Relationship between microstructure and mechanical properties of silicon nitride ceramics

The microstructure of silicon nitride ceramics is related to the
mechanical properties. It is demonstrated that the room temperature
strength and toughness is determined by size and morphology of the
Si3N4 grains and the grain boundary phase. The high temperature
properties are mainly controlled by the composition and properties of
the grain boundary. The grain boundary crystallization, as one strategy
for the development of high-temperature resistant materials, is
analysed with respect to the phase relationships between Si3N4 and the
additives and the impossibility of a complete devitrification. It is
shown that small amounts of impurities in the starting powder can be
enriched in thin grain boundary films and degrade the high-temperature
properties.