Frictional anisotropy in nonmetallic crystals

Abstract

A study is made of the effect of the crystallographic direction of sliding on the friction of the (001) surfaces of diamond, magnesium oxide and lithium fluoride crystals. The friction shows marked anisotropy and with all the crystals it is greatest in the <100> directions and least in the <110> directions. The degree and magnitude of the anisotropy is dependent upon the shape of the slider and the ease with which it penetrates the crystal surface. Sharp sliders increase the degree of brittle failure and this leads to deeper penetration and to the removal of more material during sliding. With these crystals the depth of penetration is greater in the <100> directions then in the <110> and it is this which is primarily responsible for the frictional anisotropy. An explanation of frictional anisotropy is proposed which is based on the difference in the magnitude and distribution of resolved shear stresses during sliding in various crystallographic directions. This analysis is used to predict the effect of crystallographic orientation on the frictional behaviour when a (110) surface of magnesium oxide replaces the cube (001) surface used in the other experiments. Mechanisms of deformation and fracture associated with the friction are described. Brittle behaviour predominates in diamond crystals and only cleavage cracks are observed. Appreciable plastic flow occurs in both magnesium oxide and lithium fluoride crystals. With these crystals the initial plastic deformation leads to dislocation interactions which result in cracking and fracture along the {110} planes. These interact with cleavage cracks on {100} planes which are produced by tensile stress and cause surface fragmentation and wear of the crystal. Plastic flow is the only mode of deformation observed on (001) lithium fluoride surfaces when a very smooth blunt slider is used. This causes ‘pile-up’ of material along <110> directions (as previously observed in copper crystals) but it does not produce any appreciable anisotropy in the friction.