Oxide ceramic coatings with amorphous/nano-crystalline dual-structures prepared by micro-arc oxidation on Ti–Nb–Zr medium entropy alloy surfaces for biomedical applications

Due to the presence of multi-principal elements, the multi-principal element alloys (MPEAs) are supposed to undergo competing oxidation reactions during micro-arc oxidation (MAO). In this work, Ti–Nb–Zr medium entropy alloy (MEA, MAO-Free) was MAO-treated at various applied voltage. For Ti–Nb–Zr MEA treated at 250 V (MAO-250 V), the coating microstructure was characterized by amorphous oxides of Ti, Nb and Zr from the substrate and amorphous compounds of CaO, Ca(OH)2, SiO2 and CaSiO3 from the electrolyte, which distributed homogeneously in coating matrix. Different from the microstructure of MAO-250 V group, tetragonal-ZrO2 nano-crystals dispersed in amorphous coating matrix were noticed for that treated at 300 V (MAO-300 V). For specimen treated at 350 V (MAO-350 V), the coating matrix exhibited higher crystallinity by composing of major tetragonal-ZrO2 and minor rutile-TiO2, with the presence of nano-sized amorphous regions. The pore size, porosity, surface roughness and hydrophilicity of coatings increased with the increase of applied voltage. An order of MAO-300 V > MAO-250 V > MAO-Free > MAO-350 V group was identified for the wear resistance. Moreover, the MAOed coatings exhibited comparable corrosion resistance and cyto-compatibility to MAO-Free group. Together, the results indicate that the MAO-300 V group with favorable wear resistance, corrosion resistance and cyto-compatibility is promising for biomedical applications.