Sol-gel-derived hydroxyapatite powders and coatings

Hydroxyapatite (HAP) and tri-calcium phosphate (TCP) powders and coatings with a Ca/P molar ratio from 1.56 to 1.77 were prepared by the sol-gel technique using calcium 2-ethylhexanoate (Ca(O2C8H15)2) and 2-ethyl-hexyl-phosphate as calcium and phosphorus precursors, respectively. The structural evolution and phase formation mechanisms of HAP and tri-calcium phosphate in calcined powders and coatings on Si wafer and Ti-alloy substrates (Ti-30Nb-3Al and Ti-5Al-2.5Fe) were characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The elimination of organics was studied by differential thermal analysis (DTA) and thermogravimetry (TGA). Two different formation mechanisms of crystallization are proposed. In sols with Ca/P ≤ 1.67, β-tricalcium phosphate is formed as the major phase and hydroxyapatite as a minor phase by calcination at 700°C. At 900°C these phases react to form AB-type carbonated hydroxyapatite (Ca10−2x/3[(PO4)6−x(CO3)x][(OH)2−x/3−2y(CO3)y]). A release of CO2 substituting PO43− occurs between 900°C and 1100°C yielding carbonate apatite, Ca10(PO4)6[(OH)2−2y(CO3)y], whereas CO2 substituting OH− groups in the apatite structure is released above 1200°C. In sols with Ca/P ≥ 1.70, rather than carbonate apatite, B-carbonated hydroxyapatite Ca10−2x/3[(PO4)6−x(CO3)x](OH)2 is formed, which subsequently decomposes into HAP and CaO above 1200°C. The optimum sintering conditions for coatings on Ti-alloys are found to be 600°C for 10 minutes, since, at higher temperature, oxidation of titanium and the formation of rutile (TiO2) occur. Dip coating and sintering in two cycles yielded a homogeneous and dense coated film with a thickness of 250 nm.