Microstructural characterisation of oxide formation from MCrAlY coatings on nickel-based superalloys

Abstract

Nickel-based superalloys are commonly used in industrial gas turbine engines for power generation due to their ability to withstand the arduous environments. Coating systems are employed to increase the service life of components by increasing the oxidation\\corrosion resistance and high temperature capabilities. MCrAlY coatings are a common coating system used, not only to protect the substrate from oxidation and corrosion but also to act as a bond coat for thermally insulating coatings. The ‘M’ in the MCrAlY coatings is generally Ni, Co or a mixture of the two, with Co being added to improve the hot corrosion resistance (in conjunction with Cr) and to make the coatings more ductile. These coatings work by forming oxides that act as protective layers against further oxidation (known as the thermally grown oxide, TGO). Alumina is most commonly formed oxide from the MCrAlY bond coat, which acts as a reservoir of elements for the oxide formation.It is necessary to understand the formation and nature of the TGO as a function of composition variation in the coating. This mechanistic understanding can be used to inform modelling methodologies for both service life prediction and the development of new coatings. This research is therefore concerned with the effect of compositional changes in the MCrAlY coating and their effect on the TGO formation. Various ageing temperatures and times have been studied, representative of service conditions. The alumina layer has been studied, however, the formation of other oxides, including isolated spinels, has been characterised in detail. Microstructural observations were carried out using scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis and electron backscattered diffraction. Higher resolution characterisation was carried out using a dual beam focused ion beam scanning electron microscope for site specific TEM preparation and STEM/TEM for microstructural evaluation. The results are then discussed in the light of both the bond coat composition and ageing conditions.