Chapter 6. Tensile Creep Performance of a Developmental, In-Situ Reinforced Silicon Nitride

Summary

The creep performance of a developmental, in-situ reinforced silicon nitride was evaluated at temperatures between 1300–1425°C in ambient air. The minimum creep rate as a function of tensile stress and temperature was evaluated, and the measured tensile creep performances of two different specimen geometries (buttonhead and dogbone - machined from same billet of material) were compared. This silicon nitride exhibited comparable, or better, creep resistance than other silicon nitrides described in the literature. The measured creep response of the material and lifetime were observed to be geometry dependent; the smaller cross-sectioned dogbone specimens exhibited faster creep rates and shorter lives, presumably due to faster oxidation-induced damage in this geometry. The tensile creep rates and lifetimes were found to be well-represented by the Monkman-Grant relationship between 1350 and 1425°C, with some evidence suggesting stratification of the data for the 1300°C tests and a change in dominant failure mode between 1300 and 1350°C. Lastly, values of the temperature-compensated stress exponent and activation energy for tensile creep were found to decrease by approximately 80 and 75% in compression, respectively, illustrating anisotropic creep behavior in this silicon nitride.