Preparation of fluorapatite-containing flower-like structure coatings by micro-arc oxidation combined with ion-exchange technology

Fluorapatite (FA) coatings offer superior chemical and thermal stability compared to hydroxyapatite (HA), making them attractive for biomedical applications. However, producing highly crystalline FA coatings using only micro-arc oxidation (MAO) remains challenging. Ion-exchange technology (IET) enables the substitution of OH⁻ in HA with F⁻ to form FA or fluorohydroxyapatite (FHA), thereby enhancing the long-term stability of the crystal structure. Accordingly, this study prepared FA-containing coatings on pure titanium substrates by combining MAO and IET. The MAO process was conducted at 390 V and 0.6 A for 5 min in an electrolyte containing 0.2 mol/L (CH₃COO)₂Ca·H₂O and 0.1 mol/L NaH₂PO₄·2H₂O, resulting in flower-like structure coatings containing TiO₂, DCPD (CaHPO₄·2H₂O), and HA. IET was then applied in NaF solutions at varying temperatures (25 °C, 60 °C), fluoride ion concentrations (50–100 ppm), and soaking durations (6–24 h). All the resulting coatings retained a flower-like morphology and contained F⁻ ions. At 25 °C, only FHA was formed, whereas at 60 °C, FA was produced. The MAO/IET coatings exhibited superior mechanical properties, including higher hardness (H) and elastic modulus (E), and favorable H/E and E_coating/E_substrate ratios. In addition, the MAO and MAO/IET coatings both exhibited superhydrophilicity, with contact angles approaching 0°. The bioactivity of the MAO/IET coatings was slightly lower than that of the MAO coatings. However, they demonstrated superior biocompatibility, as evidenced by more extensive MG-63 cell spreading. The findings of this study confirm that MAO/IET processing enables the fabrication of FA-containing coatings with excellent biomedical potential.