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

• Published in: Surface & coatings technology Vol. 221; pp. 163 - 172
• Main Authors: Wang, Pengfei, Hirose, Masaaki, Suzuki, Yoshinobu, Adachi, Koshi
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 25.04.2013; Elsevier
• Subjects: Applied sciences; Carbon nitride; Carbon tribo-layer; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Inert gas; Materials science; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Physics; Production techniques; Super-low friction; Surface treatment; Surface treatments; Tribo-film; Carbon tribo-layer; Super-low friction; Tribo-film; Inert gas; Carbon nitride; Mechanical properties; Surface treatments; Amorphous state; Coatings; Carbon; Rare gases; Nitrides; Friction
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2013.01.045

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 10nm 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. ► 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 10nm. ► 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.