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Author Lisa Zyga, Phys.org

(Phys.org)-In the phenomenon of super lubrication, two solid surfaces can slide against each other with almost no friction. This effect occurs when solid surfaces have a crystalline structure and their crystal lattice rotates in a way that counteracts friction. It's a bit like stacking two egg cartons. If the grids are aligned, they will lock to each other and it will be difficult to slide on the other. But if you turn an egg carton a little bit, it no longer locks in this way.

Scientists first observed the super-lubricity of graphite in 2004. So far, all experimental evidences of super-lubricity have been obtained under nano-scale and vacuum conditions. Previous research has even predicted that super lubricity will fail in a wider range. But now in a new study, scientists have proved that the super-lubricity of graphite can occur in micro-scale areas and environmental conditions, which may open the way for the practical application of micromechanical systems.

Researchers led by Zheng Quanshui from Tsinghua University in Beijing and Nanchang University in Nanchang, China, and Jefferson Zhe Liu from Monash University in Clayton, Australia, published a paper on the micro-scale superlubricity of graphite in the latest issue of Physical Review Letters. .

"We are providing evidence of ultra-lubricity on a larger scale than before-micrometers rather than nanometers-that this effect persists even under environmental conditions," Zheng told Phys.org. "At the time of the first measurement, we didn't know the previous work of predicting that the effect would collapse-maybe this was lucky, because it didn't stop us from trying!"

The most direct way to observe super-lubricity is when two solid surfaces slide against each other. In the current study, researchers have developed a new method to explore ultra-lubricity by using tungsten microtips to cut thin slices from a graphite plate or "table". After the shear is released, some slices will spontaneously return to their original positions on the table. This process of shearing and self-retraction can be repeated over and over again.

The scientists explained that self-retraction is caused by the ultra-low friction that occurs when the sheet and table surface are oriented in a distorted or uncoordinated manner.

Although the self-retracting flakes return to the same position and orientation as they were before shearing, the researcher can deliberately rotate the sheared flakes before releasing them to produce the corresponding direction, resulting in a locked state. In this case, the flakes Will not show self-retraction. These locked states occur in some specific directions with 6-fold symmetry, but self-retraction will still occur when the table is sheared in all other directions.

When investigating non-shrinking flakes, the researchers found that the color of the sheared flakes suddenly changed, while the color of the self-shrinking flakes was uniform. They believe that the color change occurs due to optical interference caused by the thickness change of the graphite mesa. Larger tables have greater thickness variations and are less likely to shrink.

As the first evidence of repeatable super-lubricity on the micrometer scale, even under environmental conditions, the results may prove to be more suitable for applications than super-lubricity on the nanometer scale. At the nanometer scale, achieving super lubrication requires complex setup and sample preparation, and this effect can easily be suppressed by various mechanisms that cause distortion and lock-up. New methods for generating super-lubricity on the microscale overcome many of these obstacles and can be used to limit friction and wear in micromechanical systems.

"There are many micromechanical devices-such as motion sensors, radio frequency generators, gyroscopes-where the relative motion of the two components is important," Liu said. "Super lubrication opens up a new way to manufacture this type of equipment."

The researchers plan to further explore the degree of super lubrication in the future.

"We are already working on several areas: studying ultra-lubricated motion in more detail, exploring its extension to larger and smaller scales, and studying the long-term robustness of effects under different physical conditions," said the co-author. Francois Gray of Tsinghua University. Further explore and discover new methods of controlling conductivity. For more information: Ze Liu, et al. "Observation of ultra-lubricity in graphite at the micro-scale." PRL 108, 205503 (2012). DOI: 10.1103/PhysRevLett.108.205503 Journal information: Physics Review Letters

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