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

MCrAlY type (M = Ni and/or Co) coatings are widely used for the protection of components in the hot sections of gas turbines at high service temperatures by forming a continuous alpha-alumina. A reliable criterion to estimate the capability of coating to form alpha-alumina is of great importance to accurately evaluate coating lifetime. However, some coatings retain the ability to form a continuous alpha-alumina scale when the concentration of Al in coatings decreases to a critical level, therefore, the empirical Al-concentration based criterion is inadequate to properly predict the formation of a continuous alpha-alumina. Thus, a new life criterion, namely the critical Al-activity criterion, is proposed. In this work, the critical Al-activity to form a continuous a-alumina was validated by Al-activity calculation using Thermo-Calc software based on survey of research results of critical Al-concentration to form alpha-alumina on binary Ni-Al and ternary Ni-Cr-Al systems. Long-term oxidation tests were performed to support the criterion: three different MCrAlY coatings coated on IN-792 superalloy substrates were oxidized at 1000 degrees C for various periods of time up to 10,000 h. The microstructural evolution of MCrAlY coatings was investigated using Scanning Electron Microscope. The near-surface Al concentration and interdiffusion behaviour between substrate and coating were measured using Energy Dispersive X-ray Spectroscopy. The new critical Al-activity criterion has been successfully adopted in alpha-alumina formation prediction, showing a good agreement with experiment results. Therefore, it can be concluded that the extrapolation of new criterion from binary and ternary systems to multi-alloyed MCrAlY system is reasonable. Furthermore, the partial pressure of oxygen (P-O2) in atmosphere has been taken into consideration by combination with Al-activity to calculate the critical chemical reaction constant (K) of formation of a-alumina. The potential applicability of the methodology to predict MCrAlY life is also discussed.Funding Agencies|Siemens Industrial Turbomachinery AB (Finspang, Sweden); Swedish Energy Agency through KME consortium [KME-703]; research school of Agora Materials and AFM Strategic Faculty Grant SFO-MAT-LiU in Linkoping University [2009-00971]