Behavior of Mirror-like Region of Sintered Si₃N₄ under Static Fatigue

Abstract

A four-point bending test was carried out on sintered Si₃N₄ at room temperature to investigate the static fatigue behavior of ceramics. The feature of the overall fractured surface after the test was similar to that after the static test if the fracture stress was the same. Mirror-like regions were observed on the fractured surfaces after both tests. The character of mirror-like region after the static fatigue test was compared with that after the static test. The characters of both regions were almost the same. The mirror-like region was roughly semicircular, and the fracture propagated radially from the mirror-like region. The size of mirror-like region, which was determined from optical micrographs at a magnification of 25, was correlated with the applied stress. Stress intensity factor at the deepest point in the region was also almost constant independent of the depth of the region. It was considered that the mirror-like region was restricted to the subcritical crack growth under the applied stress, and that the failure occurred when its size reached a critical value which was dependent on the applied stress.