The most dominant group of diamond abrasives by far are those manufactured synthetically. Carbon in graphite form serves as the raw material. Synthesis is carried out at pressures of 70 to 120 kbar and temperatures around 2000 °C in the presence of metallic catalysts. In accordance with an appropriate choice of synthesis conditions, e. g. the respective combinations of pressure, temperature and chemical features, the growth rate of the diamond crystals can be varied and adjusted across a size range of several powers. For example, an abrasive diamond of acceptable size can be gained within a few seconds at the right temperature and pressure. In this case, the growth rate is decreased by synthesis times amounting to mere thousands of a second, such that requisite crystal magnitudes are acquired for specific grinding processes. In the further processing of the grits, one proceeds in the same way as with the cBN grit.
One of the most conspicuous properties of the diamond abrasive is its extreme hardness, matched by no other material (table 3-3). Its Knoop hardness of 7000 to 8000 HK is about twice as large as that of the conventional abrasives corundum and silicon carbide. While in the case of the latter grit types the hardness is largely independent of crystal orientation, synthetic diamond exhibits a marked dependence [HOWE75]. Hardness in the (110) plane has been measured to be 123 % and in the (100) plane 138 % of the hardness in the (111) plane. There is little in the literature pertaining to the toughness of diamond grits. The synthesis process and aftertreatment can influence toughness to a limited extent.
Diamond has, in comparison with corundum or silicon carbide, a very high heat conductivity. The diamond grit thus quickly transfers the heat generated in the machining zone to the bond. This can, under extreme conditions for example, lead in the case of a synthetic resin bond to its destruction at the grit/bond interface and therefore to premature grit break-out. Coatings of the diamond grits with nickel, cobalt or composite metals are therefore used as heat retardants. Beyond this, they can enhance the adhesion between the grit and the bond.
Exposed to small pressures, diamond graphitisation is set off starting at about 900°K assuming there is enough oxygen present [GULB50].