In Operando Formation of Layered Materials for Friction Reduction

Friction and wear result in massive energy and environmental costs. The technologies nowadays available to reduce these costs are based on materials and intense research efforts are being devoted to improving the efficiency of lubricants. Among them 2D materials have emerged as promising alternative to liquid lubricants as they can provide extremely low friction at a potentially much lower environmental impact as they do not require the use of petroleum oils. Moreover, they are particularly suited for lubricating tribological systems where the use of liquid lubricants is not possible, such as those operating in vacuum, high-temperature or at the nanoscale. While the friction coefficients provided by the 2D materials can reach super-low values in mild conditions, higher pressures applications often suffer from the need of replenishment into the wear tracks. A smart solution to overcome this problem is represented by the possibility to synthesize the slippery layers in operando conditions through tribochemical reactions involving molecules made available in the tribological environment as gases, powders or additives in liquid media. The present chapter offers an overview on the state-of-the art knowledge on mechanochemical/tribochemical synthesis and the in-silico experiments based on ab initio molecular dynamics that can be performed to monitor in real time the formation of 2D tribofilms. Two case studies are also described that concerns the tribological synthesis of graphene and transition metal dichalcogenides layers.