ORIGIN OF DARKENING IN 8 WT-PERCENT YTTRIA-ZIRCONIA PLASMA-SPRAYED THERMAL BARRIER COATINGS

The orgins of darkening of 8 wt% Y2O3-ZrO2 air plasma-sprayed (APS) and low-pressure plasma-sprayed (LPPS) thermal barrier coatings (TBC) have been studied using X-ray photoelectron spectroscopy. The change of valence states of zirconium, due to the reduction of ZrO2 to Zr2O3, was responsible for darkening of TBC. Quantification of Zr3+ oxide was related both to the black color of TBC and to the spraying technologies and parameters. Furthermore, impurity (Fe, Al, Si, and Na) segregation and exsolution phenomena were monitored as a function of the air thermal treatment (up to 1473 K) and it was demonstrated not to be the origin of darkening.