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Dynamics of sliding metal surfaces as case study for complex systems

Subject Area Mechanical Properties of Metallic Materials and their Microstructural Origins
Synthesis and Properties of Functional Materials
Term from 2007 to 2013
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 53244514
 
Final Report Year 2013

Final Report Abstract

When the surfaces of two machine components are in sliding contact, the direct view at the sliding interface where friction is generated is blocked. Therefore the sliding contact is often called a “buried interface”. This hindrance makes it notoriously difficult to study and predict tribological behavior. To make it worse, physical and chemical effects on many length scales affect friction and wear properties. An analysis of the problem after an experiment has finished is often not very meaningful because tribological processes like wear are strongly non-linear and do not reflect the dynamics that occur within the tribocontact. For a better understanding of those processes it is needed to obtain time-resolved data of friction forces, wear, topography and other important quantities. The number of in-situ experiments in modern Tribology is thus constantly rising and includes the measurement of lubricant film thickness, transfer film formation, chemistry and wear. Combining a submicrometer-precise positioning system and a novel digital holographic microscope for fast measurement of topography we were able to develop an experimental platform which allows for the first time to acquire the three dimensional topography of the sliding track of a metal surface under lubrication with up to 15 frames per second. The topopgraphical resolution of this method is in the order of several nanometers while the lateral resolution is comparable to an optical microscope. Therefore we are using atomic force microscopy in addition for the measurement of topography with even higher lateral resolution. The time-resolved topography measurements allowed a view at the formation of wear particles in a lubricated metallic sliding contact. We found different mechanisms that are working in parallel within the copper/steel tribosystem. We observed that some ten to hundreds of nanometers deep voids constantly appear and vanish on the copper surface. This happens when due to the strong shear gradient within the nearsurface zone becomes nanocrystalline. At the same time crack-like features grow within the nanocrystalline zone that extend parallel to the surface and finally leave the material as wear particles. The resulting crack edges rapidly heal by plastic processes. Therefore the location where single wear particles originated from cannot be seen anymore after a few cycles of continuing sliding. The experiments that were carried out on copper also help to understand the evolution of the friction. Plowing copper out of the sliding track causes a large fraction of the initial friction. But the plowing contribution is not constant and therefore the coefficient of friction decreases quickly. The in-situ observation of the sliding track allowed us to discriminate plowing friction from sliding friction as a function of time. With adding just a few percent of zinc the behavior of the friction coefficient becomes more complicated. We found that besides the formation of a grain-refined zone underneath the sliding surfaces, also the composition of the material changes significantly. In the early stage of sliding the near surface becomes enriched with zinc oxide. Zinc oxide however is well known for its friction reducing effect. Therefore we believe that alloys can provide low friction if they contain a low friction phase that is able to segregate to the surface.

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