Atomic Prerequisites for Supra Lubrication

Dr. Volker Weihnacht, Head of Carbon Layer Department, and Stefan Makowski, Group Leader Coating Properties at Fraunhofer IWS, systematically investigated the interaction of lubricants with ta-C carbon surfaces. Unsaturated fatty acids or glycerol achieved extremely low coefficients of friction at the superlubricity level. They were astonished by two things: that this effect did not occur with the smallest changes in the molecular structure and that the friction was much higher. Thus, saturated fatty acids and alkanes did not achieve a superlubricity effect.

Prof. Michael Moseler and Dr. Gianpietro Moras at the Fraunhofer IWM explained the reason for this: “Using quantum chemical simulations, we were able to demonstrate that lubricant molecules with at least two reactive centers are able to form a chemical bond simultaneously with both ta-C-coated surfaces and are torn apart by the sliding motion and broken down into their constituent parts,” explains Prof. Moseler, head of the “Multiscale Modeling and Tribosimulation” group. This allows for the release of the lubricant’s oxygen atoms and for these to be incorporated into the ta-C film. The oxygen disturbs the three-dimensional tetrahedral carbon network and aids in the formation of graphene-like surfaces, which effectively suppress friction as well as wear and thus ensure superlubricity. Corresponding simulations with alkanes or saturated fatty acids as lubricants did not show these mechano-chemical processes because they have no or only one reactive center. “These lubricants only adhere to one surface and form a molecular brush – which reduces friction, but not at the superlubricity level,” says Prof. Moseler. The newly discovered design rule states that several reactive centers must be present in the lubricant to cause superlubricity. “Of course, this rule is not limited to the fatty acids investigated here, it can also be transferred to other lubricants,” Prof. Moseler adds.

Guidelines for the Novel Lubricants’ Design

The research team’s results allow both predicting tribological properties of ta-C surfaces lubricated with different molecule types and forming guidelines for the novel organic friction modifiers’ design. Predictions for tailoring the carbon layers themselves and the necessary supra lubricant design rules for other surfaces, such as steel or aluminum, also become conceivable projects. In 2019, the scientists at the Fraunhofer IWM and IWS will continue to work with industrial partners on translating quantum chemical findings into engineering solutions as part of the PROMETHEUS project with financial support from BMWi. Their goal: further reducing friction in combustion engines as well as other applications.

Publication:

Kuwahara, T.; Romero, P.A.; Makowski, S.; Weihnacht, V.; Moras, G.; Moseler, M.; Mechano-chemical decomposition of organic friction modifiers with multiple reactive centres induces superlubricity of ta-C, Nature Communications 10 (2019) Article number: 151; DOI 10.1038/s41467-018-08042-8

Link to the article



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