Chemical Considerations in Carbon-Fiber/Oxide-Matrix Composites

Thermochemical calculations are performed to evaluate the chemical compatibility of oxides with carbon at temperatures of the order of 1650°C and to evaluate the implications of carbon oxidation. These calculations indicate that Al₂O₃, BeO, Y₂O₃, and Ce₂O₃ are stable with carbon provided there are no cracks or pores for CO gas to escape. In the presence of cracks or interconnected pores, both the direct reduction of oxides with carbon as well as the oxidation of carbon can proceed rapidly. The calculations also indicate that even in the absence of such cracks the oxidation of carbon can generate high enough CO partial pressures to degrade the composite properties by formation of CO gas bubbles or of cracks in the oxide. The results of these calculations are confirmed by conducting experiments on samples of pyrolytic and vitreous carbon disks hot-pressed within Al₂O₃ and Y₂O₃disks and exposing them in argon and oxygen environments at 1650–1800°C. An oxidation model is then proposed that includes thermodynamic and kinetic considerations as well as creep behavior of oxides. The model quantitatively predicts the times required to initiate the formation of gas bubbles and qualitatively considers the factors influencing the growth rate of bubbles and formation of cracks.