CORROSION OF SIC MATERIALS IN N2-H2-CO GASEOUS ENVIRONMENTS .2. DURABILITY AND MECHANICAL-PROPERTIES

Three SiC materials were exposed to two gas compositions in the N2-H2-CO system at 1200-degrees and 1300-degrees-C, 133 Pa and 98.7 kPa for 50-1000 h. The durability was evaluated by fracturing bend bars at room temperature and C-rings at room temperature and 1300-degrees-C in argon. Fractography was used to correlate changes in strength with changes in flaw populations. In all three materials, surface pitting was the primary cause of degradation. Under some conditions it was found that flaw healing and new flaw formation occurred simultaneously. C-rings cut from tubes did not lose strength, although pitting occurred; however, machined bend bars lost strength under identical conditions. In the case of bend bars, following an initial loss of strength (approximately 10%-50%) after 50-150 h, there was typically no further loss observed with increased exposure time. Due to its higher P(CO), degradation in an endothermic gas was generally more severe than that in a nitrogen-based gas.