Effect of water vapour content on thermal barrier coating lifetime

Furnace cycle testing was conducted in air with 10, 50 and 90 vol.-% water vapour and compared to prior results in dry O₂. The first series of experiments examined Pt diffusion and Pt modified aluminised bond coatings on second generation superalloy N5 at 1150°C with commercially vapour deposited yttria stabilised zirconia (YSZ) top coats. Compared to dry O₂, the average lifetimes with Pt diffusion coatings were unaffected by the addition of water vapour, while the Pt modified aluminide coating average lifetime was reduced by ≥50% with 10% water vapour, but less reduction was observed with higher water contents. The second series of experiments examined MCrAlY and MCrAlYHfSi bond coatings and air plasma sprayed YSZ top coatings on superalloy X4 cycled at 1100°C. Compared to dry O₂, the addition of 10% water vapour decreased the lifetime of MCrAlY by 28%. Higher average lifetimes were observed with Hf in the bond coating.