THE EFFECT OF WATER VAPOR AND SUPERALLOY COMPOSITION ON THERMAL BARRIER COATING LIFETIME

New power generation concepts may contain higher water vapor in the turbine combustion gas due to the fuel or to steam dilution. To assess the effect of higher water vapor content on thermal barrier coaling performance, furnace cycle (lh) testing was conducted in air with 10, 50 and 90 vol.% water vapor and compared to prior results in dry O-2. The first series of experiments examined Pt diffusion (gamma+gamma') and Pt-modified aluminide (beta) bond coatings on second-generation superalloy N5 at 1150 degrees C with commercially vapor-deposited yttria-stabilized zirconia (YSZ) top coats. Compared to dry O-2, the average coating lifetimes with Pt diffusion coatings were unaffected by the addition of water vapor while the Pt-modified aluminide coating average lifetime was reduced by >50% with 10% water vapor, but less reduction was observed with higher water contents. A similar set of coatings on low Re superalloy N515 showed no debit in lifetime with Pt aluminide bond coatings exposed to 10% water vapor. Characterization of the alumina scale thickness at failure showed a thicker oxide beneath the YSZ coating (compared to the scale without a top coating) for both types of bond coatings, and an increase in the oxide thickness with the addition of 10% water vapor. These observations were further studied using analytical transmission electron microscopy. The second series of experiments examined high velocity oxygen fuel (HVOF) MCrAlY and MCrAlYHfSi bond coatings and air-plasma sprayed YSZ top coatings on X4 superalloy substrates with and without Y and La additions. Compared to a dry O-2 baseline, the addition of 10% water vapor decreased the YSZ coating lifetime for either bond coating by similar to 30% at 1100 degrees C. Substrates with Y and La additions showed no change in the average lifetime in 10% water vapor compared to standard X4. A further increase to 50% water vapor did not further decrease the average lifetime of one group of coatings. To better simulate base-load power generation, one group of specimens was cycled with 100h cycles, which substantially increased the coating lifetime. In each case, higher average lifetimes were observed with Hf in the bond coating. Initial characterization of the alumina scales formed at failure showed little effect of the water vapor addition, bond coating composition or substrate composition. For both series of coatings, the addition of 10% water vapor to the experiment reduced YSZ coating lifetime. However, increasing to 50% or 90% H2O showed no additional decrease in average YSZ lifetime.