High-temperature oxidation studies of several silicon-based systems

A mixed control oxidation model was used in Chapter 1 to reevaluate historical assumptions regarding diffusion controlled oxidation. Rather than assuming thermodynamic equilibrium at the scale/substrate interface (a fast chemical reaction), a mixed control model allows finite rates for both the diffusion process and the chemical reaction, which occur in series. This coupling of the two mechanisms by the interfacial oxygen concentration can result in kinetics that appear completely parabolic. However, the interfacial oxygen concentration may never reach the equilibrium thermodynamic value, and may in fact remain at a significant percentage of the atmospheric value, despite the apparently parabolic kinetics. In Chapter 2, siliconized silicon carbide samples were oxidized in high purity oxygen from 1250 to 1600$\sp\circ$C. The parabolic rate constants for the siliconized SiC at temperatures below 1400$\sp\circ$C agreed well with literature values for CVD-SiC and single crystal SiC. At higher temperatures, the parabolic rate constants and activation energy for siliconized SiC oxidation increased. However, the parabolic rate constants obtained in this study for oxidation of CVD-SiC at 1500 and 1600 C were consistent with a single activation energy extrapolated from lower temperature rate constants obtained in prior studies. In Chapter 3, dense, homogeneous samples in the Mo-Al-Si system were fabricated using a combination of self propagating synthesis (SPS) and hot pressing. Materials in the Mo$\sb3$Al$\sb8$-MoSi$\sb2$ pseudobinary had a strong tendency to form alumina (Al$\sb2$O$\sb3$) scales. During oxidation at 600 C for 50 days, arc melted Mo$\sb3$Al$\sb8$ was extremely oxidation resistant--forming a 170 nm thick oxide scale. Increasing the silicon content reduced the oxidation resistance at this temperature for arc-melted samples. At higher temperatures, compositions in the Mo$\sb3$Al$\sb8$-MoSi$\sb2$ pseudobinary with silicon to aluminum ratios greater than 2:1 formed scales containing both silica and alumina. The oxidation kinetics as a function of temperature were determined for hot pressed Mo(Al$\sb{0.1}$Si$\sb{0.9})\sb2$ and Mo(Al$\sb{0.01}$Si$\sb{0.99})\sb2.$ The hot-pressed Mo(Al$\sb{0.1}$Si$\sb{0.9})\sb2$ materials formed alumina scales at low temperatures (1200 C), and oxide scales composed of alumina (Al$\sb2$O$\sb3$) and mullite (3Al$\sb2$O$\sb3\sp\*$2SiO$\sb2$) at higher temperatures ($\u3e$1300 C). Hot pressed Mo(Al$\sb{0.01}$Si$\sb{0.99})\sb2$ materials formed scales consisting of cristobalite (SiO$\sb2$) and mullite. At 1500 C, the oxidation kinetics were extremely nonlinear, presumably due to the formation of a continuous mullite layer at the scale/substrate interface after $\sim$20 hours