Abnormal Oxidation Behavior of Si₃N₄-SiC-Y₂O₃ Composite at about 1000°C

Abstract

A composite with the composition of 58 mass% Si₃N₄-32 mass% SiC-10 mass% Y₂O₃ was oxidized at 900-1400°C in dry and moist air atmospheres. The weight gain rate at 1400°C in dry air was as low as 0.5mg/cm² 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°C in moist air was almost the same as that at 1400°C in moist air. The mechanism of the abnormal oxidation at 1000°C is not the same but similar to that reported by Lange et al. At 1000°C, a protective SiO₂ layer which retards the diffusion of oxygen into the composite was not formed and the SiC fine particles in the grain boundary region of the composite were oxidized. It was assumed that the volume expansion of about 30% accompanying the conversion of SiC into SiO₂ generated cracks in the grain boundary region through which oxygen can penetrate toward the unoxidized composite. Four Si₃N₄-SiC composites with different compositions also showed large weight gain at 1000°C in moist air. These results indicate that special care should be taken in handling Si₃N₄-SiC composite in moist atmospheres at high temperatures.