The aim of this study was to evaluate the potential of using Ca(EDTA)2-, together with� tri-ethyl phosphate (TEP), to regulate the hydrothermal crystallization of hydroxyapatite (HA) films. Hydroxyapatite was coated on various substrates including titanium, Ti6Al4V, grit-blasted Ti6Al4V, 316 stainless steel, and Co28Cr6Mo via hydrothermal synthesis at 200 �C utilizing a 0.232 m Ca(NO3)2 - 0.232 m EDTA - 0.187 m TEP - 1.852 m KOH - H2O chemical system. The role of film deposition processing variables on HA crystallization was studied by thermodynamic process simulation and experimental TEP hydrolysis kinetics data. Profilometer, XRD, FESEM, TEM, EDX, and adhesion testing (ASTM D3359) were used to characterize substrates and films. Kinetics and thermodynamics predicted both the release of free phosphate from TEP above 180 �C would result in HA formation, and low free Ca2+ concentrations that could result in growth controlled HA crystallization. Materials characterization results indicate that high crystallinity, (002) crystallographically oriented, passivating, HA films composed of large hexagonal faceted grains were formed on all substrates. In addition, thermodynamics predicted that the Ti-HA chemical intermediate CaTiO3 would be deposited on Ti substrates prior to TEP hydrolysis, which was confirmed by materials characterization and found to improve film-substrate adhesion. Based on these results, it is concluded that TEP/EDTA regulation allows for the growth controlled hydrothermal crystallization of HA films on numerous substrates.
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