The friction coefficient for two hexagonal boron nitride (h-BN) sheets was estimated on the basis of ab initio calculations. Several models, both cluster and periodic one, were applied to describe h-BN and the interactions between its dimers. In all cases, the gauche orientation is favoured, the interaction energies being virtually independent of the size of the model. The friction coefficient values, in contrast, converge towards the values from periodic calculations. At the lower limit of the periodic system, the friction coefficients vary between 0.226 and 0.265 depending on the orientations of the dimers. The calculations are in good agreement with an experimental study of the friction [Saito and Honda, Wear 237 (2000) 253], which suggested a friction coefficient of 0.23-0.25 for h-BN.

Friction coefficients for cubic boron nitride were determined with the use of hybrid density functional B3LYP calculations. Two cluster models and an infinite periodic model were applied. Various contacts between duplicated surfaces and different sliding paths of the surfaces with respect to each other were taken into consideration, and friction coefficients were derived. The calculations suggest that three different sliding paths contribute to the friction, the friction coefficient for the periodic model being 0.21 at its lowest. This value is in agreement with previous experimental studies, where a friction coefficient of 0.1-0.2 has been measured for cubic boron nitride.

The possibility of tribochemical reactions between methylated diamond surfaces was studied by quantum chemical HartreeFock and B3LYP methods. Lateral sliding of two diamond surfaces cleaved from the diamond crystal lattice and substituted with methyl groups was performed at planar distances of 400 and 350 pm for two models, C 28 H 48 and C 46 H 72 . In each case, the calculations suggest that tribochemical reaction is possible. At planar distance of 400 pm, the reaction occurs within a diamond plane, whereas the shorter planar distance allows the reaction to occur between two planes.

Friction between ice-Ih and hexagonal boron nitride (h-BN) has been determined by periodic ab initio calculations. Surfaces of ice-Ih and h-BN were brought into sliding contact, and the interaction energies were calculated as a function of interplanar distance and lateral displacement of the surfaces. The friction between the surfaces was calculated from the interaction energies, producing a friction coefficient of 0.140. Friction is further influenced at high loads by a tribochemical reaction between ice-Ih and h-BN.