Clarification of ultra low friction phenomena of CNx coated with Ion Beam Assisted Deposition

氮化碳(CNX)涂层具有许多吸引人的性能，特别是具有高硬度、良好的耐磨性和低摩擦系数。而非晶态氮化碳涂层已经开始用于磁性硬盘的涂层。因此，α-CNX有望成为高硬度、高耐磨性的新型涂层材料。然而，最近我们发现在氮气中氮化碳涂层与氮化硅球体的滑动摩擦系数很小，在干燥条件下摩擦系数仅为0.007，即氮气起到了润滑作用。我们认为，CNX在氮气中的这种超低摩擦是其独特的有趣的摩擦学特性。为了解氮气中氮氧化物的润滑机理，研究了氮气对氮氧化物涂层与陶瓷球摩擦时的润滑作用以及氮气润滑的机理。结果表明，氮气对陶瓷销钉滑动时氮氧化物涂层中氮气的润滑作用仅发生在Si3N4销钉上。氮气压力越高，摩擦系数越小。在氮气中摩擦系数较低的情况下，接触表面磨损颗粒较少，sp2结合增强。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。

Carbon nitride (CNx) coatings possess possibility of many attractive properties especially for high hardness, good wear durability and low friction coefficient. And amorphous carbon nitride coatings has already started to use for the coating of magnetic rigid disk. Basically, a-CNx was expected as new coating material with high hardness and wear resistance. However, recently, we found that the sliding of CNx coating against Si3N4 ball in nitrogen indicated very low friction coefficient as 0.007 under dry condition, namely nitrogen gas played a role of lubrication. We believed that such ultra low friction in nitrogen was specific interesting tribological properties of CNx. In order to know the lubrication mechanism of CNx in nitrogen, the lubrication effects of nitrogen gas for CNx coatings sliding against ceramic balls and mechanism of lubrication by nitrogen gas are investigated.As results, l it was shown that lubrication effects of nitrogen in CNx coating sliding against ceramics pin was only occurred for a Si3N4 pin. Higher pressure of nitrogen provided lower friction coefficient. In the case of low friction coefficient in nitrogen, there are few wear particles on the contact surfaces and an increase of sp2 bonding. One of the lubrication effects of nitrogen is the decrease of wear particles on the sliding surfaces. A sp2 rich structures which have low shear strength bonds is considered to cause low friction.