Quantitative Evaluation of (0001) Sapphire Recession in High-Temperature High-Velocity Steamjet Exposures

Abstract High-temperature high-velocity steam exposures of (0001) sapphire coupons were performed at temperatures of 1200 °C-1450 °C to determine the quantitative capability of the steamjet apparatus. Recession results were compared to calculated values of Al(OH)3 (g) mass transfer rates based on laminar flow models and available thermodynamic data for Al(OH)3 (g). Linear material volatilization rates and a strong gas velocity dependence on the reaction depth confirmed that the Al2O3 reaction was controlled by a gas-phase diffusion process. The temperature dependence for the steam reaction agreed with thermodynamic calculations and with the literature, confirming that Al(OH)3 (g) transport through a gas boundary layer represents the rate-limiting step for Al2O3 volatilization in steam. The steamjet experimental setup can thus be utilized for determination of steam-oxide reaction thermodynamics given known steam flow conditions. Steamjet test recession results for simple oxides are discussed for comparison with behavior of complex oxides that form porous product layers, which are not yet well understood.