Recent advances in our understanding of creep deformation in silicon nitride ceramics are reviewed and compared to two different models of creep. The classical models adopted from the metals literature are based on the assumption that creep occurs primarily by diffusion of atoms either through the grains, or along grain boundaries. The cavitation model of creep was developed specifically to explain creep in materials that consist of rigid grains with a mobile secondary phase at the grain boundaries, materials having structures similar to that of silicon nitride. Well-known effects such as creep asymmetry and a very wide range of stress exponents in the early commercial grades of silicon nitride can be fully understood within the framework of the cavitation models. The work discussed includes an identification of critical types of creep cavities in silicon nitride, the evolution of cavities with tensile strain, and an analysis of possible mechanisms involved in cavity formation. The analysis amplifies the cavitation creep model of Luecke and Wiederhorn and assumes that creep occurs via a combination of grain boundary sliding, viscous flow and solution-precipitation of the crystalline secondary phase, resulting in a redistribution of this phase among the multigrain junctions of the solid. The increase in creep resistance in the latest generation of silicon nitride materials was found to be related to the suppression of cavitation and a shift toward non-cavitation creep mechanisms. Differences between volume conservative mechanisms in tension and compression depend on the existence of different driving forces for creep: local tensile/compressive stresses and/or dilatational stresses. Increasing the viscosity of residual glassy films at the grain boundaries is believed to be an effective way to suppress cavitation and increase creep resistance. The addition of Lu+3 and N−3 to the bulk oxynitride glasses, similar to those at the grain boundary films, increases their viscosity. Thus, the suppression of cavitation and the higher creep resistance of the Lu-containing silicon nitride can be explained by the combined effect of Lu+3 and N−3 in the residual glass.

Keywords: Silicon nitride, Tensile creep, Dilatational stresses, Cavitation, Solution-precipitation, Crystalline secondary phases, Residual glass.