Observation of Behavior Presenting Ideal Friction Independent of Velocity at High Temperature

The law stating that kinetic frictional force (frictional force that acts when materials are moved) between dry materials remains constant, irrespective 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, for many types of materials, frictional force slightly changes depending on sliding velocity (the law of rate‑and state‑dependent friction), and the state completely independent of velocity, as explained in Coulomb’s law, has not been verified. Slight changes in kinetic frictional force cause oscillations and attrition upon power fluctuations, as well as unstable power transmission, and thus significantly affect the stable operation of machines. Accordingly, understanding and predicting the correlation between friction and sliding velocity has been a significant challenge in designing machines and developing lubricant materials, and the clarification of this physical phenomenon has been required.

The joint research team of 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 up to 200℃. As a result, we found that frictional responses to changes in sliding velocity become smaller with a rise in temperature and kinetic frictional force becomes irrelevant to sliding velocity at a temperature of 200℃. This phenomenon cannot be explained by conventional theories of friction, and it can be said that we have achieved ideal Coulomb friction. Observation of the test samples with an electron microscope clarified the relevance between the existence or non-existence of defects like waves called Ripplocations within mica crystals and the dependence on sliding velocity. The research for the first time revealed the fact that defects in crystals are involved in complicated frictional phenomena.

From now, we will conduct friction experiments within electron microscopes to clarify the relevance between the existence or non-existence of defects within crystals and the dependence on sliding velocity of kinetic frictional force. Furthermore, by expanding research targets to other diverse layered materials, we aim to develop a new law on friction in relation to sliding velocity. In the future, our research is expected to lead to the design of layered materials to be used as solid lubricants, and the design and development of easy-to-use and energy-saving lubricants that are free from changes in the property at various sliding velocities.

• This research was conducted by a team consisting of Dr. Hiroshi Sakuma (Principal Researcher, Environmental Circulation Composite Materials Group, Functional Materials Field, Research Center for Electronic and Optical Materials, NIMS), Dr. Diane E. Moore and Dr. David A. Lockner (Rock Physics Laboratories, the U.S. Geological Survey), and Professor Emeritus Toshihiro Kogure (School of Science, the University of Tokyo).
• This research was conducted with support under the Grants-in-Aid for Scientific Research Program by the Japan Society for the Promotion of Science (JP24K00795, JP25H00688, and JP20K04115).
• The research outcome was published on the Physical Review Letters website on July 20, 2026.