Thermodynamic investigations of the diffusion-based and oxidation-based depletion mechanisms in MCrAlY coatings

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Already today modern gas turbine suppliers are confronted with a steadily intensifying cost pressure in the development, manufacturing and service areas due to extensive economic competition among themselves and with alternative energy sources. Increasing the inlet temperatures into the turbine or extending the service life of components are established methods to increase the efficiency and profitability of gas turbine facilities. In order to protect parts from increased risk of oxidation and corrosion components in the hot gas path of a turbine are shielded for example with MCrAlY (M= Ni and/or Co) overlay coatings. Adapting MCrAlY coatings onto the more and more versatile operation conditions of modern gas turbine facilities is a necessity to ensure maximum service life of each part and the turbine. Still today the general processing route is to adapt the chemical composition and properties on an experimental trial and error basis. This, however, is a time and cost consuming process, because every single potential system has to be validated and field tested later. Experience has shown that modern thermodynamic software packages are a powerful to predict properties and structure of materials and alloys through calculation of system related state variables and simulation of diffusion processes. At the present point in time these programs can only simulate diffusion between solid and/or liquid phases, though. A straightforward consideration of the atmospheric influence is not possible due to (commercial) unavailability of mobility databases for oxygen and relevant oxides (e.g. Al2O3, Cr2O3, Y2O3). The diffusion controlled formation and growth of an oxide scale with adjacent surface near depletion, however, is crucial for the lifetime of the coating and the whole part. The research project is dedicated to investigate the tempering related oxidation and interdiffusion processes for a well known gamma/beta MCrAlY overlay coating in conjunction with IN738LC. On the basis of this data, a simulation model shall be developed and subsequently validated through oxidation trails, long-term heat treatments (12000 h), microscopic investigations, EDX, XRD, as well as available literature data. Compared to previous studies a lifetime prediction shall be given based on identified lifetime criteria and simulation results. Presuming a successful validation of the gamma/beta MCrAlY simulation model, the application onto a chemical and microstructurally different gamma/gamm prime MCrAlY overlay system is planned in a potential fourth funding year. By doing so, the universality, reliability and adaptability of the developed model shall be analysed.