Coating of HAp/CaTiO3 multilayer on titanium substrates by hydrothermal method

A multilayer material consisting of hydroxyapatite (HAp) and calcium titana te (CaTiO3) was hydrothermally synthesized on a titanium substrate. The tit anium substrate was treated with a mixed solution of CaCl2 and KOH at 150 d egrees C to form a CaTiO3 layer. The thickness of the CaTiO3 layer increase d with increasing KOH concentration, but the adhesion between the coating l ayer and the substrate diminished. In an alkaline solution such as KOH, tit anium substrates dissolved and reacted with calcium ions, and a CaTiO3 laye r precipitated on the substrate. A layer of HAp formed on the CaTiO3 coatin g layer from the mixed solution containing CaCl2, K2HPO4 and KOH, when the molar ratio of Ca/P was less than 1.67, which was the ratio of stoichiometr ic HAp. Calcium was supplied from both the reacting solution and the CaTiO3 layer, and was precipitated as HAp. Decomposition of a part of CaTiO3, and the consequent supply of Ca from the coating layer were necessary to form the HAp layer. The multilayer formed without peeling, when HAp was precipit ated on the CaTiO3 layer at 220 degrees C for 24 h with a pH of 8-10. The a dhesion force between the substrate and the CaTiO3 layer did not decrease a fter the formation of HAp, though the force decreased with increasing KOH c oncentration during synthesis of the CaTiO3 layer. After soaking the HAp/Ca TiO3 multilayer in simulated body fluid (SBF) for 35 d at 37 degrees C, the multilayer was covered with very fine needlelike particles of HAp, though neither the surface of the CaTiO3 coating layer nor the titanium substrate were altered after the same treatment.