Formation and transformation of amorphous calcium phosphates on titanium alloy surfaces during atmospheric plasma spraying and their subsequent in vitro performance.

Hydroxyapatite and 'duplex' hydroxyapatite + titania bond coat layers were deposited onto Ti6Al4V substrates by atmospheric plasma spraying (APS) at moderate plasma enthalpies. From as-sprayed coatings and coatings incubated in simulated body fluid (r-SBF) electron-transparent samples were generated by focused ion beam (FIB) excavation and investigated by STEM/TEM in conjunction with energy-dispersive X-ray analysis (EDX), electron diffraction (ED), and electron energy loss spectroscopy (EELS). Adjacent to the metal surface a thin layer of amorphous calcium phosphate (ACP) was deposited whose Ca/P ratio is determined by the presence or absence of the bond coat. No clear indication of a Ca-Ti oxide reaction layer was found at the interface titania bond coat/calcium phosphate. After in vitro incubation of duplex coatings for 24 weeks Ca-deficient defect apatite needles precipitated from ACP. During incubation of hydroxyapatite without a bond coat for 1 week diffusion bands were formed within the ACP of 1-2 microm width parallel to the interface metal/coating, presumably by a dissolution-precipitation sequence.