Founder of TriboNet, Editor, PhD (Tribology), Tribology Scientist at ASML, The Netherlands. Expertise in lubrication, friction, wear and contact mechanics with emphasis on modeling. Creator of Tribology Simulator.
In Situ Generation of Graphene
25.07.2016
Graphene has unique properties and is being extensively used in various applications. It got a deserved attention in the field of tribology as well and was reported to lead to the states of superlubricity (see Macroscale superlubricity, Reduce the Friction with Graphene Balls, Superlubricity in graphene nanoribbon – gold interface).
An international team of researchers from China and United Kingdom has recently reported a use of graphite as an additive to a Esterified Bio-Oil. Friction and wear properties were reported to be enhanced noticeably. It was found, that the graphite particles undergo a friction induced transformation into a high-quality graphene layers. This process is a mechanical exfoliation of graphene from graphite due to shear. These graphene sheets further were adsorbed or reacted to the surfaces to create a so-called tribofilm. The tribofilm has a sacrificial nature and prevent the wear of the bare materials and decreased friction. It was clearly identified, that the transformation process is highly linked to the applied conditions, namely load and sliding speed. With the increase of the applied load, smaller graphene particles were obtained and created a thicker tribofilm. This leaded to the decrease of the wear and friction. With the increase of the sliding speed, in turn, the wear increased. Although the tribofilm was found to be thicker for the higher sliding speed, the quality of it was reported to be poorer and it was destroyed more easily.
The reported in situ generation of graphene may have great prospects in tribology to reduce friction. More details of the research can be found in the original article: “Friction-Induced Transformation from Graphite Dispersed in Esterified Bio-Oil to Graphene”, Y. Xu, J. Geng, X. Zheng, K.D.Dearn, X. Hu.
Credit for image: www.rcs.org.
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The right idea, with the wrong execution.
Simple mechanical exfoliation of graphite is certainly not (bottom-up) synthesis of graphene from chemical precursors contained in the lubricant.
I’m sure there is no argument that graphene is the worlds best tribofilm material, but tribofilms don’t remove asperities! True superlubricity will only be achieved in the absence of asperities. For that scenario, an in-situ nanopolishing agent must be had to remove the source of friction on the interacting surfaces.
I would also suggest that the unexfoliated (suspended solid) graphite in oil would be met with the same objections from engineers as its graphite-in-oil predecessors were.
In short, better to actually synthesize the graphene in-situ, then permanently polish away the asperities, leaving a nice protective graphitic tribofilm in their place.
I am not really familiar with graphite lubrication, only that it has been used as a lubricant for a long time now. So I guess, graphene has been forming there all the time, its just now has been proved it forms in the contact and this formation is stress activated.
No, I don’t believe that previous attempts at in-situ mechanical exfoliation of graphite in lubricants was successful, due to the nature of both the graphite starting material (not already partially exfoliated) and a lack of understanding of the criticality of the precise ratio (O.1%) of “graphene” to base lubricant to be efficacious.
Previously, the “slippery” nature of graphite was observed and recognized, but poorly understood with regard to the existence and unique physical properties of graphene. This (in my opinion) is still poorly understood, as simple addition of partially exfoliated graphite to lubricants is far inferior to synthesis of graphene inside the lubricated system.