Scientists at the Department of Energy’s Oak Ridge National Laboratory have invented a coating that could dramatically reduce friction in common load systems with moving parts, from vehicle drive trains to wind and hydroelectric turbines. It reduces the friction of steel rubbing on steel at least a hundredfold. The new ORNL coating could help fuel an American economy that loses more than a trillion dollars each year to friction and wear and tear, equivalent to 5% of the gross national product.
“When components slide against each other, there is friction and wear,” said Jun Qu, leader of ORNL’s surface engineering and tribology group. Tribology, from the Greek word for rub, 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. By reducing wear and tear, we can extend the life of the system for better durability and reliability.”
With ORNL colleagues Chanaka Kumara and Michael Lance, Qu led a study published in Materials Today Nano on a coating composed of carbon nanotubes that imparts superlubricity to the sliding parts. Superlubricity is the property of showing virtually no resistance to sliding; its hallmark is a friction coefficient of less than 0.01. In comparison, when dry metals slide against each other, the coefficient of friction is about 0.5. With an oil lubricant, the coefficient of friction drops to about 0.1. However, the ORNL coating reduced the coefficient of friction well below the cutoff for superlubricity, down to 0.001.
“Our main achievement is that we make superlubricity feasible for the most common applications,” Qu said. “Before, you only see it in nanoscale or specialized environments.”
For the study, Kumara grew carbon nanotubes on steel plates. Using a machine called a tribometer, he and Qu made the plates rub against each other to generate shavings of carbon nanotubes.
Multi-walled carbon nanotubes coat the steel, repel corrosive moisture and act as a lubricant reservoir. When first deposited, vertically aligned carbon nanotubes lie on the surface like blades of grass. When the steel pieces slide against each other, they essentially “cut the grass”. Each leaf is hollow, but made of multiple layers of coiled graphene, an atomically thin sheet of carbon arranged in adjacent hexagons like chicken wire. Fractured carbon nanotube debris from the chip is redeposited on the contact surface, forming a graphene-rich tribofilm that reduces friction to almost zero.
The manufacture of carbon nanotubes is a multi-step process. “First, we need to activate the steel surface to produce tiny structures, on the nanometer size scale. Second, we need to provide a source of carbon to grow the carbon nanotubes,” Kumara said. He heated a stainless steel disc to form metal oxide particles on the surface. He then used chemical vapor deposition to introduce carbon in the form of ethanol so that the metal oxide particles can sew carbon into it, atom by atom in the form of nanotubes.
The new nanotubes do not provide superlubricity until they are damaged. “Carbon nanotubes are destroyed by friction, but they become something new,” Qu said. “The key part is that the fractured carbon nanotubes are pieces of graphene. These pieces of graphene are smeared and connected at the contact area, becoming what we call a tribofilm, a coating formed during the process. Then, both contact surfaces are covered by a graphene-rich coating. Now, when they rub against each other, it’s graphene on graphene.”
The presence of an oil drop is crucial to achieve super lubricity. “We tried it without oil; it didn’t work,” Qu said. “The reason is that, without oil, friction removes the carbon nanotubes too aggressively. Then the tribofilm cannot form well or survive for long. It’s like an engine without oil. It smokes in minutes, while one with oil can easily run for years.”
The superior slippery of the ORNL coating has an endurance capacity. Super lubricity persisted in tests of more than 500,000 rubbing cycles. Kumara tried continuous sliding performances for three hours, then a day and later 12 days. “We still have super lubricity,” he said. “It’s stable.”
Using electron microscopy, Kumara examined the cut fragments to show that tribological wear had cut 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 atomic bonding and crystal structure of a material
“Tribology is a very old field, but modern science and engineering provided a new scientific approach to advance technology in this area,” Qu said. “Fundamental understanding has been shallow until the last 20 years, when tribology got a new lease of life. More recently, scientists and engineers have really come together to use the most advanced material characterization technologies, which is a strong point of at ORNL. Tribology is very multidisciplinary. No one is an expert at everything. So in tribology, the key to success is collaboration.”
He added, “Somewhere, you can find a scientist with experience in carbon nanotubes, a scientist with experience in tribology, a scientist with experience in materials characterization. But they are isolated. Here at ORNL, we are together.”
ORNL’s tribology teams have done award-winning work that has attracted industry associations and licensing. 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 contributors were Qu, Huimin Luo, Sheng Dai, Peter Blau, Todd Toops, Brian West, and Bruce Bunting. The Office of Vehicle Technologies in DOE’s Office of Energy Efficiency and Renewable Energy, or EERE, sponsored the research.
Similarly, the work described in the current paper was a finalist for an R&D 100 award in 2020. And the researchers have applied for a patent on their new superlubricity coating.
“Next, we hope to partner with industry to write a joint proposal to the DOE to test, mature and license the technology,” Qu said. “A decade from now we’d like to see high-performance vehicles and improved power plants with less energy lost through friction and wear.”
The title of the paper is “Macroscale superlubricity using a sacrificial carbon nanotube coating”.
The ORNL Laboratory Directed Research and Development Seed Program provided initial support for the proof-of-concept work. The DOE EERE Office of Solar Energy Technologies and Vehicle Technologies Office then supported further research.
UT-Battelle manages ORNL for the DOE Office of Science, the leading proponent of basic physical science research in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, visit energy.gov/science. — Dawn Levy
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