Production and characterization of plasma-sprayed TiO2–hydroxyapatite functionally graded coatings

Among bioactive ceramics, hydroxyapatite (HAp) has been widely studied, especially as a coating onto metallic substrates. In clinical applications, coating delamination has been observed, close to the interface between coating and substrate. This is due to a mismatch in the thermal expansion coefficients of HAp and titanium/titanium alloy. In order to improve the adhesion, a proper bond coat may be introduced. In this work, a functionally graded coating TiO2–HAp, in which the composition gradually changed from TiO2 to HAp, was deposited onto Ti6Al4V substrates by atmospheric plasma spraying (APS). With the aim of defining the best spraying parameters to obtain the graded system, preliminary coatings of pure TiO2 and pure HAp were deposited by varying systematically the typical spraying conditions, such as the torch power and H2 flux. The preliminary coatings were characterized by means of SEM, that confirmed the strong dependence of the microstructure on the torch power, and X-ray diffraction, that showed the significant influence exerted by the hydrogen flux on the crystallinity and thermal decomposition of HAp. The results of the preliminary investigations were used to optimise the spraying conditions for the FGM deposition and, accordingly, the final graded coating was obtained and characterized. Post-deposition heat treatments were performed in order to improve further the graded coating and their effect on the mechanical properties was evaluated via Vickers micro-indentation tests. The investigation showed that, after raising the temperature, the crystallinity of HAp and the Vickers hardness increased, however, at high temperature (more than 750 °C), the stress induced by the re-crystallization promoted the propagation of cracks and weakened the interface.