Elucidating the Atomic Mechanism of Superlubricity
Superlubricity is a well-known phenomenon, but until now, an explanation for it at the atomic level has been lacking. A newly discovered design rule states that lubricant molecules must have several reactive centres in order for superlubricity to occur.
One of the most important prerequisites for sustainable and environmentally friendly mobility is minimizing friction. Superlubricity could achieve not only minor, but also extreme friction reductions. If, for example, friction in the engines and transmissions of vehicles were reduced to minimum values, such as those occurring with superlubricity, annual global CO2 emissions could be reduced by several hundred million tonnes. Two Fraunhofer Institutes have taken an important step toward this vision. In the PEGASUS II project funded by the Federal Ministry for Economic Affairs and Energy (BMWi), scientists from the Fraunhofer Institute for Mechanics of Materials (IWM) in Freiburg and the Fraunhofer Institute for Material and Beam Technology (IWS) in Dresden have uncovered the atomic mechanism behind superlubricity in a special friction partner system.
Lubricants are subjected to tribochemical decomposition
They investigated promising tribological systems in which the friction partners’ surfaces consist of special diamond-like carbon layers produced with a coating technology developed at the Fraunhofer IWS. These so-called tetrahedral amorphous carbon layers (ta-C) were combined with organic lubricants. Using unsaturated fatty acids or glycerol, extremely low coefficients of friction were achieved at the level of superlubricity. Saturated fatty acids and alkanes, on the other hand, did not achieve a superlubricity effect.
The research team found out the reason for this by performing quantum chemical simulations in the reactive centres: Lubricant molecules with at least two reactive centres, which are able to form a chemical bond with both ta-C-coated surfaces simultaneously, were torn apart by the sliding motion and broken down into their constituent parts. This way, graphene-like surfaces start to form, which are required for superlubricity. The results are presented in an article in "Nature Communications".
Guidelines for designing novel lubricants
The research team’s results will allow predictions of tribological properties of ta-C surfaces lubricated with different molecule types, and help to form guidelines for the design of novel organic friction modifiers. In 2019, the scientists at the Fraunhofer IWM and IWS will continue to work with industrial partners on implementing these quantum chemical findings into engineering solutions as part of the PROMETHEUS project, with financial support from the BMWi. Their goal is to further reduce friction in combustion engines as well as other applications.