Direct observation of the fracture process on C-plane sapphire by in-situ scratch tests

To understand the relationship between the damage mechanism of sapphire and the anisotropic characteristic of its crystal structure, in-situ scratch tests under the confocal laser scanning microscope (CLSM) were performed along different directions of C-plane sapphire wafer. The relation between in-situ images and normal/lateral forces was established. Craters in front of the contact region (CR) between the indenter and the material, and subsurface special cracks separated from CR were observed. As the scratch direction changed, these special cracks and craters initiated and propagated on the left front, front, and right front of the CR, respectively. The scratch phenomena in different scratch directions exhibited triple symmetric characteristic around the c-axis. The probability of slip/twinning activation and the theoretical stress field in front of the indenter were calculated to investigate the fracture mechanism of sapphire. Craters and those special cracks appeared in the region with the highest probability of rhombohedral twinning and basal twinning activation, respectively. According to the theoretical stress field, the material ahead of the indenter axis was separated into regions I and II. Craters in front of the CR and those special cracks separated from the CR appeared in these two regions, respectively.