Subsurface damage in scratch testing of silicon nitride

Subsurface damage in scratch tests has been investigated to highlight the relationship between damage characteristics and mechanical response of materials, such as the tangential force Ft. A progressive loading tester with a spherical indenter, on which the normal load Fn was increased with increasing sliding distance, was utilized for tests on a commercially available hot-pressed silicon nitride. The damage was visualized using a plasma etching procedure previously developed. The trend of Ft was categorized into three stages associated with the damage characteristics. In the first stage, the scratch track was barely perceptible but the presence of subsurface damage was confirmed, demonstrating that damage formation occurs prior to scratch track formation. Then, in the second stage, the scratch track was distinctly observed as the consequence of quasi plastic deformation, the presence of which was inferred from the continuous groove-like damage zone. Further increase in Fn results in both high fluctuation of Ft and significant variation in damage depth, suggesting the correlation between them.