Nanostructured titania/hydroxyapatite composite coatings deposited by high velocity oxy-fuel (HVOF) spraying

Pure nanostructured titania (TiO2) and blends with 10 and 20 wt% hydroxyapatite (HA) powders were sprayed onto Ti–6Al–4V substrates using a high velocity oxy-fuel (HVOF) system. The feedstock powders employed in this work were engineered to exhibit similar particle size distributions in order to generate similar values of particle temperature and velocity in the spray jet. By achieving these characteristics it was assumed that the differences in coating properties and microstructures produced in this study were mainly related to the nature and composition of the feedstock powders, rather than to the spraying parameters or in-flight particle characteristics. The microstructure, porosity, roughness, Vickers hardness and bond strength (ASTM C633) of these coatings were analyzed and evaluated. X-ray diffraction (XRD) patterns showed that no detectable chemical reaction occurred between the nanostructured TiO2 and HA phases during the spray process. Due to the poor mechanical performance of HA, its addition decreased the bond strength and hardness values of the coatings, especially when the content of HA was 20 wt%; however, the bond strength values were still much superior to those of HA thermally sprayed coatings. The addition of HA to nanostructured titania for producing HVOF-sprayed coatings could be very interesting for biomedical applications due to the combination of the good mechanical performance and chemical stability of nanostructured titania and a bioactive phase (HA) that can enhance the bio-performance of the coating.