Coating of HAp/CaTiO₃ Multilayer on Titanium Substrates by Hydrothermal Method

Abstract

A multilayer material consisting of hydroxyapatite (HAp) and calcium titanate (CaTiO₃) was hydrothermally synthesized on a titanium substrate. The titanium substrate was treated with a mixed solution of CaCl₂ and KOH at 150°C to form a CaTiO₃ layer. The thickness of the CaTiO₃ layer increased with increasing KOH concentration, but the adhesion between the coating layer and the substrate diminished. In an alkaline solution such as KOH, titanium substrates dissolved and reacted with calcium ions, and a CaTiO₃ layer precipitated on the substrate. A layer of HAp formed on the CaTiO₃ coating 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 stoichiometric 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 precipitated on the CaTiO₃ layer at 220°C for 24h with a pH of 8-10. The adhesion force between the substrate and the CaTiO₃ layer did not decrease after the formation of HAp, though the force decreased with increasing KOH concentration during synthesis of the CaTiO₃ layer. After soaking the HAp/CaTiO₃ multilayer in simulated body fluid (SBF) for 35 d at 37°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.