The steam oxidation of Si3N4-bonded SiC was determined at 1 000 ℃ for 50,100,150,200,250 and 300 h,respectively,according to ASTM C863-2000.The evolution of the phase composition and the microstructure as well as their relationship was investigated by XRD and SEM.The results show that the oxidation rate of Si3N4-bonded SiC is periodic.The presence of nitrogen element can impede the crystallization of SiO2 glass;the local enrichment of CaO impurities is unfavorable for the existence of fibrous SiO2.SiO2 mainly exists as cristobalite when the CaO/SiO2 ratio reaches a suitable level,but gradually transforms to quartz along with the oxidation time when the SiO2 content increases,or the CaO/SiO2 ratio decreases,due to the insufficient mineralization of CaO.The crystallization of SiO2 glass,especially the formation of quartz is the key factor leading to the volume expansion and structural stress.When the cracks extend and reach the surface,the degradation of the material accelerates.