The oxidation behaviour and its effect on the mechanical properties of fibrous monolith Si3N4/BN after exposure to air at temperatures ranging from 1000-1400 deg C for up to 20h were investigated. After exposure at 1000C, only the BN cell boundary was oxidised, forming a B203 liquid phase. with increasing exposure temperature, the Si3N4 cells began to oxidise, forming crystalline Y2Si207, Si02, and silicate glass. However, in this case a weight loss was observed due to extensive vaporisation of the B203 liquid. After exposure at 1400 deg C, large YsSi207 crystals with a glassy phase formed near the BN cell boundaries. The oxidation behaviour significantly affected the mechanical properties of the fibrous monolith. The flexural strength and work-of-fracture decreased with increasing exposure temperature, while the noncatastrophic failure was maintained.
