29.10.24 - Lars Kruse "Per Aspera ad Astra – Friction reduction with star polymers"

Per Aspera ad Astra – Friction reduction with star polymers

Towards more energy-efficient vehicles, machines and industrial plants, the reduction of friction losses in tribological systems can make a further, important contribution. From the lubricants point of view, this optimization is driven primarily by three factors: Lower viscosity, use of unconventional base oils and friction-reducing additives.

In this context, fundamental conflicts of objectives have to be solved. The trend toward ever thinner lubricating oils, leads - keeping other conditions unchanged - to a lower lubricant film thickness, associated with the risk of greater wear. This can be adressed by using low-friction, wear-resistant component coatings - or by using film-forming polymers in the lubricant.

However, this raises a new problem: Conventional polymer additives for lubricants, e.g. the well-known viscosity index improvers (VII), lead to thickening of the base oils. As a consequence, the base oils would have to be significantly "thinner" again, which in turn is limited for reasons of significantly increasing evaporation losses. A solution was found with star polymers that do not noticeably thicken the lubricating oil, but are interfacially active.

The main questions concerns the film formation of these star polymers in lubrication gaps. Here the adsorption free energy is studied atomistically via two routes: an alchemical and a geometrical. Furthermore shearing and pressurizing of the lubrication gaps gives insights about the star polymers mode of action. Finally, an optimal synthesis route of these star polymers has been found and tribological experiments could validate the simulation results.