Investigation into the microstructure evolution caused by nanoscratch-induced room temperature deformation in M-plane sapphire

Nanoscratch tests have been conducted at room temperature on (101¯0) M-plane sapphire under a ramp loading condition from 100 μN to 200 mN along a scratch length of 200 μm at a scratch velocity of 1 μm s−1 using a Berkovich nanoindenter XPS system in scratch mode. Plastic deformation features, including short shaving scratch debris, linear surface features along the scratch groove, pile-up, and fish-bone features, indicating a stick–slip mechanism, and brittle deformation features, i.e. microcracking, chipping and tearing, were observed by optical, scanning electron, and atomic force microscopy. Applying the focused ion beam (FIB) technique, site-specific cross-sectional transmission electron microscopy further revealed nanoscratch-induced deformation behavior. Basal twinning was observed in the region below the scratch. The evolution of plastic deformation mechanisms can be summarized as lattice disorder, dislocation loops, stacking faults, dislocation glide and then basal twin formation. The observed features were rationalized using elastic and residual stress field calculations.