A Na and Si co-substituted carbonated hydroxyapatite coating for carbon nanotubes coated carbon/carbon composites

A Na and Si co-substituted carbonated hydroxyapatite (NaSiCH) coating was applied on carbon nanotubes coated carbon/carbon composites using an ultrasound-assisted electrochemical deposition (UED) method. The morphology and microstructure of the coating were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The in vitro bioactivity of the coating was investigated by examining the ability of apatite formation on its surface in simulated body fluid (SBF). The results showed that the carbon nanotubes coated carbon/carbon composites could provide a porous surface structure for the deposition of NaSiCH. After the UED process, the Na could replace Ca in the structure of hydroxyapatite. The Si could enter into the hydroxyapatite structure in the form of SiO₄^4- groups that are combined with the CO ₃^2- to substitute the PO₄^3- groups. The NaSiCH coating was successfully applied on carbon nanotubes coated carbon/carbon composites. The NaSiCH crystals first enwrapped the carbon nanotubes and then infiltrated into the porous structure of the carbon nanotubes. Finally the NaSiCH coating formed a homogeneous coverage, exhibiting a belt shape with a width of 150-350 nm and a thickness less than 50 nm. The SBF tests showed that the NaSiCH coating could induce the formation of a uniform apatite layer with a wavy shape. In conclusion, NaSiCH offers great potential as a coating material for future medical application.