Atomic-scale material removal and deformation mechanism in nanoscratching GaN

Gallium nitride (GaN) is an important third-generation semiconductor material. However, due to its high hardness, high brittleness and anisotropy, the material removal efficiency of GaN during ultraprecision machining is low, and subsurface damage easily occurs. In order to achieve high-efficiency and low-damage ultra-precision machining, the model of double nanoscratches is innovatively utilized to investigate the mechanical behavioral of GaN at the atomic scale. Specifically, double nanoscratches experiments and corresponding molecular dynamics (MD) simulations were carried out on GaN (0001) crystal plane along the $\left[10\overline{1}0\right]$ and $\left[1\overline{2}10\right]$ crystal directions to reveal the material removal, deformation and subsurface damage mechanisms of GaN under anisotropic conditions. The results show that the plastic removal of GaN can be realized by setting loading force and scratch spacing. Scratching along the $\left[1\overline{2}10\right]$ crystal direction has a greater ductile-brittle transition load force than the $\left[10\overline{1}0\right]$ crystal direction. MD analysis shows that the downward-extending prismatic slip produced by scratching along the $\left[10\overline{1}0\right]$ crystal direction is the cause of crack extension to the subsurface during the experiment. As the load force increases, the surface cracks of the double scratches expand and intersect, inducing streak-like brittle fracture. The significant increase in the lateral force of the second scratch is the main reason for the brittle fracture in the double scratch experiment. After the double nanoscratches experiment, the damage presented on the GaN subsurface mainly includes atomic scale damage such as amorphization, stacked laminations, polycrystalline nanoscratch, and phase transition. The Stacking fault bands around the damage zone inhibit the damage extension. The second scratch generates a large number of dislocations in the overlap zone and attenuates the subsurface amorphization under the influence of dislocation strengthening effect.