Collaborative Research: Quantitative Prediction of Sliding Friction Using Integrated Theory and Experiments

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This research is about obtaining a quantitative understanding of energy dissipation, interfacial structure and friction of model boundary films. While theoretical methods have hitherto been able to provide qualitative insights into sliding friction, they have not yet been able to quantitatively describe the structure of a sliding interface and the associated friction forces. The approach of this research will be to combine experiments in ultrahigh vacuum on well-defined model boundary films consisting of a range of isostructural alkali halides on metal substrates with molecular dynamics simulations using tailored, high-precision interaction potentials obtained from density function theory calculations of the same systems. If successful, the benefits of this research will include an understanding of the friction properties of the boundary films that are formed by chemical reactions between lubricant additives or gas-phase lubricants at a sliding, solid-solid interface. This fundamental understanding will underpin efforts to design effective lubricants that form films with low friction or wear, thereby resulting in energy savings and longer machine lifetimes, and benefit the United States' economy. In addition, understanding sliding friction poses a unique experimental challenge because the processes occur at an inaccessible, sliding solid-solid interface that cannot be interrogated directly. This means that advances in understanding such interfaces will rely on integrating experiments and theoretical methods carried out on the same systems. Being able to achieve this for the simple systems explored in this proposal will develop the theoretical and experimental strategies for understanding more complex sliding interfaces. Finally, the integrated experimental and theoretical approaches will provide a broad-based education for students and will involve both undergraduates and under-represented minorities. Developments in theoretical methods and the refinement of experimental approaches over the past few decades have resulted in a fundamental shift in the way that science is carried out and will require both graduate and undergraduate education that integrates theory and experiment. This has been emphasized in a recent special issue of Tribology Letters co-edited by the Principal Investigators of this proposal entitled "Progress in Tribology Through Integrated Simulations and Experiments".