Long term oxidation of a SiC fiber-bonded composite in air at 1500 °C

The oxidation behavior of the SiC fiber-bonded composite (Tyrannohex™) was investigated at 1500 °C up to 4000 h in air. During heating of the samples, it was observed that an amorphous SiO2 scale was formed and that bubbles of SiO2 were generated due to the gaseous products upon oxidation. A large mass loss was also observed after 4000 h heating in spite of the condition of passive oxidation for SiC. The cause of large mass loss can be explained by destruction of oxidation protective scale due to bursting of the bubbles, and high oxidation rate due to high oxygen mobility in amorphous SiO2 scale containing aluminum ions. To prevent this accelerated oxidation of the composite, a double-layered coating using a CVD-SiC process was developed. Although the samples coated with CVD-SiC were stable in this oxidation test, the cristobalite peaks observed by XRD suggest also an inherent problem of the SiO2 scale spalling.