| 1. |
- Hansson, Kristina, 1971, et al.
(författare)
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The beneficial effect of water vapour on the oxidation at 600 and 700 °C of a MoSi2-based composite
- 2005
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Ingår i: JOURNAL OF THE EUROPEAN CERAMIC SOCIETY. ; 25:1, s. 1-11
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation characteristics of a MoSi2-based composite in O2 and O2+10% H2O at 600 and 700 °C were investigated. The effects of temperature and water vapour on oxidation were examined. The oxidation kinetics were studied using a thermobalance and furnace exposure, while the morphologies and compositions of the oxides were examined using XRD, ESEM/EDX, and SEM/EDX. We propose that oxidation proceeds by the initial formation of MoO3 crystals and amorphous SiO2 on the surface. The MoO3 is then evaporated; as volatile (MoO3)3 species in O2 and additional MoO2(OH)2 species in O2+10% H2O, which results in a porous, Mo-depleted oxide. However, the pores in the Mo-depleted SiO2 scale heal, and a protective crystalline scale is established eventually. The vapour pressures of the abovementioned volatile species increase with temperature and/or water vapour content in the atmosphere, which leads to accelerated Mo depletion from the oxide scale. A shorter time elapses before the oxide scale is transformed into the relatively Mo-free protective SiO2 scale, which results in less oxide being formed. Thus, the formed scale becomes thinner in O2+10% H2O than in O2. Thereby the Mo removal is beneficial when water vapour is added to the exposure atmosphere.
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| 2. |
- Hellström, Kristina M, 1971, et al.
(författare)
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Oxidation behaviour of a (Mo, W)Si2-based composite in dry and wet oxygen atmospheres in the temperature range 350–950◦C
- 2009
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Ingår i: Journal of the European Ceramic Society. - : Elsevier BV. - 1873-619X .- 0955-2219. ; 29:10, s. 2105-2118
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation of a (Mo, W)Si2-based composite was investigated in the temperature range (350–950 °C). The influence of temperature and water vapour on the oxidation was examined. The kinetics was studied using a thermobalance whereas the morphology and composition of the oxides were examined using X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive X-ray (EDX). Focused ion beam (FIB) milling was performed on some of the oxide scales which allowed us to look at a non-mechanically disturbed scale/oxide in cross-section. Rapid oxidation was found to occur in the 550–750 °C temperature range. The mass gains were significantly larger in O2 than in O2 + 10%H2O. The different mass changes in the two exposure atmospheres were attributed to the higher vapour pressure of the volatile MoO2(OH)2 and WO2(OH)2 species in O2 + 10%H2O than that of (MoO3)3 and (WO3)3 in dry O2. The peak mass gain was found to occur at a temperature of about 750 °C in O2 and 650 °C in O2 + 10%H2O. At temperatures above 850 °C, especially when water vapour is present, the removal of Mo and W from the oxide scales is rapid enough to allow partial healing of the silica, causing the oxidation rate to drop. At 950 °C in O2 + 10%H2O, a protective SiO2 scale could be re-established quickly and maintained, causing the oxidation to essentially cease.
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