The Influence of Microstructure on Fracture of Ceramics

: A technique has been developed for the measurement of the viscosity of the amorphous phase in the grain boundaries of silicon nitride. The technique will serve as a useful tool in the development of these ceramics since a correlation can now be established between the viscosity of the grain-boundary phase and its chemistry. Some work has been completed on building models for intergranular fracture in these materials based upon the concept of fracture in a thin viscous adhesive layer sandwiched between two hard surfaces. A simple model which considers fracture by the growth of penny-shaped voids in the adhesive layer gives sensible agreement with actual fracture measurements in silicon-nitride at elevated temperatures. The relationship between yielding and fracture in semi-brittle ceramic crystals has also been investigated. Strengthening of I-VII compounds, e.g. KCl, NaCl, and LiF, by divalent impurity additions is accompanied by an increase in fracture energy. The observed increase is explained as a result of microcrack nucleation at intersection slip bands.