Anisotropic cutting mechanisms on the surface quality in ultra-precision machining of R-plane sapphire

[Display omitted] •The DBT depth and the cutting force with type PER are larger than that with type PAR.•A subsurface crack exists under the ductile-characteristic surface with type PER.•The dislocations dominate the subsurface defect depth under PER cutting direction.•The twinnings determine the subsurface defect depth under PAR cutting direction. The machined surface-layer quality of sapphire is influenced by its anisotropic properties. This work investigates the anisotropic cutting mechanisms on the surface quality of R-plane sapphire in ultra-precision machining. Plunge-cutting experiments were conducted to reveal ductile-to-brittle transition (DBT) depth. The groove topography and profiles show that the DBT depth with machining direction perpendicular to A-plane (type PER) can reach 200 nm, while the DBT depth with machining direction parallel to A-plane (type PAR) is