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.
The objective of this project was to understand the atomic-level mechanisms of friction that control the sliding nanoscale contacts at the atomic scale, with a particular focus on the prevalent phenomenon known as "atomic stick-slip friction". Simulations at the atomic level, known as molecular dynamics (MD) simulations, and corresponding experiments, using the atomic force microscope (AFM), were conducted such that all conditions were matched as closely as possible. This included the materials being simluated, their environment, the temperature, the size and shape of the contacting materials, the forces applied, the stiffness of the spring elements connected to the materials, and most importantly, the sliding speed. Our key advance was to increase the sliding speed in the AFM experiments, and decrease the sliding speeds in the MD simuations, so that they came closer to overlapping. This allowed more realistic comparisons between the experiments and simulations, allowing us to verify and understand our results in detail and with high confidence. From this, we learned a number of new aspects about the nature of friction at the atomic scale, including: (1) atomic-scale friction for smooth, clean materials increases as a specific function of the sliding speed due to the nature of thermal vibrations, and this can be predicted by a fairly simple equation; (2) defects on a surface, like an atomic step, can increase friction, and we now understand that this effect is due to the enhanced energy of the atoms at that step; (3) extremely thin materials like graphene can show enhanced friction and adhesion with another surface due to small-scale deformations of the graphene around the point of contact; (4) when fluorine atoms are chemically connected to graphene, friction increases substantially, thanks to the increased atomic-scale roughness of the surface that is due to the high concentration of electronic charge that is associated with flourine. From the technological perspective, this work contributes to the knowledge base needed for the rational design of nanomechanical devices that involve contacting, sliding surfaces. From an educational perspective, there was significant impact through collaborative efforts between the two principal investigators. This included development of a multi-purpose demonstration module based on AFM, involvement of an undergraduate student in the research, and active participation in international collaborative cyber-network communities.
