Recession of an EB-PVD YSZ Coated Turbine Blade by CaSO_4 and Fe, Ti-Rich CMAS-Type Deposits

摘要

An in-service high-pressure turbine blade with a columnar, Y_2O_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_2O_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)_3O_(12), also known as the mineral kinizeyite. 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_2O_3, TiO_2, and SiO_2 favors the formation of kimzey-ite. The melting temperature of kimzeyite presumably defines a thermal operation limit for YSZ-based TBCs.