Yttria-Stabilized Zirconia / Gadolinium Zirconate Double-Layer Plasma-Sprayed Thermal Barrier Coating Systems (TBCs)
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Beschreibung
Thermal barrier coating (TBC) research and development is driven by the desirability of
further increasing the maximum inlet temperature in a gas turbine engine. A number of
new top coat ceramic materials have been proposed during the last decades due to limited
temperature 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 temperature
phase stability than that of the YSZ. Nonetheless, the issues related with the implementation
of pyrochlores such as low fracture toughness and formation of deleterious
interphases with thermally grown oxide (TGO, Al2O3) were reported. The implication
was the requirement of an interlayer between the pyrochlores and TGO, which introduced
double-layer systems to the TBC literature. Furthermore, processability issues of
pyrochlores associated with the di erent evaporation rates of lanthanide oxides and zirconia
resulting in unfavorable composition variations in the coatings were addressed in
di erent studies.
After all, although the material properties are available, there is a paucity of data in the
literature concerning the properties of the coatings made of pyrochlores. From the processability
point of view the most reported pyrochlore is La2Zr2O7. Hence, the goal of this
research was to investigate plasma-sprayed Gd2Zr2O7 (GZO) coatings and YSZ/GZO
double-layer TBC systems. Three main topics were examined based on processing, performance
and properties: (i) the plasma spray processing of the GZO and its impact on
the microstructural and compositional properties of the GZO coatings; (ii) the cycling
lifetime of the YSZ/GZO double-layer systems under thermal gradient at a surface temperature
of 1400 °C; (iii) the properties of the GZO and YSZ coatings such as thermal
conductivity, coecient of thermal expansion as well as time and temperature-dependent
elastic and creep deformations.
Thermal cycling results displayed that the double-layer YSZ/GZO TBC concept is able
to provide signi cant lifetime improvement at 1400 °C surface temperature compared
to the standard YSZ. The investigations on the chemical composition of the as-sprayed
GZO revealed that no signi cant gadolinia evaporation, which would compromise the
performance 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 GZO
highlighted the importance of the process parameters for achieving the desired porosity
features assisting superior lifetime performances. A signi cant insight was gained into
the elastic and creep deformation of the plasma-sprayed YSZ and GZO coatings which
play a critical role on the development of advanced TBCs. The overarching conclusion of
this work is that the GZO has the potential to increase the temperature capability of gas
turbines, if it is applied in double-layer TBC systems and if its microstructure is tailored
by adapted processing.
Autor*in
Emine Bakan