Desert Scorpions Inspire Abrasion-Resistant Surfaces

Erosion of materials by solid particles is a common failure mode in a wide variety of industries. For example, according to the Electric Power Research Institute, one of the most common causes for downtime in coal-fired power plants is the failure of boiler tubes due to erosion by fly ash. Erosion can also cause failures of turbine blades, not only in power plants, but also in aircraft engines. It's also a problem for rocket nozzles, marine propellers, and helicopter rotors.

For decades, engineers have worked to combat erosion by developing high-strength alloys, composites, and surface coatings. However, in a paper published online recently in the journal Tribology Transactions, Dr. Han Zhiwu and a team at Jilin University in China investigated a different approach. For inspiration, they turned to one of the most deadly animals in the world: androctonus australis, the yellow fat-backed scorpion.

Dr. Han and his colleagues realized that nature has had hundreds of millions of years to solve the problem of erosion for desert-dwelling creatures such as scorpions. The animals and plants that inhabit the world's deserts can survive dust storms that regularly bombard them with abrasive sand particles at velocities as high as 100 mph. How can something as fragile as a tamarisk plant another species studied by the engineers at Jilin University) or a scorpion, which a brave person can crush under his or her boot, resist conditions that highly engineered materials can't?

According to the researchers, the ability of desert scorpions to resist erosion can be attributed to the design of their bodies. In an earlier paper, published in the American Chemical Society journal Langmuir, the Jilin University team used a 3D laser scanner to generate a point cloud representation of a scorpion's back. Using NX Imageware reverse-engineering software, they converted the point cloud into a solid model. Analyzing the solid model, they found that the surface was convex and grooved. They then used ANSYS Fluentcomputational fluid dynamics software to numerically simulate how convex and grooved surfaces would respond to blowing sand. They found that both the convex shape and especially the grooves contribute to erosion resistance. In particular, the grooves cause the air to rotate, resulting in a low-speed reverse flow zone. This causes some of the sand particles that would otherwise hit the surface to be harmlessly flung away. The particles that do manage to hit the surface are moving at a lower velocity than they otherwise would. The result is a significantly lower erosion rate.

In order to test the model's prediction, the team produced steel coupons with grooved surfaces, and blasted them with sand according to a standard ASTM abrasion test. The results showed that the grooves decreased the abrasion rate compared to smooth samples.

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In their recent Tribology Transactions paper, Dr. Han and his colleagues investigated another aspect of scorpion armor. Like many other natural designs, a scorpion's carapace incorporates hard and soft elements. It consists of a series of hard, moveable plates covering a soft inner core. In order to simulate this, the researchers used wire electric discharge machining (EDM) to make millimeter-thick V-shaped plates from 6061 aluminum. The plates were set into a base of room-temperature vulcanizing (RTV) silicone rubber. They compared the performance of these specimens in the abrasion test to that of solid aluminum specimens, with and without V-shaped grooves. They found that the specimens with the silicone core had a 50% lower erosion rate than solid aluminum samples with V-grooves, and a 75% lower erosion rate than smooth aluminum samples. They believe the soft layer absorbs some of the impact energy of the sand particles that hit the surface, reducing the erosion rate.

The Chinese researchers' insight into the factors that allow desert scorpions to withstand dust storms -- curvature, grooves, and a combination of hard and soft layers -- could be used to make more erosion-resistant products for a wide variety of applications. This research is yet another example of the growing field of biomimetics, in which engineers look to the natural world for solutions to design problems. Long-time Design News readers are probably familiar with many other examples of biomimetic design (you can find some in the related posts section). It's humbling, but also exciting, to think that the world around us is full of solutions to the problems that perplex us -- if only we're willing to look.