Isotope exchange studies of oxidation mechanisms in nickel-base superalloys using FIB-SIMS techniques

Abstract

The thermo-mechanical stability of the oxide layer that grows between the metallic bond coat and the ceramic top coat on superalloy turbine blades determines the lifetime of the system. Understanding the mechanisms of oxidation of the bond coat that is applied to the superalloy is key to improving the performance of the thermal barrier coatings. FIB-SIMS (Focused Ion Beam-Secondary Ion Mass Spectrometry) techniques in conjunction with tracer diffusion experiments represent a powerful tool in characterizing this oxide layer. This paper presents the results of oxidation studies on single crystal nickel-base superalloys/bond coat systems. In these studies, a two-stage oxidation experiment is used where 18O 2 serves as a tracer element during the second stage oxidation. The aluminium oxide grown in 16O 2 during the first stage oxidation represents an initial layer of oxide. Mass spectra collected by FIB-SIMS reveal the counter mass transportation of inward oxygen diffusion and outward diffusion of aluminium. New oxide formation during the second stage oxidation under an 18O 2 enriched environment is observed at both the gas/oxide interface as well as the oxide/superalloy interface. FIB-SIMS scanning enables high-resolution isotope maps, in particular 18O −, to be captured. These confirm the existence of new oxide forming at the oxide/superalloy interface with clear indications of short circuit diffusion paths through the existing oxide. These data allow the diffusion mechanisms for different superalloy/bond coat systems to be identified and contrasted, allowing the role of alloying additions to be elucidated.