Thermal Oxidation Kinetics of MoSi ₂ ‐Based Powders

The oxidation process of MoSi 2 is very complex, and controversial results have been reported, especially for the early‐stage oxidation before the formation of passive SiO 2 film. Most oxidation studies have been carried out on bulk consolidated samples, and the early stage of oxidation has not been studied. In this investigation, very fine MoSi 2 powder with an average particle size of 1.6 μm was used. Such a fine particle size makes it easier to study the early stages of oxidation since a significant portion of the powder is oxidized before the formation of passive SiO 2 film. The oxidation kinetics of commercial MoSi 2 ‐SiC and MoSi 2 ‐Si 3 N 4 powder mixtures were also studied for comparison. Weight changes were measured at discrete time intervals at 500° to 1100°C in 0.14 atm of oxygen. X‐ray diffraction was used to identify the phases formed during oxidation. Our results show the formation of MoO 3 phase and an associated weight gain at low temperatures (500° and 600°C). At temperatures higher than 900°C, Mo 5 Si 3 phase formed first and was subsequently oxidized to solid SiO 2 and volatile MoO 3 , resulting in an initial weight gain followed by subsequent weight loss. A model based on the assumption that oxidation kinetics of both MoSi 2 and Mo 5 Si 3 are proportional to their fractions in the system describes the experimental data well.