Hydroxyapatite–Titania nanotube composite as a coating layer on Co–Cr-based implants: Mechanical and electrochemical optimization

Inadequate mechanical properties of pure hydroxyapatite (HA) coating layers make it an unsuitable candidate for many load-bearing orthopedic implants. In this study, Titania nanotubes (TNT) and HA were synthesized using Rapid Breakdown Adonization (RBA) and sol–gel methods, respectively. The sintering process at different temperatures was then conducted for the phase transformation of titanium. HA–TNT mixtures in different quantities and phases were prepared for coating on Co–Cr-based substrates. To optimize the coated HA–TNT composite layer in term of hardness, bonding strength and corrosion potential, empirical models based on Response Surface Methodology (RSM) were developed. The synthesized TNT and HA–TNT coated samples were characterized using X-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM) and Transmission Electron Microscopy (TEM). The predicted models generated by RSM were compared with the experimental results, and close agreement was observed. While the models demonstrate that TNT quantity is a more significant factor than sintering temperature in improving hardness (H), bonding strength (P) and the corrosion potential (Ecorr) of a coated substrate, sintering temperature still has a considerable effect on H and Ecorr. The optimum HA–TNT composite coating layer in terms of the mechanical and electrochemical properties were obtained with a TNT ratio of 1.07 (wt%) at a sintering temperature of 669.11 °C.