Relationship between tribological properties and sp3/sp2 structure of nitrogenated diamond-like carbon deposited by plasma CVD

The correlation between the wear resistance and the chemical states of the elements of nitrogenated diamond-like carbon (NDLC) is characterized and discussed in this paper. NDLCs have been deposited on tungsten-carbide, silicon and glass substrates by radio frequency plasma chemical vapor deposition (rf PCVD) from benzene (C6H6)-nitrogen mixtures with four different ratios. The nitrogen concentration in the films measured by electron microprobe analysis was found to be proportional to the nitrogen flow rate. The friction coefficients of the films determined by the ball-on-disk test increased with adding nitrogen contents. Such friction coefficient dependent on load was not observed in each film. The DLC showed adhesive wear. However, the wear mode of NDLCs changed from the adhesive wear to the abrasive wear at the critical load point. The film hardness measured by nano indentation decreased with increasing nitrogen. The sp3/sp2 measured by X-ray photoelectron spectroscopy (XPS) was reduced by adding nitrogen and as a result, the tribological behaviors such as the hardness, the wear resistance and the critical point were also decreased.