Novel strategies for evaluating the degradation of protective coatings on superalloys

Novel approaches involving new specimen preparation methods, O-18 tracer ex periments and secondary ion mass spectrometry have been developed to determ ine the high-temperature oxidation mechanisms and kinetics of gas turbine b lades. The industrial gas turbine blades investigated in this work consiste d of an oxide-Si-aluminide coating-directionally solidified nickel-based su peralloy system. The oxide scales were characterised after various exposure times from 0 to 15 000 h in a gas turbine environment and also after labor atory annealing in oxygen atmospheres (O-16(2) and O-18(2)) at high tempera ture. The oxide and coating layers were analysed by micro-machining cross-s ections on the surface of the blades using a Ga+ focused ion beam system (F EI-FIB200) combined with secondary ion mass spectroscopy. The powerful mill ing technique and the sub-micron resolution of the focused ion beam instrum ent, widely used within the semiconductor industry, were successfully adapt ed to our study. The FIB200 experiments provided essential results concerni ng the microstructural evolution of the oxide during high-temperature expos ure yielding the thickness, the porosity, the pathways of diffusion, the ch emical composition and the distribution of the different phases present in these layers. Further annealing experiments at high-temperature were also p erformed in an atmosphere enriched with the stable oxygen isotope O-18(2) t o determine the oxygen diffusion mechanism through the oxide scale. The oxy gen diffusion characteristics were investigated by depth profiling and imag ing (elemental mapping O-16(-) and O-18(-)) using conventional and high-res olution SIMS (ATOMIKA 6500, FIB200). An Atomic Force Microscope (Quesent Re solver) and an Interferometric Optical Microscope (Zygo) were used to measu re the SIMS craters for depth calibration. The combination of these high-re solution methods has provided a basis for a fundamental understanding of th e oxidation behaviour of the protective coatings on superalloys. (C) 1999 E lsevier Science Ltd. All rights reserved.