Bi-layer nano-TiO2/FHA composite coatings on Mg–Zn–Ce alloy prepared by combined physical vapour deposition and electrochemical deposition methods

• A nano-composite coating is fabricated on the Mg alloy using combined PVD and ED.
• The composite coating is consisted of TiO2 as inner layer and FHA as top layer.
• The corrosion resistance of the Mg alloy is improved by TiO2/FHA composite coating.
• Nano-TiO2/FHA coating showed higher pitting potential than the nano-TiO2 coating.
• TiO2/FHA coating showed more compact with less pores than the TiO2 coating.

In this study, physical vapour deposition (PVD) method was carried out to prepare nano-titania (TiO2) as an intermediate layer on Mg–Zn–Ce alloy and subsequently electrochemical deposition (ED) method was applied to develop nano-fluorine-doped hydroxyapatite (FHA) as an outer layer. X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and transmission electron microscopy (TEM) methods were used to characterize the coatings. Corrosion behaviour of coated samples was evaluated and compared to that of uncoated substrate in SBF solution using immersion test and electrochemical measurements. The resulting TiO2/FHA composite coating shows uniform, compact, and smooth surface morphology, with 10 μm thickness and crystallite size of around 75–85 nm. As a result, the bi-layer TiO2/FHA composite coating specimen presents higher corrosion resistance and superior stability in SBF solution than that of mono-layer TiO2 coated specimen. Nano-TiO2/FHA composite coating increased corrosion resistance of Mg–Zn–Ce alloys by thirty orders of magnitude owing to low solubility, thickness and compactness of the coating. The results suggested that the nano-TiO2/FHA composite coated Mg alloy could provide a suitable corrosion behaviour for use as biodegradable bone implant.