Collaborative Research: Determining the Physical Mechanisms of Atomic Stick-Slip Friction by Closing the Gap between Experiments and Atomistic Simulations

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The objective of this collaborative research project is to understand the atomic-level mechanisms of friction that control the sliding of nanoscale contacts, with a particular focus on the prevalent phenomenon known as "atomic stick-slip friction". Molecular dynamics (MD) simulations and atomic force microscopy (AFM) experiments that match tribological conditions as closely as possible will allow results from both to be directly compared. The ability to compare AFM experiments with dynamic atomistic simulations, including molecular dynamics (MD), is limited at present. One is unable see the positions and velocities of the atoms in AFM experiments, which motivates MD studies. However, MD results cannot be directly compared with those from AFM because conventional simulations must be run at speeds several orders of magnitude faster rates than AFM experiments. As well, many MD and AFM studies differ in other important conditions such as materials, load, and tip size. In this work, speeds will be matched through the concurrent use of new methods being developed collaboratively by the principal investigators specifically for atomic-scale friction studies. The use of noble metals will ensure that the interface is well-defined and reliably modeled. Other critical parameters, including stiffness, tip size and shape, environment, and crystal surface and sliding direction orientation, are also matched. If successful, this will enable the atomic structure, mechanics, and dynamics of the contact to be directly linked with the corresponding friction forces and energy dissipation. Specifically, by closing the gaps between simulation and experiment, the detailed results and mechanisms resolvable only in atomistic simulations can be validated by experiments, phenomena observed experimentally can be explained by reference to the simulations, and both can form the basis for reliable predictive models describing nanoscale frictional sliding. This will provide a deep and reliable understanding of single asperity friction, which is an important step toward fully understanding the behavior of collections of asperities that one encounters in larger-scale contacts ? a longstanding goal for nanotribology research. From the technological perspective, the work can contribute to the knowledge base needed for the rational design of nanomechanical devices that involve contacting, sliding surfaces. From an educational perspective, there will be significant impact through collaborative efforts between the two principal investigators. This includes development of a multi-purpose demonstration module based on AFM, involvement of undergraduates and high school students, organizing and delivering a short course on nanotribology to graduate students, and active participation in international collaborative cyber-network communities.