This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Through joint research with the U.S. Geological Survey and the University of Tokyo, NIMS for the first time experimentally demonstrated friction completely independent of sliding velocity—an ideal that had never been realized—by heating mica, a layered oxide also used as a lubricant material, to 200°C (392°F).
NIMS also clarified that this is caused by the disappearance of defects within crystals at high temperatures. This discovery is expected to clarify the basic principles of friction, which have remained unexplained, and lead to the design and development of energy-saving lubricants. This outcome was published in Physical Review Letters.
The law stating that kinetic frictional force (frictional force that acts when materials move) between dry materials remains constant regardless of sliding velocity (moving speed)—the Amontons–Coulomb friction law—is explained as one of the basic laws of friction in high school physics textbooks. However, in reality, frictional force changes slightly with sliding velocity for many types of materials (the law of rate-and-state-dependent friction), and a state completely independent of velocity, as described in Coulomb's law, has not been verified. Slight changes in kinetic frictional force cause oscillations and wear during power fluctuations, as well as unstable power transmission, and thus significantly affect the stable operation of machines.
Accordingly, understanding and predicting the relationship between friction and sliding velocity has been a significant challenge in designing machines and developing lubricant materials, and clarification of this physical phenomenon has been required. The joint research team from NIMS, the U.S. Geological Survey and the University of Tokyo measured changes in kinetic frictional force between single crystals of mica (KAl2[Si3AlO10](OH)2), a layered oxide, from room temperature to 200°C (392°F).
The team found that frictional responses to changes in sliding velocity became smaller as temperatures rose and that kinetic frictional force became independent of sliding velocity at 200°C (392°F). This phenomenon cannot be explained by conventional theories of friction, and the team achieved ideal Coulomb friction. Observations of the test samples with an electron microscope clarified the relationship between the presence or absence of wave-like defects called ripplocations within mica crystals and the dependence on sliding velocity.
The research revealed for the first time that defects in crystals are involved in complex frictional phenomena. The team will conduct friction experiments within electron microscopes to clarify the relationship between the presence or absence of defects within crystals and the dependence of kinetic frictional force on sliding velocity. By expanding research targets to other diverse layered materials, the team aims to develop a new law of friction related to sliding velocity.
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