Recession of an EB ‐ PVD YSZ Coated Turbine Blade by CaSO ₄ and Fe , Ti ‐Rich CMAS ‐Type Deposits

An in‐service high‐pressure turbine blade with a columnar, Y 2 O 3 ‐stabilized ZrO 2 ( YSZ ) thermal barrier coating ( TBC ) fabricated by electron‐beam physical vapor deposition was investigated to access the TBC hot corrosion mechanisms during turbine operation. The TBC exhibits a through‐thickness pore filling with anhydrite‐type CaSO 4 . Chemical analysis of the CMAS ‐type particle deposits reveals relatively low SiO 2 but high CaO contents and substantial amounts of Fe 2 O 3 and TiO 2 . The hot corrosion scenario observed at the YSZ column tips involves newly formed CaZrO 3 and the garnet‐type phase Ca 3 (Zr,Mg,Ti) 2 (Fe,Al,Si) 3 O 12 , also known as the mineral kimzeyite. The phase relationships were confirmed in laboratory experiments. CaSO 4 as well as the particle deposits prove to be effective solvents for YSZ introducing distinct solid‐state reactions. The results support the idea of a dual YSZ hot corrosion process. A first stage controlled by a SiO 2 ‐free Ca ‐source, most likely primary CaSO 4 produces a thin CaZrO 3 layer. A second, CMAS ‐type stage providing high concentrations of Fe 2 O 3 , TiO 2 , and SiO 2 favors the formation of kimzeyite. The melting temperature of kimzeyite presumably defines a thermal operation limit for YSZ ‐based TBCs .