Clarification of ultra low friction mechanism of CNx in Nitrogen and design of low friction sliding system

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Carbon nitride (CNx) coatings was basically expected as new coating with high hardness and wear resistance. However, recently, ball in N_2 gas indicated very low friction coefficient as 0.007 under dry condition, namely N_2 gas played a role of lubrication. We believed that super low friction in nitrogen was specific interesting tribological properties of CNx. After the surface analysis with SEM, Raman spectroscopy and ESCA, we found the there are few wear particles on the contact surfaces and an increase of sp^2 bonding in the case of low friction coefficient in nitrogen. One of the lubrication effects of nitrogen is the decrease of wear particles on the sliding surfaces. A sp^2 rich structures which have low shear strength bonds was considered to cause low friction. On the basis of above results, it can be considered that running-in process is important for low friction of CNx in Nitrogen. However, effect of surface roughness and transfer layer on friction is not observed.During the sliding between a-CNx and Si_3N_4 applying nitrogen as environmental gas provided very low friction as the level of 0.01 in friction coefficient. In order to know the effect of the running-in process on the reduction of the friction, the effect of surface roughness of mating surface on friction was investigated. It was shown that smooth surface in wear scar of ball provided low friction coefficient. Friction coefficient after running-in was proportional to the Ry value of wear scar of ball. Also smooth thin transferred layer was observed on the wear scar of balls with an AFM after sliding test. Those results showed the smoothing of wear scar of ball, the generating of the transferred layer from CNx was necessary for low friction.