Nucleation of biomimetic Ca–P coatings on Ti6Al4V from a SBF×5 solution: influence of magnesium

Biomimetic Calcium–Phosphate (Ca–P) coatings were applied by using 5 times concentrated Simulated Body Fluid (SBF×5) using Carbon Dioxide gas. This process allows the deposition of a uniform Ca–P coating within 24 h. A previous study of our process emphasized the importance of hydrogenocarbonate ions (HCO3−), a crystal growth inhibitor. The aim of the present study was to investigate the role of the other crystal growth inhibitor present in SBF×5, Magnesium (Mg2+), on the Ca–P coating formation. Several SBF×5 solutions were prepared with various Mg2+ and HCO3− contents. No Ca–P deposits were detected on Ti6Al4V substrate soaked for 24 h in a Mg-free SBF×5 solution, whereas by increasing HCO3− content in a Mg-free SBF×5 solution, a Ca–P coating developed on Ti6Al4V substrate. Therefore, it appeared that Mg2+ has a stronger inhibitory effect on apatite crystal growth than HCO3−. Nevertheless, Mg2+ plays also another important role as suggested by depth profile X-ray Photoelectron Spectroscopy (XPS) of the Ca–P coating obtained from SBF×5 solution. Ca2+ and Mg2+ contents increased significantly at the titanium/coating interface. Therefore, Ca2+ and Mg2+ initiated Ca–P coating from SBF×5 solution. The relatively high interfacial concentration in Mg2+ favors heterogeneous nucleation of tiny Ca–P globules onto the substrate. So physical adhesion is enhanced at the early stage of the coating formation.