The dependency of diamond lapping surface morphology on crystal orientation

In mechanical lapping, diamond exhibits a highly anisotropic tribological behavior. The dependency of wear rate on crystal orientation has long been noticed by diamond polishers. However, in the diamond manufacturing, the surface quality is another side in need of attention. In this paper, the relationship between surface morphology and crystallographic orientation under a certain lapping condition was examined. It is found that a fine surface can still be obtained at the directions that deviate a lot from the “soft” directions (near <001> crystal direction for {110} surface). But, with respect to the “hard” directions (near <110> crystal direction for {110} surface), a bad surface finish with a fracture appears. This result comes down to two aspects. First, on {110} surface, as the lapping direction from the <001> (soft direction) approaches <110> (hard direction) the phase change of diamond carbon atoms gets more difficult, which causes the depth of the amorphization layer becomes thinner. Second, the energy required by diamond amorphization is lower than that by surface fracture around the soft direction, while near the hard direction, the contrary is the case. Our work provides a further understanding of the diamond tribological properties.