Hydrothermal deposition of hydroxyapatite coatings on glass and ceramics

Abstract

The deposition of hydroxyapatite [Ca 10(PO 4) 6(OH) 2] coatings on bioactive glass (45S5) and on inert ceramic substrates (fused SiO 2 and Al 2O 3) from aqueous solutions containing calcium and phosphate ions was investigated under hydrothermal conditions at temperatures between 90°C and 150°C. Rapid deposition of hydroxyapatite coatings was achieved on 45S5 glass at pH = 12.5. Growth of the hydroxyapatite coating was preceded by the formation of an amorphous CaO-SiO 2 layer with a thickness of ∼2 μm. The hydroxyapatite coating grew rapidly, with the thickness increasing linearly with time, and with preferred alignment of the c-axis of the crystals perpendicular to the glass surface. Under the same conditions, no hydroxyapatite deposition was observed on fused SiO 2 and Al 2O 3. Successful deposition of hydroxyapatite coatings on Al 2O 3 was achieved from an aqueous solution of calcium and phosphate ions containing EDTA disodium salt as a chelating agent and hydrogen peroxide. The deposition was strongly dependent on the pH of the solution. Uniform deposition was achieved at pH = 9.8. Initial formation of the coating occurred with the hydroxyapatite crystals aligned parallel to the Al 2O 3 surface but the orientation soon changed to give growth with the c-axis perpendicular to the surface. The extent of the orientation increased with time to give nearly perpendicular orientation after ∼12 h.