GRAIN BOUNDARIES IN POLYPHASE CERAMICS

The majority of polyphase ceramics contain a residual glass phase at their grain boundaries. The stability of these phases, particularly at the two-grain boundaries, is of significance since they affect the properties of the material as a whole. Drawing analogies with soap films, the stability of a continuous intergranular phase is considered in terms of the balance between the capillarity and disjoining pressures. The individual components to the disjoining pressures are discussed. It is argued that a large structural component to the disjoining pressure is responsible for the observed constancy of the thickness of its intergranular phase in polyphase silicon nitride ceramics. Mechanisms for the de-wetting of a grain boundary containing an intergranular glass phase are also discussed.