SiCw/Sialon composite was made by pressureless sintering and HIP processing. When 5vol% SiCw added and hipped at 1973K, the fracture toughness was higher than monolithic material, but strength was the same level. On the other hand, when 10vol% SiCw added, the changing of fracture toughness was small, but strength was higher than monolithic material. The structure of these composite was observed by SEM. The structure formed with grown up β'-Sialon grain and the small grain. This "composite structure" indicated high strength and high fracture toughness. The structure of monolithic material was similar to composite, but the crack-path cross the Sialon grain. Because of this intragaranular fracture, the fractures toughness of monolithic material was low. It was considered that the effect of SiCw was control of glass phase of grain boundary.
