Isothermal and Cyclic Oxidation of Mo(Si1−x, Alx)2 up to 2048 K

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Abstract
High temperature oxidation of Mo(Si1−x, Alx)2 and MoSi2 has been examined under isothermal and thermal cyclic conditions up to 2048 K in air. Below the eutectic temperature (1868 K) in SiO2-mullite system, Mo(Si1−x, Alx)2 forms Al2O3 scales. The oxidation rate of Mo(Si1−x, Alx)2 is governed by the diffusion of Al through the layer of Mo5Si3 which forms beneath the Al2O3 scale. Above the eutectic temperature, Mo(Si1−x, Alx)2 forms a liquid scale composed of SiO2 and Al2O3. The oxidation rate of Mo(Si1−x, Alx)2 is governed by the inward diffusion of O2 molecules through the scale. Both Mo(Si0.86, Al0.14)2 and MoSi2 exhibit a good scale adherence in the cyclic oxidation at 1723 K. In the cyclic oxidation at 2023 K Mo(Si0.86, Al0.14)2 has good scale adherence, although the SiO2 scale on MoSi2 largely spalls off. The difference in spalling of the scale arises from degrees of crystallization of the amorphous SiO2 to crystobalite. The oxidation rate of Mo(Si0.86, Al0.14)2 is higher than that of MoSi2. A partial phase diagram of the Mo–Si–Al system was experimentally determined. The diagram reasonably explains the oxidation processes.

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