Microstructure and corrosion properties of AlCoCrFeNi high-entropy alloy coatings prepared by HVAF and HVOF

High entropy alloys (HEAs) represent an alternative development approach for new alloy systems. These materials have been found to yield promising properties, such as high strength in combination with sufficient ductility as well as high wear and corrosion resistance. Especially for alloys with a body-centered cubic (bcc) structure, advantageous surface properties have been revealed. However, typical HEA systems usually contain high contents of elements that are expensive or have limited availability. Consequently, applying them as coatings where the material usage is limited to the surface represents an exciting pathway to enable an economical exploitation of their superior properties. Nevertheless, there is a strong influence of processing conditions on the resulting microstructure and phase formation, which considerably influences their functional properties. In the present study, microstructural differences between high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) sprayed coatings of the alloy AlCoCrFeNi are investigated. A metastable bcc structure is formed in case of both coating processes. Precipitation reactions are suppressed by the rapid solidification during atomisation and by the relatively low thermal input during spraying. Resistance of the coatings to corrosive media was also investigated in detail, and an improved passivation behaviour was observed in the HVAF coating.