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Superlubricity

Superlubricity is a phenomenon in which friction can vanish almost completely. Superlubricity occurs when two crystalline surfaces slide over each other in dry, incommensurate contact. It is an effect that has been already suggested in 1991 but has recently been measured with great accuracy between two graphite surfaces. ^[1] One should note that the similarity of the term superlubricity with terms such as superconductivity and superfluidity is misleading; other energy dissipation mechanisms can lead to a finite (normally small) friction force.

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The atoms in graphite are oriented in a hexagonal manner and form an atomic hill-and-valley landscape, which looks like an egg-crate. When the two graphite surfaces are in registry (every 60 degrees), the friction force is high. When the two surfaces are rotated out of registry, the friction is largely reduced. This is like two egg-crates which can slide over each other more easily when they are "twisted" with respect to each other.

A state of ultralow friction is also reached when a sharp tip slides over a flat surface and the applied pressure is below a certain threshold, depending on the surface potential sensed by the tip and the stiffness of the contacting materials. ^[2] The threshold can be significantly increased by exciting the sliding system at its resonance frequency, which suggests a practical way to limit wear in nanoelectromechanical systems. ^[3]

References

^ Superlubricity of Graphite Martin Dienwiebel, Gertjan S. Verhoeven, Namboodiri Pradeep, Joost W. M. Frenken, Jennifer A. Heimberg, and Henny W. Zandbergen Phys. Rev. Lett. 92, 126101 (2004) doi:10.1103/PhysRevLett.92.126101 [1]
^ Transition from Stick-Slip to Continuous Sliding in Atomic Friction: Entering a New Regime of Ultralow Friction Anisoara Socoliuc, Enrico Gnecco, Roland Bennewitz, and Ernst Meyer Phys. Rev. Lett. 92, 134301 (2004) doi:10.1103/PhysRevLett.92.134301
^ Atomic-Scale Control of Friction by Actuation of Nanometer-Sized Contacts Anisoara Socoliuc, Enrico Gnecco, Sabine Maier, Oliver Pfeiffer, Alexis Baratoff, Roland Bennewitz, and Ernst Meyer Science 313, 207 (2006) doi:10.1126/science.1125874