Effect of CO2 partial pressure on oxidation of low-oxygen SiC fibers (Hi-Nicalon) in Ar-CO2 gas mixtures

The oxidation behavior and thermal stability of Si—C fibers (Hi-Nicalon) in Ar-CO2 gas mixtures were investigated at 1773 K, through mass change determination, XRD analysis, resistivity measurement, SEM observation and tensile tests. Mass gain and cristobalite formation were observed at pCO2 ≧ 103 Pa, showing the occurrence of passive-oxidation of the fibers. On the other hand, the active-oxidation was characterized by the mass loss, no formation of SiO2 film and a marked increase in resistivity at pCO2 ≦ 5 × 102 Pa. The oxygen potential for the active-to-passive oxidation transition in Ar-CO2 gas mixtures was nearly identical to that in Ar-O2 gas mixtures. About 50% of the strength in the as-received state was retained after the active-oxidation in Ar-CO2 gas mixtures.