The interfacial study of sol-gel-derived fluoridated hydroxyapatite coatings

Fluoridated hydroxyapatite (FHA) coatings have been investigated as an alternative for pure hydroxyapatite (HA) coatings because FHA are less soluble and are equally bioactive. In this study, Ca(NO3)2, P2O5, and HPF6 have been used to prepare FHA coatings on Ti-6Al-4V substrates by sol-gel dip-coating method. The interface between the coating and the substrate is characterized by high-resolution transmission electron microscopy (HRTEM), secondary ion mass spectroscopy (SIMS), and X-ray photoelectron spectroscopy (XPS). A TiO2 (rutile)-rich layer is found at the interface. The fluorine content fluctuates basically in sync with the oxygen content near the interface, which is ascribed to the formation of Ti(O2−xF2x). It is also found that the amount of rutile is related to the HPF6 amount in the dipping sol. The reason is ascribed to the existence of CaF2 at the interface, which provides more oxygen for rutile formation by forming hydrogen bonding (F⋯HO).