A research team at Northwestern University has developed a new coating strategy for metal that self-heals within seconds when scratched, scraped, or cracked. The novel material could prevent these tiny defects from turning into localized corrosion.

“Localized corrosion is extremely dangerous,” said Jiaxing Huang, a materials science and engineering professor who led the research. “It is hard to prevent, hard to predict, and hard to detect, but it can lead to catastrophic failure.” The project was supported financially by the U.S. Office of Naval Research.

Patent-Pending System

When damaged by scratches and cracks, Huang’s patent-pending system readily flows and reconnects to rapidly heal. The researchers demonstrated that the material can heal repeatedly, even after scratching the exact same spot nearly 200 times in a row.

While a few self-healing coatings already exist, those systems typically work for nanometer- to micron-sized damages, according to the researchers. To develop a coating that can heal larger scratches in the millimeter-scale, Huang and his team looked to fluid.

“When a boat cuts through water, the water goes right back together,” Huang said. “The ‘cut’ quickly heals because water flows readily. We were inspired to realize that fluids, such as oils, are the ultimate self-healing system.”

But common oils flows too readily, Huang explained. So his team needed to develop a system with contradicting properties: fluidic enough to flow automatically, but not so fluidic that it drips off the metal’s surface.

Lightweight Particle Network

The team met the challenge by creating a network of lightweight particles — in this case, graphene capsules — to thicken the oil. The network fixes the oil coating, keeping it from dripping. But when the network is damaged by a crack or scratch, it releases the oil to flow readily and reconnect.

Huang believes the material can be made with any hollow, lightweight particle, and not just graphene. “The particles essentially immobilize the oil film,” Huang said. “So it stays in place.”

According to the researchers, the coating sticks well even in underwater and harsh chemical environments, such as acid baths. Huang envisions that it could be painted onto bridges and boats that are naturally submerged underwater, as well as metal structures near leaked or spilled highly corrosive fluids. The coating can also withstand strong turbulence and stick to sharp corners without budging.

When brushed onto a surface from under water, the coating goes on evenly without trapping tiny bubbles of air or moisture that often lead to pinholes and corrosion.

For more information, contact: Northwestern University Department of Materials Science and Engineering, (847) 491-3537, www.northwestern.edu