High temperature oxidation behavior of Al added Mo / Mo₅SiB ₂ in-situ composites

Isothermal oxidation behavior of Mo/Mo₅SiB₂ in-situ composites containing small amounts of Al was investigated under an Ar-20%O ₂ atmosphere in the temperature range of 1073-1673 K. The Mo/Mo ₅SiB₂ in-situ composites, (Mo-8.7mol%Si-17.4mol%B) _100-xAl_x (x=0, 1, 3, and 5mol%), were prepared by Ar arc-melting, and then homogenized at 2073 K for 24 h in an Ar-flow atmosphere. Without addition of Al, Mo/Mo₅SiB₂ in-situ composite exhibits a rapid mass loss at the initial oxidation stage, followed by passive oxidation after the substrate is sealed with borosilicate glass in the temperature range of 1173-1473 K, whereas it exhibits a rapid mass gain around 1073 K. On the other hand, small Al additions, especially of 1 mol%, significantly improve the oxidation resistance of Mo/Mo₅SiB ₂ in-situ composites at temperatures from 1073-1573 K. The excellent oxidation resistance is considered to be due to the rapid formation of a continuous, dense scale of Al-Si-O complex oxides. The protective oxide scales contain crystalline oxides, and the amounts of the crystalline oxides obviously increase with Al concentration.