Influence of NaF concentration on fluorine-containing hydroxyapatite coating on carbon/carbon composites

Fluorine-containing hydroxyapatite (FHA) coatings were prepared on the surface of carbon/carbon (C/C) composites by way of combination of sonoelectrodeposition and ion exchanges. SEM, EDAX, FTIR and XRD were adopted to investigate the influence of NaF concentration on the morphology, structure and composition of the coatings. Experimental results show that treatment by NaF can promote the conversion of tricalcium phosphate (TCP) into fluor-apatite and also increase the crystallization degree of the coating. After treatment by NaF, the composition of the coating is the mixture of hydroxyapatite (HA) and F-rich apatite (or FA) in different ratios, and (3 0 0) diffraction peak shifts to a higher angle of diffraction. With the NaF concentration increasing, more and more F^- in the coating will replace the OH^- in the apatite, so HA content in the coating decreases while F-rich apatite (or FA) increases. After NaF treatment, dissolution phenomenon happens on the surface of the coating, and the plate-like crystals turn into larger ones along with the increasing porosities. When the concentration of NaF was up to 0.1 mol·L^-1, the coating turned into amorphous powder with fluorine content of 6.31 wt%.