Characterization of Powder-Precursor HVOF-Sprayed Al₂O₃-YSZ/ZrO₂ Coatings

Thermal spraying using liquid feedstock can produce coatings with very fine microstructures either by utilizing submicron particles in the form of a suspension or through in situ synthesis leading, for example, to improved tribological properties. The focus of this work was to obtain a bimodal microstructure by using simultaneous hybrid powder-precursor HVOF spraying, where nanoscale features from liquid feedstock could be combined with the robustness and efficiency of spraying with powder feedstock. The nanostructure was achieved from YSZ and ZrO₂ solution-precursors, and a conventional Al₂O₃ spray powder was responsible for the structural features in the micron scale. The microstructures of the coatings revealed some clusters of unmelted nanosized YSZ/ZrO₂ embedded in a lamellar matrix of Al₂O₃. The phase compositions consisted of γ- and α-Al₂O₃ and cubic, tetragonal and monoclinic ZrO₂. Additionally, some alloying of the constituents was found. The mechanical strength of the coatings was not optimal due to the excessive amount of the nanostructured YSZ/ZrO₂ addition. An amount of 10 vol.% or 7 wt.% 8YSZ was estimated to result in a more desired mixing of constituents that would lead to an optimized coating architecture.