The role of CN chemical bonding on the tribological behaviour of CNx coatings

The tribological performance of CNx coatings depends strongly on both the e nvironmental conditions and the nature of the coating? in relation to the d eposition process. In this paper, we present and discuss friction results i n relation to the nature, crystal structure, chemical composition and hybri dization state of CNx coatings prepared by dual ion beam sputtering under v arious conditions. The films were characterized by infrared spectroscopy, X -ray photoelectron spectroscopy and electron energy loss spectroscopy. By i ncreasing the polarization of the substrate, an increase of the N/C atomic ratio, together with a decrease of the C=N/C-N and C=N/C=C bonding ratios a re observed. The concentration of C equivalent to N triple bond is negligib le. Reciprocating pin-on-plane friction tests have been carried out in humi d ambient air and in ultrahigh vacuum. Steady-stare friction in ambient air has been found to be in the range of 0.14-0.25, increasing slightly with t he increase of the substrate polarization during deposition. The presence o f C=N double bond is associated with a lower friction coefficient. The stea dy-state friction of the film exhibiting the lowest friction in ambient air is near 0.6 in ultrahigh vacuum. The chemical composition of the topmost s urfaces both inside and outside the UHV wear tracks of the plane and the pi n was investigated by in situ XPS and AES performed at the completion of th e friction test in the analytical tribometer. The high friction in UHV is a ssociated with a significant transfer of the iron oxide top layers from the pin as wear particles inside the wear track of the plane, without any CN t ransfer onto the steel pin. (C) 1999 Elsevier Science S.A. All rights reser ved.