Slow crack growth behavior of silicon nitride ceramics in cryogenic environment

The slow crack growth behavior of Si3N4 ceramics in ambient and cryogenic environment had been investigated by dynamic fatigue testing. The fracture strength was principally dependent on stress rate at 293K while the fracture strength was comparatively independent of stress rate at 77K. The morphologies of the fracture surface and the crack tip region were examined to correlate with the slow crack growth behavior. The experimental results revealed that the Si3N4 ceramics exhibits a very high resistance to slow crack growth (SCG) in cryogenic environment. The reasons for such high resistance are likely related to the local residual stress associated with either the thermal anisotropies arising from the presence of the grain boundary glass phase, or from the crack tip shielding mechanism.