High School Physics Says Friction Doesn't Depend on Speed. It Actually Does, Everywhere, Until Japanese and USGS Scientists Heated a Mica Crystal to 200°C. Then the Defects Vanished and Coulomb's Law Came True.

Every physics textbook teaches that the friction between two dry surfaces sliding past each other does not depend on how fast they move. That rule, the Amontons-Coulomb law, is centuries old, and it has never actually been observed. In the real world friction always drifts a little with speed, a behavior engineers describe with "rate-and-state" laws that govern everything from squealing brakes to the timing of earthquakes. Now a team from Japan's National Institute for Materials Science, the U.S. Geological Survey and the University of Tokyo has made the ideal law come true, and explained why it normally fails.

The researchers, led by Hiroshi Sakuma of NIMS, measured the kinetic friction between single crystals of muscovite mica, a layered mineral used as a solid lubricant, while varying the sliding speed and the temperature from room temperature up to 200 degrees Celsius. At low temperature the friction coefficient shifted noticeably as the speed changed, just as it does for almost every material ever tested. As the crystals warmed, that sensitivity shrank. At 200 degrees it disappeared entirely: the friction was the same no matter how fast the surfaces slid. The results were published in Physical Review Letters.

That behavior cannot be explained by conventional friction theory, which attributes the speed dependence to the way tiny contact points age and strengthen while they sit still. To find the cause, the team examined the sheared samples under an electron microscope and found a clear correlation between the speed dependence and the presence of wave-like defects inside the mica crystals called ripplocations, ripples that form when layers of a crystal buckle and slide over one another. At high temperature the ripplocations vanish, and so does the velocity dependence. It is the first direct evidence that crystal defects, rather than just surface roughness, drive the complicated friction behavior that has resisted a full explanation for 300 years.

The stakes are practical. Small changes in friction with speed cause chatter, vibration and wear when a machine's load fluctuates, and they make power transmission unstable. A lubricant whose friction does not change with speed would run smoother and waste less energy. The team plans friction experiments inside electron microscopes to watch the defects directly, and to extend the tests to other layered materials such as graphite and molybdenum disulfide, with the goal of writing a new law of friction that accounts for what is happening below the surface.

The involvement of the USGS points to a second application. The rate-and-state friction laws that the new result challenges are the same ones seismologists use to model how faults lock, slip and rupture. Mica-group minerals are common in fault gouge, and knowing that temperature and crystal defects can switch the speed dependence on and off could sharpen models of how deep, hot faults behave compared with shallow, cool ones.