On the high-temperature oxidation of MCrAlY coatings

In this review, recent progress in understanding the short- and long-term oxidation behaviour of MCrAlY coatings and the instruments that can be employed to influence the oxidation behaviour of the MCrAlY's are discussed. Particular attention is given on the transient oxidation behaviour, the initial distribution of the Y in the oxide scale and how these initial oxide products influence the long-term oxide growth rate and adhesion of the oxide scale. It is shown that the growth kinetics and adhesion of the oxide scale are to a large extent determined by the amount and distribution of Y-oxide inclusions in the scale. Oxide scales with a regularly distributed and larger amount of small Y-oxide inclusions are faster growing, but perform better in thermal cycle tests. Transient oxide products like metastable aluminas (gamma- or theta-Al2O3) and spinels do not have a large influence on long-term oxidation behaviour except when the oxide scale is virtually free of Y-oxide inclusions due to Y depletion. Then, the nucleation of metastable aluminas at the onset of oxidation may be beneficial due to a slower-growing alumina scale with a larger grain size after transformation into alpha-Al2O3. Spinel oxides are considered detrimental for long-term oxidation performance, but their amount is usually limited on most types of MCrAlY coatings due to the rapid supply of Al along the numerous grain and phase boundaries in the fine-grained coatings. The key instruments that can be employed to manipulate the oxidation behaviour of MCrAlY coatings are: (i) the composition and size of the main phases in the coating (gamma-Ni and beta-NiAl), (ii) the initial distribution, chemical state (metallic or oxidized) and reservoir of the Y in the coating, (iii) the composition and roughness at the coating surface, and (iv) the oxidation conditions during post-deposition heat treatment (temperature, time, partial oxygen pressure).