The ball-on-disk friction and wear tests of CNXcoatings (CNX/CNX) were conducted under a nitrogen atmosphere with controlled relative humidity (RH) (3.4–40.0%RH) and oxygen concentration (100–21 × 104ppm) in this study. We found that the specific wear rate of CNXcoating on ball (Wb), which could give stable and low friction coefficient (<0.05), was below 3.0 × 10−8 mm3/Nm. Average friction coefficients (µa) and Wbof CNX/CNXincreased (µa: 0.02–0.33, Wb: 1.6 × 10−8–2.4 × 10−7 mm3/Nm) with increasing oxygen concentration (230–211,000 ppm) as well as RH (4.7–21.1%RH) under a nitrogen atmosphere. However, the Wbremained low value below 2.3 × 10−8 mm3/Nm regardless of oxygen concentration (100–207,000 ppm) of a nitrogen atmosphere (3.4–3.9%RH) when CNX-coated balls were slid against a hydrogenated CNX(CNX:H) coatings (CNX/CNX:H). Besides, the CNX/CNX:H achieved low and stable friction coefficient below 0.05 under a nitrogen atmosphere (10,000 ppmO2) regardless of increasing RH up to 20%RH. Raman analysis indicated that the structure of carbon on the top surface of CNXcoating was changed from as-deposited CNXcoating in the case of low friction coefficient (<0.05). Furthermore, TOF-SIMS analysis provided the evidence that the carbon derived from CNX-coated disk was considered to diffuse into the ball surface, and it mixed with the carbon derived from CNX-coated ball on the wear scar, which formed the chemically bonded carbon tribo-layer. Low friction coefficient (<0.05) with CNXcoatings under a nitrogen atmosphere was achieved due to self-formation of the carbon tribo-layer.
- Carbon nitride