Anisotropy dependence of material removal and deformation mechanisms during nanoscratch of gallium nitride single crystals on (0001) plane

Gallium nitride single crystal (GaN) is difficult to achieve high-efficiency and low-damage machining due to anisotropy, high hardness and brittleness. Nanoscratch tests of GaN single crystals was conducted on (0001) plane along different zone axes, and the anisotropy dependence of material removal and deformation behaviors were investigated systematically. The results showed that crack-free plastic deformation of GaN crystals could be acquired along different zone axes, which was dominated by phase transition, polycrystalline nanocrystals, amorphous transition, as well as close-to-atomic scale damages including stacking faults, dislocations and lattice distortions. As the stress increases, special surface radial cracks with the same orientations caused by shear stress will appear on the groove surface, and striated and step-shaped brittle fractures can be induced by the propagation and intersection of the cracks. The processing along [11-20] zone axis was more conducive to achieve the plastic removal and deformation, deeper penetration depth and less phase transformation. The higher stress along [1-100] zone axis induced more phase transition from hexagonal system to cubic system. This work will enhance the understanding of the anisotropy dependence of material removal and damage mechanisms, and provide a guide for achieving high-efficiency and low-damage machining of GaN crystals.