Effect of NaF additive on the micro/nano-structure and properties of the microarc oxidation coating on Ti6Al4V alloy

Micro/nano-structured coatings with antibacterial function were prepared by microarc oxidation (MAO) treatment on Ti6Al4V alloy in a silicate/phosphate electrolyte with a NaF additive. The microstructure, phase composition, and corrosion resistance of the coatings modified by adding NaF (0.15-0.5 M) were examined using scanning/transmission electron microscopy, energy dispersive spectroscopy, atomic force microscopy, X-ray diffraction, and potentiodynamic polarization. The results showed that the incorporation of F ion reduced the threshold voltage for electrons avalanche on the surface film of the Ti alloy, and increased the intensity and lifetime of discharge. MAO coatings with 100-500 nm nano-pores and 1-20 mu m micro-pores were formed by the modification of the NaF additive. The F ions promoted microarc discharge as well as phase transformation from the metastable anatase to the stable rutile phase. The F ions also promoted the generation of penetration cracks and bubbles in the coating. The surface roughness, phase content, and thickness of the coating were enhanced by the NaF additive. However, the corrosion resistance of the coating first increased and then decreased with the increasing F ion concentration, reaching a maximum when the NaF content was 0.25 M.