XPS INVESTIGATON OF CERIUM VALENCE STATE IN PLASMA SPRAYED PARTIALLY STABILIZED ZIRCONIA COATINGS

X-Ray photoelectron speetroscopy (XPS) was used to investlgate the valence state of cerlum in 25% CeO2-2.5% Y2O3 partially stablllzed zlrconia therrmal barrier coatings in order to establish the chemical interactions taking place between powder particles and plasma as a function of the spraying parameters. The analysis of the characteristic satellite structures, binding energies and peak shapes in the XPS spectra showed that cerium in the starting powder ls Ce (IV), after spraying CeO2 is either unaffected, partially or fulty reduced to Ce (III). Relatlve amounts of CeO2 and Ce2O3 depend on the deposition temperature: in particular high coollng rates glve rise to Ce2O3 vhereas CeO2 is formed at a hlgh deposition temperature. Moreover such amounts are comelated with microcracking patterns, as resulting from metallographic cross-sections. Effects of heat treatments on valence modifications are also evaluated. Methodologlcal aspects, such as reductlon of valence states due to argon ion beam sputtering are discussed using angle-resolved technique.