Friction and its importance in fuel economy-probing the nanoscale characteristics of surfaces in order to understand lubricant/surface interactions

In boundary lubrication, the critical aspect of lubrication is the interaction between the surfaces making up the tribocouple and the lubricant additives, yielding what is commonly known as the tribofilm. This has been widely studied in ferrous systems and the complex nature of the tribofilms, in terms of their self-healing, smart and semi-solid properties, is generally well understood. There is much emphasis by lubricant formulators and original equipment manufacturers on enhancing fuel economy of tribological systems whilst retaining good durability. To achieve this, surface engineering is often used and carbon-based coatings such as diamond-like carbon (DLC) are increasingly applied. Understanding the tribochemistry of such coatings against ferrous surfaces is not trivial, and this paper illustrates the contrast in features of tribology/tribochemistry behaviour between ferrous and a-C (non-hydrogenated) DLC coating. Whilst there is a clear link between low friction tribofilm composition (in terms of MoS2/MoO3 ratio) and friction performance for ferrous systems it is more complex for the a-C coating system; therefore an understanding of the interface microscale/nanoscale chemical and physical structure gives an insight into the interactions between friction and wear processes