Sliding processes are inevitably related to energy loss due to friction. In fact, friction losses represent about 5% of all produced energy and represent one obstacle to reach zero-carbon economy. A traditional way to minimize friction is using liquid lubricants, such as oils and greases. However, they are often produced from non-renewable resources and are related to high environmental load/risk. Solid lubricants, namely 2D materials, such as graphene or transition metal dichalcogenides, promise extremely low coefficient friction (often called superlubricity state). Unlike liquid lubricants, their frictional properties are almost independent of temperature and contact pressure. And they possess one crucial advantage – 2D materials are almost inert in normal sliding conditions, and the tribological contact is thus relatively simple being limited to a few atomic layers. Using 2D materials thus allows, perhaps for the first time, the prediction of friction at a nanoscopic contact scale by atomistic simulations (ab initio, molecular dynamics) and simulation-driven design of a new generation of solid lubricants.

We have shown that a combination of 2D materials with a large lattice mismatch, such as graphene and MoS2, leads to one of the lowest coefficients of friction ever obtained, 10-6 [1]. Using a combination of ab initio [2] and molecular dynamics simulations [3], we can predict the interaction of these 2D materials with atmosphere and/or contaminants and their effect on friction and select an optimum contact pair to reach ultra-low friction in a wide range of contact conditions. However, direct use of 2D materials is limited; they have to be fabricated elsewhere, transferred into contact, and cannot be easily replenished. Our strategy is thus to prepare tailored solid lubricant coatings with the ability to produce 2D solid lubricants during the sliding.

We will summarize the most promising solutions, typically co-sputtering transition metal dichalcogenide with an additional element, and discuss new techniques to analyze solid lubrication formation using simulations and experiments. In particular, we will shed light on the role of oxygen, traditionally considered detrimental for sliding, and use atomic force microscopy (AFM) and Raman spectroscopy mapping [4] of the worn surface to provide complete information about the sliding mechanism and 2D lubricant formation. Finally, we will discuss how HIPIMS deposition process can be explored to develop a new generation of solid lubricant coatings.

[1] M. Liao et al, Nature Materials, 21, 47 (2022).

[2] J. Missaoui et al, Physical Review Applied, 15, 064041 (2021).

[3] V.E.P. Claerbout et al, Frontiers in Chemistry, 9, 684441 (2021).

[4] A. Rapuc et al, ACS Applied Materials & Interfaces, 12, 54191 (2020).