Impact of Nano-fillers on Tribofilm-related Friction and Wear Reduction of Polymer Matrix Composites

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It has been shown recently that the tribological performance of polymer matrix composites can be improved by introducing an additional filler in the form of silica nanoparticles. The effect of friction and wear reduction was attributed to the formation of a thin tribofilm at the surface of the counter-body. The observed improvement of properties is beneficial in respect to potential applications e.g. as journal bearings. Up to now, the composition and micro/nanostructure of the tribofilm is not known and the mechanisms of friction and wear reduction are not understood. In order to find answers to these questions close cooperation between three research groups will be most effective for the following reasons: i) IVW-Kaiserslautern has already successfully prepared such composites, ii) BAM-Berlin has experience with nano-characterization of tribofilms and iii) ISPMS-Tomsk can simulate the sliding behaviour of such films.The Project will address at least three brand new issues: Firstly, silica nanoparticles will be synthesized with smaller diameters and more uniform size distributions compared to commercially available raw materials. Secondly, the thin and nano-structured surface films will be characterized by analytical transmission electron microscopy. The method of target preparation with a DualBeam instrument (combination of focused ion beam (FIB) milling and imaging with a high resolution scanning electron microscope (SEM)) has been developed recently. Thirdly, the results of nanostructural characterization will provide the input for modelling the sliding behaviour on the nanoscopic scale. Thus it will be possible to study the impact of volume fraction and distribution of nano-constituents systematically.Objectives are not only to understand how an approved sliding couple works, but also to establish a method for the development of new materials on the basis of modelling results. Our vision is to develop a system based on the formation of a stable tribofilm providing the desired friction level without any wear particle emission to the environment. We expect that this goal can be reached at least under steady state conditions which will apply for the major fraction of a bearing life time. For reaching a dynamic equilibrium matching the above demands the following prerequisites have to be fulfilled: i) Wear debris available at the surface forms a well adhering film. ii) Velocity accommodation occurs by detachment of nanoparticles from the film and granular flow within an interface layer. iii) After a certain sliding distance loose nanoparticles are re-deposited at the surface thus restoring the tribofilm. Modelling and sliding simulation will show under which conditions such processes take place. The results thus can be used to develop new material designs.