Internal Stresses in Silicon Nitride and Their Influence on Mechanical Behavior

Abstract

Internal stresses in silicon nitride are caused by the thermal mismatch between the crystalline silicon nitride matrix and the grain boundary phase, and between the matrix and free silicon. Furthermore, the crystallization of the glassy phase at triple junctions can induce additional internal stresses.

Average stresses in various phases and their fluctuations are calculated by the information theory approach. These results are used to discuss some implications for the mechanical behavior at room temperature.

The strong heterogeneities of silicon nitride ceramics require also the investigation of local stresses formed in the surroundings of triple junctions by analytical modelling and FEM technique. Based on such calculations performed elsewhere the interaction of grain boundary crystallization and transient stress development is outlined. The influence of transient stress on crystallization of the glassy phase can be described by two coupled kinetic equations involving the growing crystalline phase and the induced internal stress field. The condition for partial crystallization due to the increase of stored elastic energy is given being valid for a large stress relaxation time. Stress relaxation is taken into account by an effective viscosity which can be related to the creep viscosity. Transient internal stresses at high temperatures can influence the microstructural development. In this connection the importance of internal stresses for the high temperature mechanical behavior of silicon nitride is considered.