Abstract

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.