Fabrication and characterization of functionally graded hydroxyapatite/TiO2 multilayer coating on Ti–6Al–4V titanium alloy for biomedical applications

Single layer, double layer and functionally graded coatings of hydroxyapatite and TiO2 particles were deposited on Ti–6Al–4V titanium alloy substrate by electrophoretic deposition technique (EPD) in the acetylacetone medium. Optimum conditions to obtain stable suspensions for EPD of each type of coatings were estimated by investigating the effect of iodine concentration on the zeta-potential. In addition the effect of the applied voltage, during EPD process and polyethylenimine (PEI) concentration as binder on the coatings quality (formation of crack free coatings) was investigated. The results showed the formation of more intact coatings deposited at 20 V applied voltage from suspensions containing 0.6 g/L iodine and 4 g/L PEI additives. The coatings chemical composition, morphology and HA decomposition behavior were investigated using energy dispersive X-ray spectroscopy (EDX), scanning electron microscope (SEM) and X-ray diffraction (XRD) methods. The results showed that HA starts to decompose at lower temperature in functionally graded HA/TiO2 coating (900 °C). The adhesion strengths of the coatings were measured by shear testing and the results showed that the HA/TiO2 functionally graded coating has more adhesion strength (~31 MPa) compare to HA single layer and HA–TiO2 double layer coatings.