Effect of Al2O3 overlay on hot-corrosion behavior of yttria-stabilized zirconia coating in molten sulfate-vanadate salt

A 25 μm thick dense and continuous Al2O3 overlay has been deposited on the outer surface of the yttria-stabilized zirconia (YSZ) coating by electron-beam physical vapor deposition. Hot corrosion tests were carried out on the thermal barrier coating (TBC) with and without an Al2O3 coating in molten salt mixtures (Na2SO4+5%V2O5) at 950 deg. C for 100 h. Although the Al2O3 overlay was gradually degraded by the formation of low-melting liquid phase during exposure to the molten mixture of sulphate and vanadate, it remarkably restrained the infiltration of the molten salt into the YSZ coating. For the YSZ coating without the Al2O3 overlay, the YSZ reacted with V2O5 to form YVO4 during exposure to the molten salt, leading to the formation of a large amount of M-phase of ZrO2 due to the leaching of Y2O3 from YSZ. In contrast, the amount of M-phase in the TBC coating with Al2O3 overlay was substantially reduced