TAILORED MICROSTRUCTURES OF SILICON-NITRIDE CERAMICS

The microstructural evolution in Si3N4 ceramics is analysed by studies of pressureless and gas pressure sintered materials. Parameters controlling the size and aspect ratio of elongated grains are discussed, based on the assumption that only pre-existing beta-Si3N4 particles of the starting powder grow. Furthermore, it shown that the grain morphology is also controlled by the sintering additives due to their influence on the surface energy of Si3N4 crystals. Microstructures are also related to strength as well as toughness measurements at room temperature. It is demonstrated that strength decreases with increasing grain size, but controlled grain growth could significantly improve fracture toughness, reliability and thermal shock-behaviour.