Fundamental investigation on damage evolution and material removal anisotropic mechanism in monocrystal sapphire

Abstract

Monocrystal sapphire curved surface optical components are widely used in various fields such as optical engineering, aerospace, and scientific research due to their superior optical performance and reliable mechanical properties. However, the processing of curved components involves multiple crystal planes of sapphire (including the A, M, N, C, and R planes), and the anisotropic characteristics of material removal and damage evolution on each crystal plane have become an issue that urgently needs to be addressed. In this study, for the first time, single-abrasive scratch experiments were systematically performed on all sapphire crystal planes (A, M, N, C, R), and the crystal-orientation dependence of material removal was directly revealed through analysis of scratch morphology and force. An innovative analytical approach was developed that combines elastic stress field modeling with slip/twinning and cleavage activation probabilities to evaluate the anisotropic characteristics of damage evolution in monocrystal sapphire. Finally, transmission electron microscopy was used to characterize subsurface damage within the scratch grooves, and, based on the relationship between dislocations and cracks, the orientation dependence of damage evolution was further elucidated. We developed an innovative analytical approach that combines elastic stress field modeling with slip/twinning and cleavage activation probabilities to quantitatively assess the anisotropic characteristics of damage evolution in monocrystalline sapphire. The results indicate that the anisotropy in the material removal form of sapphire occurs during the brittle removal stage, with significant anisotropic characteristics manifested in the differences in brittle spalling shape and brittle spalling area. Furthermore, the distribution of the elastic stress field indicates that median cracks are induced by tensile stress during the loading process, while radial and lateral cracks are caused by tensile stress during the unloading process. Simultaneously, the degree of brittle fracture induced by scratching force, the stress field, and the probability of cleavage fracture activation follow the sequence R < N < C < M < A, whereas plasticity exhibits an inverse relationship with the activation probability of slip/twinning. The scratching of R plane is more conducive to the ductile machining due to its lower elastic stress and higher dislocation density. These findings provide valuable scientific insights and a solid theoretical foundation for advancing the processing of sapphire curved surface components.