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A Metal Coating That Repairs Itself

Electroplated metal could be used to make self-healing construction materials, car parts, and machinery.

Airplanes, cars, and ships that don’t corrode are the promise of self-healing paint coatings and polymer materials. Now researchers at the Fraunhofer Institute for Manufacturing Engineering and Automation and the University of Duisburg-Essen in Germany have come up with a metal coating that may be able to repair itself after sustaining damage.

Healing bubbles: Tiny fluid-filled capsules a few hundred nanometers wide are dispersed throughout a thin electroplated metal layer. The capsules could be filled with polymers to make metal coatings that repair themselves.

The self-healing metal can be electroplated, which opens up applications in construction, car manufacturing, and other industries that use or manufacture steel machines. (Nuts, bolts, and screws made of steel, which is susceptible to corrosion, are already electroplated with rustproof metals such as zinc and chromium.)

The new coating is around 15 micrometers thick and contains polymer capsules a few hundred nanometers in diameter. When the plating is scratched, the capsules should burst and release their contents - which could be a polymer capable of sealing the crack, or corrosion-inhibiting liquids.

So far, the researchers have made nanocapsule-infused coatings from metals or alloys including copper, zinc, and nickel. In principle, it should be possible to make them from any metal that can be electroplated, says Harald Holeczek, a Fraunhofer researcher who was involved in the work.

Although Holeczek and his colleagues haven’t yet demonstrated the material’s self-healing property, being able to incorporate liquid-filled nanocapsules into electroplated layers is significant, says Michael Kessler, a materials science and engineering professor at Iowa State University. “This is the first self-healing coating that can be electroplated,” he says. “The advantage is that electroplating is a widely used industrial process.”

The liquid inside the nanocapsules could be tailored to a variety of purposes. For instance, capsules in the plating of ball bearings could be filled with mineral oils to make the bearings self-lubricating. Capsules filled with colored liquids or scented oils could make metal parts that change color or release an odor when they are damaged. Better yet, several different types of capsules could be incorporated inside a metal layer, Holeczek says. For instance, it may be possible to “use color or scent in an upper layer to signal wear or damage and use some inhibition agent in a deeper layer to prevent severe damage.”

Electroplating involves passing a current through an electrolyte solution containing positive metal ions. The object that needs to be coated is given a negative charge and immersed in the electrolyte. The positive ions are attracted to the negative surface, creating a thin layer of metal.

The Fraunhofer researchers make the nanocapsules separately before adding them to an electrolyte solution. But making capsules that survive the electroplating process was not easy–the harsh electrolytes can easily degrade the capsules, Holeczek says. Additionally, “the very tiny capsules tend to stick to each other once introduced in an aqueous medium.” So the researchers had to add a proprietary mix of chemicals to the electrolyte solution, and to the capsules themselves, to prevent this from happening.

As a result, the nanocapsules can be integrated into the thin plating without affecting its hardness and other mechanical properties, Holeczek says. They are also distributed evenly through the metal layer, which means there’s a better chance that the capsules will open even when damage is minor.

Paul Braun, a materials science and engineering professor at the University of Illinois at Urbana-Champaign, has made a microcapsule healing system that can be added to a wide range of paints and protective coatings and is now being marketed. He says that making the capsules too small could defeat the purpose: “If you have a 15-micrometer-wide scratch, then you can’t release enough material to fill the crack plane.”

However, once the researchers come up with the appropriate chemistries to show that the material can heal itself, Braun says, it could “open up a whole new opportunity space.”

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