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

Abstract

Small amounts (1-5 vol%) of ammonium hydroxide (NH 4OH) were added to the ethanol - water-based hydroxyapatite (HA) sol - gel solution. After aging the sol, a Ti substrate was dipcoated and heat-treated at 500°C for 1 h 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 NH 4OH altered the sol properties significantly. Increase in the NH 4OH 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 NH 4OH addition. Such changes in pH, viscosity, and structure bands of the sol driven by the NH 4OH 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 NH 4OH addition improved the crystallization of the coatings. Within 24 h aging time, the coatings on Ti containing over 3.5 vol% NH 4OH showed characteristic HA phase and structure bands, at which period no apatite peaks were observed on the HA coating obtained without NH 4OH addition.