Passivation of water-lubricated diamond surfaces by aromatic Pandey reconstruction (orange).
Fraunhofer-Institut für Werkstoffmechanik IWM
Diamond coatings help to reduce friction and wear on cutting tools, bearings, and seals. Rubbing two dry diamond surfaces together creates enormous friction due to the bonding of reactive carbon atoms on each surface with the corresponding atoms on the opposite surface. When water or atmospheric moisture is added, the water molecules split, and hydrogen and hydroxyl groups bond with the carbon. The surface is saturated, and the friction coefficient drops sharply. However, continued rubbing can produce new reactive carbon atoms that again bind with the opposite surface. It is therefore important to achieve swift repassivation. For a more precise understanding of how friction depends on the amount of water, Prof. Dr. Moseler, head of the Multiscale Modeling and Tribosimulation group at Fraunhofer IWM, and his colleagues Dr. Takuya Kuwahara and Dr. Gianpietro Moras have carried out large-scale quantum molecular dynamics calculations with diamond surfaces lubricated by water.
Simulation reveals four different friction mechanisms
The first simulation with very few water molecules confirmed a high level of friction due to the known bonds between the rubbing surfaces, also known as cold welding. This also causes carbon amorphisation, i.e. the breakdown of the crystalline structure at the surface. Another simulation using slightly more water created a new situation. In this case, the two rubbing surfaces bonded via ether groups. This form of cold welding also results in high friction but not in amorphisation. Simulations with sufficient quantities of water confirmed the known saturation of the surfaces with hydrogen and hydroxyl groups. The researchers were, however, able to identify yet another completely unknown friction case in their simulation. "With the addition of few water molecules, an aromatisation of the structure of one of the rubbing surfaces in the form of a Pandey reconstruction was observed", Moras explains: "That means that the diamond surface passivates itself through an annular arrangement of the carbon atoms." In this case, the other surface becomes saturated with hydrogen and hydroxyl groups, also leading to a very low coefficient of friction.
The Pandey reconstruction can be viewed as a step towards a complete reconstruction of the surfaces. "In further simulations, we were able to see that the ring structure can develop into graphene domes, which further reduce friction", Moseler says. Future experiments are to investigate how the aromatisation can be selectively forced, for example, by doping the diamond surface. "Although most of our results can be transferred to other water-splitting materials, it is important to note that aromatic passivation is a speciality of carbon", Moras says. The researchers therefore assume that aromatic restructuring would also be possible with amorphous carbon surfaces.