Stability of a ZrO₂(Y₂O₃) Thermal Barrier Coating in Turbine Fuel with Contaminants

Corrosion behavior of recently developed plasma sprayed duplex thermal barrier coating systems, consisting of Udimet-500 alloy metal substrate, MCrAlY (M = Ni or Ni/Co) bond coat and ZrO₂ stabilized with 12 wt percent Y₂O₃ overcoat, has been evaluated in environments now used or anticipated in utility turbines burning liquid fuels. The base fuel, GT-2, was doped with desired concentrations of sodium, vanadium and sulfur, and burned in a gas turbine simulator which contained the coated, air-cooled specimens. The coatings were found to behave well when exposed to combustion gases obtained by burning clean GT-2 fuel oil and that containing 1 ppm Na. However in the presence of vanadium in the fuel, the ZrO₂·12 wt percent Y₂O₃ oxide coating showed severe cracking, chipping and spalling. Possible causes for such oxide coating degradation are discussed. This work shows that the thermal barrier coating concept is feasible for utility turbines; but further coating development, specifically for utility turbines burning residual fuels, is required.