ENGINEERING AND CFD

Introduction

Friction may seem like an everyday annoyance—squeaky doors or grinding gears—but managing it efficiently is vital to modern technology. Today’s demands—more compact, faster-moving and energy efficient devices—push lubricants to extreme limits, requiring insights far beyond traditional trial-and-error methods.

A team of physicists and engineers led by Prof. Michael Moseler and Dr. Kerstin Falk from the Fraunhofer Institute for Mechanics of Materials IWM took up the challenge to understand lubricant behavior under these extreme conditions. Towards this goal, research performed within the GCS regular project chfr14 on JUWELS was centered on nanoscale fluid dynamics, specifically using molecular dynamics simulations to investigate the behavior of lubricants in nanometer-thin layers under high pressures. The findings pave the way to a scientific basis for designing new lubricants and devices of the future.

Why Study Extreme Lubrication?

For reasons of efficiency, many tribological systems are operated at their load limits: lubrication gaps are becoming narrower and lubricating films have to withstand higher loads. The reliable design of such systems depends on precise calculation methods. However, conventional calculation approaches fail when it comes to so-called boundary lubrication, where gap sizes are reduced to molecular dimensions and local pressures are in the Gigapascal range. 

The ability to predict how lubricants behave at these extremes would enable engineers to choose optimal materials and designs. This project aimed to enhance these predictive capabilities by bridging the microscopic world of molecules with macroscopic engineering models. To this aim Prof. Moseler’s team searches for the appropriate constitutive laws to describe the nano-scale lubricant dynamics.

Key Results - Constitutive Laws for Nano-Lubrication

Using large-scale molecular simulations, the team studied hydrocarbon lubricants in nanoscale channels. They revealed that traditional models, such as the Reynolds equation, fail unless corrected for phenomena like wall slip, where liquid molecules slide along solid surfaces instead of adhering to them. However, after implementing a slip law that links slip velocity to pressure and shear stress in the Reynolds calculation, the extended model accurately predicts lubrication behavior at gap sizes down to 1nm and pressures up to 1GPa [1]. The importance of wall slip for the lubricant rheology has also been illustrated considering nano-channels with inhomogeneous surface properties [2]. 

Another key property of lubricants is viscosity, a lubricant’s resistance to flow, and large parts of the project were dedicated to the investigation of the physical mechanisms determining the variation of viscosity with the tribological conditions (pressure, temperature, shear rate) [3, 4]. As a main result, the team developed a parameter-free model to describe how viscosity at high temperatures changes with pressure, grounded in molecular diffusion mechanisms. This work provides insight into the connection between microscopic molecular properties and viscosity [3]. This knowledge can help to design lubricants capable of maintaining performance under extreme pressures.

Next to the focus on the nano-rheology of the lubricant, the project was also concerned with the investigation of dry contact areas, which appear in boundary lubrication when the lubricant is completely squeezed out of some high pressure contact zones. For example, the effect of different surface passivations due to degraded lubricant molecules on the dry sliding friction [5] and the effect of transfer film formation [6] were studied.

Implicatons for Engineering

Future work will refine these models and expand their application to more complex systems, such as lubricants containing a combination of different additives, or novel types of base oils as for water based lubricants, and also considering a larger range of surface materials. 

Ultimately, this research will provide a more accurate toolkit for predicting lubricant behavior, reducing reliance on costly and inefficient trial-and-error testing. Thus, it paves the way for more sustainable technologies by enabling the development of efficient, long-lasting lubricants tailored to specific applications.

References

[1] Codrignani, A.; Peeters, S.; Holey, H.; Stief, F.; Savio, D.; Pastewka, L.; Moras, G.; Falk, K. and Moseler, M.: Towards a continuum description of lubrication in highly pressurized nanometer-wide constrictions: the importance of accurate slip laws. Science Advances 9, 48 (2023) 

[2] Savio, D.; Falk, K. and Moseler, M.: Slipping domains in water-lubricated microsystems for improved load support, Tribology International 120, 269-279 (2018)

[3] Falk, K.; Savio., D. and Moseler, M.: Nonempirical free volume viscosity model for alkane lubricants under severe pressures, Phys. Rev. Lett. 124, 105501 (2020)

[4] Kruse, L. B.; Falk, K. and Moseler, M.: Calculating High-Pressure PAO4 Viscosity with Equilibrium Molecular Dynamics Simulations. Tribol. Lett. 72, 1–15 (2024)

[5] Falk, K.; Reichenbach, T.; Gkagkas, K.; Moseler, M. and Moras, G.: Relating Dry Friction to Interdigitation of Surface Passivation Species: A Molecular Dynamics Study on Amorphous Carbon. Materials 15, 3247 (2022)

[6] von Goeldel, S.; Reichenbach, T.; König, F.; Mayrhofer, L.; Moras, G.; Jacobs, G. and Moseler, M.: A Combined Experimental and Atomistic Investigation of PTFE Double Transfer Film Formation and Lubrication in Rolling Point Contacts. Tribology Letters 69, 136 (2021)