Yttrium and oxygen distributions in MCrAlY coatings after deposition, annealing and oxidation

Abstract

A novel technique was used to determine quantitatively the yttrium and oxygen distributions in MCrAlY coatings using electron microprobe X-ray analysis (EPMA). This method was used to assess the influence of annealing, pre-oxidation conditions, and long-term oxidation at high temperature on the Y and O distributions in a vacuum plasma sprayed Ni–23Co–17Cr–24Al –0.2Y (at.%) coating and in the oxide scales that developed on top of this coating. It was found that after deposition most of the Y in the coating was bonded to oxygen and preferentially enriched at the splat boundaries inside the coating. This effect became more pronounced after annealing and oxidation. Besides the internal oxidation of Y, also segregation of Y was observed near the coating surface. The amount of segregated Y near the coating surface increased with increasing oxidation temperature and time, and decreasing oxygen partial pressure. The MCrAlY coatings with a large amount of Y segregated near the surface during annealing and pre-oxidation formed oxide scales with a higher growth rate during long-term high temperature oxidation due to the presence of a larger fraction of Y oxide inclusions in the alumina scale.