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.

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."