Isothermal and Cyclic Oxidation of MoAlB in Air from 1100°C to 1400°C

Like many FeCrAl-based alloys, and some MAX phases, the atomically laminated boride, MoAlB, forms slow-growing, adherent Al2O3 scales when heated in air to 1350 degrees C. Herein the oxidation of MoAlB ceramics in air was studied in the 1100-1400 degrees C temperature range for up to 200 h. At 1400 degrees C, the oxide scale was heavily cracked and spalled. At 1100 degrees C, and up to 20 h, mass loss was recorded. At 1300 degrees C and 1350 degrees C, subparabolic, approximately cubic kinetics were observed, as a result of growth and coarsening of the Al2O3 grains in the oxide scale. At 1200 degrees C, the weight gain kinetics were nearly linear, while the oxide thickening kinetics were approximately cubic likely due to cubic growth of Al2O3 and concurrent volatility of constituents in the oxide scale. The cyclic oxidation resistance was also good for up to 125, 1-hour, cycles at 1200 degrees C. Analysis of grain coarsening and scale thickening kinetics suggest that oxygen grain boundary diffusivity is the rate controlling mechanism for the growth of Al2O3 scales at 1300 degrees C and 1350 degrees C. Dimensional changes at samples' corners after long oxidation at T > 1200 degrees C may limit the maximum operational temperature of MoAlB. (c) The Author(s) 2017. Published by ECS. All rights reserved.