Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


Puzzle about passivation of low-friction, hard carbon coatings solved


Diamond and diamond-like carbon (DLC) are used as extremely durable surface coatings in frictional contacts: from aerospace components to razors. They reduce friction and wear in bearings and valves by means of so-called passivation layers, which prevent other materials from bonding to the coating. Until now, it was unclear how these passivation layers should be designed to achieve minimal friction. Researchers at the Fraunhofer Institute for Mechanics of Materials IWM, MicroTribology Centrum µTC, have now achieved a breakthrough in understanding the relationship between passivation and friction. The unexpected results have been published in the journal "ACS Applied Materials & Interfaces".

Hard carbon coatings are in use worldwide on a large industrial scale. A particularly frequent user is the automotive industry, which uses more than 100 million coated parts per year with a market volume of several hundred million euros.

Electron density of two hydrogen-terminated (left) and fluorine-terminated (right) diamond surfaces: large fluorine atoms prevent the surfaces from interlocking and thus reduce friction.

© Fraunhofer IWM

Tribological contact of two hydrogen (top, white) and fluorine-terminated (bottom, green) diamond surfaces (grey): fluorine passivation halves friction compared to hydrogen passivation.

© Fraunhofer IWM

Other important fields of application are protective coatings for computer hard disks and recording heads as well as wear protection for cutting and forming tools in production machines or biomedical components.

"In order to achieve the desired ultra-small friction, stable passivation of unsaturated dangling bonds on the carbon surface is necessary," explains Thomas Reichenbach from the Multiscale Modeling and Tribosimulation group at the Fraunhofer IWM. There are, however, numerous substances that can be used for passivation: the most common being hydrogen. A promising, technologically relevant alternative, however, is fluorine. Both variants ensure stable, monoatomic passivation.

"We wanted to find out under which circumstances which passivation is better and, above all, why, because how surface passivation influences friction is still poorly understood," explains the simulation expert for atomistic computer models.

For this work, Thomas Reichenbach has received the Fraunhofer IWM Materials of Mechanics Prize (Werkstoffmechanik-Preis 2020). Awarded by the board of trustees, the prize comes with an endowment of 3000 Euro.

The prevailing opinion in the technical literature is that the pronounced electric charge of fluorine atoms is responsible for a large electrostatic repulsion between two fluorine-passivated surfaces that ultimately decreases friction compared to the hydrogen case. "We were skeptical about this, because the electric field caused by densely packed carbon-fluorine bonds has only a very short range. This is exactly what causes Teflon coatings in clothing or cooking utensils to be water repellent," says PhD student Reichenbach.

Interatomic force field describes friction

In order to explain the relationship between the structural and chemical properties of the surface passivation and friction, scientists at the Fraunhofer IWM developed a so-called interatomic force field. This tailor-made mathematical formalism describes how the atoms involved in the tribological contact interact with each other.

All parameters that supposedly control friction, determined by precise but computationally demanding quantum-mechanical calculations, are included in the force field – such as atomic radii, masses, charges or vibrational frequencies. On the basis of this computationally efficient force field model, they succeeded in simulating frictionalcontacts consisting of about one hundred thousand atoms on the computer, and were able to elaborate how the physical and chemical properties of the terminating species affect the friction behavior.

To find out which parameters are really decisive for friction, the research team then used a trick: "You have to imagine as if you were trying out fuses in an illuminated apartment to find the right one," says Reichenbach, illustrating the procedure.

Since passivation with fluorine roughly halves the friction on atomically flat diamond coatings compared to hydrogen passivation, they gradually interchanged the parameters of hydrogen and fluorine in the force field.

"If the parameter was really relevant for friction, we would have seen a swapping of the friction coefficients of hydrogen and fluorine in the simulation," says Reichenbach.

Only geometric parameters are relevant for passivation

The results are unexpected: for both atomic masses and atomic charges, there was no friction coefficient inversion during the parameter change - but there was one when both the atomic radii and C-H/C-F bond lengths were interchanged.

"This means that geometric parameters alone are important for optimizing friction," explains Reichenbach. With these fundamental findings, the team was also able to explain the advantages of fluorine passivation for flat diamond surfaces.

The larger atomic radii prevent two F-passivated surfaces from interlocking: the surfaces in contact slide past each other quite smoothly, as the "atomic zipper" cannot close. However, this can occur with the smaller hydrogen atoms: the surfaces can interlock and thus increase the friction.

Fluorocarbon bonds are the strongest of all chemical bonds. Due to their stability, fluorine passivation is very interesting for tribological applications. However, its disadvantage is that the larger fluorine atoms can become obstacles in non-planar DLC contacts, which would again increase friction.

"Research on the relationship between surface passivation and friction is still in its infancy. We have now opened a door with our basic research, but the results still need to be verified with high-precision experiments," says deputy group leader Dr. Gianpietro Moras.

In any case, the new "switch method" from Freiburg could easily be transferred to other frictional contacts. This would require the force-field model to be adapted to specific conditions, for example to other passivation atoms or surface geometries. The method enables the development of design rules for optimally terminated, low-friction systems.

For example, alternatives to fluorine passivation could be found. Although fluorinated carbon compounds have excellent tribological properties, they are increasingly being avoided in some areas of application due to their negative impact on health and the environment.

This research of the group Multiscale Modelling and Tribo-Simulation was performed within research projects funded by the German Federal Ministry of Economics and Energy, the German Research Foundation and the German Excellence Strategy - EXC.

Wissenschaftliche Ansprechpartner:

Thomas Reichenbach l Telephone +49 761 5142-260 l | Fraunhofer Institute for Mechanics of Materials IWM l


Reichenbach, T.; Mayrhofer, L.; Kuwahara, T.; Moseler, M.; Moras, G.: Steric Effects Control Dry Friction of H- and F-Terminated Carbon Surfaces; ACS Applied Materials & Interfaces 12, 7 (2020) 8805-8816;

Weitere Informationen: - press release online

Katharina Hien | Fraunhofer-Institut für Werkstoffmechanik IWM

More articles from Materials Sciences:

nachricht Machine learning methods provide new insights into organic-inorganic interfaces
04.08.2020 | Technische Universität Graz

nachricht Unusual electron sharing found in cool crystal
31.07.2020 | Nagoya University

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: New Strategy Against Osteoporosis

An international research team has found a new approach that may be able to reduce bone loss in osteoporosis and maintain bone health.

Osteoporosis is the most common age-related bone disease which affects hundreds of millions of individuals worldwide. It is estimated that one in three women...

Im Focus: AI & single-cell genomics

New software predicts cell fate

Traditional single-cell sequencing methods help to reveal insights about cellular differences and functions - but they do this with static snapshots only...

Im Focus: TU Graz Researchers synthesize nanoparticles tailored for special applications

“Core-shell” clusters pave the way for new efficient nanomaterials that make catalysts, magnetic and laser sensors or measuring devices for detecting electromagnetic radiation more efficient.

Whether in innovative high-tech materials, more powerful computer chips, pharmaceuticals or in the field of renewable energies, nanoparticles – smallest...

Im Focus: Tailored light inspired by nature

An international research team with Prof. Cornelia Denz from the Institute of Applied Physics at the University of Münster develop for the first time light fields using caustics that do not change during propagation. With the new method, the physicists cleverly exploit light structures that can be seen in rainbows or when light is transmitted through drinking glasses.

Modern applications as high resolution microsopy or micro- or nanoscale material processing require customized laser beams that do not change during...

Im Focus: NYUAD astrophysicist investigates the possibility of life below the surface of Mars

  • A rover expected to explore below the surface of Mars in 2022 has the potential to provide more insights
  • The findings published in Scientific Reports, Springer Nature suggests the presence of traces of water on Mars, raising the question of the possibility of a life-supporting environment

Although no life has been detected on the Martian surface, a new study from astrophysicist and research scientist at the Center for Space Science at NYU Abu...

All Focus news of the innovation-report >>>



Industry & Economy
Event News

“Conference on Laser Polishing – LaP 2020”: The final touches for surfaces

23.07.2020 | Event News

Conference radar for cybersecurity

21.07.2020 | Event News

Contact Tracing Apps against COVID-19: German National Academy Leopoldina hosts international virtual panel discussion

07.07.2020 | Event News

Latest News

Study clarifies kinship of important plant group

05.08.2020 | Life Sciences

Human cell-based test systems for toxicity studies: Ready-to-use Toxicity Assay (hiPSC)

05.08.2020 | Life Sciences

Molecular Forces: The Surprising Stretching Behaviour of DNA

05.08.2020 | Life Sciences

Science & Research
Overview of more VideoLinks >>>