RELATIONSHIP BETWEEN MICROSTRUCTURE AND MECHANICAL-PROPERTIES OF SILICON-NITRIDE CERAMICS

The microstructure of silicon nitride ceramics is related to the mecha nical properties. It is demonstrated that the room temperature strengt h and toughness is determined by size and morphology of the Si3N4 grai ns and the grain boundary phase. The high temperature properties are m ainly controlled by the composition and properties of the grain bounda ry. The grain boundary crystallization, as one strategy for the develo pment of high-temperature resistant materials, is analysed with respec t to the phase relationships between Si3N4 and the additives and the i mpossibility of a complete devitrification. It is shown that small amo unts of impurities in the starting powder can be enriched in thin grai n boundary films and degrade the high-temperature properties.