Scientists at the Department of Energy’s Oak Ridge National Laboratory have developed a coating composed of carbon nanotubes that imparts superlubricity to sliding parts. It reduces the friction of steel rubbing on steel at least a hundredfold. A paper on the coating is published in the journal Materials Today Nano.

Superlubricity is the property of showing virtually no resistance to sliding; its hallmark is a coefficient of friction less than 0.01. In comparison, when dry metals slide past each other, the coefficient of friction is around 0.5. With an oil lubricant, the coefficient of friction falls to about 0.1. However, the ORNL coating reduced the coefficient of friction far below the cutoff for superlubricity, to as low as 0.001.

For the study, researchers grew carbon nanotubes on steel plates, then with a machine called a tribometer, made the plates rub against each other to generate carbon-nanotube shavings.

The multiwalled carbon nanotubes coat the steel, repel corrosive moisture and function as a lubricant reservoir. When they are first deposited, the vertically aligned carbon nanotubes stand on the surface like blades of grass. When steel parts slide past each other, they essentially “cut the grass.” Each blade is hollow but made of multiple layers of rolled graphene, an atomically thin sheet of carbon arranged in adjacent hexagons like chicken wire. The fractured carbon nanotube debris from the shaving is redeposited onto the contact surface, forming a graphene-rich tribofilm that reduces friction to nearly zero.

Kumara et al.

The new nanotubes do not provide superlubricity until they are damaged.

The carbon nanotubes are destroyed in the rubbing but become a new thing. The key part is those fractured carbon nanotubes are pieces of graphene. Those graphene pieces are smeared and connected to the contact area, becoming what we call tribofilm, a coating formed during the process. Then both contact surfaces are covered by some graphene-rich coating. Now, when they rub each other, it’s graphene on graphene.

—Jun Qu, leader of ORNL’s Surface Engineering and Tribology group and corresponding author

The presence of even one drop of oil is crucial to achieving superlubricity.

We tried it without oil; it didn’t work. The reason is, without oil, friction removes the carbon nanotubes too aggressively. Then the tribofilm cannot form nicely or survive long. It’s like an engine without oil. It smokes in a few minutes, whereas one with oil can easily run for years.

—Jun Qu

The ORNL coating’s superior slipperiness has staying power. Superlubricity persisted in tests of more than 500,000 rubbing cycles.

The researchers have applied for a patent of their novel superlubricity coating.

ORNL’s Laboratory Directed Research and Development Seed Program provided the initial support to the proof-of-concept work. Then the Solar Energy Technologies Office and Vehicle Technologies Office in DOE’s EERE supported the follow-on research.

Resources

• Chanaka Kumara, Michael J. Lance, Jun Qu (2023) “Macroscale superlubricity by a sacrificial carbon nanotube coating,” Materials Today Nano doi: 10.1016/j.mtnano.2022.100297.