TAILORED MICROSTRUCTURES OF SILICON-NITRIDE CERAMICS

The microstructural evolution in Si3N4 ceramics is analysed by studies of pressureless and gas pressure sintered materials. Parameters contr olling the size and aspect ratio of elongated grains are discussed, ba sed on the assumption that only pre-existing beta-Si3N4 particles of t he starting powder grow. Furthermore, it shown that the grain morpholo gy is also controlled by the sintering additives due to their influenc e on the surface energy of Si3N4 crystals. Microstructures are also re lated to strength as well as toughness measurements at room temperatur e. It is demonstrated that strength decreases with increasing grain si ze, but controlled grain growth could significantly improve fracture t oughness, reliability and thermal shock-behaviour.