Nov 24 2016
Images produced from computer simulations show the response of a graphene surface as a silicon tip slides over it. Relative forces of atomic friction on the surface are shown by colors: Red points are “pushing” sites that help propel the tip along the surface, while blue points are “pinning” sites of greater friction that inhibit the tip’s motion. Credit: Courtesy of the researchers.
Graphene is a two-dimensional sheet form of carbon with a thickness of just one atom. The exceptional electrical conductivity, strength, and chemical stability have paved the way for extensive research on graphene. Despite long-term research, specific basic characteristics of graphene - such as its reaction to a material sliding on its surface - are yet not completely understood.
Scientists at MIT and elsewhere have now employed intense computer simulations to understand processes such as variation in its friction when the object sliding on graphene moves forward, instead of remaining constant as it occurs in the case of many other known materials.
Ju Li, professor of nuclear science and engineering and of materials science and engineering at MIT, collaborated with seven researchers at MIT, the University of Pennsylvania, and universities in China and Germany and reported the outcomes in an article published in the journal Nature, this week.
It is well known that graphite is a solid lubricant and a bulk material made of many layers of graphene. In other words, similar to oil, graphite can be used to reduce the friction between contacting materials. A new study has proposed that effective lubrication can be ensured by adding just one or few graphene layers.
This may come in handy for small-scale electrical and thermal contacts and similar nanoscale devices. Here, it is important to understand the friction between graphene and another material, or between two graphene pieces to retain a good electrical, mechanical, and thermal connection.
Scientists had earlier discovered that it is better to add a few more layers of graphene if there is friction reduction in one layer of graphene on a surface. Yet, a valid explanation for this phenomenon was not given earlier, Li said.
“There is this broad notion in tribology that friction depends on the true contact area,” Li says - namely, the area at which two materials are actually in contact, at the atomic level. The “true” contact area is at many instances considerably smaller than it is when observed at larger size scales.
It is critical to find out the true contact area for perceiving the friction between the graphene pieces as well as other features such as heat transfer or electrical conduction.
When two parts in a machine make contact, like two teeth of steel gears, the actual amount of steel in contact is much smaller than it appears, because the gear teeth are rough, and contact only occurs at the topmost protruding points on the surfaces. If the surfaces were polished to be flatter so that twice as much area was in contact, the friction would then be twice as high. In other words, the friction force doubles if the true area of direct contact doubles.
Robert Carpick, University of Pennsylvania
However, the condition seems more complicated than perceived by the researchers. Li and his team discovered various other features of the contact influence the way in which friction force is transferred through it. “We call this the quality of contact, as opposed to the quantity of contact measured by the ‘true contact’ area,” Li explains.
Experimental investigations revealed that when a nanoscale material slides on a single graphene layer, initially the friction force raises, but it eventually levels off. While sliding on more and more graphene sheets, this effect diminishes and there is a decrease in leveled-off friction force.
Similar phenomenon was noted even in other layered materials such as molybdenum disulfide. Earlier efforts to account for such variation in friction, which is not evident in materials other than two-dimensional materials, proved inadequate.
It is crucial to know the accurate position of each atom on each of two surfaces in contact to establish the quality of contact, which largely depends not only on how well the atomic configurations are aligned in the two surfaces but also on the synchrony of the alignments. Li stated that based on the computer simulations, these factors were more important than the conventional measure in describing the frictional behavior of the materials.
You cannot explain the increase in friction” as the material begins to slide “by just the contact area. Most of the change in friction is actually due to change in the quality of contact, not the true contact area.
Ju Li, Professor, MIT
The scientists discovered that the act of sliding allowed the graphene atoms to form better contact with the object sliding on it. The consequent enhancement in the quality of contact causes initial increase in friction when sliding proceeds and leads to eventual level-off. As a single layer of graphene is highly flexible, the atoms can progress to locations of better contact with the tip, resulting in higher frictional effect.
Li explained that many factors, such as gas molecules getting in between two solid layers, slight curvatures, and rigidity of the surfaces, can impact the quality of contact. However, if the manner in which the process takes place is clearly understood, researchers can take work toward altering the frictional behavior to match a specific intended usage of the material.
For instance, “prewrinkling” of graphene increases its flexibility and enhances the quality of contact. “We can use that to vary the friction by a factor of three, while the true contact area barely changes,” he says.
“In other words, it’s not just the material itself” that determines how it slides, but also its boundary condition - including whether it is loose and wrinkled or flat and stretched tight, he says. In addition, such principles not only apply to graphene but also to other two-dimensional materials such as boron nitride, molybdenum disulfide, or other single-molecule-thick or single-atom-thick materials.
“Potentially, a moving mechanical contact could be used as a way to make very good power switches in small electronic devices,” Li says. But this will still take some time; though graphene’s potential is being widely studied, “we’re still waiting to see graphene electronics and 2-D electronics take off. It’s an emerging field.”
Researchers have studied the unique frictional behavior of graphene for many years, but the complex mechanisms underlying these observations are still not fully understood. This paper tackles the challenge head on and provides new insights into the origins of friction on graphene that I anticipate will be applicable to two-dimensional materials in general. The authors of the paper correctly suggest that their work could be used as a foundation for ‘tuning’ friction on graphene. Actually implementing this tuning has the potential for significant impact, and an exciting next step based on this research would be to implement the proposed tuning as a first step toward controllable friction in scientific and engineering applications.
Ashlie Martini, Associate Professor, University of California at Merced
Apart from Li and Carpick, the research group included former MIT and University of Pennsylvania visiting student Suzhi Li, who is now a Humboldt Research Fellow in Germany; Qunyang Li from Tsinghua University in China; Xin Liu from the University of Pennsylvania, who is now at Intel; Peter Gumbsch from Karlsruhe Institute of Technology in Germany; and Xiangdong Ding and Jun Sun from Xi’an Jiaotong University in China.
The National Science Foundation supported this research.