Diamond puts a new shine on friction-free graphene

A novel combination of graphene wrapped around nanodiamond particles can reduce friction to near zero, according to researchers from Argonne National Laboratory [Berman et al., Science 348 (2015) 1118, http://dx.doi.org/10.1126/science.1262024]. Anirudha V. Sumant and his colleagues have reported sustained macroscale superlubricity – or an almost complete absence of friction between two sliding surfaces – with graphene and diamond-like carbon (DLC)-coated surfaces.

“We have shown, for the first time, superlucbricity at the macroscale using various combinations of carbon materials,” says Sumant. “Achieving superlubricity is very important from the practical point of view since energy is wasted due to friction in various applications in our daily life.”

Friction and wear are among the leading causes of energy loss and damage in mechanical equipment like pumps, compressors, and turbines. Reducing friction even a small amount can lead to dramatic improvements in fuel consumption. In gasoline-fueled cars, for example, nearly 17% of energy is wasted on friction, so improvements could have significant financial and environmental implications.

Superlubricity has been demonstrated on the nanoscale, using multiwalled carbon nanotubes for example, but has proved more elusive on larger scales. Now, Sumant and his colleagues have reported a dramatic reduction in friction on the macroscale using a combination of graphene flakes and nanodiamond on a SiO₂ surface. Superlubricity between the graphene- and DLC-coated surfaces was retained even when the load, velocity of movement, and temperature was varied.

Using transmission electron microscopy (TEM), the researchers discovered that the graphene flakes wrap around nanodiamond particles forming nanoscale ‘scrolls’. During sliding, the researchers believe the nanodiamond particles facilitate scroll formation for two reasons: dangling bonds on the particles adhere to the edges of the graphene flakes and the three-dimensional nature of the particles acts as a physical barrier to the sheets. The nanoscrolls decrease friction by reducing the contact area between the graphene and DLC surfaces, while van der Waals forces stabilize the structure itself.

“Graphene plays a unique role,” explains Sumant. “Being just one-atom thick, it is flexible and easily forms scrolls around nanodiamonds. Graphene is also inert, providing very low adhesion energy with DLC, over which it slides easily.”

The only limitation is humid or damp conditions, where graphene remains more strongly attached to the surface and friction increases. Water on the surface also appears to prevent the scrolling of graphene flakes during sliding.

Nevertheless, Sumant believes that achieving superlubricity in dry conditions could prove useful in bringing energy savings. Graphene/nanodiamond coatings could be applied to most surfaces using spray techniques and would be much cheaper than other solid lubricants, which require sophisticated vacuum coating systems and can only cover limited areas.

“This approach can be applied directly at engineering scales in various applications such as ball-bearings in many rotating/rolling systems, solid lubricants for use in space, wind turbines, computer hard disk drives, and micromachines,” suggests Sumant.

While James Hone of Columbia University believes that there is some way to go before that, he agrees the findings are an enormous step forward. “The breakthrough here is that no one has reported superlubricity on such a scale before,” he says. “The researchers have demonstrated a big increase in the scale on which superlubricity is seen and a big decrease in the degree of perfection that is required in the system.”

This paper was originally published in Nano Today (2015), doi:10.1016/j.nantod.2015.06.003