Influence of vacuum heat treatment parameters on the surface composition of MCrAlY coatings

In the present study the synergistic effect of vacuum heat treatment pressure and coating Y and O content on the MCrAlY surface oxide is investigated. For this purpose, free standing MCrAlY coatings with two different Y contents were exposed at 1100 °C for times between 1 and 5 h in vacuum at pressures of < 10− 3 Pa and 10− 2 Pa, and in Argon gas at 10− 1 Pa. The surface scale composition and morphology were analysed with a variety of analysis techniques. It is shown that the composition at the MCrAlY coating surface is mainly governed by two competing processes: Cr evaporation (β-NiAl formation) and Y oxidation (internal and external). The latter reaction has been observed to depend strongly on the Y reservoir in the coating. It can be observed that the chromium evaporation enhances for reducing pressure. More Y and less O in the coating result in enhanced Y rich oxide formation (Y2O3 and YAP; Yttrium Aluminium Perovskite) and decreased chromium evaporation. On rough surfaces β-NiAl enrichment in hills and yttria formation in valleys can be found. Additionally no alumina formation can be observed under all studied vacuum heat treatment conditions.