Chemical and physical origins of friction on surfaces with atomic steps

phys.org | 7/29/2019 | Staff
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Friction results from a set of complex processes that act together to resist relative motion. Despite this complexity, friction is often described using simple phenomenological expressions that relate normal and lateral forces via the friction coefficient. The defined parameter encompasses multiple, sometimes competing effects. To better understand the origins of friction, Zhe Chen and an interdisciplinary team of researchers in the departments of chemical engineering, mechanical engineering and materials research studied a chemically and topographically well-defined interface between silica and graphite using a single-layer graphene step edge setup.

The research team identified the separate contributions of physical and chemical processes to friction and showed that a single friction coefficient could be separated into two terms corresponding to these effects. The results provided insight into the chemical and topographic origins of friction as an avenue of tuning surfaces by leveraging competing frictional processes. The findings are now published on Science Advances.

Friction - Interface - Surfaces - Contact - Speeds

Friction occurs at the interface between any two solid surfaces in contact and moving at different speeds or directions. Since friction can correspond to wasted energy, scientists use the parameter to determine the efficiency and useful lifetime of all moving systems from biological to the aeronautical. Frictional force (Ff) is often linearly proportional to the applied load (L) at the microscale and the proportionality of this relationship, known as the coefficient of friction (COF) is symbolized by µ and expressed as Amonton's Law.

Adhesive forces (Fa) can become significant at the nanoscale to introduce an additional term for molecular mechanisms of tribology in thin films. While the expression is phenomenologically simple and has held value in experiments for decades, the actual mechanisms of determining the magnitude of the COF are very complicated. Physicists had previously proposed friction to have purely physical origins with related chemical processes to occur in sliding surfaces. But the...
(Excerpt) Read more at: phys.org
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