Effect of TiO₂ 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 + 0.8TiO ₂), 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 TiO ₂ 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 + TiO ₂ 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.