ABNORMAL OXIDATION BEHAVIOR OF SI3N4-SIC- Y2O3 COMPOSITE AT ABOUT 1000-DEGREES-C

A composite with the composition of 58 mass%Si3N4-32 mass%SiC-10 mass% Y2O3 was oxidized at 900-1400 degrees C in dry and moist air atmospher es. The weight gain rate at 1400 degrees C in dry air was as low as 0. 5 mg/cm(2) for 100h, revealing that the composite is highly oxidation- resistant. The weight gain in moist air was 3-7 times larger than that in dry air at all the temperatures tested, The weight gain at 1000 de grees C in moist air was almost the same as that at 1400 degrees C in moist air, The mechanism of the abnormal oxidation at 1000 degrees C i s not the same but similar to that reported by Lange et al. At 1000 de grees C, a protective SiO2 layer which retards the diffusion of oxygen into the composite was not formed and the SiC fine particles in the g rain boundary region of the composite were oxidized, It was assumed th at the volume expansion of about 30% accompanying the conversion of Si C into SiO2 generated cracks in the grain boundary region through whic h oxygen can penetrate toward the unoxidized composite,.Four Si3N4-SiC composites with different compositions also showed large weight gain at 1000 degrees C in moist air. These results indicate that special ca re should be taken in handling Si3N4-SiC composite in moist atmosphere s at high temperatures.