Thermal oxidation kinetics of MoSi sub 2 -based powders

• Published in: Journal of the American Ceramic Society 1999-10, Vol.82 (10), p.2785-2790
• Main Authors: Zhu, Y T, Stan, M, Conzone, S D, Butt, D P
• Format: Article
• Language: English
• ISSN: 0002-7820

The oxidation process of MoSi sub 2 is very complex, and controversial results have been reported, especially for the early-stage oxidation before the formation of passive SiO sub 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 sub 2 powder with an average particle size of 1.6 mu m 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 sub 2 film. The oxidation kinetics of commercial MoSi sub 2 -SiC and MoSi sub 2 -Si sub 3 N sub 4 powder mixtures were also studied for comparison. Weight changes were measured at discrete time intervals at 500-1100 deg C in 0.14 atm of oxgyen. X-ray diffraction was used to identify the phases formed during oxidation. Our results show the formation of MoO sub 3 phase and an associated weight gain at low temperature (500 deg and 600 deg C). At temperatures > 900 deg C, Mo sub 5 Si sub 3 phase formed first and as subsequently oxidized to solid SiO sub 2 and volatile MoO sub 3 , resulting in an initial weight gain followed by susequent weight loss. A model based on the assumption that oxidation kinetics of both MoSi sub 2 and Mo sub 5 Si sub 3 are proportational to their fractions in the system describes the experimental data well.