Scientists at the U.S. Department of Energy’s Oak Ridge National Laboratory (ORNL) (Oak Ridge, Tennessee, USA) have invented a novel coating that they believe could dramatically reduce friction in common load-bearing systems with moving parts. Potential applications range from vehicle drive trains to wind and hydroelectric turbines.
According to ORNL, the coating reduces the friction of steel rubbing on steel at least a hundredfold. The novel technology could help grease a U.S. economy that each year loses more than $1 trillion to friction and wear, or equivalent to 5% of the gross national product.
“When components are sliding past each other, there’s friction and wear,” says Jun Qu, leader of ORNL’s surface engineering and tribology group. Tribology, from the Greek word for rubbing, 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,” Qu adds. “If we reduce wear, we can elongate the life span of the system for better durability and reliability.”
With ORNL colleagues Chanaka Kumara and Michael Lance, Qu led a study published in the March 2023 issue of Materials Today Nano about a coating composed of carbon nanotubes that imparts superlubricity to sliding parts. Superlubricity is described as the property of showing virtually no resistance to sliding, with its hallmark being a coefficient of friction of 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 and to as low as 0.001.
“Our main achievement is we make superlubricity feasible for the most common applications,” Qu says. “Before, you’d only see it in either nanoscale or specialty environments.”
Specifics of the Study
For the study, Kumara grew carbon nanotubes on steel plates. With a machine called a tribometer, he and Qu 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.
Making the carbon nanotubes is a multistep process.
“First, we need to activate the steel surface to produce tiny structures, on the size scale of nanometers,” Kumara says. “Second, we need to provide a carbon source to grow the carbon nanotubes.”
To do this, Kumara heated a stainless-steel disk to form metal-oxide particles on the surface. He then used chemical vapor deposition to introduce carbon in the form of ethanol so that metal-oxide particles can stitch carbon there, going atom by atom in the form of nanotubes.
The new nanotubes do not provide superlubricity until they are damaged.
“The carbon nanotubes are destroyed in the rubbing but become a new thing,” Qu says. “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.”
Analysis of the Study
The presence of even one drop of oil is crucial to achieving superlubricity.
“We tried it without oil; it didn’t work,” Qu says. “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.”
The ORNL coating’s superior slipperiness has staying power. According to the researchers, superlubricity persisted in tests of more than 500,000 rubbing cycles. Kumara tested the performances for continuous sliding over three hours, then for one day and later for 12 days.
“We still got superlubricity,” he says. “It’s stable.”
Using electron microscopy, Kumara examined the mowed fragments to prove that tribological wear had severed the carbon nanotubes. To independently confirm that rubbing had shortened the nanotubes, Lance used Raman spectroscopy, a technique that measures vibrational energy. This is related to the 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 says. “The fundamental understanding has been shallow until the last maybe 20 years, when tribology got a new life. More recently, scientists and engineers really came together to use the more advanced material characterization technologies—that’s an ORNL strength. Tribology is very multidisciplinary. No one is an expert in everything. Therefore, in tribology, the key to success is collaboration.
“Somewhere, you can find a scientist with expertise in carbon nanotubes, a scientist with expertise in tribology, a scientist with expertise in materials characterization,” Qu adds. “But they are isolated. Here at ORNL, we are together.”
Research History and Future Steps
The title of the paper is “Macroscale Superlubricity by a Sacrificial Carbon Nanotube Coating,” and the work described within it was a finalist for an R&D 100 award in 2020. Since then, the researchers have applied for a patent for their novel superlubricity coating.
“Next, we hope to partner with industry to write a joint proposal to DOE to test, mature, and license the technology,” Qu says. “In a decade, we’d like to see improved high-performance vehicles and power plants with less energy lost to friction and wear.”
ORNL’s Laboratory Directed Research and Development Seed Program provided the initial funding support to the proof-of-concept work. Then the Solar Energy Technologies Office and Vehicle Technologies Office in the U.S. Department of Energy’s (DOE) Office of Energy Efficiency and Renewable Energy supported follow-on research.
The University of Tennessee and Battelle Memorial Institute co-manage ORNL for the DOE through UT-Battelle, a 50-50 limited liability partnership. UT-Battelle conducts its work for the DOE’s Office of Science, described as the single largest supporter of basic physical sciences research in the United States. More information is available at www.energy.gov/science.
Source: ORNL, www.ornl.gov.