4. Tooling for Diamond Turn Machining
4.3 Single Crystal Diamond Tools
The one single crystal material that has largely been successful as a cutting tool in the DTM process is diamond. Both artificially synthesised and naturally available diamonds have been used for fabricating cutting tools with some users indicating preference for natural diamonds. Commercial single crystal diamond (SCD) tools are available for users to buy and companies using the
Carbon atoms
Angstroms Micrometres
SCD grains
(a) (b)
FIGURE 4.2
Schematic explaining the difference between PCD and SCD. (a) Single crystal tool and (b) poly crystalline tool.
DTM process rely on such suppliers (e.g. UK-based Contour Diamond, Japan- based ALMT Diamond Corporation, etc.) for their regular use. Relapping of used/worn SCD tools are also carried out by these companies with SCD tool inserts going through multiple relaps before being discarded.
Diamonds used for cutting tools in DTM are obtained from naturally avail- able sources or synthetically made. Synthetic diamonds are made by a CVD process and are commercially available with flat surfaces of known crystal- lographic planes. Cutting tool manufacturers can then use these surfaces as datum to create desired geometric shapes for the tool. Natural diamonds used in cutting tools are usually aesthetic rejects from the jewelry industry and are usually available in a unfaceted raw form. However, certain defects present in natural diamonds also prevent successful performance as cut- ting tools. Brownish tinges in the diamond indicate the presence of residual stresses that can cause tool edges to crack and chip off during machining.
Presence of carbon spots is also an indication of hole-formation tendency in the crystals. Natural crystal growth can be irregular in many places, lead- ing to inherent property variation within the crystal. Tool manufacturers using natural diamond face such multiple challenges in picking the best dia- mond. Next challenge is to detect these prescribed crystallographic planes and directions inside the raw crystal; the diamond crystal then has to be fixtured (held) in such orientations during the processing steps needed to make the tool.
Single crystals have anisotropic properties owing to the discrete spatial arrangement of atoms and diamond is no exception. It is important to under- stand the effect of various crystallographic orientations and directions on desired properties so that, the best possible plane and direction can be cho- sen for the rake face, flank face and cutting edge of the tool.
The structure of diamond is the well-known tetrahedral arrangement of sp3 hybridised carbon atoms with a central carbon forming bonds with four adjacent atoms in the tetrahedron. Four such tetrahedra are arranged in four (of the eight) alternating sub-cubic cells inside an FCC cell arrangement of C-atoms (Figure 4.3). This set of four tetrahedra form the unit cell of diamond.
In this unit cell structure, one can examine the predominant planes and directions within each plane. Not much attempt has been made to explore the most optimum crystallographic plane for cutting tool applications. However, some main planes in diamond have been extensively explored for properties such as abrasive wear, friction, etc. The predominant planes often considered are the {111}, {110} and the {100} family of planes (Figure 4.4).
The questions that come up then are the following:
• Which of these crystallographic planes should be the rake face of the cutting tool?
• Which of these crystallographic planes should be the clearance faces?
Clearance often forms in multiple planes.
51 Tooling for Diamond Turn Machining
• In which direction on the rake face plane should the cutting edge be fabricated? Cutting edge, often being nonlinear, is normally along, i.e. tangential to, multiple directions.
Two properties: abrasive wear and friction coefficient and their crystal- lographic dependence answer these questions. The ease with which a dia- mond crystal undergoes abrasive wear is the key to its successful shaping into a cutting tool. The only way to shape the diamond, being the hardest natural substance known, is using the diamond itself. Abrasive diamond
C1 C2 C3 C4
C8 C7
C6 C5
+ Take an FCC arrangement of C
atoms; unit cell divided into 8 equal
cubes C1 to C8
Add 4 more C-atoms
C1
C7 C3
C5
C1 C2 C3
C4
C8 C7 C6
C5 One C-atom placed in sub-cube C1
One C-atom placed in sub-cube C3
One C-atom placed in sub-cube C5
One C-atom placed in sub-cube C7
Unit cell of diamond FIGURE 4.3
Schematic showing how carbon atoms are arranged in a diamond. Imagine an FCC cell of C atoms. Divide this into eight equal cubes. At the center of four cubes (C1, C3, C5 and C7 – numbered as shown), place a carbon atom and tetragonally bond it to the three nearest face centered atoms and one nearest edge atom.
C1 C2 C3 C4
C8 C7 C6 C5
C1 C2 C3 C4
C8 C7 C6 C5
C1 C2 C3 C4
C8 C1 C6 C5
Family of (111) planes Family of (110) planes Family of (100) planes FIGURE 4.4
Primary planes considered in diamond cubic structure.
powders are used to shape diamond crystals by abrasive wear (tool fabrica- tion is described in another section in this chapter). Directions that are too hard to wear out, while ideal as cutting tool surfaces require substantial and hence uneconomic processing times. For example, the family of planes {111}
is known to be the toughest plane to abrade. In this plane, the direction <110>
is the toughest to process. The softest plane is the {100} family of planes; in this plane the <100> direction is the easiest to abrade. Experimental data for these observations are readily available [32]. A summary of the various soft- est directions to abrade in various planes is shown in Figure 4.5. The key to successful fabrication of a well-performing cutting tool is to determine and hold (fixture) the diamond crystal in the desired plane and direction.
The other property of consideration is the friction coefficient. The cutting edge should be carved out of the crystal in such a way that the chip flow direction is along the one with the lowest coefficient of friction. This is a rule that cannot be applied strictly, since cutting edges are often curved and tool-normal machining process means that chip flow can occur along many directions on the rake face during DTM processing. Again, experimental data (of diamond on diamond) is available for some guidance [33]. It is not clear whether such data for rubbing of various engineering materials on dia- mond, under machining-related tribological conditions, is readily available.
Besides the nose radius geometry, another important consideration is the cutting edge radius. This portion of the cutting tool is the weakest and is subject to micro-fracture and cleavage leading to wear and edge radius enlargement. Investigations remain to be carried out to study edge geome- tries and on the best way (e.g. crystallographic planes, directions, geometries
(100)
(111)
(110)
FIGURE 4.5
Schematic summarising the various soft crystallographic directions (shown with black arrows) to abrade diamond.
53 Tooling for Diamond Turn Machining
such as chamfer, land, etc.) to shore up the edge strength against chipping.
Fabrication challenges need to be addressed to incorporate such fine edge control in diamond crystals.
The other consideration in the diamond crystal is the surface finish that is imparted on the various tool surfaces. The crystallographic dependence on resistance to abrasive wear is also responsible for varied roughness patterns on the diamond surface. Ideally, the diamond tool rake face should be very smooth (sub-nanometric roughness) and be isotropic – this way any chip flow direction can be supported. However, attaining this requires several steps of polishing processes.
Sub-surface damage imparted to the crystal by the abrasive fabrica- tion process is also an important consideration in the performance of the cutting tool in the DTM process. Sub-surface damage can be detected by etching the diamond with suitable chemicals with thermal assist in an atmosphere rich in oxygen. Each abrasive process leaves behind its signa- ture in the form of surface lay patterns and also sub-surface damage and stresses. Polishing should be undertaken in various steps so that the dam- age induced in each step is removed by the subsequent processing step.