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- Hansson, Kristina, 1971, et al.
(författare)
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On the Mechanism of MoSi2 Pesting in the Temperature Range 400–500°C
- 2000
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Ingår i: Ceramic Engineering and Science Proceedings. - 0196-6219 .- 1940-6339. ; 21:4, s. 469-476
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation of a MoSi2 composite was studied in the temperature range 400–500°C. The peak pesting temperature was identified and the oxidation kinetics was observed up to a holding time of 4000 hours. The reaction kinetics was tracked using thermogravimetric analysis :IS well iis oxide thickness nieasurenients. A detailed analysis of the morphology and composition of the oxide was performed using SEM and EDX. The peak pesting temperature was found to be 470°C. The reaction kinetics in static air could be described with two consecutive parabolic oxidation curves, one up to 500 hours and another between 500–4000 hours. With a higher water vapor coiitent the kinetics showed linear behwior. MoO3 evaporation took place during the entire duration of the oxidation. The oxide appeared to grow by inward diffusion of oxygen to the oxidehulk interface.
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- Hansson, Kristina, 1971, et al.
(författare)
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Oxidation behaviour of a MoSi2-based composite in different atmospheres in the low temperature range (400–550 °C)
- 2004
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Ingår i: Journal of the European Ceramic Society. - : Elsevier BV. - 1873-619X .- 0955-2219. ; 24:13, s. 3559-3573
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation characteristics of a MoSi2-based composite within the temperature range of 400–550 °C were investigated. The effects of temperature and water vapour on oxidation were examined. The oxidation kinetics were studied using a thermobalance, while the morphology and composition of the oxides were examined using XRD, ESEM/EDX, and SEM/EDX.The peak oxidation rates in dry O2 and View the MathML source were found to occur at temperatures of approximately 510 and 470 °C, respectively. Within the temperature range of accelerated oxidation (400–500 °C), the oxidation rate in View the MathML source was substantially higher than that in dry O2. At higher temperatures, the oxidation rate decreased, and the magnitude of the decrease was steeper and occurred at a lower temperature for View the MathML source (510 °C) than for O2 (550 °C). Furthermore, the rate of depletion of molybdenum (Mo) from the oxide scales during oxidation increased with increasing temperature and water vapour content. It appears that Mo loss is a key process influencing the protective properties of the oxide layer on the MoSi2 composite. A potential mechanism for the different oxidation behaviours in O2 and View the MathML source is proposed.
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- Hansson, Kristina, 1971, et al.
(författare)
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The influence of water vapor on the oxidation of MoSi2 at 450 °C
- 2001
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Ingår i: Materials Science Forum. - 1662-9752 .- 0255-5476. ; 369-372, s. 419-426
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation of a MoSi2 composite was studied in dry air, oxygen and oxygensaturated with 10% water vapour at 450°C. The kinetics were investigated using TGA as well as oxide thickness measurements. Detailed analyses were performed on the morphology and composition of the oxide using XRD, ESEM, SEM, and EDX. It is shown that the oxidation rate increases drastically in the presence of water vapour, and the growth of Mo03 crystals on the oxide surface increases considerably. The different regions in the oxide cross-section are Mo-depleted compared with the corresponding regions in the bulk when oxidised in oxygen saturated with 10% water vapour. However, the samples oxidised in dry oxygen only shows Mo-depletion in some outer parts of the oxide. Accelerated growth of the MoSi2-oxide layer during exposure in 02+10%H20 compared to that in 02 can be related to the fact that more volatile Mo-species form in the presence of water vapour, resulting in a substantial loss of Mo03 from the inner part of the oxide. The voids left behind are not healed by the silica at this low temperature, which leaves the oxide with an open structure. As a result, the oxidation rate increases.
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- Tang, Jun Eu, 1974, et al.
(författare)
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An Investigation of the Microstructure in the Pest Oxide of a MoSi2- Based Composite
- 2000
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Ingår i: Ceramic Engineering and Science Proceedings. - Hoboken, NJ, USA : John Wiley & Sons, Inc.. - 0196-6219 .- 1940-6339. ; 21:4, s. 477-484
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Tidskriftsartikel (refereegranskat)abstract
- The pesting of MoSi2 was investigated by performing detailed microanalysis on the pest oxide layer. The studied material was a MoSi2-Mo5i3-clay composite which had undergone pest-oxidation for 4000 hours at 450°C. Results from detailed SEM and TEM studies, including quantitative EDX analysis, on the various features in the pest oxide are presented. It was found that MoSi2, as well as the MosSi3, transforms immediately into an oxide mixture of MoO3 nanocrystals and amorphous SiO2 with significant loss in molybdenum upon oxidation. With time, part of the oxide mixture cluster into lamellar MoO3 aggregates. These aggregates disappear from the oxide after even longer times, leaving voids in the oxide structure. This allows even quicker depletion of MoO3 from the oxide, leaving dark grey regions containing mostly SO2.
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- Tang, Jun Eu, 1974, et al.
(författare)
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Microanalysis on the oxidation and sulfate attack of partially stabilized zirconia thermal barrier coating
- 2001
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Ingår i: Ceramic Engineering and Science Proceedings. - 0196-6219 .- 1940-6339. ; 22:4, s. 463-470
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Tidskriftsartikel (refereegranskat)abstract
- This aim of this investigation is to study the effect of the presence of Na2SO4 deposits and water vapor on the oxidation of an air plasma-sprayed TBC composed of a partially stabilized ZrO2 top coat with an underlying NiCoCrAlY bond coat. XRD and SEM/EDX were used to analyze the changes in the coatings after oxidation at 1000degreesC for 72 hours in dry or humidified (containing 50% H2O) O-2 atmosphere, with or without Na2SO4 deposited on the top coat. When oxidized in oxygen, bond coat oxide regions, consisting of almost pure aluminum oxide, were formed at the top coat / bond coat and bond coat / substrate interfaces and in the bond coat around the splat lines. When water vapor was present, the top coat / bond coat interface oxide was marginally thicker and included small regions with more chromium, cobalt and nickel. The addition of the salt deposits resulted in some destabilization in the outermost regions of the top coat. The salt deposits also caused the formation of thicker oxide comprising two regions, though this formation was observed only at the top coat / bond coat interface. The first type was a thin inner (i.e. bordering the bond coat) oxide that was mostly aluminum oxide. The other was a much thicker oxide containing higher levels of chromium, cobalt and nickel along with aluminum. In this oxide region, the aluminum level was higher when the chromium level was lower and vice versa.
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- Tang, Jun Eu, 1974
(författare)
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The Influence of Water Vapour on the Oxidation Behaviour of High-temperature Materials Microstructure investigation using analytical electron microscopy
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis covers the analytical electron microscopy work carried out on samples exposed to high-temperature oxidation in O2 atmospheres with the intention of investigating the effect of oxygen and water vapour. SEM/EDS, TEM/EDS and diffraction techniques, complemented by FIB precision milling, were employed to characterise the formed oxide scales as well as the material directly under the scale in order to understand the mechanisms that influence the corrosion process. Two distinct categories of high-temperature materials - Fe-Cr stainless steels and MoSi2-based refractory composites - were studied in this research, both showing behaviour affected by the selective volatilisation of species from the oxide scale by oxygen and water vapour. On Fe-Cr stainless steels, water vapour heightened Cr vaporisation (through the formation of volatile CrO2(OH)2) from the originally Cr-rich oxide scale even at temperatures as low as 600°C. Below a critical Cr concentration, transition to a non-protective oxide scale occurred, followed by rapid breakaway oxide growth. The severity of corrosion was a function of the degree of Cr vaporisation (intensified by an increase in pH2O and oxidant gas flow rate) and the Cr supply from the underlying steel. The latter, in turn, depended on the Cr content and the microstructure near the steel surface. Comparing type 304L (18Cr10Ni) and 310 (25Cr19Ni) austenitic stainless steels, the higher Cr content of the latter provided it with the capability to withstand a higher degree of Cr vaporisation. As austenitic steel grains are relatively large (~ 50 µm), the breakaway tended to be localised, occurring far away from high diffusivity paths (i.e. efficient Cr supply) such as steel grain boundaries. The local breakaway resulted in oxide nodules, each consisting of an outward growing Fe-rich island and an inward growing multi-phase subscale crater region. In contrast, the X20 (CrMoV 11 1) ferritic/martensitic steel microstructure, with more rapid bulk diffusion and high grain boundary density, was more efficient at maintaining the Cr concentration in the oxide scale initially, only to suffer rapid and widespread breakaway later due to its lower overall Cr content. MoSi2-based refractory composites oxidised in O2/H2O in the 400-600°C range, producing MoO3 nanocrystals in amorphous SiO2. Mo vaporisation resulted in a more open structure in the silica scale, hence facilitating oxygen access. The oxidation rate peaked at around 500°C; at higher temperatures, silica healing could take place. Above 600°C, the quicker MoO3 removal and silica healing resulted in a protective pure silica scale, thus preventing further oxidation. The presence of water vapour expedited Mo vaporisation through the formation of more volatile MoO2(OH)2. It was thus detrimental at lower temperatures, but beneficial at higher temperatures where silica healing was possible. The oxidation of (Mo,W)Si2 produced WO3 in addition to MoO3 and SiO2. The more sluggish vaporisation kinetics of W meant that the peak oxidation temperature was higher (around 750°C) and protective silica scale formation was possible only above 950°C.
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