Physicists are one step closer to understand lubrication at nanoscale

Author of the article

The article was written by Dr. Aydar Akchurin

The famous picture of transportation of an Egyptian statue to the grave of Tehuti-Hetep, El-Bersheh indicates that the concept of lubrication was already known to ancient Egyptians. The picture below shows how the slaves are dragging a large statue along sand. To help the slaves, one man, standing on the sledge supporting the statue, pours a liquid (oil/water) as a lubricant in order to reduce friction between sledge and ground/sand.

Since then advance in engineering and science revealed the mechanisms of lubrication, at least at macroscopic level. The famous work of Professor Reynolds (1886) was a break through and is a foundation of the current lubrication practices. However, new challenges linked to the development of sustainable economy, reduced emissions  and  increased energy efficiency dictate the need in further understanding of the mechanisms of lubrication.

In recent decades,  lubrication field has seen a great interest in friction and wear processes occurring at nanoscale. With the development of the field, it became clear that the many mysteries of lubrication, friction and wear at the macroscopic level can be studied via the processes at micro and nanoscale. The effects of surface energy, wetability, adhesion, capillary pressure and many other phenomena become influential at nanoscale and may significantly influence macroscopic behavior of the lubricated system.

Recently researchers from the Technische Universität Kaiserslautern (TUK) have addressed the lubrication and heat dissipation problem at nanoscale. They have conducted a systematic study to reveal the mechanisms of the solid−fluid interaction energy impact on nanoscopic scratching processes. The researchers have used detailed molecular dynamic simulations to explore in very fine details the mechanism of indentation and scratching of a surface in presence of a lubricant. The focus of their work was on the investigation of the thermal effects on scratching.

The difficulty in exploring nanoscale lubrication lies in the difficulty of experimental observation. The tools available for such experiments are very scarce, but also give very limited data for the analysis. In contrast, molecular dynamic simulations give a very detailed information about each and every aspect of the processes occurring in the simulated contact. One can for example obtain the information on how the energy is dissipated during the scratch between various processes, like plastic and elastic deformation (found to be around 7% of the total energy), heating of the solids and lubricants.

The simulations revealed that 81% of the total energy input goes into the heating up and deforming the solids. The rest of the energy is dissipated via the lubricant or the substrates. The lubricant helps the system to dissipate heat in a better way so that the heat impact is significantly reduced. In this particular case the heat input into the substrate was reduced 20% in presence of the lubricant as compared to the dry scratching case. This reduction occurs in 2 ways. First, the lubricant reduces friction coefficient and consequently reduces the total energy input. Second, the lubricant absorbs part of the energy and can carry it away from the contact.

According to the researchers, this work is a prove of concept in using a molecular dynamic approach to study lubrication at molecular level. The next steps are needed to improve the model and particularly a more accurate representation of lubricant molecules and surfaces. Since molecular dynamic simulations require significant amount of computing power, researchers have to rely on supercomputers and have big hopes on the increasing computational power.

Further information can be found here: Simon Stephan, Maximilian Dyga, Herbert M. Urbassek, Hans Hasse, The Influence of Lubrication and the Solid–Fluid Interaction on Thermodynamic Properties in a Nanoscopic Scratching Process, https://doi.org/10.1021/acs.langmuir.9b01033