High‐Temperature Oxidation of Molybdenum Disilicide

Abstract

The oxidation of MoSi, in air at atmospheric pressure was studied by electron diffraction, Xray diffraction, and thermogravimetric analyses. The oxidation process occurs in two parts: (1) formation of MooI and SiOz at temperatures below the boiling point of Moo3, and (2) formation of Mo6Si3 and SiOz at higher temperatures. Evidence is presented which indicates that oxygen permeation through a silica layer, which may be of a mixed crystalline-glassy nature, controls reaction rate at high temperatures and that Mo& is present directly beneath the protective oxide. The activation energy for oxidation of MoSiz above 1200°C was calculated as 81.3 kcal mole-'.

Oxidation behavior of molybdenum silicides and their composites

“…Of the potential candidate systems, Mo silicides are particularly attractive owing to their high melting points. Molybdenum disilicide exhibits particularly good mechanical strength properties [1][2][3], high thermal conductivity, high electrical conductivity [4] and promising oxidation resistance at elevated temperature [5][6][7][8]. However, the material has been reported to exhibit high creep rates at temperatures > 1200"C [9] and degradation by a phenomenon known as "pesting" in the temperature range of 400-600"C [10][11][12][13].…”

MoSi2 Oxidation in 670–1498 K Water Vapor

“…(2), thus leading to passivation above 873 K. 6,8,5 Similarly, it is argued here, at 670-773 K, the MoO 2 (OH) 2 volatility allows for a more passive oxide formation, primarily SiO 2 . In a previous study, the hydroxide volatility was displayed as linear mass loss of Mo exposed to steam as low as 823 K where in air MoO 2 remained a stable oxide on the surface of the Mo sample.…”

“…(1) above. 6 MoO 3 behavior dominates the response of Mo oxidation in air and water vapor, as recently shown by Nelson and Sooby et al 7 At temperatures below roughly 873 K, MoO 3 will remain a solid and form whiskers throughout the material. This causes expansion of the bulk material and exposure of unoxidized MoSi 2 , preventing the SiO 2 from forming a protective layer.…”

Oxidation features of SiOC/MoSi2 composite at low temperatures

“…Also, no pesting was observed during isothermal oxidation at 450 o C for 100 hr, in spite of the large amount of MoSi2. It is known that MoSi2 oxidizes to SiO2, accompanying the formation of protective Mo5Si3 (above 800 o C) or the evaporation of highly volatile Mo-oxides such as MoO3, which has excessive vapor pressure (below 800 o C) [6]. SiOC oxidizes to protective SiO 2 [5], possibly accompanying CO(g) evolution [7].…”