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Near-frictionless carbon coating nears commercial applications

DOE/Argonne National Laboratory

Four years and more than 3,000 phone calls and e-mail contacts later, Argonne's "Near-Frictionless Carbon" coating stands on the brink of commercialization.

A flurry of calls from just about every engineer who works with moving parts followed the announcement in 1997 of a new coating with the lowest coefficient of friction ever measured.

Not only is the material slick, it's extremely wear-resistant. A sample of the coating on a sapphire substrate, placed in a standard testing machine, survived 17.5 million passes of a steel ball pressed against its surface. After 32 days, the testing machine failed, but the steel ball had left only a barely visible track on the shiny black coating. Publicity about the coating led to a flurry of calls from engineers across the country, who wanted to test the coating on everything from artificial-hip sockets to rocket-sled rails.

The development led to R&D 100 and Discover awards, invited talks and invitations for keynote speeches for materials scientist Ali Erdemir of Argonne's Energy Technology Division (ET) and national recognition for Argonne and its tribology program.

But as the initial clamor died down, Erdemir and his fellow tribologists (scientists who study lubrication and friction) John Woodford, Layo Ajayi and George Fenske (all in ET's Tribology Section) turned their efforts to learning how the coating worked - and converting the laboratory curiosity into something industry could use.

"Turning the coating into an engineering application was not that straightforward," Erdemir said. "When you venture into specific applications, you have to be able to tailor the material to very specific conditions. We needed to figure out how something like this works and under what conditions it works."

Dozens of companies sent parts to be coated and tested for applications such as diesel fuel systems, bearings, manufacturing equipment and compressors. The coating performed well on many of these parts.

"Companies liked the coating, and then they'd ask how we could coat 100,000 parts per year," Erdemir said. "With the original lab equipment, we could coat a few tens of small pieces. But for the coating to be commercially viable, you have to process parts by the hundreds, if not thousands. That was the biggest stumbling block."

Argonne's Office of Technology Transfer secured a cooperative research and development agreement with CemeCon USA, a subsidiary of CemeCon Germany, which makes industrial coating systems. CemeCon provided one of their best coating systems to Argonne, where it is being adapted to produce the NFC coating.

"It's the Cadillac of coating systems," Erdemir said. "We can coat hundreds of small parts per day."

Although Argonne's tribology group is able to produce the NFC coating and adapt it to various industrial uses, it wasn't until very recently that they began to understand why the stuff is so hard and slick. The answer seems to be that the carbon atoms in the coating are benefiting from an overdose of hydrogen.

Black magic

NFC coating is made in a plasma chamber. Parts to be coated are mounted on a fixture that sits on a rotating table inside. Air is pumped out of the sealed chamber, which is then refilled with a mixture of hydrocarbon gases, such as methane. High voltage creates intense plasma around the parts, breaking apart the methane molecules into its constituent carbon and hydrogen, which begins to coat the parts.

The ability of carbon atoms to bond in many ways is both a blessing and a curse. It allows for exotic forms like "buckyballs" and "nanotubes," but can be a nuisance when friction is a problem. When two surfaces with regular carbon coatings come in contact, for example, carbon atoms from each surface bond at the contact point. The relative motion of the surfaces then rips bonded atoms from each surface, causing high friction and wear.

In the NFC coating, the carbon atoms lie down in flat layers, just like a conventional carbon coating. However, due to the hydrogen-rich mix of gasses in the chamber, any available bond on the coating surface may attract a hydrogen atom. Erdemir believes the hydrogen atom loses its electron to the carbon atom's outer shell, leaving the positively charged hydrogen nucleus exposed. Some carbon atoms could even support two hydrogen atoms.

This may explain the super-slick properties of the coating, especially when two NFC-coated parts come in contact: The hydrogen atoms' positive charges repeal each other. The surfaces are essentially gliding past each other like maglev trains.

"No matter how hard you press them together, there is a repulsive force overcoming the 'sticktion,'" Erdemir said.

And since the hydrogen-carbon bond is extremely strong, more so than even a carbon-carbon bond, the surface is highly wear-resistant.

The tribology group is planning to use scanning tunneling microscopy - a technique capable of resolving individual atoms - to study the coating's atomic structure directly and confirm his hypothesis. The group also plans to use Argonne's Advanced Photon Source and Intense Pulsed Neutron Source to study NFC's microstructure and chemical bonding.

The nation's first national laboratory, Argonne National Laboratory conducts basic and applied scientific research across a wide spectrum of disciplines, ranging from high-energy physics to climatology and biotechnology. Since 1990, Argonne has worked with more than 600 companies and numerous federal agencies and other organizations to help advance America's scientific leadership and prepare the nation for the future. Argonne is operated by the University of Chicago as part of the U.S. Department of Energy's national laboratory system. -- Dave Jacque