On the critical roles of initial plastic deformation and material transfer on the sliding friction between metals

It is well known that the friction level rapidly shifts during the initial sliding between two unlubricated metals. Often this change is so rapid that the initial friction level is not even noticed, owing to initial surface contamination or roughening of the contacting surfaces, either due to plastic deformation and wear, or transfer. The present work uses a unique experimental set-up – the Uppsala Load Scanner – to generate detailed information on friction and surface modification in initial sliding contact between pairs of crossed metal cylinders. A wide range of loads, each exclusively coupled to a well-defined contact area on both contacting bodies, is evaluated in a single test. By performing repeated scans, intertwined with high resolution FEG-SEM studies of the contact surfaces, investigations of the couplings between local load, friction, surface modification, topography, transfer, etc. is facilitated and the progress of deformation and friction can be followed. The SEM studies also include local FIB cross sectioning of selected samples to learn more about the sub-surface modifications. The materials selected include a PM tool steel, a case-hardened component steel and an aluminium alloy. All tests were conducted in dry contact. The roles of the large initial surface roughening or flattening, the following material transfer and other gradual surface modifications in determining the friction level, are discussed.