Super Lubricating Coating Reduces Friction

Super Lubricating Coating Reduces Friction

Scientists at Oak Ridge National Laboratory have invented a coating that can significantly reduce friction in common load-bearing systems with moving parts, from vehicle drivetrains to wind and hydroelectric turbines. The coating reduces steel-on-steel friction by at least a hundred times. ORNL's new coating could help the U.S. economy, which loses more than $1 trillion annually to friction and wear—equivalent to 5% of gross domestic product. "Friction and wear occur when components slide over one another," said Jun Qu, leader of ORNL's Surface Engineering and Tribology group. Tribology, derived 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, we can extend the life of the system for better durability and reliability." Working with colleagues Chanaka Kumara and Michael Lance from ORNL, Qu led a study published in Materials Today Nano about a coating made of carbon nanotubes that imparts superlubricity to sliding parts. Superlubricity is the property of exhibiting almost no resistance to sliding, distinguished by a friction coefficient below 0.01. In contrast, when dry metals slide over one another, the friction coefficient is around 0.5. With a lubricant, the friction coefficient drops to approximately 0.1. However, ORNL's coating has reduced the friction coefficient well below the threshold established for superlubricity, down to 0.001. "Our main achievement is making superlubricity applicable for the most common applications," said Qu. "Previously, you could only see this at the nanoscale or in special environments." For the study, Kumara grew carbon nanotubes on steel plates. He and Qu rubbed the plates against each other using a machine called a tribometer to produce carbon nanotube shavings. Multiwalled carbon nanotubes coat the steel, repel corrosive moisture, and function as a lubricant reservoir. When first deposited, the vertically aligned carbon nanotubes stand on the surface like blades of grass. As steel parts slide past each other, they essentially "mow the grass." Each blade is hollow but made of multiple rolled graphene layers—an atomically thin carbon layer arranged in adjacent hexagons like chicken wire. Carbon nanotube debris broken during shaving is redeposited on the contact surface, creating a tribofilm rich in graphene that reduces friction to near zero. Making carbon nanotubes is a multistep process. "First, we need to activate the steel surface to produce small structures at the nanometer scale. Second, we need to provide a carbon source to grow the carbon nanotubes," said Kumara. He heated a stainless steel disk to create metal oxide particles on the surface. He then used chemical vapor deposition to add carbon in ethanol form, allowing the metal-oxide particles to build carbon atom-by-atom into nanotubes. The new nanotubes don't provide superlubricity until they are damaged. "The carbon nanotubes disappear during friction but become something new," said Qu. "The key part is that these broken carbon nanotube pieces are graphene. These graphene pieces contaminate and bind to the contact area and become what we call a tribofilm, a coating that forms during the process. Then both contact surfaces are coated with a graphene-rich coating. Now, when they slide against each other, it's graphene on graphene." The presence of even a drop of oil is critical to achieving superlubricity. "We tried without oil; it didn't work," said Qu. "This is because without oil, friction strips away the carbon nanotubes too aggressively. Then the tribofilm cannot form nicely or survive for long. Like an engine without oil. It smokes in minutes, whereas an oiled engine can run smoothly for years." ORNL's coating has permanent superlubricity. Superlubricity persisted through more than 500,000 friction cycles in testing. Kumara tested performance for three hours continuously, then one day, and later for 12 days of continuous sliding, and "We still have stable superlubricity," he said. Kumara used an electron microscope to examine cut samples to prove that tribological wear broke apart the carbon nanotubes. To independently confirm that friction shortened the nanotubes, ORNL coauthor Lance used Raman spectroscopy, a technique that measures vibrational energy related to a material's atomic bonding and crystal structure. "Tribology is a very old field, but modern science and engineering has provided a new scientific approach to advance technology in this area," said Qu. "The fundamental understanding is packed into perhaps the last 20 years, when tribology has had a new life. Recently, scientists and engineers really came together to use more advanced material characterization technologies—this is a strength of ORNL. Tribology is very multidisciplinary. No one is an expert in everything. Therefore, the key to success in tribology is collaboration. You might find in one place a scientist who is an expert in carbon nanotubes, a scientist who is an expert in tribology, a scientist who is an expert in material characterization. But these are isolated. Here, at ORNL, we are together." ORNL's tribology teams have conducted award-winning work that has attracted industrial partnerships and licensing interest. In 2014, ORNL won an R&D 100 award for an ionic anti-wear additive for fuel-efficient motor oils developed with General Motors, Shell Global Solutions, and Lubrizol. ORNL's collaborators on that work were Qu, Huimin Luo, Sheng Dai, Peter Blau, Todd Toops, Brian West, and Bruce Bunting. Similarly, the work described in the current article was a finalist for an R&D 100 award in 2020. The researchers have filed for a patent on the 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," said Qu. "Within a decade, we want to see high-performance vehicles and power plants that lose less energy to friction and wear." Academic Reference: Chanaka Kumara et al, Macroscale superlubricity by a sacrificial carbon nanotube coating, Materials Today Nano (2022). DOI: 10.1016/j.mtnano.2022.100297 Source