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Dr Johann Schnagl (l.) and Prof. Dr Michael Moseler are researching diamond-like carbon coatings and explain the details in a BINE interview.
© Dr. Matthias Unbehaun/K&S GmbH
Interview with Johann Schnagl (BMW Group) and Michael Moseler (Fraunhofer IWM)
08.04.2015

Comparison of bevel drive gears: Bevel drive gears in series production, one with a specially smoothed surface and the other with a diamond-like coating.
© BMW

Superlubricity reduces friction

The forces of friction play a central role in vehicles. They inevitably lead to higher fuel consumption and wear in and on all moving parts. Researchers are therefore searching for ways to keep the forces of friction as low as possible. One area of research is diamond-like carbon coatings (DLC). In an interview, Dr Johann Schnagl, Project Manager in the BMW Group and Professor Dr Michael Moseler from Fraunhofer IWM explained this area of research.

BINE Information Service: In the Pegasus I research project you have examined diamond-like carbon coatings. What is this material?

Dr Johann Schnagl: Diamond-like carbon can be viewed as a type of amorphous diamond. It has a similar degree of hardness. With a special coating process we can deposit thin coatings onto components that are subjected to heavy friction loads. The layers are deposited from carbonic gases or graphite under a vacuum. In conjunction with matching lubricants, these DLC coatings achieve extremely low coefficients of friction. In addition to exceptionally low friction, the material also has anti-adhesion properties, is chemically stable and also has excellent wear resistance in many cases.

How large is the coefficient of friction compared to classical lubrication?

Prof. Dr Michael Moseler: Prof. Dr Michael Moseler: When steel rubs against steel without lubrication then this results in a coefficient of friction within the range between 0.4 to 1.2. Unlubricated DLC coatings have a value of approximately 0.1. Using a lubricant improves the value for steel-steel contact to approximately the same value of 0.1.
The friction of DLC coatings cannot be improved very much using standard lubricants. This is due to the fact that lubricants have been optimised for metallic surfaces over the last 100 years. Closer examination of the interaction between lubricants and coated surfaces has only been performed in recent years. The superlubricity effect occurs unexpectedly when tetrahedral amorphous DLC coatings are used in conjunction with very special chemical lubricants. This results in extremely low coefficients of friction in the region of 0.01.

Reduction of friction – A task with many variable parameters

Classical tribology, the science of friction, lubrication and wear, is already highly developed. But how is it possible to reduce friction?

Schnagl: A fundamental fact is that friction and wear are system characteristics. The coefficient of friction from rubbing surfaces results from the composition of the components. To achieve low friction, the two surfaces, their coatings and lubricant between them must be exactly matched to suit each other. However, this alone is not enough. You must also configure many other influencing parameters: the progression of temperature, velocity and pressing force over time and the associated running characteristics of the tribological system.

Are there many different variable parameters?

Moseler: Yes, you need to adjust more than one parameter in order to achieve the optimum results for the system. It is important to recognise the fact that you cannot solve this complex optimisation task with traditional trial-and-error cycles in an application-related system. You must attempt to understand the active friction and wear mechanisms with carefully abstracted model experiments and multi-scale simulations. Only then do you have the chance of steering the system into the target corridor leading to ultra-low friction.

Can you explain the most important parameters using a current example?

Schnagl: You usually want to operate a bearing in a hydrodynamic environment. This means that a lubricant film ensures that the surfaces do not come into contact with each other. In this case the friction is dominated by the viscous losses of the lubricant. These losses can be reduced by using thinner lubricants but this also moves the working point towards a mixed friction system. The surfaces have points of contact, which also results in higher wear. You can compensate for this through a more wear-resistant surface, such as a special coating for example.

Does the roughness of the surface play a major role?

Schnagl: Yes, smooth surfaces cause less mixed friction and thus less wear. The transition from mixed friction to liquid friction can be very uncomfortable, for example when aquaplaning occurs due to worn tyres on a wet road.
You require experts on lubricants and coatings and also people with knowledge of surface machining or hydrodynamics in order to correctly adjust the interplay between the surfaces and the lubricant. Regardless of how much effort you make, for example in smoothing the surfaces, especially in the starting phase you cannot rule out the possibility of boundary friction and mixed friction. You therefore also need experts in plasticity and surface chemistry who are able to set a low level of boundary friction.

Part 2 of the BINE interview is here ...

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Addresses

Project management
BMW Group

Atomistic simulations
Fraunhofer IWM

In interview

Dr Johann Schnagl performs research at BMW where he is responsible for initial development and innovation management.

Prof. Dr Michael Moseler heads the “Multi-scale modelling and tribological simulation” workgroup at the Fraunhofer Institute for Mechanics of Materials IWM.

Info tip

Superlubricity in vehicles
BINE-Projektinfo 11/2012