Increasing the hot-corrosion resistance of thermal barrier coating systems by the addition of oxides of Cr and Mg
The hot corrosion of model thermal barrier coating (TBC) systems was studied at 9000C using either Na2SO4 or Na2SO4-CaSO4-MgSO4 eutectic as the corrosive salt. The yttria-stabilized zirconia (YSZ) top coat was deposited via air plasma spraying and the top coat modification methods investigated involved (1) infiltration with aqueous solutions of water-soluble compounds of chromium (e.g., chlorides, nitrates, acetates) and alkaline earth elements (e.g., MgCrO4·xH2O) followed by an oxidizing conversion treatment and (2) co-depositing Cr2O3 with the YSZ. The goal of these modifications was to deliver alkaline earth and/or chromium oxides for reaction with the molten salt, thereby inhibiting its penetration through the top-coat and/or its reactivity with the substrate alloy. It was found that these modifications could have a favorable effect on the chemistry of the salt and the hot-corrosion resistance of the TBC system. It was further found that the corrosiveness of a given salt was very dependent on the type of test conducted, i.e., thin-film pre-deposit of the salt onto the sample or submersion of the sample into molten salt.