THE LOW-TEMPERATURE HEXAGONAL TO ORTHORHOMBIC TRANSFORMATION IN SI3N4REINFORCED BAS MATRIX COMPOSITES

The amount of Si3N4 in a Si3N4-BaO . Al2O3 . 2SiO(2) (BAS) composite a nd the time of sintering was examined in the context of the low temper ature hexagonal to orthorhombic transformation in BAS. It was found th at with increasing amounts of Si3N4 in the composite, the temperature of the hexagonal to orthorhombic transformation was decreased. As the sintering time was increased for a given composite composition, a drop in the temperature of the hexagonal to orthorhombic transformation wa s observed, which was linked to an increase in the beta-Si3N4 content in the composite. In addition, as the Si3N4 content in the composite w as increased, a resultant drop in the coefficient of thermal expansion of the composite occurred. The extent of the linear dimensional chang e of the composite during the BAS hexagonal to orthorhombic transforma tion is reported.