Biomechanical compatibility and electrochemical stability of HA/Ta2O5 nanocomposite coating produced by electrophoretic deposition on superelastic NiTi alloy

In this paper, natural hydroxyapatite (nHA) and nHA/20 wt% Ta₂O₅ coatings were electrophoretically deposited on NiTi. Phase analysis and microstructural characterizations of coatings were accomplished using X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM). Micro-indentation examination results revealed that nHA/Ta₂O₅ coating has the most compliant elastic modulus (32.85 GPa) to that of the adjacent bone tissue. Adhesion strength of coatings that was determined with tensile as well as nano-scratch tests affirmed the better adhesion properties of nHA/Ta₂O₅ compared with nHA coating on NiTi. Fracture toughness calculations from nano-scratch data exhibited the highest value for nHA/Ta₂O₅ coating (1 MPa m^1/2). Degradation behaviors of nHA and nHA/Ta₂O₅ coated NiTi in Phosphate Buffered Saline (PBS) solution were characterized by electrochemical impedance spectroscopy (EIS) technique during 30 days of immersion. Time-dependent evolution of open circuit potentials and impedance characteristics' results clarified the positive role of Ta₂O₅. Reasons for this improvement were explained by the lower amounts of open porosities and easier blocking of pores in the presence of Ta₂O₅.