24th International Colloquium Tribology - January 2024 165 Per Aspera ad Astra Design of Friction Reducing Star Polymers from Computer Simulation to Lubricant Application. L. B. Kruse 1* , K. Falk 1 , M. Moseler 1,2 , D. Markert 3 , R. Klein 3 , J. Rausch 4 , R. Luther 4 1 MikroTribologie Cetrum µTC - Fraunhofer IWM, Freiburg, Germany 2 University of Freiburg, Freiburg, Germany 3 Fraunhofer Institutefor Structual Durability and System Reliability LBF, Darmstadt, Germany 4 Fuchs Lubricants GmbH, Mannheim, Germany * Corresponding author: Lars.Kruse@iwm.fraunhofer.de 1. Introduction The Presentation covers results of a novel star polymer (Fig.1b), which enables low viscous base oils (Fig. 1a, blue arrow), by delaying boundary lubrication through surface adsorption (Fig.1a, green arrow). The topic was addressed by three project partners, Fraunhofer LBF, Fraunhofer IWM and Fuchs Lubricants GmbH, which worked on the respective topics of synthesis, simulation and tribological experiments. Figure 1: Schematic Stribeck curve (a), divided into boundary, mixed and fluid lubrication and a MD-snapshot of a four-arm star polymer (b), with a carbon backbone (orange) and functional groups (blue). 2. Synthesis A synthesis via the controlled polymerization was found by the Fraunhofer LBF to be a successful choice for the novel star polymers. In contrast to the radical polymerization, a defined star shaped structure is obtained by linking of multiple polymer chains. In this way the molar mass of the star polymer can be easily adjusted by the initiator/ monomer-ratio with a very narrow chain length distribution. Figure 2: Schematic representation of the synthesis of a 4-arm star polymer via controlled polymerization (a) and the laboratory setup (b). 3. Simulation Accompanying atomistic simulations are carried out by the Fraunhofer IWM to calculate structure-property relationships, in order to support the synthesis with design rules. To analyze the surface attachment of the star polymers, the adsorption process is simulated via steered molecular dynamics. A linear trend in the free energy of adsorption with respect to the pulling distance allows an adsorption energy scale up (or scale down) with the number of functional groups. The tribological behavior of the star polymers was analyzed in squeezeout- (Fig.2a) and shear- (Fig.2b) simulations. In both simulations, the star polymers showed a great surface attachment, preventing solid contact, while non-thickening the low viscous base oil. Finally microscopic friction results could be combined with contact theory (Persson), to calculate macroscopic friction coefficients for the boundary lubrication regime. 166 24th International Colloquium Tribology - January 2024 Per Aspera ad Astra Figure 2: Snapshots of a squeezeout- (a) and a shear- (b) molecular dynamics simulation, for conditions of T-=-100-°C and P load = 200MPa. 4. Experiment Tribological experiments by Fuchs Lubricants GmbH, allowed investigations on the lubricating film structure, thickening effect, surface adhesion, friction and shear strength. Results of Ball-on-disc experiments (Fig.3a) show the superiority of the novel star polymers, compared to commercial additives mixed in a model base oil (Fig.3b), with an agreement of the experimental and simulation results. Further experiments proved the star polymers mode of action also for a fully additivated engine oil (Fig.3c). Figure 3: Schematic tribological ball-on-disc setup (a), friction coefficient results for various rolling speeds and additives mixed in a model base oil (b) and mixed in a fully additivated engine oil (c).