Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Diamond friction: simulation reveals previously unknown friction mechanisms at the molecular level

01.09.2017

Diamond coatings help reduce friction and wear on tools, bearings, and seals. Lubricating diamond with water considerably lowers friction. The reasons for this are not yet fully understood.

The Fraunhofer Institute for Material Mechanics IWM in Freiburg and the Physics Institute at the University of Freiburg have discovered a new explanation for the friction behavior of diamond surfaces under the influence of water. One major finding: in addition to the known role played by passivation of the surfaces via water-splitting, an aromatic passivation via Pandey reconstruction can occur. The results have been published in the journal Physical Review Letters.


Passivation of water-lubricated diamond surfaces by aromatic Pandey surface reconstruction (orange)

Image: © Fraunhofer Institute for Mechanics of Materials IWM

Diamond coatings are commonly used today to protect tools and machine components that are subjected to high wear, and thus to extend their service life. It is known that 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.

If atmospheric moisture is added, the water molecules split and hydrogen and hydroxyl groups bond with the reactive carbon. The surface is saturated and the friction coefficient drops sharply. However, continued rubbing can produce new reactive carbon atoms, which again bind with the opposite surface. It is thus important to achieve swift repassivation.

“To make progress on this point, we wanted to understand more precisely how the friction depends on the amount of water,” says Prof. Dr. Moseler, Head of the Multiscale Modeling and Tribo-Simulation Group at Fraunhofer IWM. He and his colleagues Dr. Takuya Kuwahara and Dr. Gianpietro Moras therefore carried out large-scale quantum molecular dynamics calculations with water-lubricated diamond surfaces. To be able to identify precisely the friction mechanisms, different quantities of water molecules were added.

Simulation shows four different friction mechanisms

The simulation yielded surprising results. The first trial, with very few water molecules, confirmed the presence of covalent bonds between the dry friction partners, also called cold welding, which result in high levels of friction. This also causes the amorphization of carbon, i.e., the breakdown of the crystalline structure at the surface. Another simulation using slightly more water resulted in a different situation. In this case, the two rubbing surfaces bonded via ether groups.

This form of cold welding also results in high friction, but no amorphization occurs. Simulations with sufficient quantities of water confirmed the known saturation of the surfaces with hydrogen and hydroxyl groups. However, in their simulation the researchers were able to identify yet another completely unknown friction case.

"With the addition of few water molecules, an aromatization of the structure of one of the rubbing surfaces in the form of a Pandey reconstruction was observed,” explains Gianpietro Moras. “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. As a result, a very small coefficient of friction develops.

The results can be transferred to other materials

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,” says Moseler. Future experiments will investigate how the aromatization 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 specialty of carbon,” adds Moras. The researchers therefore assume that aromatic restructuring is also possible with amorphous carbon surfaces.

Kuwahara, T.; Moras, G.; Moseler, M.: Friction regimes of water-lubricated diamond (111): Role of interfacial ether groups and tribo-induced aromatic surface reconstructions, Physical Review Letters 119 (2017) 096101, doi: 10.1103/PhysRevLett.119.096101

Weitere Informationen:

http://www.iwm.fraunhofer.de/en/press/press-releases/01_09_17_diamondfrictionunk... - link to press release
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.096101 - link to publication in Physical Review Letters

Katharina Hien | Fraunhofer-Institut für Werkstoffmechanik IWM

More articles from Materials Sciences:

nachricht Epoxy compound gets a graphene bump
14.11.2018 | Rice University

nachricht Automated adhesive film placement and stringer integration for aircraft manufacture
15.11.2018 | Fraunhofer IFAM

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

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

Im Focus: UNH scientists help provide first-ever views of elusive energy explosion

Researchers at the University of New Hampshire have captured a difficult-to-view singular event involving "magnetic reconnection"--the process by which sparse particles and energy around Earth collide producing a quick but mighty explosion--in the Earth's magnetotail, the magnetic environment that trails behind the planet.

Magnetic reconnection has remained a bit of a mystery to scientists. They know it exists and have documented the effects that the energy explosions can...

Im Focus: A Chip with Blood Vessels

Biochips have been developed at TU Wien (Vienna), on which tissue can be produced and examined. This allows supplying the tissue with different substances in a very controlled way.

Cultivating human cells in the Petri dish is not a big challenge today. Producing artificial tissue, however, permeated by fine blood vessels, is a much more...

Im Focus: A Leap Into Quantum Technology

Faster and secure data communication: This is the goal of a new joint project involving physicists from the University of Würzburg. The German Federal Ministry of Education and Research funds the project with 14.8 million euro.

In our digital world data security and secure communication are becoming more and more important. Quantum communication is a promising approach to achieve...

Im Focus: Research icebreaker Polarstern begins the Antarctic season

What does it look like below the ice shelf of the calved massive iceberg A68?

On Saturday, 10 November 2018, the research icebreaker Polarstern will leave its homeport of Bremerhaven, bound for Cape Town, South Africa.

Im Focus: Penn engineers develop ultrathin, ultralight 'nanocardboard'

When choosing materials to make something, trade-offs need to be made between a host of properties, such as thickness, stiffness and weight. Depending on the application in question, finding just the right balance is the difference between success and failure

Now, a team of Penn Engineers has demonstrated a new material they call "nanocardboard," an ultrathin equivalent of corrugated paper cardboard. A square...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

“3rd Conference on Laser Polishing – LaP 2018” Attracts International Experts and Users

09.11.2018 | Event News

On the brain’s ability to find the right direction

06.11.2018 | Event News

European Space Talks: Weltraumschrott – eine Gefahr für die Gesellschaft?

23.10.2018 | Event News

 
Latest News

Purdue cancer identity technology makes it easier to find a tumor's 'address'

16.11.2018 | Health and Medicine

Good preparation is half the digestion

16.11.2018 | Life Sciences

Microscope measures muscle weakness

16.11.2018 | Life Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>