(Top) Illustration of a mesa being partially sheared with a micromanipulator to form a self-retracting flake on a graphite platform. When the microtip is raised to release the flake, it automatically returns to its original position on the mesa. (Middle) Observation of this process in a vacuum in a SEM. (Bottom) Observation of the same process under ambient conditions with an optical microscope. Image credit: Liu, et al. ©2012 American Physical Society
In the phenomenon of superlubricity, two solid surfaces can slide past each other with almost no friction. The effect occurs when the solid surfaces have crystalline structures and their lattices are rotated in such a way as to cancel out the friction force. A bit like stacking two egg cartons, if the lattices are aligned, they lock in to each other and it is hard to slide one over the other. But rotate one egg carton a bit, and it no longer locks in this way.
Scientists first observed superlubricity in graphite in 2004, and so far all experimental evidence of superlubricity has been obtained on the nanoscale and under vacuum conditions. Previous research even predicted that superlubricity breaks down on larger scales. But now in a new study, scientists have demonstrated that superlubricity in graphite can occur over microscale areas and under ambient conditions, which could open the way toward practical applications in micromechanical systems.
The researchers, led by Quanshui Zheng from Tsinghua University in Beijing and Nanchang University in Nanchang, China, and Jefferson Zhe Liu from Monash University in Clayton, Australia, have published their paper on the microscale superlubricity in graphite in a recent issue of Physical Review Letters.
“We are providing evidence of superlubricity on a much larger scale than previously – micro rather than nano – and the effect persists even in ambient conditions,” Zheng told Phys.org. “We were not aware of the previous work predicting breakdown of the effect at the time the first measurements were done – perhaps that was fortunate, as it did not dissuade us from trying!”
via Friction almost vanishes in microscale graphite.
