Volcanic Ash‐Induced Decomposition of EB ‐ PVD Gd 2 Zr 2 O 7 Thermal Barrier Coatings to Gd ‐Oxyapatite, Zircon, and Gd , Fe ‐Zirconolite

The resistance of EB ‐ PVD Gd 2 Zr 2 O 7 thermal barrier coatings against high‐temperature infiltration and subsequent degradation by molten volcanic ash is investigated by microstructural analysis. At 1200°C, EB ‐ PVD Gd 2 Zr 2 O 7 coatings with silica‐rich, artificial volcanic ash ( AVA ) overlay show a highly dynamic and complex recession scenario. Gd 2 O 3 is leached out from Gd 2 Zr 2 O 7 by AVA and rapidly crystallizes as an oxyapatite‐type solid‐solution ( Ca , Gd ) 2 ( Gd , Zr ) 8 ( Si , Al ) 6 O 26 . The second product of Gd 2 Zr 2 O 7 decomposition is Gd 2 O 3 fully stabilized ZrO 2 ( 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 Gd 2 O 3 from Gd ‐ FSZ followed by destabilization to the monoclinic ZrO 2 polymorph. Finally, zircon ( ZrSiO 4 ) 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 CaZrTi 2 O 7 ), exhibiting an almost full substitution Ca 2+ + Ti 4+ <> Gd 3+ + Fe 3+ . As all Gd 2 Zr 2 O 7 decomposition products with the exception of ZrSiO 4 exhibit considerable solid solubility ranges, it is difficult to conclusively assess the resistance of EB ‐ PVD Gd 2 Zr 2 O 7 coatings versus volcanic ash attack.