In this work, plasma electrolytic fluorination (PEF) was performed on AZ91 alloy in a non-aqueous NH4F-ethylene glycol (EG) electrolyte. The coating growth and the plasma discharge behaviour were investigated to understand the mechanism of PEF process. Results illustrated that the non-aqueous electrolyte determined the intrinsic characteristics of the PEF process, which resulted in the coating growth characteristics and plasma discharge behaviour distinct from those of a conventional PEO process. MgF2 compound formed from the combination of the oppositely charged ions Mg2+ from the substrate and F- form the electrolyte. The deposition of MgF2 facilitated the growth of the PEF coating with an inward-growth characteristic. The discharge was characterised by a very small size and a weak intensity of the sparks, with an electron temperature (Te) range of 3400∼3600 K. The three-type discharge model of conventional PEO processes was introduced to understand the mechanism of the PEF process.
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