Characterisation of the flattening behaviour of modelled asperities

In metal-forming processes tribological conditions between tool and workpiece are of greatest importance for process quality and process feasibility. This is even truer for microforming applications, where at least two dimensions of the workpiece are in the sub-millimetre range, due to increasing friction when process dimensions are scaled down. This effect can be explained by the model of open and closed lubricant pockets characterising the workpiece surface and the invariance of topography to scaling. As the number of workpiece asperities contacting the tool is drastically reduced the flattening behaviour of single asperities is of major interest for characterising tribology in microforming processes in more detail. Especially, a topography emerging on top of flattened asperites, the so-called nanotopography and its impact on the friction conditions has to be considered. Modelled asperities represented by pyramids with a base area of 120 μm × 120 μm and a height of 32 μm are flattened with a high-resolution experimental setup which enables in situ observation of the contact area. In-process measurement is complemented by post-process analysing the topography by confocal microscopy and scanning probe microscopy. This paper will show that a nanotopography on top of flattened asperities can emerge under certain geometrical conditions and that it has an impact on the friction conditions in the tool/workpiece interface. The detailed knowledge about the evolution of surface topography is relevant in particular to microforming but also for an improved understanding of tribological phenomena in general.