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

Super-low friction of amorphous carbon nitride (CNx) coating in inert gas environment has generally been attributed to the formation of a uniform and homogeneous carbon-rich transfer film (denoted as tribo-film) on the mating ball surface in the ball-on-disk tribo-system. However, the role of tribo-film in terms of its appearance (visibility and amount), composition and structure in the low friction mechanisms of CNx coatings is still not clearly understood. In this study, the key parameters of tribo-film for achieving low friction coefficients of CNx coatings in inert gas environments are systematically investigated by classifying the wear scars on the ball surfaces. The wear scars are classified into three modes according to the amount of tribo-film, such as no, partial, and almost full, accumulated inside of the contact area on the ball surface. As a result, it is confirmed that the amount as well as composition and structure of tribo-film on the wear scar of the ball surface can control the friction behavior of CNx coatings. Most importantly, a thin amorphous carbon layer (denoted as carbon tribo-layer) with a thickness of approximately 10 nm has been found as sub-surface of the wear scar on the ball surface where tribo-film is not observed using naked eye as well as confocal microscopy. It is strongly argued that the formation of this carbon tribo-layer on the wear scar of the ball surface is enough for achieving stable and low friction coefficients (less than 0.05) of CNx coatings in inert gas environments. On the other hand, the tribo-film is not necessary and should be avoided from both viewpoints of low and stable friction.