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

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.