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

氮化碳（CNx）涂层作为一种高硬度、高耐磨性的新型涂层被人们所期待。然而，最近在N_2气体中的球在干燥条件下显示出很低的摩擦系数，为0.007，即N_2气体起润滑作用。我们认为，超低摩擦在氮气中是特定的有趣的摩擦学性能的CNx。通过SEM、拉曼光谱和ESCA分析发现，在氮气中摩擦系数较低的情况下，摩擦副表面的磨粒较少，sp^2键增多。氮的润滑作用之一是减少滑动表面上的磨损颗粒。具有低剪切强度键的富含sp^2的结构被认为导致低摩擦。基于以上结果，可以认为磨合过程对于CNx在氮气中的低摩擦是重要的。在a-CNx和Si_3N_4之间的滑动过程中，施加氮气作为环境气体提供了非常低的摩擦系数，其水平为0.01。为了了解磨合过程对摩擦降低的影响，研究了配合表面粗糙度对摩擦的影响。结果表明，磨痕处的光滑表面提供了较低的摩擦系数。磨合后的摩擦系数与磨痕的Ry值成正比。滑动试验后，用原子力显微镜在磨痕上观察到光滑的薄转移层。结果表明，磨痕的平滑化和CNx转移层的形成是降低摩擦的必要条件。

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.