Methionine as a Friction Modifier for Tungsten Carbide-Functionalized Surfaces via in Situ Tribo-Chemical Reactions

This work presents a novel method for generating in situ low friction tribofilms in lubricated contacts using α-amino acid l-methionine as an additive. Methionine is an environmentally acceptable natural organosulphur compound that is commonly used in the food industry. Our approach relies in the use of steel surfaces functionalized with tungsten carbide particles that are tailored to interact with methionine via a tribo-chemical reaction. The results show that after an induction period, the friction drops dramatically by 60% down to values of 0.06 when methionine was used as an additive in lubricated tungsten carbide-functionalized surfaces. The low friction could only be achieved by the coexistence of tungsten from the functionalized surfaces and sulfur from methionine, which led to the presence of tribo-chemically generated tribofilms. Ab initio simulations indicate that the tribo-chemical reaction for forming tungsten disulfide is energetically favorable, thus attributing the observed friction reduction mechanism to the in situ formation of this compound during the sliding process. The concept of functionalizing surfaces to react with specific additives opens up a wide range of possibilities, which allows tuning surfaces to target specific additive interactions. This synergy can be exploited for using novel green additive technology, thus allowing more environmentally friendly formulations with outstanding tribological performance.