The mechanical response characteristics of sapphire under dynamic and quasi-static indentation loading

Sapphire is widely used as optical materials and substrate materials due to its excellent physical and chemical properties. The mechanism of crack propagation and fracture damage evolution has important significance for improving the manufacturing quality and application performance of sapphire parts. In this study, dynamic and quasi-static indentation tests have been performed on the c-plane and a-plane of sapphires by Hopkinson pressure bar tester and continuous indentation tester, respectively. The crack propagation path in sapphire has been captured by High-speed camera and the crack velocity has been calculated. The crack propagation and fracture damage evolution has been analyzed based on the fracture morphology of specimen. It was found that the bearing capacity of sapphire is related to the loading velocity, while the crack propagation is affected by the crystal orientation. Under the indentation loading, the cracks in sapphire first propagate steadily, and then the cracks begin to propagate uncontrollably after reaching the critical conditions, where the crack propagation velocity obviously increases, typically from 204 m/s to 1006 m/s (dynamic indentation) or from 0.0032 m/s to 820 m/s (quasi-static indentation). And the crack propagation velocity depends on the loading speed at stable stage. The r-planes of sapphire are weaker than other crystal planes and are prone to crack propagation.