Microscopic origin of ultra-low friction coefficient between the wear track formed on carbon nitride coating and transfer layer on sliding ball in friction tests under dry N2

Abstract Carbon nitride (CN x ) coatings have been found to exhibit an ultra-low friction coefficient (less than 0.01) after repeated sliding against a silicon nitride (Si 3 N 4 ) ball in a dry nitrogen gas (N 2 ) atmosphere. However, the mechanism for this has not yet been fully elucidated with respect to analyzing changes in the space structure of CN x . In the present study, we examined the microscopic structural changes in the near-surface structural transition layers of the wear track in the CN x coating and the transfer layer of the Si 3 N 4 ball, using scanning transmission electron microscopy and electron energy-loss spectroscopy. We investigated the nitrogen concentration, density, and curvature of fragmented graphitic planes in the structural transition layers of both the CN x coating and the transfer layer of the Si 3 N 4 ball by analyzing the plasmon peak and carbon K-edge. In addition, we theoretically examined the chemical bonding states of nitrogen doping in various model clusters using an ab initio molecular orbital method to clarify the role of nitrogen in the ultra-low friction behavior. The mechanism behind the ultra-low friction behavior is explained by nano-graphitization of the fragmented carbon structure with optimal nitrogen concentration passivating the contacting counter surfaces of the coating and ball formed during sliding.