Morphology changes accompanying creep of sintered Si₃N₄ for hot turbine engine application

Using analytical and high resolution transmission electron microscopes, scanning electron microscope and energy dispersive spectroscopy, we have studied the microstructural and compositional changes associated with the high temperature creep of a sintered silicon nitride which is considered a candidate for hot turbine engine application. First we studied the microstructures and composition of the virgin sample and found dislocation networks and amorphous grain boundaries and triple junction phase of the additives of alumina, magnesia and yttria. The amorphous phase remains after the creep test but gaps between silicon nitride grains were formed because of creep, which implies the occurrence of grain boundary sliding during creep process. We observed dislocation pile-ups and dislocation arrays besides the strain whorl phenomenon in the crept samples, suggesting the deformation of some silicon nitride grains. We consider this to be only a secondary deformation mechanism in the creep of this material since the stress component as calculated based on creep experiments is close to unity, which means that the creep is dominated by the diffusional mechanism. SEM fractography shows no cavitation. Initial studies on the thermally soaked samples show that surface deterioration of the samples occurred at high temperatures in corrosive atmosphere and the scales thus formed are depleted of Si and Mg, indicative of material loss because of chemical encroachment. Protective surface coating to promote the high temperature performance of the material is recommended for further investigations.