Fundamental study on damage-free machining of sapphire: Revealing damage mechanisms via combining elastic stress fields and crystallographic structure

This paper aimed to reveal the damage mechanisms of sapphire by single grit scratch tests on C-plane, which would establish the basis of damage-free machining. The deformation and damage evolution experienced five stages with increasing cutting depth resulting from the combined effect of principle stresses and activated slip/twining systems. It was proved that subsurface and surface damages occur in ductile removal regime of sapphire, which were dominantly induced by high residual stress and the rhombohedral twinning on (101‾2) and (1‾102), respectively. Three critical cutting depths for suppressing subsurface damage, surface damage and surface roughness were determined, respectively. These conclusions were also supported by the acoustic emission (AE) features both in time and frequency domains during scratching. This paper also undertook the theoretical study. An improved elastic stress field model has been established to understand the deformation and damage evolution in relation to cutting depth.