Effect of TiO 2 addition on surface microstructure and bioactivity of fluorapatite coatings deposited using Nd:YAG laser

To study the effect of titania (TiO₂) addition on the surface microstructure and bioactivity of fluorapatite coatings, fluorapatite was mixed with TiO₂ in 1:0.5 (FA + 0.5TiO₂), 1:0.8 (FA + 0.8TiO₂), and 1:1 (FA + TiO₂) ratios (wt%) and clad on Ti-6Al-4V substrates using an Nd:YAG laser system. The experimental results show that the penetration depth of the weld decreases with increasing TiO₂ content. Moreover, the subgrain structure of the coating layer changes from a fine cellular-like structure to a cellular-dendrite-like structure as the amount of TiO₂ increases. Consequently, as the proportion of TiO₂ decreases (increase in fluorapatite content), the Ca/P ratio of the coating layer also decreases. The immersion of specimens into simulated body fluid resulted in the formation of individual apatite. With a lower Ca/P ratio before immersion, the growth of the apatite was faster and then the coating layer provided a better bioactivity. X-ray diffraction analysis results show that prior to simulated body fluid immersion, the coating layer in all three specimens was composed mainly of fluorapatite, CaTiO₃, and Al₂O₃ phases. Following simulated body fluid immersion, a peak corresponding to hydroxycarbonated apatite appeared after 2 days in the FA + 0.5TiO₂ and FA + 0.8TiO₂ specimens and after 7 days in the FA + TiO₂ specimen. Overall, the results show that although the bioactivity of the coating layer tended to decrease with increasing TiO₂ content, in accordance with the above-mentioned ratios, the bioactivity of all three specimens remained generally good.