Role of chemical vs. physical interfacial interaction and adsorbed water on the tribology of ultrathin 2D-material/steel interfaces

Lubrication of steel using two-dimensional (2D)-materials has been a growing interest in recent years at the macro/microscale for a wide range of applications including lubricating 440 C-steel for satellite and automotive components. This work takes a new approach of comparing the tribological behavior of ultrathin-graphite and ultrathin-MoS2 at varying humidity against a custom-fabricated 440 C-steel counter-surface using friction force microscopy. The presence of oxides on the 440 C-steel counter-surface is found to form stronger chemical interactions with MoS2, leading to higher friction, interfacial-shear-strength and adhesion as compared to physically interacting steel/ultrathin-graphite interface. While water increases friction and adhesion for steel/ ultrathin-graphite interface, an opposite trend is observed for steel/MoS2 interface, where water act as a temporary protective film to suppress the chemical interaction.