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

Abstract

<p>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 ZrO<sub>2</sub> solution-precursors, and a conventional Al<sub>2</sub>O<sub>3</sub> spray powder was responsible for the structural features in the micron scale. The microstructures of the coatings revealed some clusters of unmelted nanosized YSZ/ZrO<sub>2</sub> embedded in a lamellar matrix of Al<sub>2</sub>O<sub>3</sub>. The phase compositions consisted of γ- and α-Al<sub>2</sub>O<sub>3</sub> and cubic, tetragonal and monoclinic ZrO<sub>2</sub>. 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/ZrO<sub>2</sub> 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.</p>