Chemical adsorption-induced distinct friction behaviors of supported atomically thin nanofilm

Graphene, with its excellent mechanical properties and friction-reducing capabilities, functions as a solid lubricant and protective coating. However, environmental contamination, consisting of various compounds, elements, and molecules, can degrade these properties and is challenging to characterize. We address this difficulty to unravel the impact of contamination on graphene's tribological performance by adsorbing six different chemical reagents on graphene supported by silicon substrates. Through friction experiments, six distinct frictional behaviors were observed on these contaminated graphene samples. Specifically, benzyl alcohol, toluene, and ethanol all increased the surface friction, adhesion, and friction coefficient of graphene to varying degrees, resulting in positive frictional hysteresis. In contrast, acetone, 1-pentanol, and 1-pentane had the opposite effect to different extents. Notably, 1-pentane significantly reduced the friction coefficient of graphene, achieving superlubricity, while benzyl alcohol damaged thin layers of graphene, causing them to completely disappear. Finally, through MD simulations, we demonstrated that hydrogen bonds formed by hydroxyl groups and the carbon chain structure of the chemical contaminants cause variations in the contact area and stress/strain distribution within it, thus leading to varied surface friction. The evolution of these factors during the loading-unloading process was the primary reason behind these six distinct hysteretic friction behaviors.