Investigation of Pure Sliding and Sliding/Rolling Contacts in a DLC/Cast Iron System when Lubricated in Oils Containing MoDTC-Type Friction Modifier

Diamond-like carbon (DLC)/cast iron (CI) systems have been widely investigated due to their important application in engine components such as cylinders, pistons and more specifically for the cam/follower interface. The pure sliding contact of the DLC/CI system has traditionally been the focus of research; consequently less is understood about sliding/rolling contact systems. In addition, the tribological and tribochemical characteristics of the Molybdenum Dialkyl Dithiocarbamate (MoDTC) as a lubricant additive in such sliding/rolling contacts are not fully understood.

In this study, a Mini Traction Machine (MTM) was used to run the experiments using alloy steel balls coated with 15 atomic percent (at. %) hydrogenated DLC (a-C: 15H) rubbing against uncoated cast iron discs. Results showed that the sliding/rolling ratio affects friction, wear and tribochemistry in CI/DLC systems; pure sliding enhances MoDTC activation. MoDTC decomposes to form MoS₂, FeMoO₄ and not MoO₃. In addition, it was observed that MoS₂/FeMoO₄ ratio depends on test conditions and affects to the friction performance.