Microstructural Evolution and Failure Analysis for 8YSZ/(Y0.5Gd0.5)TaO4 Double-Ceramic-Layer Thermal Barrier Coatings on Copper Substrate

The main purpose of this work is to suppress the rate of thermal and oxidative corrosion of copper substrates using double-ceramic-layer thermal barrier coatings (TBCs). Herein, the orthogonal spray experiment was employed to optimize the spraying parameters for TBCs consisting of Cu/NiCoCrAlY/8YSZ/(Y0.5Gd0.5)TaO4. The thermal cycling and average mass loss rate of TBCs prepared by atmospheric plasma spraying (APS) with optimum spraying parameters correspond to 20 cycles and 0.56‰, respectively. The thermal conductivity (0.39 W·m−1·K−1 at 900 °C) of (Y0.5Gd0.5)TaO4 is 71.68% and 52.7% lower than that of (Y0.5Gd0.5)TaO4 bulk and 8YSZ, respectively. Meanwhile, the bond strength increased from 8.86 MPa to 14.03 MPa as the heat treatment time increased from 0 h to 24 h, benefiting from the heat treatment to release the residual stresses inside the coating. Additionally, the hardness increased from 5.88 ± 0.56 GPa to 7.9 ± 0.64 GPa as the heat treatment temperature increased from room temperature to 1000 °C, resulting from the healing of pores and increased densification. Lastly, crack growth driven by thermal stress mismatch accumulated during thermal cycling is the main cause of coating failure. The above results demonstrated that 8YSZ/(Y0.5Gd0.5)TaO4 can increase the service span of copper substrate.