Enhanced corrosion resistance and in vitro bioactivity of NiTi alloys modified with hydroxyapatite-containing Al2O3 coatings

Hydroxyapatite (HA)-containing Al2O3 coatings were prepared on NiTi alloys through a cathodic plasma electrolytic deposition (CPED) process, followed by hydrothermal treatment (HT). The surface morphology, elemental distribution and phase composition of each coating were studied by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD). SEM observations revealed that the coatings exhibited a “crater-like” porous structure, and that the thickness of the coating could be up to 100 μm. The results of EDS demonstrated that the Ca and P concentrations in the Al2O3 coatings could be increased to 8.2% and 4.7%, respectively, and that the Ca/P ratio could reach 1.72, which was close to that of natural HA. The results of XRD analysis showed that the CPED coating mainly comprised α-Al2O3 and γ-Al2O3, and that HA crystallization was induced by the subsequent HT. The corrosion resistance of the samples was investigated by potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS). The results showed that the corrosion resistance of the sample with the CPED-HT coating was enhanced by more than two orders of magnitude compared to that of the uncoated NiTi alloy. Ni ion release in simulated body fluid (SBF) was characterized by inductively coupled plasma-atomic emission spectroscopy, and the results showed a significant drop in the concentration of released Ni ions from 18.0 μg/mL for the uncoated NiTi alloy to 0.7 μg/mL for the CPED-HT coating. In addition, the SBF soaking test showed that HA precipitated on the CPED-HT coating and that the HA layer became thicker and larger with increasing soaking time, indicating that the CPED-HT coating exhibited excellent apatite-inducing ability. Thus, the introduced CPED-HT treatment provides a new and effective route for the surface modification of NiTi alloys for use as biomedical materials.