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

Abstract

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.