A snake generates friction at the points needed to move forwards only and prevents its scales from being worn off by too much friction. Researchers of KIT have found a way to transfer this feature to components of movable systems. In this way, durability of hip prostheses, computer hard disks or smartphones might be enhanced.
“Friction and wear are two of the biggest challenges in systems of several individual components,” Christian Greiner of the Institute for Applied Materials says. A solution is found in nature: Snakes, such as the ball python, or lizards, such as the sandfish skink, use friction to move forwards, but can reduce it to a minimum thanks to their scales. Together with Michael Schäfer, Greiner developed a process to transfer the scale structure of reptiles to components of electromechanical systems: With a fiber laser, they milled scales into a steel bolt of 8 mm in diameter.
With the help of two different structures, the materials researchers tested whether the distance of the scales influences friction behavior. “The distance between the rows in our experiment was the smallest possible distance we could produce with the laser. The structure, hence, does not entirely correspond to that of the sandfish skink,” Greiner says. In the future, however, the researchers plan to produce structures that are closer to the original in nature.
Every scale is 5 µm high and has a lateral dimension of 50 µm. In nature, the scales of a snake are about 300 to 600 nm. The scales of a sandfish have a size of 2 x 3 mm. Another project of the KIT scientists revealed that the change of the size does not necessarily lead to the artificial structure being less effective than the natural one. To find out whether scales reduce friction, they fixed the surface of the bolts to a rotating plate. The experiments were carried out without and with a lubricant (1 ml of mineral oil). For the experiments with oil as lubricant, the scientists used steel disks. Under dry sliding conditions, sapphire disks were applied.
>> Experiments under lubricated conditions revealed that both narrow and wide arrangements of the scales increase friction compared to the unstructured bolt: By the wide scales, friction is increased by a factor of 1.6. The narrow scales increase friction by a factor of 3. In the non-lubricated state, the wide scale structure reduced friction by more than 40%, while friction was reduced by 22% in case of a narrow scale structure. “We assumed that the narrow structure is more effective, as it is closer to nature,” Greiner says. Overall it was concluded that the scales reduce friction,” Now, they plan to test how friction of the bolt is changed when the size of the scales is varied or harder material is used. http://www.alphagalileo.org/ViewItem.aspx?ItemId=155498&CultureCode=en
This image shows vertically and horizontally overlapping scale structures. Credit: Interferometer images: Christian Greiner, KIT
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