Dependence of tribological behavior of GaN crystal on loading direction: A molecular dynamics study

In order to investigate the tribological property of the gallium nitride (GaN) crystal at the nanoscale, a series of molecular dynamics nanoscratch simulations are carried out on the surfaces of c-GaN, a-GaN, and m-GaN. The key factors of scratch depth and scratch direction that greatly influence the deformation behavior are explored by analyzing the mechanical response, surface wear, and subsurface dislocation nucleation. The friction coefficient, wear rate, and total length of dislocations are all found to increase with the increase of scratch depth. A clear directional dependence could be recognized for c-GaN, where the friction coefficient along the [101¯0] direction is always lower than that along the [12¯10] direction, and the wear rate along the [101¯0] direction is higher than that along the [12¯10] direction, regardless of the scratch depth. On the contrary, the directional dependence of the wear rate and friction coefficient is unclear for a-GaN and m-GaN. For scratches at a specific depth, dislocations in c-GaN are smallest in length and occupy shallow positions close to the surface, while widely distributed dislocations could be observed in m-GaN.