The reactivity of diamond with transition metals such as nickel and iron is a major limitation to the use of diamond as an abrasive for machining and grinding these materials. Thornton and Wilks [1978, 1979] showed that certainly in single-point turning of mild steel with diamond, chemical wear was excessive and exceeded abrasive mechanical wear by a factor of 104. Hitchiner and Wilks [1987] showed that difference when turning nickel was >105. Turning pearlitic cast iron, however, the wear rate was only 102 greater. Furthermore, the wear on pearlitic cast iron was actually 20 times less than that measured using CBN tools. Much less effect was seen on ferritic cast iron, which unlike the former material contained little free carbon; in this case, diamond wear increased by a factor of 10 when turning workpieces of comparable hardness.
5.6.18 Grinding Steels and Cast Irons with Diamond
It is generally considered, as the before-mentioned results imply, that chemical-thermal degradation of the diamond prevents it being used as an abrasive for steels and nickel-based alloys, but that under certain circumstances free graphite in some cast irons can reduce the reaction between diamond and iron to an acceptable level. For example, in honing of automotive cast iron cylinder bores, which is performed at very similar speeds (2 m/s) and cut rates to that used in the turning experiments mentioned above, diamond is still the abrasive of choice outperforming CBN by a factor of 10. However, at the higher speeds (80 m/s typical) and temperatures of cylindrical grinding of cast iron camshafts, the reverse is the case.
5.6.19 Thermal Properties
Diamond has the highest thermal conductivity of any material with a value of 600 to 2,000 W/mK at room temperature, falling to 70 W/mK at 700°C. These values are 40 times greater than the thermal conductivity of alumina. Much is written in the literature of the high thermal conductivity of both diamond and CBN, and the resulting benefits of lower grinding temperatures and reduced thermal stresses. Despite an extremely high thermal conductivity, if the heat capacity of the material is low it will simply get hot quickly! Thermal models for moving heat sources, as shown by Jaeger [1942], employ a composite transient thermal property. The transient thermal property is к. p.c, where k
is the thermal conductivity, p is the density, and c is the thermal heat capacity.
The value of в for diamond is 6 x 104 W/m K compared to 0.3 to 1.5 x 104 W/m K for most ceramics, including alumina and SiC, and for steels. Copper has a value of 3.7 x 104 W/m K due in part to a much higher heat capacity than that of diamond. This may explain its benefit as a cladding material and wheel filler material.
Steady-state conditions are quickly established during the grain contact time in grinding. This is because the heat source does not move relative to the grain. The situation is similar to rubbing a finger across a carpet. It is the carpet that sees the moving heat source and stays cool, rather than the finger that sees a constant heat source and gets hot! In grinding, the abrasive grain is like the finger and the workpiece is like the carpet. In this case, it is the thermal conductivity of the grain that governs the heat conducted by the grain rather than the transient thermal property [Rowe et al. 1996]. For nonsteady conduction, a time-constant correction is given by Rowe and Black [Marinescu et al. 2004, Chapter 6]. The application of thermal properties to calculation of temperatures is discussed in more detail in Chapter 17 on external cylindrical grinding.
The coefficient of linear thermal expansion of diamond is 1.5 x 10-6/K at 100°C increasing to
4.8 x 10-6/K at 900°C. The values are significant for bonded wheel manufacturers who must try to match thermal expansion characteristics of bond and grit throughout the firing cycle.
For further details on the properties of diamond, see Field [1979, 1983].