Simple mechanics model and Hertzian ring crack initiation strength characteristics of silicon nitride ceramic ball subjected to thermal shock

This study investigated the ring crack initiation strength characteristics of silicon nitride ceramic balls subjected to thermal shock. Sphere indentation tests were conducted on ceramic balls following water quenching and furnace cooling operations performed in air and vacuum. The ring crack radius was significantly smaller because of the very thin silicon oxide (SiO2) layer formed owing to heat treatment in air at high temperatures. In addition, the ring crack initiation load and Weibull shape parameter of water-quenched ceramic balls decreased with increasing temperature difference and remained relatively unaffected by the high-temperature oxidation. This phenomenon was mainly due to microscopic damage, which occurs near the surface of ceramic balls, caused by transient thermal stress developed during water quenching. A simple mechanics model based on the constant energy release rate criterion has been proposed for comparing experimental results obtained in this study against theoretical predictions, and results of the said comparison adequately verify strength characteristics of the aforementioned ceramic balls subjected to thermal shock.