Friction between moving parts can significantly degrade mechanical components and devices. The introduction of graphene-enabled superlubricity, a state of ultra-low friction between two surfaces, could reduce wear and tear and enable longer-lasting, more durable dry-lubricated machinery. This could be particularly important for the next generation of wind turbines, electrical switches, and micro- and nano-electromechanical systems.
We interviewed Costas Galiotis, Graphene Flagship Work Package Leader for Composites, based at Graphene Flagship partner FORTH, Greece, who speaks about his research and emphasises the potential of superlubric graphene as a new and effective dry lubricant for moving components.
What is superlubricity and how does it arise?
Superlubricity is a state of motion in which friction is very low, or even vanishes – although there is not a strict definition from a quantitative perspective. Roughly speaking, for superlubric behavior, the friction coefficient needs to be less than 0.01.
Superlubricity arises when two crystalline surfaces are in an incommensurate stacking state: in other words, the two surfaces slip and slide against each other under dry conditions, with very low friction.
Why superlubricity a useful property?
When two solid surfaces in contact are subjected to relative motion, friction between them converts kinetic energy to thermal energy. Friction can cause serious wear problems by damaging moving components.
Superlubricity is a state of motion that generates ultra-low friction and, therefore it significantly suppresses friction-related wear and tear problems. Superlubricity has important implications for practical applications, such as in the dry lubrication of mechanical drive components like micro/nano-electromechanical systems (MEMS/NEMS), in electrical switches and in wind energy components.
How can graphene enable new superlubric components?
If we can better understand the interlayer shear stress reduction mechanism in a graphene–graphene system, such as bilayer graphene, then we can better exploit it as a superlubricant for state-of-the-art applications to minimize both energy loss and damage from wear and tear – which would improve the lifetime of the products and components as well.
Is it a challenge to introduce superlubricity to a graphene-based material such as bilayer graphene?
Yes. In graphite, which is considered a lubricant, there are many commensurate stacking domains, which lead to mechanical interlocking between the graphene layers – meaning there is high friction between them. Equally, in bilayer graphene, the two layers are strongly bonded to each other, and the friction is much higher – confirmed by the presence of a single characteristic 2D peak in the Raman spectrum.
Do you have any recent advances in this area?
We recently investigated how graphene could enable superlubric components for mechanical devices. We tested bilayer graphene in an incommensurate state, where graphene layers are stacked randomly by sequential transferring.
In bilayer graphene, the two layers are normally strongly bonded to each other, meaning the friction is higher. When the two layers are randomly stacked, they slide against each other with minimal friction. We confirmed this by measuring the interlayer shear stresses, which were very low, and found that the friction between the graphene layers drops to almost zero – as long as they are randomly stacked.
These graphene layers exhibited superlubricity.
What causes this behavior?
Using Raman spectroscopy, we found that in graphene with disordered stacking, less than half of the total strain applied to the bottom layer is transferred to the top layer, meaning the layers can freely slip and slide against each other. After verifying the mechanism using computational molecular dynamics simulations, we demonstrated the phenomenon in practice by coating two surfaces with disordered single-layer graphene. In line with our predictions, friction between them substantially decreased when we applied a strain. This persists even for large graphene monolayers produced by chemical vapour deposition (CVD) – as long as they are randomly stacked, paving the way for us commercial applications that use CVD graphene sheets.
What applications could graphene-based superlubricity have?
This technology could lead to a new generation of dry lubricants based on graphene and layered materials. The lubricants will also play a key role in the Graphene Flagship's Circuit Breakers project. One of a series of eleven new industry-focused Spearhead Projects, the initiative aims to develop maintenance-free circuit breakers for new, flexible and smart power distribution systems. The circuit breakers' moving components will be coated with graphene and layered material lubricants, enhancing their durability, lifetime and more.
"Tunable macroscale structural superlubricity in two-layer graphene via strain engineering," C. Androulidakis, E.N. Koukaras, G. Paterakis, G. Trakakis and C. Galiotis., Nature Comm., 11, 1595 (2020), DOI: 10.1038/s41467-020-15446-y