Understanding the formation mechanism of supersonic atmospheric plasma sprayed in-situ hypereutectic Al-25 wt%Si coating with nanostructured coupled eutectic: From powder, in-flight droplet, splat to coating

Abstract A comprehensive study on the formation mechanism of supersonic atmospheric plasma sprayed hypereutectic Al-25 wt%Si coating was performed by investigating microstructural evolution from powder, in-flight droplet, splat, to coating. Results show that feedstock Al-20 wt%Si powder exhibits sub-micron eutectic and micro-sized primary Si. Elements loss, surface oxidation, breakup and chemical diffusion occur when in-flight droplets are exposed to plasma jet. The splat exhibits an ideal structure of nano-sized coupled eutectic. The coating consists of featureless (~84.7 vol%) and ultrafine (~14.2 vol%) zones. Four Si morphologies coexist in the coating from scale of angstrom to sub-micron: Si atoms/clusters in supersaturated solid solution; round/rod-shape Si precipitations (~6.23 nm); nano-Si particulates (~34.37 nm) in featureless zone; and sub-micron Si particulates (~107.32 nm) in ultrafine zone. Further, physical/chemical changes mechanisms of in-flight droplets, rapid solidification mechanisms of splat and coating, formation mechanisms of featureless and ultrafine zones, precipitation and growth mechanisms of Si phase are extensively elucidated. Accordingly, the coating demonstrates ultra-high hardness (~259.8 HV0.1, better than previously reported values of hypereutectic Al-Si alloys with similar silicon content), and superior wear resistance. The unique microstructure with solid solution and grain refinement strengthening mechanisms, and in-situ fabricating higher Si-content Al-Si alloys contribute to the exceptional mechanical properties.