Effect of Oxygen on the Self-formation of Carbonaceous Tribo-layer with Carbon Nitride Coatings under a Nitrogen Atmosphere

The ball-on-disk friction and wear tests of CN_Xcoatings (CN_X/CN_X) 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 CN_Xcoating on ball (W_b), which could give stable and low friction coefficient (<0.05), was below 3.0 × 10⁻⁸ mm³/Nm. Average friction coefficients (µ_a) and W_bof CN_X/CN_Xincreased (µ_a: 0.02–0.33, W_b: 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 W_bremained 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 CN_X-coated balls were slid against a hydrogenated CN_X(CN_X:H) coatings (CN_X/CN_X:H). Besides, the CN_X/CN_X: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 CN_Xcoating was changed from as-deposited CN_Xcoating in the case of low friction coefficient (<0.05). Furthermore, TOF-SIMS analysis provided the evidence that the carbon derived from CN_X-coated disk was considered to diffuse into the ball surface, and it mixed with the carbon derived from CN_X-coated ball on the wear scar, which formed the chemically bonded carbon tribo-layer. Low friction coefficient (<0.05) with CN_Xcoatings under a nitrogen atmosphere was achieved due to self-formation of the carbon tribo-layer.