Silicon nitride-silicon carbide micro/nanocomposites: A review

This paper reviews investigations of silicon nitride-silicon carbide micro-nanocomposites from the original work of Niihara, who proposed the concept of structural ceramic nanocomposites, to more recent work on strength and creep resistance of these unique materials. Various different raw materials are described that lead to the formation of nanosized SiC within the Si3N4 grains (intragranular) and at grain boundaries (intergranular). The latter exert a pinning effect on the amorphous grain boundary phases in the silicon nitride and also act as nucleation sites for beta-Si3N4, which limits grain growth during sintering. This finer microstructure results in strengths higher than for the monolithic silicon nitride. Intragranular SiC particles enhance strength and fracture toughness as a result of residual compressive thermal stresses within the nanocomposites. High temperature strength and creep resistance are also much higher than for monolithic silicon nitride and a few investigations of these topics are briefly reviewed and the proposed mechanisms are described. Within the context of other studies cited, work on Si3N4-SiC micro-nanocomposites by the current authors describes an aqueous processing route for better dispersion of commercial powders prior to sintering.