The influence of crystal orientation on the surface finish in lapping of diamond

Diamond exhibits a highly anisotropic tribological behavior. The dependency of wear rate on crystal orientation of diamond has long been noticed. In this paper, the relation of surface morphology and crystal orientation under certain lapping condition is examined. Angle of lapping direction from [001] on a (110) surface changes in our experiments, while the lapping velocity and pressure are fixed. It is found that a fine surface finish can still be obtained at the 60° angle from [001] crystal direction. But, with respect to the near 90° a bad surface with nano-fracture appears. This result comes down to two aspects. The first is the anisotropic amorphization that is the depth variation of phase change of diamond to sp² and sp carbon atoms on a crystal surface. The second is that there is a potential maximum scratch depth when abrasive grains on the diamond wheel slide on the diamond sample surface. On (110) surface, as the lapping angle from the [001] approaches [110], the phase change of diamond gets more difficult, which causes the amorphization depth becomes thinner. For a smaller amorphization depth, the bulk bodies of the diamond sample and abrasive grains can impact each other with a high possibility. Under this condition the surface fracture is likely to happen. The explanation provided in this paper gives a further understanding on the relation of material removal mechanism of diamond and lapping direction.