Source: TU Wien post

VIENNA — A seemingly paradoxical effect: friction normally causes more damage at higher speeds. But at extremely high speeds, it is the other way around.

When two metal surfaces slide against each other, a variety of complicated phenomena occur that lead to friction and wear: Small crystalline regions, of which metals are typically composed, can be deformed, twisted or broken, or even fuse together. It is important for industry to understand such effects. After all, wear can destroy machinery and cost a lot of money.

Typically, the faster the two surfaces slide past each other, the greater the wear. But at extremely high speeds, comparable to the muzzle velocity of a firearm, this can be reversed: Above a certain speed, the wear decreases again. This surprising and seemingly contradictory result has now been explained using computer simulations by the Research Unit Tribology at TU Wien and the Austrian Excellence Center for Tribology (AC2T research GmbH) in Wiener Neustadt in cooperation with Imperial College in London.

Simulations on high-performance computers

“In the past, friction and wear could only be studied in experiments,” says Stefan Eder (TU Wien, AC2T research GmbH). “Only in recent years have supercomputers become so powerful that we can model the highly complex processes at the material surface on an atomic scale.”

Stefan Eder and his team recreate various metal alloys on the computer – not perfect single crystals, with a strictly regular and defect-free arrangement of atoms, but an alloy that is much closer to reality: a geometrically complicated arrangement of tiny crystals that can be offset from each other or twisted in different directions, manifesting as material defects. “This is important because all these defects have a decisive influence on friction and wear,” says Stefan Eder. “If we were to simulate a perfect metal on the computer, the result would have little to do with reality.”

Surprising results

The research team calculated how the sliding speed affects wear: “At comparatively low speeds, in the order of ten or twenty meters per second, wear is low. Only the outermost layers change, the crystal structures underneath remain largely intact,” says Stefan Eder.

If you increase the speed to 80–100 meters per second, the wear increases – that is to be expected, after all, more energy is then transferred into the metal per time unit. “You then gradually enter a range where the metal behaves like a viscous liquid, similar to honey or peanut butter,” says Stefan Eder. Deeper layers of the metal are pulled along in the direction of the passing surface, and the microstructure in the metal is completely reorganized. The individual grains that make up the material are twisted, broken, pushed into each other and finally pulled along.

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