Evolution of material removal modes of sapphire under varied scratching depths

Scratching experiments were conducted on C-plane of sapphire along the [011̅0] direction to investigate the evolution of material removal modes under varied scratching depths. The results indicated that the material removal experienced three stages depending on the increase of scratching depth. Pure plastic flow was firstly observed as the scratching depth was small enough, characterized by smooth scratching groove, curled shavings and tiny stick-slip lines. Plastic deformation and brittle fracture coexisted in the second stage, where the initial radial cracks, tearing and segmental chips appeared. Brittle fracture dominated in the final stage: irregular debris and spalling occurred due to the intersection of lateral and radial cracks. In addition, the mechanism of plastic deformation and the cracks initiation/evolution of sapphire is discussed by combining the scratching patterns with the contact-induced deformation theory. Finally, the scratching-induced Acoustic Emission (AE) signals were processed based on fast Fourier transform and wavelet packet decomposition. The characteristics of the AE signals corresponding to the three material removal stages are discussed in frequency domain.