The frictional anisotropy of diamond

Abstract

If a diamond stylus is slid over a flat diamond surface in air the friction may depend significantly on the direction of sliding. The most marked effect is on the {100} faces. Along the 〈100〉 direction the friction is high (coefficient of friction μ ≃ 0.15) while along the 〈110〉 direction it is low (μ ≃ 0.07). This has been variously attributed to surface roughness (Coulomb friction) or to processes involving surface adhesion. (Some of these processes and related ideas are discussed in ref. 1.) We describe here some recent experiments which show that frictional anisotropy occurs only when the contact pressure exceeds a critical value. In these conditions cathodoluminescence studies of the diamond flat indicate that the frictional anisotropy is largely due to surface and sub-surface damage produced in preferred crystallographic directions by the sliding process itself. However, the results do not, as yet, provide a detailed account of the mechanism of energy dissipation.