Abstract
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.
Highlights
Sprayed ceramic coatings are typically used in the aerospace industry for their low thermal diffusivity and high-temperature erosion resistance
The coatings adhered well to the substrate, and in all hybrid coatings bimodality was achieved with in situ synthesized yttria-stabilized zirconia (YSZ)/ZrO2 particles of \ 200 nm embedded between the well-melted Al2O3 splats
By combining solution and powder feedstock in situ, it is possible to combine oxides into coatings jointly with other oxides, hard metals or metals of virtually any combination without the limitations of powder or Preliminary results on the characterization of coatings prepared via a hybrid powder-precursor HVOF spray process were presented in this study
Summary
Sprayed ceramic coatings are typically used in the aerospace industry for their low thermal diffusivity and high-temperature erosion resistance. E.g., in components in the process industry, such as center rolls and dewatering elements for paper machines, mechanical seals and process valves (Ref 1) In these components, combined wear- and corrosion resistance is the key factor and the main reason for choosing ceramic coatings over other material options. A vast amount of research has gone into improving the fracture toughness of Al2O3 with additions of ZrO2, with the intent of strengthening the composite This can be achieved by the well-known toughening effect of the phase transformation of tetragonal ZrO2 to monoclinic and the following volume change as well as the ferroelastic domain switching in tetragonal ZrO2 (Ref [6,7,8,9,10]). An undesirable side effect of the phase change is a large volume increase, which deteriorates the
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