Study on Damage of 4H-SiC Single Crystal through Indentation and Scratch Testing in Micro-Nano Scales

In this paper, a series of indentation tests in which the maximum normal force ranged from 0.4 to 3.3 N were carried out to determine the fracture toughness of 4H-SiC single crystals. The results indicated that an appropriate ratio of the distance from the indentation center to the radial crack tip to the distance from the indentation center to the indentation corner is significant to calculate fracture toughness of 4H-SiC single crystals. The critical condition with no cracks on the edge of the indentation was obtained through a fitting method. The surface morphologies of the groove were analyzed by scanning electron microscopy (SEM). Plastic deformation was observed and characterized by the smooth groove without cracks and ductile chips on the edge of the groove in the initial stages of scratch. With increased normal force, median cracks, radial cracks, and microcracks appeared in turn, followed by the crack system no longer being able to stably extend, causing the brittle fracture to dominate the material removal. The size of the edge damages were measured through SEM and the experimental data highly agreed with the predicted curve. A modified calculation model considering elastic recovery of the sample by the indenter during the scratching process was suggested. These results prove that elastic recovery of 4H-SiC single crystals cannot be ignored during ultra-precision machining.