High-Temperature Corrosion of EB-PVD Yttria Partially Stabilized Zirconia Thermal Barrier Coatings with an Artificial Volcanic Ash Overlay

High-temperature interaction of sol-gel-derived artificial volcanic ash (AVA) matching the bulk composition of the April 15, 2010 Eyjafjallajokull (Iceland) volcanic eruption with a standard 4 mol% (7 wt%) Y2O3-stabilized ZrO2 (YSZ) electron-beam physical vapor deposition (EB-PVD) thermal barrier coating and a corresponding YSZ powder is investigated in order to access possible implications of similar volcanic ashes on the performance of coated turbine engine airfoils. Up to 900 degrees C, AVA deposits and EB-PVD YSZ do not show significant interaction. Viscous flow above the glass transition of AVA (T-g similar to 930 degrees C) yields proceeding wetting of EB-PVD YSZ coatings. At 1100 degrees C, the YSZ surface is covered by a dense glaze-like AVA overlay. At 1200 degrees C, AVA is mostly infiltrating the coating, leaving a crystalline plagioclase- and hematite-type residue at the interface. Moreover, some ZrSiO4 is formed at the expense of YSZ. The overall thermochemical effects on short-term exposure of the EB-PVD YSZ coating to a small AVA load were moderate, in particular before complete infiltration. On the other hand, AVA acts as a solvent for the stabilizing Y2O3 beyond 1000 degrees C and a progressive depletion of Y2O3 in the YSZ is observed at the AVA/YSZ interface. Detrimental effects on YSZ phase stability and hence coating lifetime cannot be ruled out for long-term exposure and higher AVA loads.