Phase transition and plastic deformation mechanisms induced by self-rotating grinding of GaN single crystals

Despite being the most promising third-generation semiconductor materials, the deformation and removal mechanisms of gallium nitride (GaN) single crystals involved in the ultra-precision machining process are not well revealed and few investigations on the grinding of GaN crystals have been reported, which hinders the development of high-efficiency and ultra-precision manufacturing of GaN components. Self-rotating grinding tests of GaN crystals were performed, and the results indicated that abrasive size had a significant influence on the surface morphology and roughness, in comparison with wheel rotational speed and feed speed. As the abrasive size decreased from 18 mu m to 1.6 mu m, the brittle fracture-dominated surface gradually changed to a full plastic surface without brittle fractures and cracks. An ultra-smooth surface with a roughness of 1 nm in Sa was acquired using #8000 grinding wheels and a spark-out time of 10 min, which indicated that the machining technology of "grinding instead of polishing" of GaN crystals was achieved in this work. The plastic deformation mechanism of GaN crystals induced by ultra-precision machining was investigated using a cross-sectional TEM method and MD simulation, and both experimental and simulation results indicated that the plastic deformation involved in the scratching process was caused by the formation of polycrystalline nanocrystals, high-angle lattice misorientations, and close-to-atomic-scale defects, including stacking faults, dislocations and serious lattice distortions, along with a small amount of amorphous and phase transitions. There was an obvious delamination phenomenon in the plastic deformation zone. This research enhances the understanding of the deformation and damage mechanisms of GaN crystals involved in the ultra-precision machining process and is of significance for achieving the high-efficiency and high-accuracy manufacturing of GaN components.