Volcanic Ash-Induced Decomposition of EB-PVD Gd2Zr2O7 Thermal Barrier Coatings to Gd-Oxyapatite, Zircon, and Gd, Fe-Zirconolite

Abstract

The resistance of EB-PVD Gd2Zr2O7 thermal barrier coatings against high-temperature infiltration and subsequent degradation by molten volcanic ash is investigated by microstructural analysis. At 1200°C, EB-PVD Gd2Zr2O7 coatings with silica rich, artificial volcanic ash (AVA) overlay show a highly dynamic and complex recession scenario. Gd2O3 is leached out from Gd2Zr2O7 by AVA and rapidly crystallizes as an oxyapatite-type solid-solution (Ca,Gd)2(Gd,Zr)8(Si,Al)6O26. The second product of Gd2Zr2O7 decomposition is Gd2O3 fully stabilized ZrO2 (Gd-FSZ). Both reaction products are forming an interpenetrating network filling open coating porosity. However, first-generation Gd-oxyapatite and Gd-FSZ are exhibiting chemical evolution in the long term. The chemical composition of Gd-oxyapatite does evolve from Ca,Zr enriched to Gd-rich. AVA continuously leaches out Gd2O3 from Gd-FSZ followed by destabilization to the monoclinic ZrO2 polymorph. Finally, zircon (ZrSiO4) is formed. In addition to the prevalent formation of Gd-oxyapatite, a Gd-, Zr-, Fe-, and Ti-rich oxide is observed. From chemical analysis and electron diffraction it is concluded that this phase belongs to the zirconolite-type family (zirconolite CaZrTi2O7), exhibiting an almost full substitution Ca2+ + Ti4+ <> Gd3+ + Fe3+. As all Gd2Zr2O7 decomposition products with the exception of ZrSiO4 exhibit considerable solid solubility ranges, it is difficult to conclusively assess the resistance of EB-PVD Gd2Zr2O7 coatings versus volcanic ash attack.