Anisotropy-driven dislocation slip and crack initiation mechanisms in enhancing ductile removal of sapphire via oblique cutting

Sapphire's high hardness and low fracture toughness make stable ductile cutting highly challenging. This study proposes oblique cutting as a simple and effective approach to improve sapphire's ductile removal capability. To elucidate the brittle-to-ductile transition (BDT) behavior and the mechanisms of crack initiation during oblique cutting, a model based on stress intensity factor theory and incorporating tool geometric parameters is established. The model reveals that the minimum stress intensity in the cutting zone occurs at 45° tool inclination, corresponding to the lowest probability of crack initiation. Experiments demonstrate that introducing tool inclination markedly increases the BDT depth. Oblique 30° cutting along the 180° orientation achieves near-micrometer BDT depths, while a 45° inclination yields the most pronounced overall enhancement across four crystallographic orientations. Combined experimental observations and simulations further reveal that the plastic removal behavior of sapphire is governed by its crystallographic anisotropy, primarily manifested through variations in prismatic dislocation slip. When the tool-induced shear stress is collinear with the Burgers vector of prismatic dislocations, coplanar slip is activated, leading to stable and continuous plastic removal, a significant increase in BDT depth, and outward propagation of radial cracks without disruption of the plastically deformed region. In contrast, misalignment between the shear stress direction and the Burgers vector promotes cross-slip, destabilizes plastic flow, and leads to localized tearing, thereby deteriorating surface quality. Overall, oblique cutting, by adjusting the tool inclination and leveraging sapphire's crystallographic anisotropy, provides a simple and effective approach to enhance BDT depth and achieve high quality SPDT. • A simple and effective oblique cutting strategy is proposed for ultra-precision machining of sapphire. • Stress intensity factor model incorporating tool geometric parameters is established to predict crack initiation. • Plastic removal behavior of sapphire is governed by crystallographic anisotropy through prismatic dislocation slip. • Through the combined effects of tool inclination and sapphire anisotropy, a near-micrometer BDT depth was achieved.