When two metal surfaces slide over each other, a variety of complex phenomena that lead to friction and corrosion occur: the small crystalline regions, of which the metals are usually formed, can be deformed, warped or broken, or even fused together. It is important for the industry to understand such implications. After all, wear can destroy machines and cost a lot of money.
Usually, the faster the two surfaces slide off each other, the greater the wear. But at very high velocities, comparable to the muzzle velocity of a firearm, this can be reversed: above a certain velocity, wear decreases again. This surprising and contradictory result has now been explained using computer simulation by the Tribology Research Unit at TU Wien and the Austrian Center of Excellence for Tribology (AC2T Research GmbH) at Wiener Neustadt in collaboration with Imperial College London.
Simulation 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 very complex processes in matter Surface on an atomic scale.”
Stefan Eder and his team have recreated many metallic alloys on the computer — not perfect single crystals, with a perfectly regular and flawless arrangement of atoms, but an alloy closer to reality: a geometrically complex arrangement of tiny crystals that can be skewed apart or twisted in different directions. Different, manifested in the form of material defects. “This is important because all of these defects have a critical impact on friction and wear,” says Stefan Eder. “If we were to simulate a perfect metal on a computer, the result would have little to do with reality.”
The research team calculated how slip velocity affects the erosion: “At relatively low speeds, on the order of ten or twenty meters per second, the erosion is low. Only the outer layers change, and the crystal structures underneath remain largely intact,” says Stefan Eder. .
If you increase the speed to 80-100 meters per second, the wear increases – this is to be expected, after all, more energy is transferred to the metal per unit time. “You then gradually enter a range where the mineral behaves like a viscous liquid, similar to honey or peanut butter,” says Stefan Eder. The deeper layers of the mineral are pulled towards the passing surface, and the microstructure in the mineral is completely reorganized. The individual granules that make up the material are twisted, broken, pushed together and finally pulled out.
However, the team encountered a surprise when they moved to higher speeds: more than 300 meters per second – which roughly corresponds to the maximum speed of an aircraft in civil aviation – the wear decreased again. The microstructure of the metal just below the surface, which was completely destroyed at medium velocities, is now largely intact again.
“This has been great for us and the tribology community,” says Stefan Eder. “But literature research has shown us: This effect has been observed by other scientists in experiments – it is not well known because such high speeds rarely occur. However, the origin of this effect has not yet been clarified.”
Surface melt protects the deeper layers
More detailed analyzes of the computer data have now shed light on how this effect is possible: at very high speeds, friction generates a lot of heat – but in a very uneven way. Only individual spots on the surfaces of the two metals sliding against each other touch, and these small areas can reach thousands of degrees Celsius. Between them, the temperature is much lower.
As a result, small portions of the surface can dissolve and then crystallize again a split second later. And so the very outer layer of the metal changes dramatically, but this is exactly what protects the deeper areas of the material: only the outer layers of the material feel the wear, and the crystal structures under them change slightly.
“This effect, which has not been discussed much until now, occurs with different materials,” says Stefan Eder. “Wherever friction occurs at high to very high speeds, this will need to be taken into account in the future.” This applies, for example, to bearings and modern high-speed transmissions in electronic mobility, or to machines that grind surfaces. The better understood effect now plays a role in the stability of minerals in a car crash or in the impact of small particles at altitude.Speed Planes.
The study was published in Today’s Applied Material.
SJ Eder et al, Does it kill speed or make friction better? – Material design for high speed gliding, Today’s Applied Material (2022). DOI: 10.1016 / j.apmt.2022.101588
Vienna University of Technology
the quote: Researchers Report the Counterintuitive Friction Effect (2022, September 6) Retrieved September 6, 2022 from https://phys.org/news/2022-09-counterintuitive-friction-effect.html
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