The oxidation of zone melted specimens of ZrB2 and HfB2 was studied at temperatures of 1200°–2000°K and at oxygen partial pressures of 1–700 Torr in helium. Parabolic rate behavior was observed. For ZrB2 above 1400°K an activation energy of 77±5 normalkcal/normalmole was determined. Below 1400°K the activation energy is much lower, and the mechanism of oxidation is different, probably because of the presence of boron oxide in the condensed phase. Below 1300°K, the parabolic rate constant appears to be directly proportional to oxygen partial pressure; at 1830°K, the rate constant is independent of oxygen pressure. For HfB2 in the range 1480°–1870°K an activation energy of 47±4 normalkcal/normalmole was observed. Around 1970°K, the transition temperature for the monoclinic to tetragonal phase change in HfO2 , there is a sharp increase in the rate of oxidation. The parabolic rate constant for total oxygen consumption increases as the 0.4±0.1 power of the oxygen partial pressure at 1630° and 1760°K.

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.

The oxidation of molybdenum and tungsten was investigated mass spectrometrically at temperatures between 1500° and 2600°K and oxygen pressures between 10—4 and 10—2 Torr. The principal products of oxidation were found to be the gaseous dioxide and trioxide, MO2(g) and MO3(g) (M=Mo or W). The trioxide is the more important species at low temperatures; the dioxide is the dominant product at high temperatures. Small amounts of polymeric oxides, (MO3)2(g) and (MO3)3(g) were formed at the lowest temperatures and highest oxygen pressures. The gaseous oxidation products are not in equilibrium with a solid surface oxide.

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