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Super-slick coating nears commercialization
by Dave Jacqué
Four years and more than 3,000 commercial phone contacts later, Argonne's "Near-Frictionless Carbon" coating stands on the brink of commercialization.
The material is slick and 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.
 SLICK SCIENCE – Ali Erdemir checks a super-slick coated component. |
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, invitations for talks and keynote speeches for materials scientist Ali Erdemir, 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 in the Energy Technology Division's Tribology Section turned their efforts to learning how the coating worked and converting the laboratory curiosity into something industry could use.
The tribologists coated and tested parts from 100 manufacturers of such products as diesel engines, artificial-hip sockets and refrigeration system compressors. The coating performed well on many of these parts.
"With the original lab equipment, we could coat only a few small pieces," Erdemir said. "But for the coating to be commercially viable, you have to process parts by the hundreds, if not thousands."
A cooperative research and development agreement with CemeCon USA, a subsidiary of CemeCon Germany, and Argonne produced a machine that coats hundreds of small parts a day with the NFC coating.
Computer controls on the machine can customize the surface for various applications. At least five companies are interested in licensing either the system or its control software to manufacture coated parts by the millions.
Argonne materials scientists have only recently begun to understand why the stuff is so hard and slick – the carbon atoms in the coating are benefiting from an overdose of hydrogen.
NFC coating is made in a plasma chamber. Parts to be coated are mounted 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 an intense plasma around the parts, breaking apart the methane molecules into their constituent carbon and hydrogen. The carbon 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 repel each other. The surfaces are essentially gliding past each other like magnetically levitated 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.
Scanning tunneling microscopy – a technique capable of resolving individual atoms – will be used to reveal the coating's atomic structure directly and confirm his hypothesis. The group also plans to use the Advanced Photon Source and Intense Pulsed Neutron Source to study NFC's microstructure and chemical bonding.
For more information, please contact Don Knight at dknight@anl.gov.
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