A new hydrocoating method (the thermal substrate method) has been proposed for coating calcium phosphates, such as hydroxyapatite, onto titanium substrates in an aqueous solution. The influences of several solution properties on the thermal substrate method were examined. The solutions used included 3 mmol dm(-3) Ca(H(2)PO(4))(2) and 7 mmol dm(-3) CaCl(2) as a reference concentration solution. The ion concentration was changed from 0.1 to 2 times with respect to the reference concentration. The experimental studies were conducted under the following conditions: temperature = 140 degrees C, heating time = 10-20 min., pH = 4-8 and Ca/P = 0.0167-16.7. The type of precipitate changed, depending on the pH and ion concentration. In the reference solution with pH > 6, predominantly hydroxyapatite was precipitated onto titanium. By contrast, only CaHPO(4) was formed in the solution of pH 4. In the solution with an ion concentration of one-tenth the reference solution, CaHPO(4) was also precipitated. The addition of H(3)PO(4) to the 0.1-times solution accelerated the precipitation rate of HA. It is suggested that the PO(4) (3-) concentration was insufficient to form HA in the Ca/P = 1.67 solution.
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