Surface microstructure and bioactivity of hydroxyapatite and fluorapatite coatings deposited on Ti-6Al-4V substrates using Nd-YAG laser

Hydroxyapatite (HA) and fluorapatite (FA) coatings were deposited on Ti-6Al-4V substrates with an Nd-YAG laser and then immersed in simulated body fluid (SBF) for up to 21 days to evaluate their bioactivity. Prior to SBF immersion, the coating layer of the HA specimen had a coral-like structure, and was mainly composed of Ti, CaTiO₃, TiO₂, Al₂O₃, and Ca₂P₂O₇, whereas that of the FA specimen had a dense cellular-like structure, and was mainly composed of Ti, CaTiO₃, TiO₂, Al₂O₃, and residual FA. The Ca/P ratios of the HA and FA coating layers were 7.61 and 2.12, respectively. After 21 days of immersion in SBF, only a very small amount of precipitates, mainly consisting of CaCO₃ with some hydroxycarbonated apatite (HCA) and HA, formed on the HA coating layer, whose Ca/P ratio retained a high value of 6.34. In contrast, a dense accumulation of granulated precipitates, mainly consisting of HCA, formed on the FA coating layer after just 7 days of SBF immersion, with a corresponding Ca/P ratio of 1.63. The SBF immersion test shows that FA coatings produced via an Nd-YAG laser cladding technique on a Ti-6Al-4V substrate have better bioactivity than that of their HA counterparts.