisms at different range of temperatures are further worn by the abrasion from the rotating scaife. The material removal rate is in atomic level and thus a highly sharp cutting edge radius of 10 nm or less can be achieved. Considering the machining efficiency and production cost, the mechanical lapping and thermo-mechanical lapping are the best choice to fabricate nanoprecision diamond cutting tools.

4. Mechanisms of material removal in ultraprecision machining with diamond tool

Ultraprecision machining is achieved by reducing the scale of machining process which in turn brings the “size effect” into picture. Difference between conventional machining and ultraprecision machining lies in the ratio of uncut chip thickness and edge radius (see Figure 8). Unlike conventional machining process, diamond turning process with a highly sharp single crystal diamond tool exhibits the size effect due to its reduced scale of material removal. Cutting tool edge radius (r) becomes significant with respect to uncut chip thickness during diamond turning process (see Figure 8(b)). Resisting shear strength of the material increases considerably as the chip thickness is reduced.

Figure 8.

Schematic of (a) conventional and (b) ultraprecision machining [3].

At the macroscale in conventional cutting, while the uncut chip thickness is larger than the tool edge radius, the tool is able to remove cluster of grains. The shearing action takes place along the easy slip plane. The easy slip plane is governed by the stress concentration along the grain boundaries, defects and vacancies in the material. When the uncut chip thickness is reduced from few microns to a few nanometers, cutting within a grain takes place. It requires more energy to cut the solid grain with round edge than to remove a number of grains with sharp edge tool (see Figure 9). Therefore, the specific energy is higher in the ultraprecision machining process like diamond turning compared to the conventional cutting. With the reduction in chip thickness up to few nano meters, plastic deformation viz. rubbing and burnishing become more dominant than cutting due to very small effective cutting zone. In diamond turning, the tool edge radius is continuously deteriorated and increased with constant uncut chip thickness, which in turn deteriorates the machining process.

Figure 9.

Size effect in ultraprecision machining.

5. Cutting mechanism and material deformation in diamond turning

The cutting edge radius of the diamond tool in ultraprecision machining affects the cutting mechanisms. The uncut chip thickness (a) is of usually comparable size as the tool edge radius (r) in diamond turning and their ratio governs the cutting mechanism viz. shear plane cutting, plowing and sliding. These mechanisms in turn decides the resulting surface quality. There exists a ‘minimum uncut chip thickness’ below which no material removal takes place due to cutting. The minimum uncut chip thickness depends on the tool geometry which includes edge radius, rake & flank plane angle and the workpiece material properties. Sliding at the flank face due to elastic recovery of the workpiece material and plowing due to edge radius of the tool becomes dominant in ultraprecision machining.

5.1 Ductile machining

Figure 10 shows the chip formation and various regions along the chip profile in ultraprecision diamond turning. In the chip profile, ratio of uncut chip thickness (a) to toll edge radius (r) varies from zero to some maximum value. The cutting mechanisms vary with different regions of a/r ratios. At a/r ratio sufficiently higher than 1 (region III), maximum amount of material removal takes place along the rake face of the tool and thus facilitates pure cutting. As a/r ratio approaches to 1 (region II), the tool edge radius and uncut chip thickness become comparable which makes the effective rake angle of the tool negative. The negative rake angle leads to extrusion like squeezing action. As a/r ratio becomes less than 1 (region I), the tool is unable to remove the material as a chip and only sliding and elastic deformation takes place.

Figure 10.

Cutting mechanism along the chip profile for the workpiece machined with round nose diamond tool.

As the uncut chip thickness is decreased thrust force increases in comparison to the cutting force and mechanisms like plowing, rubbing and burnishing become dominant. This is because as the chip thickness is reduced, the tool edge radius becomes significant in comparison to chip thickness and the effective rake angle thus becomes negative. Higher negative rake angle restricts the chip to flow along rake face and therefore the chip flows sideways of the tool. It can be described as tool plows through the workpiece. Further reduction on the uncut chip thickness causes the tool to rub or burnish the work surface. The mechanism of material removal in diamond turning, thus, primarily depends upon the uncut chip thickness and tool edge radius. Since, the uncut chip thickness along the chip profile varies, the mechanisms tend to vary along it. The tool edge sharpness also degrades during the course of machining, which also affects the cutting mechanisms.

5.2 Ductile regime machining of brittle materials

In conventional macroscale machining process, the brittle materials are subjected to fracture as a result of median and lateral cracks [14] as shown in Figure 11. This brittle failure can be prevented using a very low uncut chip thickness. Machining of brittle materials takes place in ductile mode at very low uncut chip thickness and therefore it is known as ductile regime machining. Ductile regime machining of brittle materials leads to crack-free mirror finished surfaces. In case of brittle materials, there exists a critical uncut chip thickness below which ductile regime machining takes place and above which the brittle machining takes place. Blackley and Scattergood [15] proposed a machining model depending on critical depth of cut and subsurface damage depth which indicated that fracture takes place above the critical uncut chip thickness (d_c) and is subsequently propagated in the workpiece material. The chip thickness varies from zero at the tool centre to a maximum at the top of the uncut portion as shown in Figure 11. If the fracture does not penetrate into the cutting plane, the brittle mode machining will take place. Thus the critical chip thickness is a deciding parameter between the two machining modes.

Figure 11.

Schematic of brittle and ductile regime machining in brittle materials [15].

Smooth, damage free and optical quality surface is generated if the uncut chip thickness along the tool nose is below critical chip thickness. This transition of mechanism from brittle to ductile machining is also known as brittle - ductile transition. According to another ductile regime machining model proposed by Nakasuji et al. [16], the brittle to ductile transition takes place from in the diamond turning process of brittle materials. For the larger uncut chip thickness, the stress zone is such that it contains a number of defects which nucleate to expand the cracks and cause brittle fracture. With the extremely small uncut chip thickness, the defects are too less in the stress zone to form cracks and therefore, it enables the material removal in ductile mode (see Figure 12).

Figure 12.

Model of chip removal with a size effect in terms of defects distribution [16].

Shimada et al. [17] used a different model to elaborate the brittle-ductile transition and suggested that there are two ways of material removal: one is the ductile machining due to plastic deformation in the slip direction on the characteristic slip plane and the other is the brittle mode machining owing to cleavage fracture on the characteristic cleavage plane. When the resolved shear stress τ_slip in the slip direction on the slip plane exceeds a certain critical value τ_c inherent to the workpiece material, plastic deformation occurs in a small stressed field in the cutting region of a specified scale. On the other hand, a cleavage occurs when the resolved tensile stress normal to the cleavage plane σ_cleav exceeds a certain critical value σ_c. The mode of material removal depends on which criteria dominates or precedes τ_slip > τ_c or σ_cleave > σ_c for the stress state under a particular machining condition (see Figure 13).

Figure 13.

Ductile and brittle regime models of chip removal [17].

6. Diamond tool and workpiece material interactions during machining

With the tool wear, the cutting edge radius at every section of the tool edge interacting with the work material increases and it changes the a/r ratio which in turn shifts the range of these regions. Therefore, it is not only the chip thickness which varies along the chip profile but the tool edge radius also continuously changes with increasing tool wear. Figure 14(a-d) shows a schematic which shows the variation in cutting edge sharpness (r) at any uncut chip thickness (a) as the tool wear takes place. Figure 14(b-d) shows the condition of tool edge at three different stages of tool wear. In initial cutting stages when the tool is fresh, it possesses a sharp cutting edge with a very small radius (r₁) for an uncut chip thickness ‘a’ as shown in Figure 14(b). During the intermediate stages of cutting, as the tool wears and the edge becomes blunt, the cutting edge radius increases from ‘r₁’ to ‘r₂’ as shown in Figure 14(c). The increased edge radius causes a decrease in the a/r ratio. This is a stage where uncut chip thickness and edge radius become approximately comparable i.e. a/r ~ 1 at that section. With a further increase in cutting edge radius from ‘r₂’ to ‘r₃’, the a/r ratio decreases to a value less than 1. The material removal becomes difficult and thereby no chips are removed from the workpiece material at the final stages of cutting (Figure 14 (d)).

Figure 14.

Schematic of tool edge condition and cutting mechanism with tool wear [3].

6.1 Wear characteristics of the diamond tool

Tool wear influences the cutting forces, chip formation and the surface roughness. Tool wear in diamond turning of ductile and non-ferrous materials is considerably low whereas it is very high in case of hard and brittle materials. Wear on the diamond tool deteriorate the machined surface quality. In general, flank wear is observed to be the most dominant wear in single crystal diamond tool. There are various effects of the tool wear on the machining parameters which are concluded in Table 3.

No.	Parameters	Effects
1	Surface roughness	Increases
2	Form error	Increases
3	Force	Increases
4	Mechanism	Increases
5	Chip thickness	Decreases
6	Brittle to ductile transition	Increases
7	Surface quality	Decreases
8	Material properties	Decreases
9	Plowing/side flow	Increases

Table 3.

Effect of tool wear on various parameters [18].

Tool wear in machining vary depending upon the tool and workpiece material combination. In ultraprecision diamond turning, the tool being used is diamond, and the workpiece material is the solely responsible for change in mechanisms. Diamond tool wear is different for different class of materials. In machining the class of materials are categorized in easy to machine and difficult to machine materials. The easy to machine materials are Al, Cu, brass, and Al & Cu alloys. For instance, the single crystal diamond tool is very useful in machining nonferrous and easy to cut materials such as aluminum, copper, gold, brass, electroless Nickel and plastics. In these materials, diamond tool does not wear out for a few hundreds of kilometers of cutting distance and still achieves a surface finish in the order of few nanometers. In these materials, diamond tool wear out slowly and abrasive wear take place more predominantly. The wear mechanisms become complicated while machining difficult to cut materials. The wear pattern consists of crater and flank wear. Impurities in alloys causes initial microchipping on the tool edge. The dominant wear mechanism in easy to machine materials is abrasive type. Table 4 shows the wear pattern and wear mechanisms for easy to machine materials in diamond turning.

Materials	Wear patterns	Mechanisms / Causes
Aluminum	Flank wear; Crater wear	Mechanical abrasion
Copper	Flank wear; Crater wear	Mechanical abrasion
Al alloy	Flank wear; Crater wear; Microchiping	Mechanical abrasion Impurities
Al359	Flank wear	Abrasive wear
RSA 905	Flank wear; Crater wear	Abrasion Thermo-chemical erosion
Cu-Cr-Zr	Chipping; Grooving	Mechanical abrasion Thermo-chemical wear

Table 4.

Wear patterns and mechanisms in easy to cut materials [18].

Wear mechanism of a single crystal diamond tool is highly complicated which may involve one or multiple mechanism types. These can be chemical, physical, thermal and mechanical interactions between the diamond tool and the workpiece. Depending on the tool and workpiece material combination, the wear mechanisms can vary. Diamond tool wear can be characterized into mechanical, chemical and physical wear. Mechanical wear comprise of abrasive wear, fatigue, and adhesive wear. Chemical wear includes chemical reaction, graphitization, amorphization, and diffusion. Physical wear is comprised of thermo-chemical wear, tribo-electric wear, anisotropy, and defect.

6.2 Wear mechanism in difficult-to-machine materials with diamond tool

When it comes to cutting difficult to machine materials with diamond tool, ferrous materials are first among several. Diamond turned ferrous materials are generally used as precision dies and molds. The other class of materials are glass, crystal materials like silicon, silicon carbide, germanium, selenide, and zinc sulfide. Rapid tool wear rate limits the diamond turning applications in machining of difficult to machine materials. In machining ferrous metals for high precise and complex surfaces, diamond tools are subject to catastrophic wear, which subsequently downgrades surface quality. The diamond tool wear rate is very high in case of machining ferrous materials and it is 10,000 times higher than that in case of machining brass [19].

In difficult to machine materials, the wear pattern is flank wear assisted with micro/nano grooves. Initially the microchipping is also observed. In Difficult to machine materials dominant wear mechanism is chemical wear based on the interface temperature. The interface temperature decides the type of wear mechanism which is predominantly categorized into diffusion, graphitization and oxidation. Table 5 shows the wear pattern and wear mechanisms for difficult to machine materials in diamond turning.

Materials	Wear pattern	Mechanisms / Cause
Si	Chipping; Microcracking; Micro/nanogrooves	SiC Formation; Abrasion; Thermochemical wear; Graphitization; Diffusion
SiC	Grooves	Abrasion; Graphitization
Titanium	Micro-Chipping; Flank wear	Graphitization; Adhesion
Copper beryllium	Micro-Chipping; Flank wear	Abrasion; Amorphization
Glass	Uniform wear; Micro grooves	Thermochemical wear; Diffusion; Abrasive wear
Steel	Ridges; Grooves	Chemical wear; Diffusion; Abrasion; Graphitization

Table 5.

Wear patterns and mechanisms in difficult to machine materials [18].

7. Conclusions

In this chapter, the application of diamond tool in ultraprecision machining has been covered. The chapter highlights the applicability of single crystal diamond tool in ultraprecision machining. The cutting mechanisms are explained for both ductile and brittle materials. The tool wear mechanisms are elaborated for different class of materials. Following conclusions can be drawn from the chapter:

1. Single crystal diamond tool is the best cutting tool in terms of hardness, strength and wear resistance for ultraprecision machining.

2. In ultraprecision machining with the diamond tool, the material removal mechanism varies from pure cutting to plowing and then sliding depending upon the ratio of uncut chip thickness to cutting edge radius. In brittle materials, there is brittle to ductile transition when the uncut chip thickness is below the critical chip thickness.

3. Tool wear mechanisms are distinct for different class of materials. In case of easy to machine materials, the dominant mechanism is abrasion, whereas in case of difficult to machine materials, the dominant wear mechanism is chemical which could be diffusion, graphitization and oxidation.

Acknowledgments

Author would like to thank his supervisor during Ph.D. ‘Dr. R. Balasubramaniam’ from BARC Mumbai, under whom the knowledge in ultraprecision machining and single crystal diamond tool was gained.

Conflict of interest

The author declares no conflict of interest.