Nucleation and growth of hydroxyapatite on titanium pretreated in NaOH solution: experiments and thermodynamic explanation.

Titanium was submitted to chemical attack with sodium hydroxide solution under hydrothermal (SBF) conditions and then kept for 4 weeks in simulated body fluid after heat treatment. The resultant coating titanium samples were characterized regarding nucleation and growth of hydroxyapatite on their surfaces using scanning electron microscopy and energy dispersive spectroscopy, as well as low angle X-ray diffraction. In order to obtain a thermodynamic explanation of same results, Eh-pH diagrams of Na-Ti-H2O and Ca-Ti-H2O systems at 25, 100, 200, and 300 degrees C were built for selected activities of the species in aqueous solutions. Values of pairs corresponding to the predominance limit of the species in solution at equilibrium with 0.21 atm of oxygen pressure were taken from these Eh-pH diagrams for subsequent building of the pNa-pH and pCa-pH diagrams of the same systems at each referred temperature (pi = -log10ai). In addition, the titanate-apatite free energy of formation was estimated and then a pCa-pH diagram of the Ca-P-Ti-H2O system at 25 degrees C was built. Examination of the resultant diagrams could elucidate the thermodynamic viability of the process.