Abstract
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.
Highlights
The sliding friction between metals is a complex process, often involving several mechanisms that evolve and vary with increasing sliding distance, number of passages, etc
The present investigation has illuminated the complexity of unlubricated sliding between metals, and presented interesting exam ples of the many possible mechanisms involved in determining the friction levels, the material transfer and the roughening of the surfaces
Based on the findings in the present investigation of highly loaded, unlubricated sliding of three selected material combinations, (PM tool steel/aluminium, PM tool steel/case hardened steel and PM tool steel/ PM tool steel), the following conclusions can be drawn:
Summary
The sliding friction between metals is a complex process, often involving several mechanisms that evolve and vary with increasing sliding distance, number of passages, etc. In a short-time perspective, the friction coefficient will typically fluctuate and shift quite rapidly, often in a seemingly random way. If starting out from smooth, polished metal surfaces, the unlubricated friction coefficient may initially be quite low, let us say in the 0.1–0.2 region. This is relatively close to typical levels for corresponding boundary lubricated contacts. This low level typically does not last long Often it is so short-lived that it is not even noticed in a typical friction test [1,2,3,4,5]. A third mechanism is the initial transfer that roughens the “receiving” surface [1,6,7,8]
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