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

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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.