Friction of Hexagonal Boron Nitride in Various Environments

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Abstract
The mechanisms of friction of hot-pressed h-BN sliding against itself in an AES/XPS analytical tribometer have been studied under high vacuum of 10−8 Pa; under a low partial pressure of 10−3 Pa of different gases, i.e., air, CO, C3H8 and H2O; under a higher partial pressure of 10 Pa of N2, O2, air and C3H8; and in ambient air, 50 percent humidity, at an atmospheric pressure of 105 Pa, respectively. Two crystallographic orientation combinations of basal plane alignments were tribotested, namely the flat/flat configuration and the edge/edge configuration, where the specimens were directionally fabricated from the anisotropic molds. The major finding is that h-BN basal plane slip appears to be the only deformation mode which is associated with low friction in the current test conditions. Generally, fracture, shear and amorphization of the BN crystallites give higher friction. The mechanism of friction-induced basal plane slip in h-BN is thought to be related mainly to the presence of condensed liquid water in crystal defects, leading to a decrease of the surface tension and, therefore, of the critical resolved shear strength. The mechanism of the friction-altering action of hydrocarbon adsorbates is different: a lubricating phase of carbon seems to be formed by a tribochemical reaction. The results suggest that hybrid combinations of h-BN and carbon interacting synergistically could act as a better tribomaterial in a wide-environmental regime.