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

Amorphous carbon nitride (CN _x ) 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 N ₂ /Ar flow ratios. The structural properties (i.e. deposition rate, composition and bonding structure, residual stress, hardness, and surface morphology) of those CN _x coatings were greatly affected by the N ₂ /Ar flow ratio. However, the friction behaviors of the CN _x coatings in air and N ₂ gas stream showed less dependency on the N ₂ /Ar flow ratio. The friction coefficients of CN _x coatings sliding against AISI 52100 pins in air and N ₂ 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 CN _x 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 N ₂ gas stream are due to the directly sliding between CN _x coating and steel pin surface.