Diamond polishing: the dependency of friction and wear on load and crystalorientation

Results are presented of friction and wear measurements during diamond poli shing using new instrumentation designed and built in our laboratory. The w ear rates are found to be highly anisotropic, depending both on the crystal lographic plane and on the direction along which sliding occurs. Friction m easurements performed during polishing on a {100} plane in the [100] direct ion (the direction of the highest wear rate on the plane) and the [110] dir ection (the direction of the lowest wear rate) exhibit similar trends to th ose occurring in friction at low speed on the {100} face, with the [100] di rection having a higher friction coefficient than the [110] direction. It w as further found that the variation of friction with pressure in the [100] and the [110] directions are different, pointing to the fact that two wear mechanisms operate in the two principal directions. The results are consist ent with, and provide macroscopic evidence in support of, a model of mechan ically induced transformation of diamond to sp(2) hybridized carbon occurri ng on the diamond surface during polishing along the direction in which the wear rate is high. In contrast to this, the wear process proposed for the [110] direction is one of micro-fracture, in agreement with earlier studies of the polished surface and the wear debris. By explaining the wear mechan ism in terms of two different types of processes operating in the two direc tions, we are able to understand the high anisotropy observed.