Novel strategies for evaluating the degradation of protective coatings on superalloys |
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| Affiliation: | 1. Institute for Powertrain and Vehicle Engineering, University of Kassel, Germany;2. Institute for Materials Engineering, University of Kassel, Germany;1. CIEFMA — Departament de Ciència dels Materials i Engenyeria Metal·lúrgica, ETSEIB, Universitat Politècnica de Catalunya, Barcelona 08028, Spain;2. Nanostructured Materials, Department of Physics, Chemistry, and Biology (IFM), Linköping University, Linköping 58183, Sweden;3. CRnE — Center for Research on Nanoengineering, Universitat Politècnica de Catalunya, Barcelona 08028, Spain;4. SECO Tools AB, Fagersta 73782, Sweden;5. Departament de Física Aplicada i Òptica, Universitat de Barcelona, Barcelona 08028, Spain;1. Aerospace Research Institute of Materials & Processing Technology, Beijing 100076, PR China;2. Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, United Kingdom;3. China Academy of launch Vehicle Technology, Beijing 100076, PR China |
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| Abstract: | Novel approaches involving new specimen preparation methods, 18O tracer experiments and secondary ion mass spectrometry have been developed to determine the high-temperature oxidation mechanisms and kinetics of gas turbine blades. The industrial gas turbine blades investigated in this work consisted of an oxide–Si-aluminide coating–directionally solidified nickel-based superalloy system. The oxide scales were characterised after various exposure times from 0 to 15 000 h in a gas turbine environment and also after laboratory annealing in oxygen atmospheres (16O2 and 18O2) at high temperature. The oxide and coating layers were analysed by micro-machining cross-sections on the surface of the blades using a Ga+ focused ion beam system (FEI-FIB200) combined with secondary ion mass spectroscopy. The powerful milling technique and the sub-micron resolution of the focused ion beam instrument, widely used within the semiconductor industry, were successfully adapted to our study. The FIB200 experiments provided essential results concerning the microstructural evolution of the oxide during high-temperature exposure yielding the thickness, the porosity, the pathways of diffusion, the chemical composition and the distribution of the different phases present in these layers. Further annealing experiments at high-temperature were also performed in an atmosphere enriched with the stable oxygen isotope 18O2 to determine the oxygen diffusion mechanism through the oxide scale. The oxygen diffusion characteristics were investigated by depth profiling and imaging (elemental mapping 16O− and 18O−) using conventional and high-resolution SIMS (ATOMIKA 6500, FIB200). An Atomic Force Microscope (Quesent Resolver) and an Interferometric Optical Microscope (Zygo) were used to measure the SIMS craters for depth calibration. The combination of these high-resolution methods has provided a basis for a fundamental understanding of the oxidation behaviour of the protective coatings on superalloys. |
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