ELABORATION CONDITIONS, MICROSTRUCTURE AND PROPERTIES RELATIONSHIP IN SILICIUM NITRIDE CERAMICS.

Silicium-Based Ceramics were investigated to determine the effect of microstructural parameters and densification on thermal stress resistance to fracture initiation during thermal shock testing . The different materials and microstructures were obtained by changing parameters such as the type of powder, additive, forming process and sintering condition. Critical temperature differences ((delta)Te), after water quenching, are discussed in relation to change in Young s module of elasticity, a variable affecting thermal shock. The maximum thermal shock resistance of dense Si3N4 has been achieved after completing conversion (alpha) (right arrow) (beta), the changes in grain morphology toward elongated grain and the relative crystallisation of the second phase. These characteristics are produced by a higher percentage of the powder in the (alpha)-phase, high Y203, and the sintering condition at a higher temperature (2000°C), longer soaking times (1h) and load application at the beginning of thermal cycle.