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Tiny Capsules Pack Friction-Free Punch

Tuesday, October 23, 2012

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A new coating filled with tiny lubricant capsules could be the next advancement in self-healing coatings for metal surfaces that fall victim to friction.

Researchers from the Scandinavian research organization known as SINTEF and the Norwegian University of Science and Technology (NTNU) are testing a coating that may prevent friction when applied where two metal surfaces come in contact.

NTNU

Researchers are working on a self-healing coating for metal using lubrication capsules.

Such a coating could provide the constant lubrication that metal surfaces need to keep from drying out, becoming damaged, and even failing, the team says.

Wind turbines, for example, have high maintenance costs, 30 percent of which comes from overhauling their mechanical components. Other cost savings for that industry alone involve energy consumption, personnel, lubrication expenses, and maintenance and spare parts.

Tribology Technology

The researchers have spent two years developing the coating technology as part of an internal project at the Tribology Gemini Center. The center is a strategic collaboration between Oslo-based SINTEF and NTNU dedicated to studying the physical processes behind friction, wear, and lubrication.

Researchers say the new coating is formed of both hard particles and capsules filled with liquid lubricant.

“We apply the lubricant using a thermal spray technique, where powder and capsules are fired at the surface using a flame,” says Sergio Armada, of SINTEF Materials and Chemistry.

“When the metal surfaces come into contact with each other, the coating is broken down in a controlled manner, releasing the contents of the capsules, and the lubricant will then prevent further friction.”

Applications and Testing

The method “has many potential applications, and is suitable for a wide range of coatings,” says Armada. “It will be possible to apply the coating to large surfaces and components, and the actual material of the coating can also be varied—from metals to polymers and ceramics.”

In the medical sector, for example, the coating could be sprayed onto hip and knee replacements before they are inserted. These parts have surfaces that move against each other, and grating can occur if there is not enough fluid in the joint.

The researchers have carried out several tests on slide bearings in industrial settings, measuring friction on surfaces with and without the capsules. When a coating without capsules was applied to the slide bearing, the friction coefficient was 0.7, compared to 0.15 in bearings coated with a layer of capsules.

After completing the initial project, the parties have written a new proposal and applied to the EU to be able to continue their work on thermal spraying and biocompatible coatings.

Challenges in Past Research

Research in self-healing coatings for metals has been underway for years at many facilities worldwide, but those efforts have encountered drawbacks.

SINTEF

The capsules must be designed to be only a fraction of the size of the coating.

Two years ago, materials researchers from Fraunhofer and the University of Stuttgart presented an article on coating metal using an electroplated layer lubrication method. SINTEF and NTNU researchers say the downfall of this method is applying the lubrication with electricity, which allows for only a single metallic layer.

Previous research in Japan focused on developing a self-healing polymer capable of conducting electrical currents to protect metal.

Another challenge in similar coatings was that the coating capsules in the coating were too big. A surface layer should only be 15–20 micrometers thick, so the capsules must be a fraction of that size so they do not release too quickly.

“In our case, the capsules must be smaller than 10 micrometers, so that they do not block the nozzles and cause problems during the spraying process,” says Christian Simon, a SINTEF researcher. “We have now created capsules as small as two micrometers.”

Simon’s colleague, Ruth Schmid, is producing capsules with an extremely low friction coefficient that could have even better tribological properties, SINTEF says.