Characterization of as-plasma-sprayed and incubated hydroxyapatite coatings with high resolution techniques

To improve the reliability of thermally sprayed coatings for human implants application it is vital to understand the changes that morphology, and chemical and phase compositions of the hydroxyapatite coatings undergo when brought in contact with living tissue. In this study, hydroxyapatite (HAp) coatings were deposited onto Ti6A14V substrates by atmospheric plasma spraying (APS) followed by incubation for up to 84 days in revised simulated body fluid (rSBF) to assess their possible biofunctional performance. Morphology, phase and chemical compositions of as-sprayed coatings were investigated by synchrotron x-ray powder diffraction (XRD), laser-Raman spectroscopy (LRS), and nuclear magnetic resonance spectroscopy (NMR). In addition, incubated coatings were characterized by scanning transmission electron microscopy (STEM) and proton-induced x-ray emission (PIXE). The results indicate that during plasma spraying hydroxyapatite transforms to crystalline oxyapatite and beta-tricalcium phosphate as well as amorphous calcium phosphate (ACP). On incubation crystallization of ACP occurs via fluid flow of solution along cracks and fissures of the coating. The composition develops towards fully crystalline but porous defect apatite containing residual water.