The Effect of Coarse-Grained and Ultrafine-Grained Ti13Nb13Zr Alloy Substrates on the Structure and Properties of Calcium Phosphate Coatings Formed by Micro-Arc Oxidation

This study investigates the formation, structure, phase composition, and performance of oxide–phosphate coatings produced by micro-arc oxidation (MAO) on coarse-grained and ultrafine-grained Ti13Nb13Zr alloy substrates. The effects of MAO treatment conditions and substrate structural state on coating morphology, thickness, elemental and phase composition, tribological behaviour, and microhardness were evaluated. All investigated coatings exhibited a similar crystalline phase composition dominated by α-Ti and TiO2 in the anatase and rutile forms, while the relative intensities of the oxide reflections varied with processing conditions. The tribological response depended on both the MAO mode and substrate state: the lowest mean friction coefficient was obtained for the ultrafine-grained substrate in Mode 1 (μ = 0.149), whereas the highest value was observed for the coarse-grained substrate in Mode 3 (μ = 0.723). The highest microhardness in the reported dataset, 700 HV0.1, was obtained on the ultrafine-grained substrate in Mode 3 using the electrolyte containing 1.0 g/L TiO2. SEM/EDS observations further showed that the substrate’s structural state influenced the uniformity of oxide-layer development and elemental distribution. These results demonstrate that both the electrical treatment mode and the initial microstructural state of Ti13Nb13Zr affect the characteristics of MAO-derived oxide–phosphate coatings.