High‐Temperature Strength Behavior of Hot‐Pressed Si 3 N 4 : Evidence for Subcritical Crack Growth

The high‐temperature strength of commercial hot‐pressed Si 3 N 4 was obtained for (1) two materials with different impurity contents, (2) the weak and strong material directions, (3) air and Ar ambients, and (4) different stressing rates. Strength degradation occurred at a lower temperature for the less pure material; both material directions exhibit the same rate of strength degradation. The testing ambient did not affect strength. The strength at temperatures ∼1200°C depended strongly on stressing rate. The presence of rough, crack‐shaped topographical features on the fracture surface and the observation of large cracks that formed during stressing are reported as evidence for subcritical crack growth at high temperatures. It is hypothesized that accelerated creep caused by grain‐boundary sliding at preexisting crack fronts is the mechanism responsible for the observed subcritical crack growth.