Papers by Keyword: Thermal Cyclic Oxidation

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Authors: Yong Hwan Kim, K.T. Kim
Abstract: The microstructure and thermal cyclic oxidation resistance of the wide-gap region brazed with different filler metal powder (BNi-3 and DF 4B) comparing with that of Ni-based IN738 alloy were investigated. The microstructure characterization showed that Cr borides with a blocky morphology were existed in the brazed region in both filler metal powder. The normalized weight gain with cyclic oxidation showed that weight loss of the specimen brazed with BNi-3 filler metal occurred after 600 cycles. However, the specimen brazed with DF 4B filler metal had no obvious weight loss until 700 cycles. It was observed that the oxidation kinetics of the all oxidized specimens followed the quasi-parabolic law, and the oxide layer was mainly composed of NiO, Al2O3 and NiCr2O4.
894
Authors: Raphaël Rolland, Henri Buscail, Christophe Issartel, Frédéric Riffard, Françoise Rabaste, Sébastien Perrier
Abstract: In isothermal oxidation condition, water vapour has little effect on the oxidation rate and scale composition of a nickel-based SY 625 alloy oxidized at 1100°C. The scale is composed of an outer Cr2O3 and an internal CrNbO4 scale. The oxide scale morphology differs between dry and wet conditions. Under dry conditions the oxide scale appears to be compact and chromia pegs are observed at the internal interface. Under wet conditions, porosities are observed spread inside the scale and the chromia grain size is smaller. At this temperature some scale spallation is observed under dry and wet conditions. Under cyclic oxidation conditions the oxide scale adherence is slightly improved in wet environment. The chromia scale is adherent during the 4 first oxidation cycles. In dry air, spallation occured after the first cycle. In dry and wet conditions, after the chromia scale spallation has started, NiO and NiCr2O4 form first. NiMoO4 forms later on the alloy surface during the cycling test. The best resistance of the alloy under thermal cycling conditions under wet conditions is related to the presence of a more plastic and adherent scale owing to a higher scale porosity and smaller chromia grain size compared to dry conditions.
333
Authors: Wen Ma, Yue Ma, Sheng Kai Gong, Hui Bin Xu, Xue Qiang Cao
Abstract: Lanthanum-cerium oxide (La2Ce2O7, LC) is considered as a new candidate material for thermal barrier coatings (TBCs) because of its low thermal conductivity and high phase stability between room temperature and 1673K. The LC coatings with different La2O3 contents were prepared by air plasma spraying (APS) and their lifetime was evaluated by thermal cyclic testing from room temperature to 1373 K. The structures of the coatings were characterized by XRD and SEM and the deviation of the composition from the powder was determined by EDS analysis. Long time annealing for the freestanding coating at 1673K reveals that the near stoichiometric LC coating is stable up to 240h, and the stability decreases with increasing the deviation from stoichiometric LC composition. During thermal cyclic testing, spallation was observed within the top coat near the bond coat. It is considered that the effect of intrinsic stress caused by the coefficient of thermal expansion (CTE) mismatch between top coat and bond coat is larger than that of thermally grown oxide (TGO) and the bond adherence of top coat with TGO.
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