Chemical impurities make carbon surfaces superslippery

Engineers often treat impurities as a problem to eliminate in order to improve material performance. But new research from Osaka Metropolitan University and Fraunhofer Institute for Mechanics of Materials IWM suggests that in some cases, a little chemical messiness is exactly what helps materials slide more smoothly.

When two surfaces slide or rub against each other, friction occurs. Whilst friction is essential for many everyday applications, it also wears down machines, wastes energy, and limits the lifespan of moving parts. Therefore, research has focused on achieving superlow friction, or superlubricity, in which surfaces can slide past one another with exceptionally low resistance.

“While graphene- or graphite-like structures are known to enable nearly frictionless sliding, creating and maintaining such structures in practical systems remains challenging,” said Takuya Kuwahara, lecturer at Osaka Metropolitan University’s Graduate School of Engineering and lead author of the study.

Carbon has a lot of different structural forms including graphene, graphite, diamond, and amorphous carbon. However, the forms vary in their ability to slide.

Graphite is made of stacked graphene layers that can slide easily over each other, resulting in extremely low friction, whereas graphene consists of atomically thin carbon sheets. In contrast, diamond forms a rigid three-dimensional structure that makes it exceptionally hard and difficult to slide, whereas amorphous carbon lacks an ordered atomic arrangement.

Amorphous carbon interested the research because it can transform into graphitic, aromatic structures at points of contact between sliding surfaces.

This process, called shear-induced aromatization, raised the possibility of coatings that could form and even restore their own low-friction interfaces.

Yet, one question remained: Why does this transformation happen in some cases but not others?

To investigate, the researchers conducted a large-scale computational study using quantum-mechanical molecular dynamics simulations. They found that it was affected by chemical impurities.

“While impurities have often been associated with reduced material performance, we found that chemical impurities play a key and previously underappreciated role in enabling the formation of superlow-friction interfaces in amorphous carbon,” Kuwahara said.

The results of 1,000 simulations of sheared amorphous carbon containing different impurity elements showed that impurities with low valency, meaning they form fewer than four chemical bonds, consistently promoted the formation of graphitic, aromatic structures. Hydrogen and oxygen, in particular, enabled the emergence of stable low-friction interfaces. In contrast, pure carbon and silicon-doped systems failed to develop the same structures.

The researchers found that these impurities help stabilize tiny voids within the carbon network. Under continued mechanical stress, surrounding carbon atoms reorganize into aromatic ring structures resembling graphene or graphite. At the same time, the impurities prevent the material from reverting to harder, diamond-like arrangements, allowing slippery interfaces to persist.

The findings challenge the conventional view that impurities mainly degrade material performance and point to a new design strategy: carefully tuning the type and concentration of impurities to control how carbon coatings reorganize under stress. Instead of relying solely on external lubricants or pre-engineered graphitic coatings, future materials might generate low-friction surfaces autonomously during operation.

The researchers plan to test the mechanism under more realistic conditions, with combinations of multiple impurity elements, and under varying operating environmental factors such as pressure and temperature. Experimental validation of the predicted atomic-scale processes will also be an important step.

“Our ultimate goal is to contribute to the development of design strategies for carbon-based materials that can form and maintain ultralow-friction interfaces under real-world conditions,” Kuwahara said.  “Such materials could reduce wear, improve durability, and cut energy loss in mechanical systems across a wide range of technologies.”

The findings were published in Advanced Science.

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