If components are to be movably mounted in a machine, the bearings must be well lubricated. This is the only way to sufficiently reduce friction between moving parts, for example in a sleeve bearing. However, grease or oil are not always the best choice, explains Prof. Dr. Jürgen Molter, head of the Competence Center for Tribology at Mannheim University of Applied Sciences:

“These classic lubricants leave residues on the machines, which are not wanted in production sectors with high demands on hygiene and cleanliness, such as food production or medical technology.” But even in automotive manufacturing, people try to avoid greases and oils if possible, he explains, “Because in combination with dust and road grime, they form a kind of grinding paste.”

To replace greases and oils, tribology experts are therefore working on so-called solid lubricants such as graphite, certain polymers, or very fine ceramic particles. Since solid substances cannot be applied dropwise, they are often applied in the form of powder or lubricant varnish. In other cases, the bearing consists entirely of the lubricant material, i. e. the bearing counterpart rotates in a sleeve of solid lubricant. The goal of Jürgen Molter and his team is to develop such a component within the PTFE Transferfilme project.

The polymer PTFE, commonly known as Teflon^Ⓡ, is one of the most slippery materials we know: nothing sticks to it. It is non-flammable, does not dissolve in any known solvents, does not swell, and is generally characterized by remarkable chemical stability. These properties would make PTFE a perfect lubricant, says project team member Andreas Keller – but they are all undone by mechanical instability: “Unfortunately, PTFE is about six times more sensitive to pressure than other polymers,” explains the young scientist. “A component made of this material would simply slip away under pressure. Therefore, it has to be embedded in a matrix of a more stable material.”

Sounding out the optimum conditions for good transfer films

In the PTFE Transferfilme project, Molter and Keller seek to determine the optimal conditions for such a component: Which materials are suitable as a matrix, what is the best mixing ratio of matrix and PTFE, and what actually happens within a PTFE sleeve bearing? The only thing that is clear so far is that a thin film of PTFE transfers to its counterpart in the bearing – the so-called transfer film that gave the project its name. “We want to know exactly when and how this happens, because there is only a narrow window of ideal conditions in which good transfer films form,” explains Jürgen Molter.

But what is a “good” transfer film anyway? This question is widely discussed in the scientific literature. One of the most popular models suggests that PTFE is not deposited as a homogeneous film covering the entire surface, but rather in the form of “islands”, and that the PTFE molecules arrange themselves in lamellae that easily slide past each other. This model seems pretty close to reality: Molter and Keller got the best results in their experiments with those compounds that formed a kind of PTFE patchwork on the bearing's counterpart.

The conclusion from the project is therefore: The best transfer film is one that is not completely closed, and it must also not be too thin or too thick. If it is too thick, there is too much wear in the bearing; if it is too thin, then the mating piece cannot move without friction. The surface of the counterpart also plays an important role: In basic research, people often work with very smooth surfaces, but Molter and his team have found that the PTFE sleeve bearing is not particularly well suited to such surfaces. On the contrary, the counterpart that rotates in the bearing must have a certain roughness to allow the PTFE transfer film to adhere to it.

A new material and a lot of know-how

In over 300 experiments with different PTFE mixtures under various conditions, the project team has now worked out the optimum composition for a new PTFE material. The project partner, GGB Heilbronn GmbH, will be able to make use of those findings by launching new products. For Jürgen Molter, however, the analytical and methodological progress the project has brought is equally important: “We have accumulated a lot of know-how on friction, wearing, and film formation, and we have also been able to find some evidence that partially confirms the existing models.”

Of course, when you have learnt so much from a research project you don't just stop working on the topic afterwards. Molter and his team have already made plans for what to do with the extensive data they have gained. During the project, they have successfully constructed and implemented their own analysis software which provides the characteristic values of various PTFE compounds. Based on this software, Molter says, they could develop an AI-based tool that accurately predicts the behaviour of different materials. To this end, however, the team still needs to build up a much larger database from other projects. Otherwise, however, things are running pretty smoothly at the Mannheim Competence Centre for Tribology – just like a well-oiled bearing.