Assessment of applied voltage on the structure, pore size, hardness, elastic modulus, and adhesion of anodic coatings in Ca-, P-, and Mg-rich produced by MAO in Ti–25Ta–Zr alloys

This work aims to modify the surface of the Ti-25Ta-xZr (x = 0, 25, and 50wt.%) system alloys through micro-arc oxidation (MAO) to incorporate calcium, phosphorus, and magnesium ions in the oxidized layer deposited. The MAO surface modification was performed by changing the limited voltage applied to the system (200, 250, and 300V). Structural characterizations by X-ray diffraction and X-ray photoelectron spectroscopy showed that the formed MAO films comprise TiO2, Ta2O5, ZrO2, and CaCO3 oxides. Voltage inhibits the formation of Ta2O5 and CaCO3 phases and decreases the incorporation capacity of Ca, P and Mg ions. The MAO layers' topography, thickness, and roughness, analyzed by scanning electron microscopy and confocal microscopy techniques, showed that increasing the voltage tends to increase pore sizes and reduces film thickness; conversely, it decreases the number of pores per μm2. Regarding hardness and elastic modulus, adding Zr increases the MAO layer's hardness due to the formation of ZrO₂. However, increasing the voltage decreases the MAO films' hardness due to the formation of the compound CaCO3. The MAO process increased the elastic modulus of the Ti-25Ta-xZr bulk alloys. Finally, the adhesion of the films was analyzed by the Rockwell-C adhesion test. The results showed a good adhesion of MAO films to Ti-25Ta-xZr substrates.