Molecular dynamics simulations were used to investigate surface material removal and subsurface damage at the nanoscale to the atomic scale during ultrasonic vibration-assisted (UVA) nanoscratching of single-crystal AlN with a single-point diamond tip. The simulation results suggest that UVA-scratching results in a lower tangential force, normal force, and friction coefficient than ordinary scratching at the same scratch depth. UVA-scratching results in a greater material removal rate than ordinary scratching because of the vibration-induced rise in the local temperature, which facilitates atomic bond breakage and the lateral extension of stacking faults in the superficial layer. Uniform monolayer removal consisting of the outermost Al atoms and the connected N atoms is easier to achieve in the scratching path with the UVA-scratching mode than with the ordinary scratching mode. UVA-scratching produces a smoother scratched surface. For example, the root mean square of the surface after UVA-scratching is only approximately one-third of that after ordinary scratching at the same scratch depth. Furthermore, ultrasonic vibration can reduce scratching-induced material pile-up and subsurface damage, which primarily consists of dislocations and stacking faults. This is because vibration can reduce the stress distribution range and restrain the stress concentration. This work can provide useful knowledge for high-quality and efficient ultraprecision surface machining for hard-brittle materials.

Mechanochemical reactions of the GaN–Al₂O₃ interface offer a novel principle for scientific and technological merits in the micro-/nano-scale ultra-precision surface machining. In this work, the mechanochemical reactions on Ga- and N-faced GaN surfaces rubbed by the Al₂O₃ nanoasperity as a function of the environmental humidity were investigated. Experimental results indicate that the N-face exhibits much stronger mechanochemical removal over the relative humidity range of 20%–80% than the Ga-face. Increasing water molecules in environmental conditions significantly promotes the interfacial mechanochemical reactions and hence accelerates the atomic attrition on N-face. The hypothesized mechanism of the selective water-involved mechanochemical removal is associated with the dangling bond configuration, which affects the mechanically-stimulated chemical reactions via altering the activation energy barrier to form the bonding bridge across the sliding interface. These findings can enrich the understanding of the underlying mechanism of mechanochemical reactions at GaN–Al₂O₃ interface and a broad cognition for regulating the mechanochemical reactions widely existing in scientific and engineering applications.