Superlubricity coating could reduce economic losses due to friction and wear

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ORNL’s vertically aligned carbon nanotubes reduce friction to nearly zero to improve energy efficiency. Credit: Chanaka Kumara/ORNL, US Department of Energy

Scientists at the Department of Energy’s Oak Ridge National Laboratory have invented a coating that could dramatically reduce friction in common load-bearing systems with moving parts, from vehicle transmissions to wind and hydroelectric turbines. It reduces the friction of steel rubbing on steel by at least a hundred times. The new ORNL coating could help fatten up a US economy that loses more than $1 trillion each year to friction and wear, equal to 5 percent of gross national product.

“When components slide past each other, there is friction and wear and tear,” said Jun Qu, leader of ORNL’s surface engineering and tribology group. Tribology, from the Greek word for friction, is the science and technology of interacting surfaces in relative motion, such as gears and bearings. “If we reduce friction, we can reduce energy consumption. If we reduce wear and tear, we can extend system life for better durability and reliability.”

With ORNL colleagues Chanaka Kumara and Michael Lance, Qu led a study published in Nano Materials Today on a coating composed of carbon nanotubes which gives superlubricity to the sliding parts. Superlubricity is the property of exhibiting virtually no creep resistance; its hallmark is a coefficient of friction of less than 0.01. By comparison, when dry metals slide past each other, the coefficient of friction is about 0.5. With a lubricating oil, the coefficient of friction drops to about 0.1. However, the ORNL coating reduced the coefficient of friction well below the limit for superlubricity, down to 0.001.

“Our main achievement is to make superlubricity possible for the most common applications,” Qu said. “Before, you only saw it in the nanoscale or in special environments.”

For the study, Kumara grew carbon nanotubes on steel plates. With a machine called a tribometer, he and Qu made plates rub against each other to generate carbon nanotube chips.

Multi-walled carbon nanotubes coat the steel, repel corrosive moisture and act as a lubricant reservoir. When first deposited, the vertically aligned carbon nanotubes sit on the surface like blades of grass. When the steel parts slide over each other, they essentially “cut the grass”. Each blade is hollow but made up of multiple layers of coiled graphene, an atomically thin sheet of carbon arranged in adjacent hexagons like a wire mesh. Fractured carbon nanotube debris from shaving is redeposited on the contact surface, forming a graphene-rich tribofilm that reduces friction to almost zero.

Making carbon nanotubes is a multi-step process. “First, we need to activate the surface of the steel to produce tiny, nanometer-sized structures. Second, we need to provide a carbon source to grow the carbon nanotubes,” said Kumara. She heated a stainless steel disc to form metal oxide particles on the surface. So she used chemical vapor deposition to introduce the carbon in the form of ethanol so that the metal oxide particles could stitch the carbon back together there, atom by atom in the form of nanotubes.

ORNL researchers used a tribometer for friction tests to show that carbon nanotubes in the presence of even a single drop of oil could sustain superlubricity for more than 500,000 cycles. Credit: Carlos Jones/ORNL, US Department of Energy

A stainless steel disc was heated to create particles of iron and nickel oxide on its surface. The particles catalyzed the growth of carbon nanotubes during chemical vapor deposition. Credit: Carlos Jones/ORNL, US Department of Energy

The new nanotubes don’t provide superlubricity until they are damaged. “Carbon nanotubes are destroyed by rubbing but become a new thing,” Qu said. “The key part is that those fractured carbon nanotubes are pieces of graphene. Those pieces of graphene are smeared and bonded to the contact area, becoming what we call a tribofilm, a coating formed in the process. So both contact surfaces are covered in a little graphene rich coating. Now, when they rub against each other, it’s graphene on graphene.”

The presence of even a single drop of oil is essential to obtain superlubrication. “We tried it without oil; it didn’t work,” Qu said. “The reason is, without oil, friction removes carbon nanotubes too aggressively. So the tribofilm can’t form well or survive for long. It’s like an engine without oil. It smokes within minutes, while one with oil can easily run for years.”

The ORNL coating’s superior slipperiness has staying power. Superlubricity persisted in tests of over 500,000 rub cycles. Kumara tested performance for continuous scrolling for three hours, then one day, and then for 12 days. “We still have superlubricity,” she said. “It’s stable.”

Using electron microscopy, Kumara examined the cut fragments to demonstrate that tribological wear had severed the carbon nanotubes. To independently confirm that rubbing had shortened the nanotubes, ORNL co-author Lance used Raman spectroscopy, a technique that measures vibrational energy, which is related to a material’s atomic bonding and crystal structure.

“Tribology is a very old field, but modern science and engineering have provided a new scientific approach to advance the technology in this area,” Qu said. “The fundamental understanding was shallow until the last maybe 20 years when tribology got a new life. More recently, scientists and engineers have really come together to use the most advanced materials characterization technologies, which is a strength of ORNL. Tribology is very multidisciplinary. No one is an expert in everything. Therefore, in tribology, the key to success is collaboration.”

He added, “Somewhere, you can find a carbon nanotube scientist, a tribology scientist, a materials characterization scientist. But they’re isolated. Here at ORNL, we’re together.”

ORNL’s Jun Qu shows stainless steel discs before (silver) and after (black) coating with carbon nanotubes that provide superlubricity. Credit: Carlos Jones/ORNL, US Department of Energy

ORNL’s Chanaka Kumara used a chemical vapor deposition system, in the background, to coat a stainless steel disk, in the foreground, with carbon nanotubes. Credit: Carlos Jones/ORNL, US Department of Energy

ORNL’s tribology teams have delivered award-winning work that has attracted industry partnerships and licenses. In 2014, an ionic antiwear additive for fuel-efficient motor lubricants, developed by ORNL, General Motors, Shell Global Solutions and Lubrizol, won an R&D 100 award. ORNL’s contributors were Qu, Huimin Luo, Sheng Dai, Peter Blau , Todd Toops, Brian West and Bruce Bunting.

Similarly, the work described herein was a finalist for an R&D 100 award in 2020. The researchers have applied for a patent for their new superlubricating coating.

“After that, we hope to work with industry to write a joint proposal to DOE to test, mature and license the technology,” Qu said. “In a decade we would like to see improved high-performance vehicles and power plants with less energy lost to friction and wear and tear.”

More information:
Chanaka Kumara et al, Macroscale Superlubricity by a Carbon Nanotube Sacrificial Coating, Nano Materials Today (2022). DOI: 10.1016/j.mtnano.2022.100297

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