Growth kinetics and mechanism of microarc oxidation coating on Ti–6Al–4V alloy in phosphate/silicate electrolyte

Microarc oxidation (MAO) is an effective surface treatment method for Ti alloys to allow their application in extreme environments. Here, binary electrolytes consisting of different amounts of sodium phosphate and sodium silicate were designed for MAO. The surface morphology, composition, and properties of MAO coatings on Ti–6Al–4V alloy treated in 0.10 mol/L electrolyte were investigated to reveal the effect of \rm PO_4^3- and \rm SiO_3^2- on the growth kinetics of the MAO coatings, using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and potentiodynamic polarization. The results showed that \rm PO_4^3- is beneficial for generating microarcs and forming pores within the coating, resulting in a thick but porous coating. \rm SiO_3^2- facilitates the blocking of pores in the outer deposition layer and impedes the generation of microarcs, resulting in a thin dense coating. The thickness, density, phases content, and polarization resistance of the MAO coatings are primarily affected by the intensity of microarcs for low \rm SiO_3^2- contents, and by the number of microarcs when the \rm SiO_3^2- content is sufficiently high. The thickness of MAO coatings obtained in P/Si electrolytes shows a piecewise linear increase with increasing process time during the three stages of microarc discharge. \rm SiO_3^2- is beneficial to the density increase of the coating formed in the previous stage of microarcs discharge, but slows down the growth of the coating formed in the next stage.