Difference between revisions of "Superlubricity"
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Examples exhibiting superlubrication: | Examples exhibiting superlubrication: | ||
* two coplanar sheets of graphene rotated to one another to minimally mesh | * two coplanar sheets of graphene rotated to one another to minimally mesh | ||
| − | * two appropriately chosen tightly fitting coaxial nanotubes | + | * two appropriately chosen tightly fitting coaxial nanotubes (experimantally demonstrated) '''[Todo: add ref]''' |
* [[diamondoid]] molecular bearings and other [[diamondoid molecular elements|DMEs]] with sliding interfaces. | * [[diamondoid]] molecular bearings and other [[diamondoid molecular elements|DMEs]] with sliding interfaces. | ||
Revision as of 11:49, 30 December 2013
Superlubrication is a state of extremely low friction that occurs when two atomically precise surfaces slide along each other in such a way that the "atomic bumps" do not mesh or more precisely when the lattices distances projected in the direction of movement are maximally incommensurate. It works at all (non destructive) temperatures includung 300K room temperature. Superlubricating bearings are generally wear free. Their dominating damage mechanism is ionizing radiation or thermal destruction.
Examples exhibiting superlubrication:
- two coplanar sheets of graphene rotated to one another to minimally mesh
- two appropriately chosen tightly fitting coaxial nanotubes (experimantally demonstrated) [Todo: add ref]
- diamondoid molecular bearings and other DMEs with sliding interfaces.
If AP surfaces are designed or aligned to not mesh then the "perceived bumps" (the bumps that the surfaces perceive as a whole) become lower and their spacial frequency becomes higher (more bumps per length). If the surface pressure isn't extremely high the characteristic thermal energy kBT can become a lot higher than the bumps energy barriers. Thus the friction becomes so low that e.g. an unconstrained DMME bearing can be activated thermally and may starts turning randomly in a Brownian fashion.
Oxygen or sulfur with their two bonds in a plane parallel to the relative sliding direction are a good choice for surface termination of bearing interfaces since this configuration gives maximal stiffness in sliding direction. If the two bonds of the atoms are instead in a plane normal to the sliding direction the lower stiffness may lead to higher energy dissipation (friction). Singly bonded hydrogen fluorine or chlorine passivations have low stiffness too.
Note that superlubrication is akin to superconuctivity only in name:
- Superlubrication is reached by decrease of degree of intermeshment while superconductivity is reached by decrease of temperature.
- There is not a sharp cutoff in friction when decreasing the dergree of intermeshment like the cutoff in superconductivity when decreasing temperature.
DM bearings
Interestingly Van der Waals forces allow for stable designs in which the axle in DM bearings is pulled outward in all directions instead of compressed inward allowing even lower friction. At high operation speeds resonant vibrations might occur though.
