Carbon tribo-layer for super-low friction of amorphous carbon nitride coatings in inert gas environments

► Low friction coefficients of less than 0.05 are achieved in inert gas. ► Carbon tribo-layer is found on the wear scar where tribo-film is not observed. ► The thickness of the carbon tribo-layer is approximately 10 nm. ► This carbon tribo-layer is enough for low friction of CNx coatings in inert gas. ► Tribo-film is not necessary and should be avoided for stable and low friction.