Microstructure of the mullite - X-phase composites made by reaction of β-Sialon and mullite is the mixture of needle shaped particles (15μm) and fine particles (1μm). The amount of liquid phase in the composites decrease with increasing the amount of X-phase. Neither cracks nor liquid phase is obserbed at the boundary between X-phase and mullite. The flexural strength of the composites at 1300°C increases in proportion to the amount of β-Sialon addition and the composite with 10vol% β-Sialon keeps the same flexu ral strength thatmullite shows at room temparature. The fracture toughness of the composites at room temperature does not improved compered with that of mullite.
