Characterisation of a duplex TiO2/CaP coating on Ti6Al4V for hard tissue replacement

An initial TiO2 coating was applied on Ti6Al4V by electrochemical anodisation in two dissimilar electrolytes. The secondary calcium phosphate (CaP) coating was subsequently applied by immersing the substrates in a simulated body fluid (SBF) with three times concentration (SBF×3), mimicking biomineralisation of biological bone. Electrochemical impedance spectroscopy and potentiodynamic polarisation assessments in SBF revealed that the anodic TiO2 layer is compact, exhibiting up to four-folds improvement in in vitro corrosion resistance over unanodised Ti6Al4V. X-ray photoelectron spectroscopy analysis indicates that the anodic Ti oxide is thicker than air-formed ones with a mixture of TiO2−x compound between the TiO2/Ti interfaces. The morphology of the dense CaP film formed, when observed using scanning electron microscopy, is made up of linked globules 0.1–0.5 μm in diameter without observable delamination. Fourier transform infrared spectrometry with an attenuated total internal reflection analysis revealed that this film is an amorphous/poorly crystallised calcium-deficient-carbonated CaP system. The calculated Ca:P ratios of all samples (1.14–1.28) are lower than stoichiometric hydroxyapatite (1.67). These results show that a duplex coating consisting of (1) a compact TiO2 with enhanced in vitro corrosion resistance and (2) bone-like apatite coating can be applied on Ti6Al4V by anodisation and subsequent immersion in SBF.