Crystal-orientation-dependent nanoscale machining mechanisms in ultrasonic vibration-assisted scratching sapphire

Ultrasonic vibration-assisted grinding (UVAG) is a promising, low-damage, high-efficiency and environmentally friendly technique for machining sapphire, yet its atomistic mechanisms and orientation dependence remain poorly understood. In this work, we employ molecular dynamics (MD) simulations to compare ultrasonic vibration-assisted scratching (UVAS) with conventional scratching (CS) on the A/C/M/R-planes of sapphire. Applying ultrasonic vibration dramatically reduces the scratching force by redistributing the stress field and activating cyclic deformation mechanisms, with the force-reduction sequence A≈ M > C > R. Surface-topography analysis shows that chip pile-up modes depend on crystal orientation; ultrasonic vibration not only lowers the pile-up height but also makes it more uniform. Moreover, ultrasonic vibration mitigates subsurface damage by suppressing tangled dislocation networks on the A- and M-planes and by promoting the nucleation of dislocations and twinning on the C- and R-planes, with the C-plane experiencing the least damage. These results systematically clarify the coupled effects of ultrasonic vibration and sapphire anisotropy, providing valuable guidance for selecting crystal orientations and vibration parameters during ultra-precision grinding of electronic devices, such as sustainable clean-energy LEDs.