Molecular dynamics simulation for orientation dependence of deformations in monocrystalline AlN during nanoindentation

Molecular dynamics simulations were performed for the nanoindentations using a virtual cylindrical indenter on monocrystalline aluminum nitride (AlN) with the indentation surface orientations of [0001], [101¯0], [1¯21¯0] and [1¯012¯], respectively, to investigate the orientation dependence of the material. Vashishta potential was used to model the interactions between Al-Al, N-N, and Al-N atoms in the specimens. Simulation results indicated that the deformation mechanism varies with surface orientations at the initial inelastic stage. In the specimens with the surface orientations of [0001] and [101¯0] phase transformation plays a predominant part, in the case of [1¯21¯0] dislocation slip dominates the inelastic deformation, whereas in the case of [1¯012¯] both phase transformation and dislocation slip act a leading role. However, the phase transformation and dislocation slip occur in all the samples during the further indentation. We found two paths of B4 to B1 phase transitions. Path I includes two steps: Al and N atoms move along the [0001] axis by an anti-parallel vertical motion, followed by horizontal relative movement of the two types of atoms. Path II is similar to Path I, but the sequence of relative movement of Al and N atoms is different. Path I occurs in the cases of [0001] and [1¯012¯] orientations, while Path II in the cases of the other two orientations.