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

氮化碳(CNX)涂层是一种具有高硬度、高耐磨性的新型涂层。然而，最近在N_2气体中的球体在干燥条件下的摩擦系数很低，仅为0.007，即N_2气体起到了润滑作用。我们认为，CNX在氮气中的超低摩擦是其独特的有趣的摩擦学特性。通过扫描电子显微镜、拉曼光谱和ESCA分析发现，在氮气中摩擦系数较低的情况下，接触表面磨粒较少，sp2键增多。氮气的润滑作用之一是减少了滑动表面的磨粒。具有低剪切强度键的富sp2结构被认为是导致低摩擦的原因。根据以上结果，可以认为，磨合工艺对氮气中CNX的低摩擦是重要的。在a-CNX和Si3N4之间的滑动过程中，氮气作为环境气体提供了很低的摩擦力，摩擦系数仅为0.01。为了了解磨合过程对减摩效果的影响，研究了摩擦副表面粗糙度对摩擦的影响。结果表明，磨痕表面光滑，摩擦系数低。磨合后的摩擦系数与钢球磨痕的Ry值成正比。用原子力显微镜观察到滑动试验后的磨痕表面有光滑的薄层转移层。结果表明，磨球磨痕的平整、CNX过渡层的生成是实现低摩擦的必要条件。

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.