Abstract:The microstructure, physical and mechanical, and chemical properties of micro-arc calcium phosphate (CaP) coatings deposited under different process voltages in the range of 150-400 V on the commercially pure titanium (Ti) and Ti-40%Nb (Ti-40Nb) (mass fraction) alloy were investigated by the SEM, TEM, XRD and EDX methods. The coating thickness, roughness, and sizes of structural elements were measured and showed similar linear character depending on the process voltage for the coatings on both substrates. SEM results showed the porous morphology with spherical shape structural elements and rough surface relief of the coatings. XRD and TEM studies exhibited the amorphous structure of the CaP coating. With increasing the process voltage to 300-400 V, the crystalline phases, such as CaHPO₄ and β-Ca₂P₂O₇, were formed onto the coatings. The annealing leads to the formation of complex poly-phase structure with crystalline phases: CaTi₄(PO₄)₆, β-Ca₂P₂O₇, TiP₂O₇, TiNb(РО₄)₃, TiO₂, NbO₂, and Nb₂O₅. The applied voltage and process duration in the ranges of 200-250 V and 5-10 min, respectively, revealed the coating formed on Ti and Ti-40Nb with optimal properties: thickness of 40-70 mm, porosity of 20%-25%, roughness (R_a) of 2.5-5.0 mm, adhesion strength of 15-30 MPa, and Cа/Р mole ratio of 0.5-0.7.

Key words: calcium phosphate coating; micro-arc oxidation; Ti-40%Nb alloy; commercially pure titanium; microstructure