Effects of NaF treatment on the bioactive apatite coating for C/C composites

The bioactive apatite coatings were prepared on the surface of carbon/carbon composites using the method of sono-electrodeposition. The effects of NaF treatment on the morphology, structure and composition of the as-received coatings were investigated by SEM, EDAX, FTIR and XRD. The results show that NaF promotes the change of tricalcium phosphate to fluoridated hydroxyapatite together with increasing the crystallization degree of the coating. After being treated with NaF, the composition of the coating is a mixture of hydroxyapatite(HA) and F-rich apatite (FHA). The coatings are both plate-like crystals before and after treatment, however, the size of the crystals increases after treatment. The bonding strength between the coating and C/C matrix increases slightly after being treated with NaF, the bonding strength can reach 4.08MPa, and the fluorine content of the coating was 4.59wt%. In addition, the reaction mechanism of hydroxyapatite transformed to fluorine-containing hydroxyapatite was also discussed. To obtain fluorine-containing hydroxyapatite coatings with the treatment by NaF, it is necessary to introduce phosphate in the immersion solution in order to maintain the pH.