7-8 wt. yttria-stabilized zirconia (YSZ) is current standard ceramic material in thermal barrier coating (TBC) applications, mainly in gas turbine engines. However, for high temperature (>1200°C) gas turbine engine operations, YSZ has limitations of poor phase stability and high sintering rates along with its susceptibility to CMAS (calcium magnesium alumino silicates) attack. These challenges effect the durability of TBCs based on YSZ top coat. A wide range of both academic and industrial researches has been done to improve TBC performance by opting for advanced TBC materials and appropriate TBC microstructure. Amongst different proposed materials, pyrochlores have shown excellent properties such as lower thermal conductivity and resistance to CMAS attack compared to YSZ. However, pyrochlores have the drawback of inferior thermo-mechanical properties than YSZ.

To address this issue, double layered TBCs with YSZ as the base layer and pyrochlores as top layer were proposed to take advantages of high fracture toughness of YSZ and high temperature capability of pyrochlores. In this study, double layer TBC consisting of gadolinium zirconate, Gd2Zr2O7 (here after referred to as GZ), as the top layer and YSZ as the base layer were deposited. Three double-layered GZ/YSZ TBCs with different layer thickness, i.e. 100GZ/400YSZ, 250 GZ/250 YSZ and 400GZ/100YSZ (the prefixed numbers before GZ and YSZ denote corresponding layer thickness in µm) were deposited by suspension plasma spray (SPS). The intention behind varying thickness of YSZ was to alter the fracture toughness at probable failure location of double-layered TBCs (GZ/YSZ interface). The deposited TBCs were subjected to burner rig test (BRT) and thermal cyclic fatigue (TCF) test.

It was shown that the double layer TBC with lower YSZ thickness had higher thermal cyclic fatigue life and burner rig lifetime (also known as thermal shock lifetime) compared to 100GZ/400YSZ and 250 GZ/250 YSZ. Failure analysis of the TCF specimens and burner rig tested specimens were investigated using a scanning electron microscope (SEM)/energy-dispersive spectroscopy (EDS). The failure modes in BRT were similar for the three double layered TBC variations i.e. at the GZ/YSZ interface. In the case of TCF, failure was observed in the TGO. Porosity analysis and hardness test were employed to compare the sintering behaviour of GZ and YSZ by analysing TBC specimens before and after exposure to TCF test.