Yttria-Stabilized Zirconia / Gadolinium Zirconate Double-Layer Plasma-Sprayed Thermal Barrier Coating Systems (TBCs)

摘要

Thermal barrier coating (TBC) research and development is driven by the desirability offurther increasing the maximum inlet temperature in a gas turbine engine. A number ofnew top coat ceramic materials have been proposed during the last decades due to limitedtemperature capability (1200 °C) of the state-of-the-art yttria-stabilized zirconia (7 wt. %Y2O3-ZrO2, YSZ) at long term operation. Zirconate pyrochlores of the large lanthanides((Gd La)2Zr2O7) have been particularly attractive due to their higher temperaturephase stability than that of the YSZ. Nonetheless, the issues related with the implementationof pyrochlores such as low fracture toughness and formation of deleteriousinterphases with thermally grown oxide (TGO, Al2O3) were reported. The implicationwas the requirement of an interlayer between the pyrochlores and TGO, which introduceddouble-layer systems to the TBC literature. Furthermore, processability issues ofpyrochlores associated with the dierent evaporation rates of lanthanide oxides and zirconiaresulting in unfavorable composition variations in the coatings were addressed indierent studies.After all, although the material properties are available, there is a paucity of data in theliterature concerning the properties of the coatings made of pyrochlores. From the processabilitypoint of view the most reported pyrochlore is La2Zr2O7. Hence, the goal of thisresearch was to investigate plasma-sprayed Gd2Zr2O7 (GZO) coatings and YSZ/GZOdouble-layer TBC systems. Three main topics were examined based on processing, performanceand properties: (i) the plasma spray processing of the GZO and its impact onthe microstructural and compositional properties of the GZO coatings; (ii) the cyclinglifetime of the YSZ/GZO double-layer systems under thermal gradient at a surface temperatureof 1400 °C; (iii) the properties of the GZO and YSZ coatings such as thermalconductivity, coecient of thermal expansion as well as time and temperature-dependentelastic and creep deformations.Thermal cycling results displayed that the double-layer YSZ/GZO TBC concept is ableto provide signicant lifetime improvement at 1400 °C surface temperature comparedto the standard YSZ. The investigations on the chemical composition of the as-sprayedGZO revealed that no signicant gadolinia evaporation, which would compromise theperformance of the coating, takes place in the examined spray current range (300 A-525 A). The detailed examination of microstructural properties of the as-sprayed GZOhighlighted the importance of the process parameters for achieving the desired porosityfeatures assisting superior lifetime performances. A signicant insight was gained intothe elastic and creep deformation of the plasma-sprayed YSZ and GZO coatings whichplay a critical role on the development of advanced TBCs. The overarching conclusion ofthis work is that the GZO has the potential to increase the temperature capability of gasturbines, if it is applied in double-layer TBC systems and if its microstructure is tailoredby adapted processing