The roles of amorphous grain boundaries and the β– α transformation in toughening SiC

Controlled development of the ceramic microstructure has produced silicon carbide (SiC) with a toughness three times that of a commercial SiC, Hexoloy–SA, coupled with >50% improvement in strength. Al, B and C were used as sintering additives, hence the designation ABC–SiC. These additives facilitated full densification at temperature as low as 1700°C, the formation of an amorphous phase at the grain boundaries to enhance intergranular fracture, and the promotion of an elongated microstructure to enhance crack deflection and crack bridging. Comparisons of microstructures and fracture properties have been made between the present ABC–SiC, Hexoloy–SA and other reported SiC ceramics sintered with YAG or Al 2O 3. The AlO chemistry of the amorphous phase in the ABC–SiC accounted for the intergranular fracture vs the transgranular fracture in Hexoloy–SA. An interlocking, plate-like grain structure developed during the β to α transformation without limiting densification. The combined microstructural developments improved both strength and toughness.