Abstract
The mechanism of oxidation of Si-C-O fibers has been investigated. The oxidation rates of two different types of Nicalon fibers (NL400 and NL200) were measured with a thermo-balance in O2 gas stream at temperatures from 873K to 1773K. The reaction products were examined by SEM observation and X-ray diffraction. The results were compared with those of Tyranno fiber. The oxide film was amorphous silica at the beginning of oxidation and at low temperatures, and tended to crystallize on prolonged heating and at high temperatures. The formation of cristobalite increased the oxidation rate. The oxide film suppressed the pyrolysis of the amorphous core of Nicalon. The oxidation rate was analyzed by the parabolic rate law. The temperature dependence of the rate constant k was complicated, because there was a wide variation in the morphology of the oxide film. It is considered that the oxidation of Nicalon was controlled by the diffusion of gas through pores in the oxide film. The oxidation of Tyranno was slower than that of Nicalon at high temperatures, because of the dense SiO2-TiO2 film of Tyranno.
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