Tailoring Surface Forces to Control the Frictional Properties of Graphene

A pressing financial and environmental challenge is the impact of friction and wear on energy usage, economic costs, and greenhouse gas (GHG) emissions. Globally nearly ¼ of the world’s total energy is consumed at moving contacts, with 20% of that total used to overcome frictional forces. To combat the negative effects of friction and wear, thus mitigating economic spending on energy losses and reducing GHG emissions, new lubrication schemes need to be developed. Effective lubrication solutions will need to be compatible with a host of sliding conditions, including a diverse range of surface chemistries and structural features. One highly adaptable material capable of meeting this challenge is graphene, a two-dimensional sheet of carbon atoms with excellent electronic, optical, and thermal properties. Graphene additionally exhibits exceptional friction reducing and wear-resistant properties, although it is difficult to implement as a practical lubricant because its mechanical behavior strongly depends on its interactions with the top and bottom contacts within an interface. To effectively capitalize on the lubricating potential of graphene, a thorough investigation into the tribological responses of graphene in controlled sliding contacts is required. Towards this goal, this dissertation includes research into graphene that is in sliding contact with molecularly modified interfaces, dynamically oscillating on rough surfaces, and covalently immobilized to the supporting substrate. Adhesion and friction measurements on graphene with molecularly functionalized atomic force microscopy (AFM) tips demonstrated how both chemical functionality and shear strain can be used to tune the tribological response of the graphene-molecule sliding interface. Dynamic measurements of graphene on rough surfaces were exploited to examine the relationship between energy dissipation and friction at different frequencies. Pinning graphene to the supporting surface further showed how the physical properties of graphene can be manipulated at interfaces. By understanding how tailored adhesion, modulated out-of-plane forces, and localized pinning concertedly impact the tribological performance of graphene, the development of targeted lubricant technologies can take advantage of graphene’s sensitivity to different sliding conditions. Designer boundary lubrication schemes incorporating graphene can then play a central role in overcoming the challenges associated with energy losses at tribological contacts