For this project, Oak Ridge National Laboratory (ORNL) has three tasks. The first task is to study the effect of higher water vapor contents during thermal cycling. Results show the average thermal barrier coatings (TBCs) life-time data for three specimens of each coating type at 1,150 degrees Celsius (°C) in dry oxygen (O2) and air with 10, 50, and 90 percent by volume water vapor for two different types of diffusion bond coatings. Lifetime is being defined as the time to 20 percent spallation of the low thermal conductivity yttria-stabilized zirconia (YSZ) top coating of the TBC. The addition of water vapor had a dramatic effect on the platinum (Pt)-modified aluminide coating, especially with 10 percent water vapor, but no statistical effect on the average lifetime of Pt-diffusion coatings. The second ORNL task is to quantify the benefit of adding yttrium (Y) and lanthanum (La) dopants to nickel (Ni)-base superalloys on TBC lifetime. Superalloy coupons were coated with nickel-cobalt-chromium-aluminum-yttrium (NiCoCrAlY) and NiCoCrAlY-hafnium-silicon (NiCoCrAlYHfSi) bond coatings using a thermal spray high velocity oxygen fuel (HVOF) process. Ten percent water vapor had a negative effect on coating lifetime at 1100 °C, but similar lifetimes were observed for the substrates with and without Y and La. The third task is characterization of the microstructure and microchemistry of these TBC systems to assist in mechanistic understanding of the roles of dopants and water vapor on coating lifetime. The initial results have demonstrated that titanium (Ti) from the superalloy can diffuse through the NiCoCrAlYHfSi coating and become incorporated into the thermally-grown alumina (aluminum oxide) scale.
This project will perform three tasks that will help understand the role of water vapor and dopants on thermal barrier coatings (TBC) lifetime. Turbine materials research seeks to improve coating materials that will allow for higher temperature operation and increased durability leading to increased turbine efficiency and reduced maintenance. Specifically, this project will study the effect of higher water vapor contents on TBC life during thermal cycling, quantify the benefit of adding yttrium (Y) and lanthanum (La) dopants to nickel (Ni)-base superalloys on TBC lifetime, and characterize the microstructure and microchemistry of these TBC systems to assist in mechanistic understanding of the roles of dopants and water vapor on coating lifetime.
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