Creep Mechanisms in Silicon Nitride Ceramics

The mechanism responsible for creep in glass containing ceramics such as silicon nitride are reviewed. It is suggested that two basic creep regimes are observed. In some materials, especially at intermediate temperatures, viscous flow in the amorphous phase controls the creep. This is a transient process leading to exhaustion after a strain of the order of h/d, where h is the grain boundary film thickness and d is the grain diameter. Typically this yields strains of about 0.3%. In other materials, and especially at higher temperatures creep continues beyond this strain due to a combination of cavity growth and dissolution/precipitation creep. These steady-state processes must be avoided by materials under service conditions. It is shown that a more complete understanding of these mechanisms is required in order to develop strategies for developing materials with improved high temperature capability.