Scientists at the US-based Argonne National Laboratory demonstrated a method that reduces friction between two surfaces to almost zero on macroscopic scales, achieving superlubricity. It was done by combining nanodiamonds with sheets of graphene, which curl around the nanodiamonds to form ‘nanoscrolls’ that lubricate the two surfaces. As friction is the cause of massive energy waste in various devices, this discovery can be hugely beneficial for saving energy and money in a multitude of fields.
Superlubricity required two perfectly flat surfaces with incompatible crystal structures to slide past each other. It has only been detected in extremely small samples, however, as larger surfaces have imperfections that tend to get stuck as they slide. This is why creating superlubricity in a large sample is so unique.
The researchers assumed 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 quite manage to fall into the superlubricity category. Further testing showed why - small sheets of graphene had peeled off one of the surfaces and rolled up, creating 'scrolls' in the debris that ended up between the two surfaces. In order to give the graphene more staying power, the team turned to a robust carbon form: diamonds. The scientists expected that the diamonds would act as tiny ball bearings, allowing the graphene 'scrolls' to roll while the two surfaces slid past each other.
The team coated one surface with graphene, another with diamond-like carbon, and scattered nanodiamonds in between. This resulted in the coefficient of friction dropping down to near zero, achieving superlubricity. Electron mictrographs of the surface revealed that, as expected, graphene sheets had wrapped themselves around the nanodiamonds.
Superlubricity was also retained through testing of various conditions, like different temperatures, varying load on the surfaces, and increased velocity. The only exception came when they increased the relative humidity to 30%, which caused friction to increase dramatically, so this technique currently only works in dry, inert conditions. It seems water vapor can make its way into the space between the two surfaces, creating transient bonds that need to be broken to shift the surfaces.
The scientists are working on solving this issue, but even now this is considered a major find as this is the first time that superlubricity's been demonstrated for something other than two microscopic, defect-free surfaces. It could be useful for many applications, such as space technology or electronic systems where the environment can be controlled.