Lifetime modelling for MCrAlY coatings in industrial gas turbine blades

Abstract

A novel theoretical and experimental approach for lifetime modelling of MCrAlY coatings for stationary gas turbines has been undertaken using the Inverse Problem Solution (IPS) technique. With this technique feasible experimental data acquired after a defined experimental time t e are used as input values for the model parameters estimation. In the first stage of the approach a model, based on the oxidation and diffusion processes (Fick's first and second law) was assumed, which considers the Al concentration profile across the coating. The measured average Al concentration profiles in the two-phase g+b and g - regions of coating as well as base metal were used as input values for the model parameters estimation and calculational prediction of the long term diffusion and oxidation behavior of the coating was performed. The time, when the b-NiAl phase is completely consumed was assumed as the coating lifetime end. Exposure experiments were carried out with a NiCoCrAlY coating (200 micron thickness) with 8% Al in air at 900 °C and 950 °C, currently up to 10000 h. The oxide scale is growing continuously and no other oxides were observed. The average and b-NiAl phase concentration profiles of Al across the coating thickness were determined by electron microprobe and image analysis systems in the initial state after 700 and 10000 h of oxidation. The concentration profile measured after 700 h was used as input values for the model parameters estimation in order to calculate the Al and b-NiAl phase concentration profiles after 10000 h. The computational forecast for 10000 h at 950 °C and 900 °C are in good agreement with the measured data. The approach was applied for NiCoCrAlY (200 micron thickness) coating lifetime modelling at 950 °C and 900 °C as well as for different coating thicknesses at 950 °C

Mechanical Properties and Microstructure of VPS and HVOF CoNiCrAlY Coatings

“…Toscano et al (Ref 28) have reported the coexistence of the c phase with the r-(Co,Cr) phase in samples of a similar CoNiCrAlY coating that had been annealed at 800°C for 300 h. However, since the Co-to-Cr ratio in the Cr-rich phase was very low, it was assumed that it was a-Cr rather than r-(Co,Cr) which would have had a higher Co-to-Cr ratio. Toscano et al carried out their annealing process on an attached coating, so that interdiffusion between substrate and coating may have changed the coating composition ( Ref 25,29,30). Also their annealing process was carried out in air which will cause some oxidation of the coating and hence loss of elements, such as Al, Cr, etc.…”

Slurry aluminizing of IN‐800HT austenitic stainless steel and pure nickel. Correlations between experimental results and modelling of diffusion

“…Secondly, the goal was to use the results of experimental studies in an attempt to estimate the model parameters allowing to ascertain the lifetime of the coating through the inverse problem solution (IPS) approach. The IPS was successfully employed in the past to estimate the lifetime of MCrAlY coatings all by considering the phase transformations occurring in the coating and its oxidation . Usually the diffusion coefficients in metals are not accurately known for particular conditions (alloy composition and temperature), which obstruct the modeling and the estimation of lifetime.…”

“…Only the concentration of the main species is calculated. Oxidation in this work is not considered but it can be taken into account as outgoing flux calculated assuming, for instance, parabolic growth law .…”

Long-term oxidation of MCrAlY coatings at 1000 °C and an Al-activity based coating life criterion

“…During high temperature exposure, a thin layer of protective α-Al2O3 formed on the coating surface, which serves as a diffusion barrier between the environment and underlying alloy [6]. The depletion rate of β phase in MCrAlY has been used as a criterion by many investigators [7][8][9][10][11] to evaluate the end of coating lifetime. However, the complete depletion of β phase doesn't necessarily mean the end of coating life.…”