The atomic layers of MXene can move relative to one another, reducing friction. Image: TU Wien.
The atomic layers of MXene can move relative to one another, reducing friction. Image: TU Wien.

You can lubricate a bicycle chain with oil, but what do you do with a Mars rover or a red-hot conveyor belt in the steel industry? In a new study, researchers from the Vienna University of Technology (TU Wien) in Austria, together with colleagues from Purdue University, Saarbrücken University in Germany and the Universidad de Chile in Santiago, Chile, have shown that one solution could come in the form of a class of nanomaterial known as MXenes.

MXenes (pronounced 'maxenes') have caused quite a stir in recent years in connection with novel battery technologies. But it now turns out they can also make an excellent solid lubricant that is extremely durable and performs its task even under the most difficult conditions. The researchers report these remarkable properties of MXenes in a paper in ACS Nano.

Just like the carbon material graphene, MXenes are 2D materials: their properties are essentially determined by the fact that they comprise single atomic layers without strong bonds to the layer above or below.

"You first start with so-called MAX phases, which are special layer systems consisting of titanium, aluminium and carbon, for example," explains Carsten Gachot, head of the Tribology Group in the Institute of Engineering Design and Product Development at TU Wien. "The crucial trick is to etch out the aluminium with hydrofluoric acid."

What remains is a stack of atomically thin layers of titanium and carbon that lie loosely on top of each other, much like sheets of paper. Each layer is relatively stable on its own, but the layers can easily be shifted against each other.

This displaceability of the atomic layers makes the material an excellent dry lubricant, allowing it to generate extremely low-resistance sliding without abrasion. In their study, the researchers found that MXenes could reduce friction between steel surfaces to just one sixth the original value. They also had exceptionally high wear resistance: even after 100,000 movement cycles, the MXene lubricating layer still functioned without problems.

This is perfect for use in challenging conditions. For example, while lubricating oil would evaporate immediately in a vacuum during space missions, a fine powder of MXene would make an effective, long-lasting lubricant.

"Similar things have been tried with other thin-film materials, such as graphene or molybdenum disulphide," says Gachot. "But they react sensitively to moisture in the atmosphere. Water molecules can change the bonding forces between the individual layers. With MXenes, on the other hand, this plays a lesser role."

Another decisive advantage is the heat resistance of MXenes. "Many lubricants oxidize at high heat and lose their lubricity," says Gachot. "MXenes, on the other hand, are much more stable, and can even be used in the steel industry, where mechanically moving parts can sometimes reach a temperature of several hundred degrees Celsius."

"There is also already great interest in these materials on the part of industry," he adds. "We assume that such MXenes can soon be produced on a larger scale."

This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.