Friction, wear and N2-lubrication of carbon nitride coatings: a review

Recent results of tribological properties of carbon nitride (CNx) coatings are reviewed. CNx coatings of 100 nm thickness were formed on Si-wafer and Si3N4 disks by the ion beam mixing method. Friction and wear tests were carried out against Si3N4 balls in the environments of vacuum, Ar, N2, CO2, O2 or air by a ball-on-disk tribo-tester in the load range of 80–750 mN and in the velocity range of 4–400 mm/s. It was found that friction coefficient μ is high (μ=0.2–0.4) in air and O2, and low (μ=0.01–0.1) in N2, CO2 and vacuum. The lowest friction coefficient (μ<0.01) was obtained in N2. It was also found that N2 gas blown to the sliding surfaces in air effectively reduced the friction coefficient down to μ≈0.017. Wear rate of CNx coatings varied in the range 10−9–10−5 mm3/N m depending on the environment. The wear mechanisms of CNx in the nanometer scale were studied by abrasive sliding of an AFM diamond pin in air. It was confirmed that the major wear mechanism of CNx in abrasive friction was low-cycle fatigue which generated thin flaky wear particles of nanometre size.