Dependence of material removal on crystal orientation of sapphire under cross scratching

The cross scratching experiments were carried out on C- and M-planes of sapphire to investigate the crystal-orientation dependence of crack initiation and damage mechanism. The observed scratching patterns showed that the shallow lateral cracks were dominant along [1¯100] direction, while both lateral and radial cracks occurred along [011¯0] direction on C-plane. However, less surface fractures appeared on M-plane, but with greater scratching affected zone caused by deep lateral cracks in subsurface. The anisotropic scratching patterns were closely related to the activated slip/twinning systems, which accommodate dislocation and provide fracture nucleation during scratching. In addition, the acoustic emission (AE) signals were processed based on Fast Fourier Transform (FFT) and Wavelet Packet Decomposition (WPD), and the results indicate that the anisotropic material deformation under scratching can be identified by the frequency bandwidth and the intensity of special frequency components of the AE signals for the C- and M-planes of sapphire.