Mechanism of tetraborate and silicate ions on the growth kinetics of microarc oxidation coating on a Ti6Al4V alloy

Abstract

The growth kinetics mechanism of microarc oxidation (MAO) coatings on Ti6Al4V alloy was studied by designing a binary mixed electrolyte with various SiO₃²⁻ and B₄O₇²⁻ ion ratios via scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and potentiodynamic polarization. When the ratio of B₄O₇²⁻ in the electrolyte is 100%, B₄O₇²⁻ dissolves molten TiO₂ at a high temperature, exposing nano-scale filamentary channels in the barrier layer of MAO coating, resulting in the repeated nucleation of microarc in the same area. When the ratio of SiO₃²⁻ in the binary mixed electrolyte reaches 10%, the amorphous SiO₂ formed by SiO₃²⁻ at high temperatures from discharge blocks discharge channels and induces microarc nucleation in other regions, thus inhabiting the discharge cascade phenomenon. When the ratio of SiO₃²⁻ in the binary mixed electrolyte increases from 15% to 50%, the amounts of molten oxides cover some of the pores formed by the primary microarc discharge, so part of the secondary discharge preferentially generates in the uncovered pores. Finally, the discharge cascade phenomenon occurs. Moreover, the thickness of the MAO coating obtained in the binary mixed electrolyte containing B₄O₇²⁻ and SiO₃²⁻ shows a power function with time.