Superlubricity in layered nanostructures

Abstract

Interaction between two surfaces in relative motion can give rise to energy dissipation and hence sliding friction. A significant portion of the energy is dissipated through the creation of non-equilibrium phonons. Recent advances in material synthesis have made the production of specific single layer honeycomb structures and their multilayer phases, such as graphene, graphane, fluorographene, MoS2 and WO2. When coated to the moving surfaces, the attractive interaction between these layers is normally very weak and becomes repulsive at large separation under loading force. Providing a rigorous quantum mechanical treatment for the 3D sliding motion under a constant loading force within Prandtl-Tomlinson model, we derive the critical stiffness required to avoid stick-slip motion. Also these nanostructures acquire low critical stiffness even under high loading force due to their charged surfaces repelling each other. The intrinsic stiffness of these materials exceeds critical stiffness and thereby thematerials avoid stick-slip regime and attain nearly dissipationless continuous sliding. Remarkably, layered WO2 a much better performance as compared to others and promises a potential superlubricant nanocoating. The absence of mechanical instabilities leading to conservative lateral forces is also confirmed directly by the simulations of sliding layers. Graphene coated metal surfaces also attain superlubricity and hence nearly frictionless sliding through a charge exchange mechanism with metal surface. © Springer International Publishing Switzerland 2015.