EFFECT OF ROUGHNESS ON THE FRICTION OF DIAMOND ON CVD DIAMOND COATINGS

Reciprocating sliding tests in air have been performed on polycrystalline diamond coatings with a range of surface roughnesses. Both the coatings and the single crystal diamonds used as sliders were abrasively worn during the tests. This wear reduced the roughnesses of the coatings, and was most severe at the beginning of the tests. The friction coefficient decreased as the coating surface was smoothed. Friction coefficients depended on both the roughness of the coating and the stylus: values ranged from 0.5 for rough styluses on rough coatings (R(a) > 200 nm), to 0.03 for smooth styluses on smooth coatings (R(a) almost-equal-to 1 nm), the latter being comparable to the lowest value obtained with polished single crystal diamond. The higher initial friction coefficients could be eliminated by mechanical or chemical polishing. Roughness parameters relevant to loaded elastic solids are discussed, and related to a model for the dependence of friction on the angle of interlocking asperities. Wear behaviour and estimates of wear rates indicate that the wear resistance of the coatings is comparable to that of natural, single-crystal diamond, despite the polycrystalline nature of the coatings.