Using a real-time setup, scientists at Sheffield University's NanoLAB Centre, UK, are studying how ultra-thin layers of amorphous carbon, just 10–100 atoms thick, can be changed into different forms of carbon by simply rubbing the two surfaces together.

In the journal Nanotechnology, the authors report how surface layers of amorphous carbon can be radically altered by mechanical damage. Fracture by a single impact can form long filaments of amorphous carbon. However, if a carbon surface is rubbed more gently over a longer time period, the carbon layer can break up and form ordered carbon structures such as graphite.

Onion-shaped features

In essence, the transfer of energy during persistent nanoscale sliding energises the carbon atoms enabling them to reorder to form new structures. The sliding of a 20 nm thick carbon layer on a clean gold surface was even observed to result in the formation of a carbon onion.

The behaviour of carbon is important because it is present throughout the environment, and objects left out in air gradually accrete an ultra-thin layer of carbon-rich material, often less than 10 atoms thick, on their surface. It is also well known in industry that some forms of carbon, such as amorphous films and graphite, can reduce friction, making it easier to slide materials across each other. For machines with moving parts, huge amounts of energy can be dissipated at surfaces during operation, so surface carbon may help to reduce energy loss and increase machine lifetimes.

TEM triboprobe

The real-time observation of the transformation of carbon by mechanical deformation has been enabled by the development of a novel miniaturized machine, which fits inside an electron microscope. This has been funded by a Basic Technology grant from the EPSRC.

Dubbed the TEM triboprobe, the assembly can bring together two surfaces <10 nm in size and slide them across each other. The unit will have many future applications in tribology (the study of nanoscale processes at sliding surfaces). Uses include the analysis of surface films, lubricants and nanoscale mechanical devices.