Thermally grown oxide in water vapor on coated and uncoated SiC
Benjamin A. Kowalski, Bryan J. Harder
Abstract
Abstract Coupons of Hexoloy ® , CVD SiC, and SiO 2 were thermally cycled in a flowing steam atmosphere of 90%/10% H 2 O/O 2 at 1426°C in order to simulate the water vapor partial pressure of a turbine environment. The paralinear model for oxidation and volatilization is examined and a condensed version is provided that allows for extraction of the oxidation ( k p ) and volatilization ( k l ) rates from only a few measured data points across a small time window. Due to high Si(OH) 4 (g) volatility rates, SiO 2 scale thickness approached nearly invariant paralinear limiting values (~3‐6 μm) for all conditions, including cycle frequency or material. However, a disparity still exists between weight changes measured and the thickness of the resulting oxide as well as the contribution from the material properties to the oxidation and volatilization rates. Comparisons are made for the oxidation and volatilization rates, specific weight changes, and oxide thicknesses for a number of cycle times for both uncoated and environmental barrier coated SiC.