THE ROLES OF AMORPHOUS GRAIN-BOUNDARIES AND THE BETA-ALPHA TRANSFORMATION IN TOUGHENING SIC

Controlled development of the ceramic microstructure has produced sili con carbide (SiC) with a toughness three times that of a commercial Si C, 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 170 0 degrees C, the formation of an amorphous phase at the grain boundari es to enhance intergranular fracture, and the promotion of an elongate d microstructure to enhance crack deflection and crack bridging. Compa risons of microstructure and fracture properties have been made betwee n the present ABC-SiC, Hexoloy-SA and other reported SiC ceramics sint ered with YAG or Al2O3. The Al-O chemistry of the amorphous phase in t he ABC-SiC accounted for the intergranular fracture vs the transgranul ar fracture in Hexoloy-SA. An interlocking, plate-like grain structure developed during the beta to alpha transformation without limiting de nsification. The combined microstructural developments improved both s trength and toughness. (C) 1998 Acta Metallurgica Inc.