A new model for tensile creep of silicon nitride

The tensile creep rate of most commercial grades of Si3N4 increases strongl y with stress. Although the usual power-law functions can represent the cre ep data, the data often show curvature and systematic variations of slope w ith temperature and stress. In this article, we present a new approach to u nderstanding the creep of ceramics, such as Si3N4, where a deformable secon d phase bonds a deformation-resistant major phase. A review of experimental data suggests that the rate of formation and growth of cavities in the sec ond phase controls creep in these materials. The critical step for deformat ion is the redistribution of the second phase away from the cavitation site to the surrounding volume. The effective viscosity of the second phase and the density of active cavities determine the creep rate. Assuming that the hydrostatic stresses in pockets of the second phase are normally distribut ed leads to a model that accurately describes the tensile creep rate of gra des of Si3N4, In this model, the creep rate increases exponentially with th e applied stress, is independent of Si3N4 grain size, is inversely proporti onal to the effective viscosity of the deformable phase, and is proportiona l to the cube of the volume fraction of the deformable phase.