The ball-on-disk friction and wear tests of CNX coatings (CNX/CNX) were conducted under a nitrogen atmosphere with controlled relative humidity (RH) (3.4-40.0% RH) and oxygen concentration (100-21 x 10(4) ppm) in this study. We found that the specific wear rate of CNX coating on ball (W-b), which could give stable and low friction coefficient (<0.05), was below 3.0 x 10(-8) mm(3)/Nm. Average friction coefficients (mu(a)) and W-b of CNX/CNX increased (mu(a): 0.02-0.33, W-b: 1.6 x 10(-8)-2.4 x 10(-7) mm(3)/Nm) with increasing oxygen concentration (230-211,000 ppm) as well as RH (4.7-21.1% RH) under a nitrogen atmosphere. However, the W-b remained low value below 2.3 x 10(-8) mm(3)/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 ppmO(2)) regardless of increasing RH up to 20% RH. Raman analysis indicated that the structure of carbon on the top surface of CNX coating was changed from as-deposited CNX coating 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 CNX coatings under a nitrogen atmosphere was achieved due to self-formation of the carbon tribo-layer.