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

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 <143.51 nm. However, the TEM images show a crack of 560 nm depth existing in the subsurface of type PER groove although its surface exhibits ductile characteristics. Besides, HRTEM results reveal that the dislocation defects determine the affected-layer depth for type PER groove, while the twinning defects dominate the affected-layer depth for type PAR groove. Moreover, overlap-cutting experiments highlight that the cutting force with type PER is larger than that with type PAR. MD simulation results indicate that the plastic deformation depth for type PER groove is higher than that for type PAR groove. In regards to the precision machining of sapphire, comprehensive evaluation of DBT depths, subsurface damage and cutting forces is essential for optimizing the process parameters.