There are a variety of complicated phenomena that occur when two metal surfaces slide against each other. Industry needs to understand the effects. Wear can cause machinery to be destroyed and cost a lot of money.
The faster the two surfaces move past each other, the more wear they will accumulate. The wear can be reversed if the speed is high enough. The 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.
There are simulations on high- performance computers.
Friction and wear could only be studied in experiments. We can model the highly complex processes at the material surface on an atomic scale, thanks to the power of the computer.
A metal alloy that is 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 "All these defects have a big influence on wear and friction," says Eder. The result on the computer would have little to do with reality.
Results that were unexpected.
The research team calculated how the sliding speed affects wear. The crystal structures under the outermost layers are mostly intact.
The wear increases if the speed is increased to 100 meters per second. The metal behaves like a liquid similar to honey or peanut butter. The metal is pulled along in the direction of the passing surface in order to reorganize the structure. The individual grains are twisted and pushed into each other.
The team experienced a surprise, however, when they moved on to even higher speeds: Above some 300 meters per second, which corresponds to the top speed of aircraft in civil aviation. The metal just below the surface is mostly intact again.
It was great for the tribology community. The effect has been observed by other scientists in experiments, but it's not very well known. The origin of this effect hasn't been explained.
Deeper layers are protected by the melting of the surface.
More detailed analyses of the computer data have shown how this effect can be achieved at high speeds. There are only a few small patches on the surface of the two metals that are in contact. The temperature is not as warm in between.
Small parts of the surface can melt and re-crystallise in a fraction of a second. The deeper regions of the material are protected because only the outermost layers of the material feel the wear.
The effect, which has not been discussed, occurs with different materials. It will be important to take this into account when it comes to high to extremely high speeds. This applies to modern, high-speed bearings and transmissions in E-mobility. The effect plays a part in the stability of metals in a vehicle crash or in the impact of small particles on high-speed aircraft.
The study was published in a journal.
More information: S.J. Eder et al, Does speed kill or make friction better?—Designing materials for high velocity sliding, Applied Materials Today (2022). DOI: 10.1016/j.apmt.2022.101588
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