Effect of heat treatments on apatite-forming ability of NaOH- and HCl-treated titanium metal
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Titanium (Ti) metal was soaked in HCl solution after NaOH treatment and then subjected to heat treatments at different temperatures. Their apatite-forming abilities in a simulated body fluid (SBF) were discussed in terms of their surface structures and properties. The nanometer scale roughness formed on Ti metal after NaOH treatment remained after the HCl treatment and a subsequent heat treatment below 700°C. Hydrogen titanate was formed on Ti metal from an HCl treatment after NaOH treatment, and this was converted into titanium oxide of anatase and rutile phases by a subsequent heat treatment above 500°C. The scratch resistance of the surface layer increased with the formation of the titanium oxide after a heat treatment up to 700°C, and then decreased with increasing temperature. The Ti metal with a titanium oxide layer formed on its surface showed a high apatite-forming ability in SBF when the heat treatment temperature was in the range 500–700°C. The high apatite-forming ability was attributed to the positive surface charge in an SBF. These positive surface charges were ascribed to the presence of chloride ions, which were adsorbed on the surfaces and dissociated in the SBF to give an acid environment.