Micromechanism of high temperature fatigue crack growth in silicon nitride

High temperature fatigue crack growth behavior of silicon nitride has been investigated. Fatigue crack growth resistance decreased with increasing temperature and with decreasing loading rate at high temperature above the glass softening temperature. Microvoids were nucleated at triple points due to grain boundary sliding in the near crack tip region. The microvoid grew along a grain boundary accommodated by sliding of the adjacent boundaries to form a microcrack. A fatigue crack propagated by coalescing to the microcracks. The stress shielding due to bridging formed in the crack wake had also a significant influence on the fatigue crack growth behavior at high temperature.