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

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 different evaporation rates of lanthanide oxides and zirconia resulting in unfavorable composition variations in the coatings were addressed in different 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, coefficient 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 significant 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 significant gadolinia evaporation, which would compromise the performance of the coating, takes place in the examined spray current range (300 A525 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 significant 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.