More important than hardness is mechanical wear resistance. This is also a difficult property to pin down because it is so dependent on load, material, hardness, speed, and so on. Wilks and Wilks [1972] showed that when abrading diamond with diamond abrasive, wear resistance increases with hardness but the differences between orientations are far more extreme. For example, on the cube plane, the wear resistance between the [100] and the [110] directions varies by a factor of 7.5, giving good correlation with wear data of needle diamonds reported in Figure 5.14. In other planes, the differences were as great as a factor 40, sometimes with only relatively small changes in angle. Not surprisingly, diamond gem lappers often speak of diamond having “grain”-like wood. Factors regarding the wear resistance of diamond on other materials in a machining process such as grinding, however, must include all possible attritious wear processes including thermal and chemical.
5.6.14 Strength of Diamond
Diamond is very hard and brittle. It can be readily cleaved along its four (111) planes. Its measured strength varies widely due in part to the nature of the tests, but also because it is heavily dependent on the level of defects, inclusions, and impurities present. Not surprisingly, small diamonds (with smaller defects) give higher values for strength than larger diamonds. The compressive strength of top-quality synthetic diamond (100#) grit has been measured at 1,000 kg. mm-2.
5.6.15 Chemical Properties of Diamond
The diamond lattice is surprisingly pure, as the only other elements known to be incorporated are nitrogen and boron. Nitrogen is present in synthetic diamonds at up to 500 parts per million in
single substitutional sites and gives the stones their characteristic yellow/green color. Over an extended time at high temperature and pressure, the nitrogen migrates and forms aggregates, and the diamond becomes the colorless stone found in nature. Synthetic diamond contains up to 10% included metal solvents, while natural diamond usually contains inclusions of the minerals in which it was grown (e. g., olivine, garnet, and spinels).