Characterization and colorization of microarc-oxidized layers of binary titanium alloys

This study aimed to confer esthetic appearance on ash gray-colored titanium-based implant materials. Binary Ti alloys were prepared by arc-melting of titanium and one of the transition metals (Au, Cr, Cu, Mn, Mo, Nb, V, and Zr). Ti alloys containing 1, 2.5, 5 and 10 wt% alloying elements were microarc-oxidized in an electrolyte containing Ca and P ions under a constant current using a DC power supply. During the microarc oxidation (MAO) treatment, the potential vs. time curves were strongly affected by the alloying element type and content. The pore size, thickness, and crystallinity of the oxide layer increased as the final potential was increased. Colored oxide layers were obtained for the Ti alloys containing 3d (Cr, Cu, Mn, and V) and 5d (Au) transition metals. Their colors were dependent on the alloying metal type and content. In the L*a*b coordinate for color measurement adopted by the Commission Internationale d’Eclairage (CIE), the oxide layer of Ti-5Cr/MAO exhibited the highest average b* value of 19.81 (p < 0.05). The oxide layer of Ti-2.5Cu/MAO showed the next highest average b* value of 16.73. The oxidation state of 3d transition metals contributed to colorization of the microarc-oxidized Ti alloys. The characteristic colors of MAO-treated Ti alloys were rendered by the chemical binding state of alloying elements: yellow color for Cr3+, red color for Cu+, yellow color for Mn4+ and yellow color for V5+. The purple color of the MAO-treated Ti-Au alloy surface originated from Au nanoparticles. The 4d transition metals (Zr, Nb, and Mo) did not affect color revelation of the MAO-treated Ti alloys.