Tribological Performance of Different Coating-lubricant Systems

Abstract: Diamond-like carbon (DLC) coatings on steel are used to protect mechanical components, to improve lubrication properties such as anti-wear and friction reduction under certain operating conditions. The objectives of this work are (1) to evaluate the compatibility of lubricant with DLC in the boundary lubrication regime by laboratory tribo-test (2) to investigate lubrication mechanism using instrumental analysis. The coatings tested are hydrogen-free/sp3-rich/amorphous carbon with/without after treatment, hydrogenated amorphous carbon with WC-doping, Cr-doped DLC with hydrogenated amorphous carbon top layer. The base oil was a poly-alpha olefin containing model friction modifier additives: glycerol mono-oleate, oleylamine and oleylamide. The concentration of additive was 1 mass %. Commercially available fully formulated oil was employed as a reference. The tribological properties were evaluated according to ASTM D 6425. A contact stress of 0.1 GPa, a frequency of 50 Hz, a stroke of 1 mm were applied in the tribo-test. The resultant worn surfaces were studied by an SEM-EDX and a 3D-profilometer.
Among four DLC, two types of hydrogen-free/sp3-rich/amorphous type coating provided outstanding lubrication properties with fully formulated oil. None of the model additives in base oils achieved good lubrication performance corresponding to the reference. It should be noted that oleylamine improved the running-in process of the reference oil. A model for low friction and low wear under the test conditions were proposed on the basis of laboratory tribo-tests as well as instrumental analyses.