Heterogeneous Thermal Barrier Coatings with Engineered Gradient Interfaces

Recent development of plasma spraying with liquid feedstocks enabled deposition of novel thermal barrier coatings (TBCs) with top-coats incorporating various desirable features, such as columnar microstructure or ultrafine porosity. Moreover, different materials may be relatively easily combined, e.g., by alternating feedstocks in the feed line or using feedstock mixtures. Coatings with gradient change of chemistry/microstructure towards the free-surface can be also prepared by gradual change of the feedstock composition, which may be potentially beneficial for example to mitigate stresses at the macroscopic interfaces between TBC layers (typically bond-coat/top-coat or within layered top-coat). In this study, three experimental TBCs with gradient top-coats were successfully deposited by hybrid water/argon-stabilized WSP-H plasma torch, i.e., one version of coating with 8 wt.% yttria stabilized zirconia transitioning into gadolinium zirconate Gd₂Zr₂O₇ (YSZ→GZO) and two versions of Al₂O₃ transitioning into YSZ (Al₂O₃→YSZ). Thermal cycling fatigue (TCF) test with peak temperature of 1100 °C showed outstanding thermal shock resistance of the YSZ→GZO coating and mediocre to poor resistance of both Al₂O₃→YSZ coatings.