Preparation and tribological characterization of amorphous carbon nitride coatings in a RF PECVD–DC PVD hybrid coating process

Amorphous carbon nitride (CNx) coatings were prepared on Si (100) substrates using a unique hybrid coating process with the combination of radio frequency plasma enhanced chemical vapor deposition (RF PECVD) and DC magnetron co-sputtering of graphite target at different N2/Ar flow ratios. The structural properties (i.e. deposition rate, composition and bonding structure, residual stress, hardness, and surface morphology) of those CNx coatings were greatly affected by the N2/Ar flow ratio. However, the friction behaviors of the CNx coatings in air and N2 gas stream showed less dependency on the N2/Ar flow ratio. The friction coefficients of CNx coatings sliding against AISI 52100 pins in air and N2 gas stream were in the ranges of 0.17–0.19 and 0.33–0.42, respectively. The analyses of the worn surfaces from optical microscopy and Raman spectroscopy suggested that the friction behaviors of the CNx coatings are mainly controlled by the tribo-films on the pin surfaces other than the composition and bonding structure of the coatings. Specifically, the moderate friction coefficients (0.17–0.19) in air are attributed to the formation of a polymer-like carbon tribo-film, whereas the high friction coefficients (0.33–0.42) in N2 gas stream are due to the directly sliding between CNx coating and steel pin surface.