High-Temperature Oxidation

Abstract

The oxidation of MoSi2 , Mo5Si3 , and Mo3normalSi has been studied at temperatures of 1300°–2000°K, and at oxygen partial pressures of 2–20 Torr. Below 1970°K, at oxygen pressures around 10 Torr, measurements of the rate of total oxygen consumption vs. time show an initial period of high linear rate, an intermediate region where the rate declines sharply with time, and a final plateau region of virtually zero oxidation rate. For each of the silicides, the total oxygen consumed prior to the attainment of the plateau decreases with temperature, and under given experimental conditions is largest for Mo3normalSi , less by a factor of about six for Mo5Si3 , and less by another factor of six for MoSi2 . The oxidized samples display a smooth glassy outer oxide layer that bridges across the tops of the cracks in the alloys and an inner oxidized zone, consisting of several irregular phases. Electron micro‐probe analysis and x‐ray photomicrographs of the oxidized specimens show that the outer protective oxide is pure SiO2 , within the limits of detectability, i.e., the outer oxide contains less than 0.1% Mo. The inner oxide layer contains SiO2 , molybdenum rich phases, including the terminal solid solution phase, and the original alloy phase. Evidence is presented that suggests that the oxide layers grow by inward diffusion of oxygen to the oxide/alloy interface where preferential oxidation of silicon occurs. The excellent oxidation resistance of the molybdenum silicides derives from the formation of this continuous layer of pure silica. Above 1970°K, at oxygen pressures above 10 Torr, an increased oxidation rate is observed due to formation of normalSiOfalse(normalgfalse) at the alloy/oxide interface, which causes rupture of the protective oxide. Below 1970°K, at oxygen pressures near 2 Torr, the rate of oxidation of MoSi2 is linear at all times since the oxygen partial pressure is insufficient to maintain a protective SiO2 layer on the alloy surface.

Oxidation behavior of Mo–12.5Si–25B alloy at high temperature

“…For decades, MoSi 2 has been extensively investigated and found to have excellent oxidation resistance at elevated temperatures (up to 1700 • C). Although MoSi 2 has excellent high temperature oxidation resistances, it has a high creep rate above 1200 • C [1] and shows poor oxidation resistance at moderate temperature (about 500 • C) because of "pest oxidation" [2][3][4]. These make it unsuitable as a high temperature structural material.…”

Thermal Oxidation Kinetics of MoSi2‐Based Powders

“…12,13 However, for simplicity, we will only mention MoO 3 in this paper, as did other researchers. 2,5,6,10,11,14 Thermogravimetric analysis (TGA) is a powerful tool for studying oxidation kinetics. However, the phase transition of MoO 3 makes it difficult to do accurate high-temperature oxidation studies on MoSi 2 using commercial TGA instruments.…”

High-Temperature Oxidation

“…A known mixture of helium and oxygen was passed through the reference side of a thermal conductivity cell (6) and over an inductively heated carbide pellet supported on ThO2 fingers by three point contact. The signal from the thermal conductivity cell in this case was therefore not directly related to the rate of oxygen consumption as in previously studied systems (6)(7)(8), but instead reflected the difference between the rate of total oxygen consumption and the rate of formation of CO(g). The effluent gas was therefore depleted in oxygen, but enriched in CO(g) and CO2(g).…”

“…Due to the evolution of permanent gases, the thermal conductivity method described in previous publications (6)(7)(8) had to be modified to study the oxidation. Due to the evolution of permanent gases, the thermal conductivity method described in previous publications (6)(7)(8) had to be modified to study the oxidation.…”