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Snake scales inspire new technique for friction reduction

18 August 2015

Researchers of KIT have found a way to transfer the special features of a snake's belly scales to steel components subject to friction in machines.

Inspired by nature and milled into a steel bolt by a laser: a 3D imageof the scale-like structure, courtesy of Christian Greiner, KIT

A snake moves without legs by the scales on its belly gripping the ground. It generates friction at the points needed to move forwards only and prevents its scales from being worn off by too much friction. Researchers at the Karlsruhe Institute of Technology (KIT) are looking at ways to transfer this feature to the moving parts of machines.

KIT's Christian Greiner and Michael Schäfer have developed a process to transfer the scale structure of reptiles using a fibre laser to mill 'scales' into an 8mm diameter steel bolt.

With the help of two different structures, the materials researchers tested whether the distance of the scales influenced friction. In the first structure, the scales overlap and are located very closely to each other, such as the scales on the belly of a ball python.

The second structure consists of scales arranged in vertical rows at a larger distance, such as the skin of a sandfish skink. “The distance between the rows in our experiment was the smallest possible distance we could produce with the laser," says Greiner.

While the structure doesn't entirely correspond to that of the sandfish skink (each scale is 5µm by 50µm, whereas in nature, they are closer to 300 by 600nm), the researchers plan to produce structures that are closer to the original in nature. In their work, however, the KIT scientists noted that the size difference  does not necessarily lead to the artificial structure being less effective than the natural one.

To find out whether scales reduce friction, Greiner and Schäfer fixed the structured surface of the bolts to a rotating plate. The experiments were carried out with and without 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. The disk diameter was 50mm.

Experiments under lubricated conditions revealed that both narrow and wide arrangements of the scales increase friction compared with the unstructured bolt. With the wider scales, friction was increased by a factor of 1.6. The narrower scales increased friction by a factor of 3. In the non-lubricated state, the wide scale structure reduced friction by more than 40 percent, while friction was reduced by 22 percent in the narrow scale structure.

The finding that the narrow scale structure increases friction under both lubricated and non-lubricated conditions had not been expected by the researchers: “We assumed that the narrow structure is more effective, as it is closer to nature,” Greiner says.

The possibility that the smaller friction was caused by a surface modification due to the laser was discounted. “On and between the scales, the material was softer than the untreated material," Greiner adds. "Only the edges cut by the laser were harder and they had no contact with the rotating disk. Hence, we concluded that the scales were responsible for the reduced friction.”

The researchers now plan to test how the bolt friction is changed when the size of the scales is varied or harder material is used.


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