Thermal Barrier Coating Validation Testing for Industrial Gas Turbine Combustion Hardware

As gas turbine (GT) temperatures have increased, thermal barrier coatings (TBCs) have become a critically important element in hot section component durability. Ceramic TBCs permit significantly increased gas temperatures, reduced cooling requirements, and improve engine fuel efficiency and reliability. TBCs are in use throughout the GT hot section with turbine blades, vanes, and combustion hardware, now being designed with TBCs or upgraded with TBCs during component refurbishment (Miller, 1987, “Current Status of Thermal Barrier Coatings,” Surf. Coat. Technol., 30(1), pp. 1–11; Clarke et al., 2012, “Thermal-Barrier Coatings for More Efficient Gas-Turbine Engines,” MRS Bull., 37(10), pp. 891–898). While the industry standard 6–9 wt. % yttria stabilized zirconia (7YSZ) has been the preferred ceramic composition for the past 30+ yr, efforts have been underway to develop improved TBCs (Stecura, 1986, “Optimization of the Ni–Cr–Al–Y/ZrO₂–Y₂O₃ Thermal Barrier System,” Adv. Ceram. Mater., 1(1), pp. 68–76; Stecura, 1986, “Optimization of the Ni–Cr–Al–Y/ZrO₂–Y₂O₃ Thermal Barrier System,” NASA Technical Memorandum No. 86905). The principal development goals have been to lower thermal conductivity, increase the sintering resistance, and have a more stable crystalline phase structure allowing to use above 1200 °C (2192 °F) (Levi, 2004, “Emerging Materials and Processes for Thermal Barrier Systems,” Curr. Opin. Solid State Mater. Sci., 8(1), pp. 77–91; Clarke, 2003, “Materials Selection Guidelines for Low Thermal Conductivity Thermal Barrier Coatings,” Surf. Coat. Technol., 163–164, pp. 67–74). National Aeronautics and Space Administration (NASA) has developed a series of advanced low conductivity, phase stable and sinter resistant TBC coatings utilizing multiple rare earth dopant oxides (Zhu and Miller, 2004, “Low Conductivity and Sintering-Resistant Thermal Barrier Coatings,” U.S. Patent No. 6,812,176 B1). One of the coating systems NASA developed is based on Ytterbia, Gadolinia, and Yttria additions to ZrO₂ (YbGd-YSZ). This advanced low conductivity (low k) TBC is designed specifically for combustion hardware applications. In addition to lower thermal conductivity than 7YSZ, it has demonstrated thermal stability and sintering resistance to 1650 °C (3000 °F). The Electric Power Research Institute (EPRI) and cincinnati thermal spray (CTS) have teamed together in a joint program to commercialize the YbGd-YSZ TBC coating system for GT combustion hardware. The program consists of validation of coating properties, establishment of production coating specifications, and demonstration of coating performance through component engine testing of the YbGd-YSZ TBC coating system. Among the critical to quality coating characteristics that have been established are (a) coating microstructure, (b) TBC tensile bond strength, (c) erosion resistance, (d) thermal conductivity and sintering resistance, and (e) thermal cycle performance. This paper will discuss the coating property validation results comparing the YbGd-YSZ TBC to baseline production combustor coatings and the status of coating commercialization efforts currently underway.