Yttrium oxides in vacuum-plasma-sprayed CoNiCrAlY coatings

Abstract

To improve the oxide scale adherence on MCrAlY-type alloys, active elements such as yttrium are added in amounts of 0.1–1 wt.%. To understand the mechanism behind this improvement it is useful to characterize the distribution of active elements in the microstructure. The purpose of this investigation was to study the distribution and explain the formation of yttrium-rich precipitates in vacuum-plasma-sprayed CoNiCrAlY coatings. The CoNiCrAlY coatings were prepared by vacuum plasma spraying (VPS) in a full-scale production unit and subsequently vacuum heat-treated (1120°C for 2 h and 845°C for 24 h). The yttrium-rich precipitates were identified by scanning and transmission electron microscopy. The structure and chemical composition of those precipitates was determined by electron diffraction and energy dispersive X-ray analysis. Yttria particles were found at four different locations in the CoNiCrAlY coatings: (1) as fine dispersions, 30–300 nm in size, throughout the coating structure, (2) as a ten-times-coarser distribution randomly distributed throughout the coating structure, and as course particles located along (3) the alumina-coating and (4) the coating-substrate interfaces. The structure of the dispersed yttria precipitates was b.c.c. with the Ia3 space group.