TBC experience in land- based gas turbines
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TBC Experience in Land-Based Gas Turbines W.A. Nelson and R.M. Orenstein This paper summarizes prior and on-going machine evaluations of thermal barrier coatings (TBC) for power generation, that is large industrial gas turbine applications. Rainbow testing of TBCs on turbine nozzles, shrouds, and buckets are described along with a test of combustor liners. General Electric Power Generation has conducted more than 15 machine tests on TBC turbine nozzles with various coatings. TBC performance has been quite good, and additional testing, including TBCs on shrouds and buckets, is continuing. Included is a brief comparison of TBC requirements for power generation and aircraft turbines.
Keywords gas turbine, rainbow tests, thermal barrier coating
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1. Introduction YTTR1A-STAB1LIZEDzirconia (YSZ) ceramic thermal barrier coatings (TBC) on superalloy components are being used successfully in heavy duty gas turbines and aircraft engines. At General Electric Power Generation (GEPG), TBCs applied by air plasma spray (APS) are used in the combustor and turbine sections, at ceramic thickness up to 0.51 mm (0.02 in.). Table l summarizes GEPG production experience with TBCs. A thermal barrier coating is typically comprised of two layers: ceramic oxide top coat and metallic bond coat. The low thermal conductivity ceramic top coat provides thermal insulation, while the bond coat provides a suitable interlayer to improve adherence of the ceramic top coat and environmental protection to the underlying superalloy substrate. GEPG state-of-the-art TBC systems use top coats of 6 to 8 wt% YSZ, while the bond coats are based on Ni(Co)-Cr-AI-Y alloys. Bond coat material development has been successful in improving the high-temperature oxidation resistance and thermal cycle lives of plasma-sprayed TBCs. In general, the aircraft engine industry has led turbine technology improvements, and these improvements were later adapted in power generation machines. This is also true in the area of TBCs. Experience with TBC usage in aircraft turbine applications is greater than in power generation equipment. While operating conditions in aircraft turbines, especially the peak temperature and number of cycles, are typically more severe than in power generation equipment, the time requirements are much longer in power generation equipment. Table 2 highlights some of the differences between aircraft and power generation requirements for TBCs. The aircraft engine duty cycle is highly cyclic, with only a small percentage of its time at maximum temperature conditions, that is, during takeoff and climb. Conversely, power generation equipment operates under different duty cycles, varying from one cycle per day for peaking power applications to one cycle per year for baseload machines. Coating life, that is, time to refurbish aircraft turbines, is approximately 8000 h; only a small portion, 5 to 15%, of the total coating lifetime of 8000 h is at maximum conditions. In contrast, component life for power generation turbine applications W.A. Nelson and R.M. Orens
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