Laser-Raman and Nuclear Magnetic Resonance (NMR) studies on plasma-sprayed hydroxylapatite coatings: Influence of bioinert bond coats on phase composition and resorption kinetics in simulated body fluid

The structure and phase composition of HAp coatings deposited onto Ti6A14V coupons (50x20x2mm) by atmospheric plasma spraying (APS) were studied by laser-Raman spectroscopy, 31 P-and 1 H-MAS-NMR and 2D- 31 P/ 1 H HETCOR-CP-NMR spectroscopy, and XRD with Rietveld refinement. The samples investigated comprised APS HAp coatings with and without a TiO 2 bond coat as well as coatings incubated for different lengths of time (up to 12 weeks) in simulated body fluid (SBF) under physiological conditions. In APS coatings the presence of a bond coat increased the proportion of well-ordered crystalline HAp at the expense of distorted apatite-like structures such as oxyHAp and oxyapatite, and thermal decomposition products such as tricalcium phosphate (TCP) and tetracalcium phosphate (TTCP), and also decreased the amount of amorphous calcium phosphate (ACP). Incubation in SBF further advanced the proportion of crystalline HAp since the disordered structures, the thermal decomposition products, and ACP exhibit substantially higher solubility.