Abstract: Hydroxyapatite coatings were formed on titanium electrode by using the hydrothermal electrodeposition method in an autoclave. The electrolyte contained 0.010 5 mol/L Ca(NO₃)₂, 0.006 3 mol/L NH₄H₂PO₄ and 0.1 mol/L NaNO₃. The electrolyte temperature was controlled at 60—200 ℃ and pH value was adjusted at 4.6. After loading of a constant current at 0.4 mA/cm² for 2 h, the deposits were characterized by XRD, FTIR, environment scanning electron microscopy and energy dispersive X-ray spectroscopy. The experiment results show that the deposit crystals are rods in shape, the edge face of rod is a defined flat hexagon and the width and length of the crystal rods increases continuously with increasing the electrolyte temperature. The component of coatings is calcium-deficient hydroxyapatite [Ca_10-x(HPO₄)_x(PO₄)_6-x(OH)_2-x, 0≤x≤1], which decomposed into the mixture of hydroxyapatite and β-calcium phosphate after sintering at 800 ℃. The n(Ca)/n(P) ratio of the coatings increased and was close at 1.67 with the electrolyte temperature increasing. These results suggest that the component of coating is continuously close to stoichiometric hydroxyapatite with the electrolyte temperature increasing. Furthermore, mass gain of the coatings and bonding strength of coatings increase with electrolyte temperature increasing up to 160 ℃. The bonding strength of coatings after sintering at 800 ℃ for 6 h attained 16.7 MPa when the electrolyte temperature was controlled at 160 ℃.

Key words: Hydroxyapatite, Hydrothermal electrodeposition, Coating, Binding strength