Super-slick material makes steel stronger

Accelerated corrosion test, in which unmodified stainless steel (right)and the lower part of the SLIPS sample with a 600-nm-thick porous TO film on steel (left)were exposed to corrosive Glyceregia stainless steel etchant
Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a way to make steel stronger, safer and more durable. Their surface coating, made from rough nanoporous tungsten oxide, is claimed to be the most durable anti-fouling and anti-corrosive material to date, capable of repelling any kind of liquid even after sustaining intense structural abuse.

While various grades of steel have been developed over the past 50 years, steel surfaces have remained largely unchanged. It is as prone as ever to the corrosive effects of water, salt and abrasive materials such as sand.

The material joins a portfolio of other non-stick, anti-fouling materials developed in the lab of Professor Joanna Aizenberg of the Wyss Institute for Biologically Inspired Engineering at Harvard University. Prof Aizenberg’s team developed Slippery Liquid-Infused Porous Surfaces (SLIPS) in 2011 and since then has demonstrated a range of applications for the super-slick coating.

“Our slippery steel is orders of magnitude more durable than any anti-fouling material that has been developed before,” said Prof Aizenberg.“This research shows that careful surface engineering allows the design of a material capable of performing multiple, even conflicting, functions, without performance degradation.”

The material could be used in applications including non-fouling medical tools and devices, such as implants and scalpels, nozzles for 3D printing and, potentially, larger-scale applications for buildings and marine vessels.

Prof Aizenberg said the biggest challenge in the development of this surface was to figure out how to structure the steel to ensure its anti-fouling capability without mechanical degradation. The team solved this by using an electrochemical technique to grow an ultrathin film of hundreds of thousands of small and rough tungsten-oxide islands directly onto a steel surface.

“If one part of an island is destroyed, the damage doesn’t propagate to other parts of the surface because of the lack of interconnectivity between neighbouring islands,” said Alexander Tesler, former postdoctoral fellow at SEAS. “This island-like morphology combined with the inherent durability and roughness of the tungsten oxide allows the surface to keep its repellent properties in highly abrasive applications, which was impossible until now.”

Electrochemical deposition is already a widely used technique in steel manufacturing. The goal, Prof Aizenberg said, is to be scalable, but not disruptive to current industry practices.

The team tested the material by scratching it with stainless steel tweezers, screwdrivers, diamond-tipped scribers, and pummelling it with hundreds of thousands of hard, heavy beads. Then, the team tested its anti-wetting properties with a variety of liquids, including water, oil, highly corrosive media, biological fluids containing bacteria and blood. Not only did the material repel all the liquid but the tungsten oxide also made the steel stronger.

Tom Austin-Morgan

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