Alternate activation-annihilation of dislocations realizes the plasticity of sapphire during indentation

A "renaissance" of sapphire is regarded as one of the most significant tendencies of modern materials science. But sapphire has almost no plasticity at room temperature, making its surface finishing a formidable challenge. Realizing the plasticity of sapphire and revealing its plastic deformation mechanism are crucial for the micronano and ultra-precision machining of sapphire. In this work, the deformation behavior of sapphire during indentation is revisited using molecular dynamics simulation. It finds that sapphire possesses a diverse plastic deformation behavior during the nanoindentation. Systematical investigation into the evolution of dislocation confirms that sapphire realizes plasticity during indentation through alternate activation-annihilation of dislocation. In light of these results, it is reasonable to believe that an improved machining quality can be obtained by precisely controlling the machining depth to keep sapphire in the dislocation activation stage. Furthermore, our simulations provide a possible physical essence of the ductile-brittle transition during material removal observed in experiments.