Microstructure and Thermal Properties of the Early-Stage Experimental Thermal Barrier Coatings Deposited by Hybrid Plasma Spraying

Hybrid plasma spraying combines plasma spraying of dry coarse powders and liquids (suspensions or solutions) potentially providing high deposition rates and coatings with novel microstructures and amended functionality. Such composites may be potentially interesting also for the thermal barrier coatings (TBCs). In this study, the first experimental TBCs with hybrid ceramic top-coats were deposited by hybrid water-/argon-stabilized plasma (WSP-H) technology. Coarse dry YSZ (yttria-stabilized zirconia: ZrO2-Y2O3) powder was selected to form the top-coat matrix. Suspensions of YSZ, Al2O3 (alumina), YAG (yttrium aluminum garnet: Y3Al5O12), or GZO (gadolinium zirconate: Gd2Zr2O7) were the source of the additional miniature phase. Uniform functionalized coatings with TBC-relevant thickness and microstructure were successfully deposited. Hybrid and reference coatings deposited also with WSP-H torch and with YSZ feedstocks showed comparable thermal diffusivity regardless of the spraying conditions but the addition of Al2O3 and YAG increased it by similar to 65%, whereas GZO decreased it by similar to 20%. Thermal cycling resistance of the coatings was evaluated by thermal cycling fatigue (TCF) test. Hybrid coating with the addition of YSZ miniature phase endured on average 640 TCF cycles, which was 250-300 cycles less than the reference YSZ coatings deposited without liquid feedstocks but still comparable to the currently used YSZ-based industrial TBCs. Coatings with dissimilar chemistry of additional miniature splats showed inferior mean TCF lifetimes: GZO 438 cycles, Al2O3 272 cycles, and YAG 173 cycles, opening space for further coating optimization. All hybrid coatings failed above the bond-coat/top-coat interface. Comparison of coatings before and after the thermal exposure by scanning electron microscopy (SEM) and x-ray diffraction (XRD) revealed possible changes within the coating microstructure, namely sintering (typical for top-coat consisting only of YSZ), conversion of metastable and amorphous phase (top-coats with the addition of Al2O3 and YAG) or interaction between the matrix and secondary phase (top-coat with the addition of GZO).