(Nanowerk News) Scientists at the Department of Energy’s Oak Ridge National Laboratory have discovered a coating that can dramatically reduce friction in common load-bearing systems with moving parts, from train-driven vehicles to wind turbines and hydroelectric generators. This reduces the friction of the steel rubbing against the steel by at least a hundred times. The new ORNL lining could help lubricate the US economy which loses more than $1 trillion annually to friction and wear and tear — the equivalent of 5% of gross national product.
“As components slide past one another, friction and wear occur,” says Jun Qu, ORNL Tribology and Surface Engineering group leader. Tribology, from the Greek word for rubbing, is the science and technology of surfaces interacting 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 greater durability and reliability.”
With ORNL colleagues Chanaka Kumara and Michael Lance, Qu led the research published in Materials Today Nano (“Macroscale superlubricity with a sacrificial carbon nanotube layer”) about the coating consisting of carbon nanotubes which provide super lubrication to the sliding parts. Superlubricity is a property that exhibits almost no resistance to shear; its distinctive feature is the friction coefficient of less than 0.01. In comparison, when dry metals are sliding past one another, their coefficient of friction is about 0.5. With oil lubrication, the friction coefficient drops to about 0.1. However, the ORNL coating reduces the friction coefficient well below the limit for super lubrication, to as low as 0.001.
“Our main achievement is making superlubricity feasible for the most common applications,” said Qu. “Before, you would only see it in nanoscale or custom environments.”
For the research, Kumara grew carbon nanotubes on steel plates. With a machine called a tribometer, he and Qu made the plates rub against each other to produce carbon-nanotube shavings.
Multi-coated carbon nanotubes coat the steel, repel corrosive moisture, and serve as a lubricant reservoir. When first deposited, the vertically aligned carbon nanotubes stand on the surface like blades of grass. As the steel sections slide past one another, they are essentially “cutting the grass.” Each blade is hollow but made of multiple layers of rolled graphene, thin sheets of carbon atomically arranged in adjacent hexagons like chicken wire. Carbon nanotube debris cracked from shearing is deposited back onto the contact surface, forming graphene-rich tribofilms that reduce 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 structures, on the nanometer size scale. Second, we need to provide a carbon source to grow carbon nanotubes,” said Kumara. He heated a stainless steel disc to form metal-oxide particles on its surface. Then he used chemical vapor deposition to incorporate the carbon in the form of ethanol so that the metal-oxide particles could stitch the carbon there, atom by atom in the form of nanotubes.
New nanotubes don’t provide super lubrication until they break. “Carbon nanotubes crumble when rubbed but are a novelty,” said Qu. “The key part is that the cracked carbon nanotubes are bits of graphene. Those graphene pieces are smeared and connected to the contact areas, becoming what we call tribofilms, the layers that form during the process. Then the two contact surfaces are covered by several layers of graphene rich. Now, when they rub against each other, it’s graphene on graphene.”
The presence of even one drop of oil is essential to achieve superlubricity. “We tried it without oil; it doesn’t work,” said Qu. “The reason is, without oil, friction removes carbon nanotubes too aggressively. Then the tribofilm cannot form properly or last long. Like an engine without oil. It smokes within minutes, whereas one with oil can easily run for years.
The superior slippery properties of the ORNL coating provide durability. Superlubricity stands the test of more than 500,000 scrubbing cycles. Kumara tested the gliding performance continuously for three hours, then one day and then 12 days. “We still have superlubricity,” he said. “It’s stable.”
Using an electron microscope, Kumara examined the cut pieces to prove that tribological wear had broken the carbon nanotubes. To independently confirm that scrubbing shortens nanotubes, co-author ORNL Lance used Raman spectroscopy, a technique that measures vibrational energy, which is related to atomic bonds and the crystal structure of a material.
“Tribology is a very old field, but modern science and engineering are providing new scientific approaches to advancing technology in this field,” said Qu. “The fundamental understanding was shallow until maybe the last 20 years, when tribology got a new lease on life. It’s only recently that scientists and engineers have really come together to use more advanced materials characterization technologies — that’s the strength of ORNL. Tribology is highly multidisciplinary. No one is an expert at everything. Therefore, in tribology, the key to success is collaboration.”
He adds, “Somewhere you can find a scientist with expertise in carbon nanotubes, a scientist with expertise in tribology, a scientist with expertise in material characterization. But they are isolated. Here at ORNL, we are together.”
ORNL’s tribology team has done award-winning work that has attracted industry partnerships and licenses. In 2014, an ionic anti-wear additive for fuel-efficient engine lubricants, developed by ORNL, General Motors, Shell Global Solutions and Lubrizol, won the R&D 100 award. ORNL collaborators are Qu, Huimin Luo, Sheng Dai, Peter Blau, Todd Toops, Brian West and Bruce Bunting.
Similarly, the work described in the current paper is a finalist for the R&D 100 award in 2020. And the researchers have filed a patent for their new superlubricity coating.
“Next, we look forward to partnering with the industry to write a joint proposal to the DOE to test, mature, and license the technology,” said Qu. “In a decade we want to see increased vehicle and power plant performance with less energy lost to friction and wear and tear.”