Tensile creep and rupture of silicon nitride

We have characterized the tensile creep, rupture lifetime, and cavitation b ehavior of a commercial, gas-pressure-sintered silicon nitride in the tempe rature range 1150 degrees to 1400 degrees C and stress range 70 to 400 MPa. Individual creep curves generally show primary, secondary, and tertiary cr eep, The majority of the primary creep is not recoverable. The best represe ntation of the data is one where the creep rate depends exponentially on st ress, rather than on the traditional power law. This representation also re moves the need to break the data into high and low stress regimes. Cavitati on of the interstitial silicate phase accompanies creep under all condition s, and accounts for nearly all of the measured strain. These observations a re consistent with a model where creep proceeds by the redistribution of si licate phase from cavitating interstitial pockets, accommodated by grain-bo undary sliding of silicon nitride.