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 Al 2 O 3 scales when heated in air to 1350°C. Herein the oxidation of MoAlB ceramics in air was studied in the 1100–1400°C temperature range for up to 200 h. At 1400°C, the oxide scale was heavily cracked and spalled. At 1100°C, and up to 20 h, mass loss was recorded. At 1300°C and 1350°C, subparabolic, approximately cubic kinetics were observed, as a result of growth and coarsening of the Al 2 O 3 grains in the oxide scale. At 1200°C, the weight gain kinetics were nearly linear, while the oxide thickening kinetics were approximately cubic likely due to cubic growth of Al 2 O 3 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°C. Analysis of grain coarsening and scale thickening kinetics suggest that oxygen grain boundary diffusivity is the rate controlling mechanism for the growth of Al 2 O 3 scales at 1300°C and 1350°C. Dimensional changes at samples' corners after long oxidation at T > 1200°C may limit the maximum operational temperature of MoAlB.