Effect of water vapor on the 1100 °C oxidation behavior of plasma-sprayed TBCs with HVOF NiCoCrAlX bond coatings

With the goal of investigating the reported detrimental effect of water vapor on thermal barrier coating (TBC) performance, furnace cycle experiments were conducted in dry O2 and air with 10 and 50% water vapor at 1100 °C. The TBC systems evaluated were air plasma-sprayed (APS), yttria-stabilized zirconia (YSZ) top coatings with high velocity oxy fuel (HVOF)-deposited NiCoCrAlY or NiCoCrAlYHfSi bond coating. Average TBC lifetime was reduced by ~ 30% in air with 10% water vapor compared to cycling in dry O2, using 1 h cycle durations. Superalloy substrates with Y and La additions also were investigated but showed no statistical change in the average TBC lifetime compared to the base CMSX4 superalloy. In all cases, the bond coating with Hf and Si additions increased YSZ lifetime by 20% or more. Experiments that increased water vapor to 50% showed no further decrease in TBC lifetime. Increasing the cycle frequency to 100 h resulted in a large increase in TBC lifetime, especially for the NiCoCrAlYHfSi bond coatings. Co-doping the NiCoCrAl bond coat with Y and Hf was beneficial to TBC lifetime, but did not mitigate the detrimental impact of water vapor.