Anisotropic mechanism of material removal and ductile-brittle transition in sapphire scratching based on acoustic emission signal

Sapphire exhibits typical anisotropy due to its unique crystal structure, and there are significant property differences in different processing areas during the machining of curved sapphire component, making it essential to investigate the anisotropy in the removal process of sapphire materials for efficient curved surface machining. In this study, the force and acoustic emission (AE) signals were monitored at different scratch stages and orientations, and the anisotropic characteristics of ductile-brittle transition (DBT) depths, material removal morphology, and subsurface crack propagation direction were analyzed. The results indicate that the anisotropy of force signals and DBT depths is mainly reflected in the differences between crystal planes. In addition, the removal morphology of materials with different crystal orientations showed smooth scratch grooves with unclear anisotropic characteristics during the ductile removal (DR) stage, while the crack propagation direction with different scratch orientations during the DBT stage showed significant anisotropic characteristics. The brittle removal (BR) stage is characterized by brittle spalling in the form of shell-like, blocky, and fishbone spalling, and there is a significant anisotropic difference in the spalling shapes on both sides of the scratch. The original AE signals, Fast Fourier Transform (FFT), and Short Time Fourier Transform (STFT) results show that the AE frequencies of different crystal planes are different, and the differences in different scratch orientations on the same crystal plane are reflected in the peak amplitude. Finally, the propagation directions of subsurface cracks with different directions are anisotropic due to different scratching crystal planes, and propagate mainly along directions of 15°, 29°, 32°, 58°, and 105°.