The properties of fluorine-containing diamond-like carbon films prepared by pulsed DC plasma-activated chemical vapour deposition

Diamond-like carbon films containing up to 23.1 at. % of fluorine (F-DLC), were deposited onto silicon substrates by low-frequency, pulsed DC, plasma-activated, chemical vapour deposition (PACVD). The influence of fluorine on plasma current density, deposition rate, composition, bonding structure, surface energy, hardness, stress and biocompatibility was investigated and correlated with the fluorine content. X-ray photoelectron spectroscopy (XPS) analysis revealed the presence C–C, C–CF and C–F for F-DLC films with a low fluorine concentration (1.5–12.1 at. %), however for films with a higher fluorine content (23.0 at. %) an additional peak due to CF2 bonding was detected. The addition of fluorine into the DLC film resulted in lower stress and hardness values. The reduction in these values was attributed to the substitution of strong C=C by weaker C–F bonds which induces a decrease in hardness. Ion scattering spectrometery (ISS) measurements revealed the presence of fluorine atoms in the outmost layer of the F-DLC films and there was no evidence of surface oxygen contamination. The water contact angle was found to increase with increasing fluorine content and has been attributed to the change of the bonding nature in the films, in particularly increasing CF and CF2 bonds. Biocompatibility tests performed using MG-63 osteoblast-like cell cultures indicated homogeneous and optimal tissue integration for both the DLC and the F-DLC surfaces. This pulsed-PACVD technique has been shown to produce biocompatible DLC and F-DLC coatings with a potential for large area applications.

Research Highlights►Films of diamond-like carbon containing fluorine (F-DLC) were deposited on silicon substrates by pulsed dc plasma-activated chemical vapour deposition (PACVD). ►We report on the composition of the outmost atomic layer of F-DLC films and the results have provided clear evidence that F atoms are exposed on the surface of the film. The outer layer predominantly determines the surface energy of the films which is very important in designing a coating for a specific application, (e.g. improving antibacterial activity). ► We report on the correlation between the properties of the films and the fluorine content (properties such as mechanical, structure, surface energy and composition). ►The contact angle values were maintained over a period of 12 weeks at a humidity of 50–65%. The water contact angle was found to increase with increasing fluorine content and has been attributed to the change of the bonding nature in the films, in particularly increasing CF and CF2 bonds. ►This pulsed DC-PACVD technique has been shown to produce biocompatible DLC and F-DLC coatings with a potential for biomedical applications.