Carbon nitride films prepared at different N₂/Ar ratios by closed field unbalanced reactive magnetron sputtering

Carbon nitride (CN_x) thin films have been deposited onto Si(100) (for structural and mechanical analyses) and M42 high-speed-steel (for tribological measurements) substrates at room temperature by closed-field unbalanced magnetron sputtering. The mechanical and tribological properties of these films were highly dependent on the N/C concentration ratio that was adjusted by the F(N₂)/F(Ar) flow-rate ratio at fixed substrate biasing of -60 V during deposition. The films were characterized by employing scanning electron microscopy (SEM), atomic force microscopy (AFM), nano-indentation measurements, X-ray photoelectron spectroscopy (XPS), Roman scattering and Fourier transform infrared (FTIR) spectroscopy, pin-on-disc tribometer, scratch tester, and Rockwell-C tester. The results showed that the N content in the films increased with the N₂ pressure. However, the maximum N/C ratio obtained was 0.25. The nanohardness was measured to be in the range of 11.7-20.8 GPa depending on the N/C ratios. The XPS N Is spectra showed the existence of both N-C sp² and N-C sp³ bonds in films. Roman and FTIR spectra exhibited that N-C bonds were fewer when compared to other N-C bonds. The friction coefficient of the film deposited onto steel substrate with N/C=0.26 was measured to be -0.08 and for film with N/C=0.22 a high critical load of 70 N was obtained. The tribological data also showed that the wear rates of these films were in the range of -10^-16 m³/Nm, indicating excellent wear resistance for CN_x films.