Oxidation and hot corrosion of hot-pressed SiC containing either $Al_2O_3$ or $Y_2O_3$ were investigated.
For oxidation experiment, specimens hot pressed at 2050℃ were oxidized at the temperatures between 1000 and 1500℃ up to 50hr in a static air atmosphere. The weight gain of the SiC hot-pressed with $Y_2O_3$ additive was higher than that of the SiC hot-pressed with $Al_2O_3$ additive. The diffusion of yttrium ion from grain boundary to surface and the formation of yttrium silicate compound enhanced the weight gain of SiC hot-pressed with $Y_2O_3$ additive, but the formation of mullite in SiC hot-pressed with $Al_2O_3$ additive appeared to retard the diffusion of oxygen and the weight gain of SiC hot-pressed with $Al_2O_3$ additive.
For hot corrosion experiments, specimens hot pressed at 2050℃ were coated with $Na_2SO_4$ and corroded at 1000℃ up to 8hr in a static air atmosphere. X-ray diffraction result showed that the intensity of the cristobalite peak of SiC hot-pressed with $Y_2O_3$ additive was higher than that of SiC hot-pressed with $Al_2O_3$ additive. This result suggests that $Y_2O_3$ promotes the crystallization of $SiO_2$ in the melt formed during the hot corrosion process. Since the diffusion of oxygen through cristobalite is slower than that of oxygen through melt, it was expected that the weight gain of SiC hot-pressed with $Al_2O_3$ additive was higher than that of SiC hot-pressed with $Y_2O_3$ additive. however, the weight gain of SiC hot-pressed with $Al_2O_3$ additive was similiar to that of SiC hot-pressed with $Y_2O_3$ additive. The formation of silicate compound via the reaction of $SiO_2$, oxygen, and yttrium diffused out to surface was responsible for relatively high weight gain of SiC hot-pressed with $Y_2O_3$ additive.
Four-point bending test results revealed 12% strength degradation after hot corrosion experiment in the case of SiC hot-pressed with $Y_2O_3$ additive and 35% strength degradation in the case of SiC hot-pressed with $Al_2O_3$ additive. This difference resulted from relatively large pits of SiC hot-pressed with $Al_2O_3$ additive.
