Improvement of hydroxyapatite sol-gel coating on titanium with ammonium hydroxide addition

Small amounts (1-5 vol%) of ammonium hydroxide (NH4OH) were added to the ethanol-water-based hydroxyapatite (HA) sol-gel solution. After aging the sol, a Ti substrate was dip-coated and heat-treated at 500degreesC for 1h in air. The sol properties were monitored in terms of pH, viscosity, and structure changes with aging time; also, the coating phase and structure on Ti were investigated. During aging, the viscosity of the sol increased while the pH decreased, confirming the polymerization and gelation of the sol. The addition of NH4OH altered the sol properties significantly. Increase in the NH4OH concentration increased the pH and viscosity of the Sol considerably, especially at short aging period, followed by a saturation with further aging. The Fourier transformed-infrared analysis also confirmed a gradual structure change of the sol with NH4OH addition. Such changes in pH, viscosity, and structure bands of the sol driven by the NH4OH addition were attributed to the improved polymerization and gelation of the sol. The improved gelation shortened the aging time needed for crystallization of the apatite coating, i.e., increase in the NH4OH addition improved the crystallization of the coatings. Within 24 h aging time, the coatings on Ti containing over 3.5 vol% NH4OH showed characteristic HA phase and structure bands, at which period no apatite peaks were observed on the HA coating obtained without NH4OH addition.