Friction is an important fact of life, robbing efficiency from anything where two surfaces interact with each other, such as engines and wheels. Lubrication can reduce the amount of friction, but it's never possible to get rid of it entirely.

In some rare cases, however, it's been possible to get the coefficient of friction to drop dramatically. A phenomenon called superlubricity occurs when two perfectly flat surfaces with incompatible crystal structures slide past each other. It's only been observed in extremely small samples, however, as larger surfaces have imperfections that tend to get stuck as they slide around.

Now, researchers have managed to create superlubricity in a large sample. They do so by getting graphene to wrap around nanoscopic diamonds, creating something akin to tiny ball bearings.

The authors of the new paper, a team from Argonne National Lab, were initially intending to study the traditional type of superlubricity. They reasoned that graphene and diamonds would have incompatible surfaces, and hoped that coating two surfaces with them would allow them to slide with minimal friction. Although friction was low, it didn't fall into the superlubricity category.

By looking at the surfaces afterwards, however, they found that small sheets of graphene had peeled off one of the surfaces and rolled up, creating scrolls in the debris. However, as graphene is only a single atom thick, these scrolls weren't very robust, and they ended up crunched between the two surfaces. In order to give the graphene more staying power, the team then turned to a rather robust substance: diamonds. The authors expected that the diamonds would act like tiny ball bearings, allowing the graphene scrolls to roll while the two surfaces slid past each other.

Diamonds have two other properties that make them an excellent choice. For one, they provide the same sort of surface that the authors had already reasoned would slide past graphene with minimal resistance. It's also possible to create incredibly small diamonds, which the authors refer to as nanodiamonds.

So, the authors coated a surface with graphene, coated another with diamond-like carbon, and sprinkled nanodiamonds in between. This dropped the coefficient of friction down to near zero, indicating the superlubricity had been achieved. Electron micrographs of the surface revealed that, as expected, graphene sheets had wrapped themselves around the nanodiamonds, which acted a bit like a ball bearing.

The authors tried a variety of conditions, changing the temperature, varying the load on the surfaces, and increasing their relative velocity. In all cases, the nanodiamonds retained superlubricity. The only exception came when the increased the relative humidity to 30 percent, which caused friction to increase dramatically. Apparently, water vapor can make its way into the space between the two surfaces and act as a bridge between them, creating transient bonds that need to be broken to shift the surfaces.

This is the first time that superlubricity's been demonstrated for something other than two microscopic, defect-free surfaces. So, in that sense, it represents significant progress, and may point a way forward to getting rid of some of the friction that robs us of energy.

Science, 2015. DOI: 10.1126/science.1262024 (About DOIs).