Prediction of pop-in load for sapphires with different crystal orientations

Sapphire has wide applications due to its excellent physical properties. However, it is difficult to manufacture due to its brittleness and hardness and the determination of the load at which ductile regime changes to brittle regime is of great importance in abrasive machining of sapphire. The occurrence of pop-in events at micro-scale always presents ductile-brittle transition. In this study, a mechanical model has been developed for pop-in event of sapphire based on indentation test results obtained by a Rockwell indenter with a tip radius of 4.5 mu m. Critical stress value was applied to predict critical loads at which pop-in events took place with spherical indenters of variable tip radii. Sequential loading-unloading indentation test using a spherical tip with radius 50 mu m was carried out to verify the proposed mechanical model. The predicted pop-in loads showed good agreement with experimental values for sapphires with different crystal orientations. Moreover, this model had certain flexibility and could predict the load at which pop-in events occurred under different loading conditions, which could be used to optimize the parameters of manufacturing process.