Microstructural characterization of carbon films and CNx films produced by N+ implantation

We report on a study of the electronic, chemical and lattice structure of carbon films and of the microstructural modifications induced by N+ implantation. Correlations were made with the new mechanical properties of the implanted films. The C films were r.f. magnetron sputtered from a graphite target with Ar discharge. N+ implantations were performed with energies ranging from 30 to 160 keV and at a fluence of 2 x 10(17) N+ cm(-2). The effects of N on the structural and chemical properties of the C films were studied by means of x-ray photoelectron spectroscopy (XPS) and Raman scattering. Nuclear reaction analysis (NRA) and elastic recoil detection (ERD) were used to determine the atomic composition and density of the films. The carbon films consisted of two phases: microcrystalline graphitic domains dispersed in an amorphous matrix. The ratio of the hardness to the Young modulus, H/E, is comparable to that of diamond. N+ implantation produces new bondings in these films, enhances the amorphicity and increases the disorder of the graphitic phase. The diamond-like features of the electronic valence band (strong s-p-orbital mixing) are preserved in the implanted films, together with the Young modulus of the network. Only the top of the valence band was affected by the ion irradiation. The new microstructure of the surface region of the implanted films resulted in an improved friction behaviour. N+ implantation decreased the ratio H/E to a value similar to that of metallic hard coatings.