Effects of high water vapor pressures on the oxidation of SiC-based fiber-reinforced composites

The oxidation of SiC-fiber-reinforced SiC composites at high temperature (1200°C) and elevated total water vapor pressures (up to 1.5 atm) was studied. Substantial degradation of SiC composites was observed under these exposure conditions, particularly when BN (used as a fiber coating) was a composite constituent. Besides accelerated SiC recession due to oxidation, the reactions of oxygen and H₂O with boron and between boria and SiO₂ led to extensive borosilicate glass formation as well as some volatile H_xB_yO_z products. The extent of these reactions and the rate of overall degradation were compared for different as-processed structures by exposing composites fabricated by processes such as chemical-vapor- and silicon melt-infiltration. Microstructural and compositional factors such as the amount of interconnected porosity and total boron content in the composite had important influences on the rate of degradation. Based on the present results, the overall oxidation behavior of the different composites at high water vapor pressures can be explained based on the concurrent surface reactions involving the formation of volatile reaction products, borosilicate glass, and silica.