Long-Term Oxidation of Ti₂AlC in Air and Water Vapor at 1000–1300°C Temperature Range

The long-term isothermal oxidation behavior of bulk Ti 2 AlC in air and 100% water vapor has been investigated in the 1000–1300°C temperature range. The kinetics, for oxidation up to 120 hours, follows a cubic rate law in both environments – air and water vapor. The kinetics was found to be slightly faster for hydrothermal oxidation compared to oxidation in air, especially below 1100°C. However, there is little variation between the activation energies for oxidation in air (approximately 279 kJ/m 3 ) and in 100% water vapor (approximately 261 kJ/m 3 ). Scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction show that oxidation of Ti 2 AlC forms mostly a continuous and stable layer of α-Al 2 O 3 , along with a thin surface layer of rutile-TiO 2 in both environments. However, the thin TiO 2 layer volatilizes by forming gaseous TiO(OH) 2 in the presence of water vapor at high temperatures (>1200°C). At short-term oxidation and lower temperatures, diffusion of hydroxyl ions is proposed to be the rate limiting mechanism under hydrothermal conditions. Above 1100°C, the diffusion of oxide ions through Al 2 O 3 layer becomes the rate limiting step and humidity has little effect on the overall oxidation kinetics of Ti 2 AlC.