Comparing Tribochemical Film Formation and Durability at Steel and CrN Coating Interface in Boundary Lubrication

One of the main challenges that engine oil manufacturers are now facing is to reduce friction losses while at the same time increasing the durability of engine components. New materials are being used for engine components, alongside and to replace conventional ferrous materials. More components are being coated with low friction and highly durable coatings. Lubricants are required to lubricate these surfaces as effectively as conventional materials, with no environmental impact. Knowing that the additive package in today’s lubricants is optimised for lubricating conventional ferrous materials it is of paramount importance to understand how effective the current additives are in lubricating non-ferrous materials. The aim of the current paper is to understand the formation and durability of tribofilms on a Chromium Nitride (CrN) coating formed from two conventional additives; Zinc Dialkyldithiophosphate (ZDDP) and Molybdenum Dialkyl Dithiocarbamate (MoDTC) additives. Tribofilm formation was studied by testing model lubricants containing ZDDP and MoDTC additives on the CrN coating in boundary lubrication, taking the performance on a ferrous system as a reference. The durability was assessed by assessing the tribocouple friction performance in base oil once the tribofilm was formed. Wear scars were fully chemically characterised using X-ray Photoelectron Spectroscopy (XPS). The study shows that the chemical nature of the tribofilms formed on steel and CrN is similar. In both materials, tribofilms were shown to have comparable durability. The factors which affect tribofilm durability and formation, and the reason why ferrous systems and CrN show similarities are discussed.