No surfaces are perfectly smooth, so the actual, i.e. real contact area between asperities is always smaller than the nominal contact area. Understanding the real contact area is crucial for predicting actual conditions in engineering contacts, particularly those with relative motion – sliding, where it also affects friction. This doctoral thesis studies the real contact area and friction behaviour of multi-asperity contacts between nominally flat surfaces under engineering-relevant sliding conditions. To determine the influence of load, surface topography and material properties, an in-situ optical test rig was developed for detailed analysis of the entire multi-asperity contact during sliding at submicron scale, which was previously not available in the literature. Results show the real contact area of hard metal contacts increases from static to sliding conditions. Real contact area and friction are primarily governed by surface roughness and asperity deformations depending on the applied normal load. The influence of material properties is lesser, with similar contact behaviour observed for all tested metals. Experimental results were compared to well-known theoretical real contact area and friction models, including classical friction laws, showing deviations due to model assumptions and highlighting the importance of considering surface roughness at the appropriate scale as a key variable.
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