It has been demonstrated that intermetallic thin films usually show different oxidation rates compared to those of bulk materials. Within the intermetallic phases, RuAl thin films have not been thoroughly investigated. Thereby, new studies of these systems are needed. Single-phase RuAl was found to be a promising candidate for protective coating material in applications that demand oxidation resistance. An important advantage of this system over other B2-aluminides arises on the coefficient of thermal expansion (CTE), which is substantially lower than that of FeAl, CoAl and NiAl, and it is closer to that of the α-Al2O3, thus increasing the adherence to the alumina protective layer, by decreasing the CTE mismatch between each other. In the present work, the isothermal oxidation behaviour of single-phase RuAl thin films deposited onto austenitic stainless steel substrates was studied in ambient air at 750 and 900 °C for short times (up to 60 min.). Scanning transmission electron microscopy and X-ray diffraction were performed for the subsequent analysis of the oxide scale. A protective α-Al2O3 scale was formed and in comparison to other aluminides, no evidence of the formation of transient alumina was found even at temperatures as low as 750 °C, being this fact an advantage over the most studied aluminides. The presence of particles at the metal/oxide interface is an indication that the oxide growth is dominated by the outward diffusion of Al cations. Moreover, this growth is described by a parabolic law showing an oxidation rate kx = 1.3 × 10−13 cm2/s at 900 °C and the corresponding oxidation activation energy is 116.4 ± 7.5 kJ/mol. The sufficient Al flux to the oxidation front, in combination with the narrow inter-nuclei spacing lead to a faster formation of continuous α-Al2O3 compared to bulk RuAl, as a consequence of the high density of grain boundaries.
B2-RuAl Aluminides Oxidation Thin films
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This study was funded within a research project MU 959/24-1 of the Deutsche Forschungsgemeinschaft (DFG). The authors would like to thank the EFRE Funds of the European Commission for support of activities within the AME-Lab project. The authors are also grateful to Prof. Seidel, from the Department of Mechatronics, Saarland University, for the use of the magnetron sputtering device; and to Dr. F. Soldera, Dr. C. Gachot, Dipl.-Ing. N. Souza, Dipl.-Ing. C. Pauly and Dipl.-Ing. Sebastián Suarez for the usefully comments and discussion. A.Guitar is grateful to the German Academic Exchange Service (DAAD) for the financial support.