Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

Thermal Barrier Coating Validation Testing for Industrial Gas Turbine Combustion Hardware

[+] Author and Article Information
Jeffery Smith

Material Processing Technology, LLC,
Norton Shores, MI 49441

John Scheibel

Electric Power Research Institute,
Palo Alto, CA 94304

Daniel Classen, Scott Paschke, Kirk Fick

Cincinnati Thermal Spray Inc.,
Cincinnati, OH 45242

Shane Elbel

Cincinnati Thermal Spray Inc.,
Rocky Point, NC 28457

Doug Carlson

Carlson Consulting, LLC,
Cedar Crest, NM 87008

Contributed by the Manufacturing, Materials, and Metallurgy Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 9, 2014; final manuscript received August 21, 2015; published online October 28, 2015. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(3), 031508 (Oct 28, 2015) (7 pages) Paper No: GTP-14-1350; doi: 10.1115/1.4031448 History: Received July 09, 2014; Revised August 21, 2015

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.

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Fig. 1

Periodic table of the elements highlighting new oxide composition elements being evaluated for low k TBCs

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Fig. 2

NASA Laser high gradient testing of baseline 7 wt. % YSZ TBC versus NASA 10 mol. % YbGd-YSZ TBC [17] (courtesy of NASA)

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Fig. 3

Transition duct being plasma sprayed with a TBC (left) and a TBC coated combustion liner (right) (courtesy of CTS)

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Fig. 4

Thermal spray configuration for property validation test specimens

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Fig. 5

Baseline TBC systems to serve as benchmarks for comparing the YbGd-YSZ TBC properties

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Fig. 6

Baseline coating microstructures APS NiCrAlY/7YSZ

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Fig. 7

YbGd-YSZ TBC target architecture and microstructure

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Fig. 8

YbGd-YSZ TBC target architecture for powder supplier validation run series H, I, and J

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Fig. 9

YbGd-YSZ TBC microstructures for powder supplier validation run series H, I, and J

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Fig. 10

BC roughness values for 7YSZ baseline and YbGd-YSZ TBCs

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Fig. 11

Tensile bond strength testing in conformance with ASTM C633

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Fig. 12

TBC tensile bond strength—baseline 7YSZ and YbGd-YSZ TBCs

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Fig. 13

CTS room temperature erosion test rig

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Fig. 14

Comparison of sintering resistance of 7YSZ and YbGd-YSZ following a 1400 °C 100 hrs exposure

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Fig. 15

CTS FCT for assessing TBC performance

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Fig. 16

Results of FCT series #2. YbGd-YSZ demonstrate excellent FCT life comparable to the baseline 7YSZ TBCs with similar thickness and structure.




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