MoDTC Tribochemistry in Steel/Steel and Steel/Diamond-Like-Carbon Systems Lubricated With Model Lubricants and Fully Formulated Engine Oils

This work focuses on the tribochemistry of molybdenum dithiocarbamate (MoDTC) oil additive to improve friction behavior of diamond-like-carbon (DLC) coated systems lubricated in boundary regime. Raman microscopy has been used to investigate surface tribolayers formed on coated (hydrogenated a-C:H and non-hydrogenated ta-C) and steel surfaces when lubricated with model lubricants and commercial engine oils. The effect of the additive package and the type of DLC played a crucial role in the development and composition of the tribolayer and the friction performance. The additive package contained in the fully formulated (FF) oils limited the friction reduction capabilities of MoDTC additive for every material pair. Accelerated a-C:H coating wear related to MoDTC tribochemistry was found. For the first time, it has been shown that a distinctive MoS₂-containing tribolayer can be formed on the ta-C surface, leading to a coefficient of friction lower than 0.04. The underlying mechanisms of MoDTC/surface interactions and their effect on friction and wear are discussed.