High temperature oxidation behaviour of CVD β-SiC seal coated SiCf/SiC composites in static dry air and combustion environment

A composite with SiC matrix and 40 vol% of SiC fibres with a beta-SiC coating (SiCf/SiC) was prepared by the isothermal chemical vapour infiltration process. Oxidation of the SiCf/SiC composite was investigated at 1200 degrees C, 1300 degrees C and 1400 degrees C, for up to 100 h in static dry air, and for up to 24 h in combustion environment. In static dry air, the composite showed mass gain with nearly parabolic oxidation kinetics. The uniform oxide scale developed cracks as oxidation progressed. Oxidation in combustion environment was performed using an oxy-acetylene flame containing CO2 and H2O gases. The chemical erosion by the reaction of H2O with oxide scale was the likely ablation mechanism. At 1200 degrees C and 1300 degrees C, the oxidation rate was higher than that of ablation and the composite showed a net mass gain. However, at 1400 degrees C, significant ablation was observed. Localised attack by the combustion gases caused pitting of the beta-SiC seal coating. The present study shows that beta-SiC seal coating remains intact at relatively lower temperatures (below 1300 degrees C) for a shorter period of time (similar to 24 h) and is lost it at higher temperatures (similar to 1400 degrees C) by ablation.