MXenes make durable solid lubricants

A thin coating of 2-D nanosheets has proved to be the most wear-resistant solid lubricant yet identified, according to researchers who put the titanium carbide–based material through its paces (ACS Nano 2021, DOI: 10.1021/acsnano.1c01555). “In terms of durability, we outperform all the other solid lubricants,” says Philipp G. Grützmacher of TU Vienna, one of the researchers who led the work.

The coating is part of a large family of materials known as MXenes. These transition-metal carbides or nitrides contain layers just a few atoms thick that are covered with oxygen, fluorine, or hydroxyl groups. Like their 2-D cousin graphene, MXenes are being tested in applications such as sensing, catalysis, and energy storage.

Now there is growing interest in using MXenes as lubricants. Graphite and molybdenum disulfide are already widely used as solid lubricants, and their 2-D forms offer even lower-friction coatings, thanks to the way their layers slide easily against one another. But these 2-D materials often wear off too quickly to be useful in practical applications. In contrast, MXenes could be robust enough to act as solid lubricants in high-temperature industrial processes or in space-based applications where oil isn’t compatible with the high-vacuum conditions.

Grützmacher and his colleagues made their low-friction coating by stirring titanium aluminum carbide in hydrofluoric acid, which strips out aluminum and leaves an MXene powder. Then they used an electrospraying process to cover a small piece of stainless steel with a 100 nm thick covering of the MXene.

The researchers slid pea-sized balls of silicon nitride against this coated surface under dry air, rubbing them back and forth 100,000 times—a workout that Grützmacher says is the most extensive test of an MXene solid lubricant to date.

They found that the MXene coating offered a six-fold reduction in friction between the stainless steel and the balls relative to untreated surfaces, which puts it in the same low-friction ballpark as graphene and molybdenum disulfide. Crucially, the MXene coating maintained its performance throughout the 100,000 cycles, offering at least twice the wear life of rival 2-D materials. “We get low friction for a very extended amount of time,” Grützmacher says.

Electron microscopy revealed that the wear tests did actually strip away some of the MXene coating, creating piles of MXene flakes at each end of the abrasion tracks. But these piles acted as reservoirs that continuously supplied fresh lubricant to the contact points, keeping friction low.

“I think it’s very important work, because it shows the potential of MXenes,” says Vadym N. Mochalin of Missouri University of Science & Technology. Mochalin has also studied the lubricating effects of titanium carbide MXenes, recently reporting even lower friction—albeit with poorer wear resistance—in a coating rubbed against diamond-like carbon under a nitrogen atmosphere (Materials Today Advances 2021, DOI: 10.1016/j.mtadv.2021.100133).

The sheer variety of possible MXenes means that even better lubricants could be just around the corner, and Grützmacher’s team is using computational techniques to identify other low-friction MXenes for testing in the lab. “The number of MXenes is huge, and we’re just making the first steps into that territory,” Mochalin says. “We’re discovering so many interesting properties faster than we can explain them.”