On the Role of Material Transfer in Friction Between Metals : Initial Phenomena and Effects of Roughness and Boundary Lubrication in Sliding Between Aluminium and Tool Steels

In the absence of a lubricant, the friction we measure in sliding contact between metals is typically high and quite erratic, with rapid fluctuations. If we filter out these rapid fluctuations, we can typically also notice slower trends, which can lead to quite dramatic friction changes. Unless careful studies are performed, the cause to this behaviour cannot be understood. How come a material couple cannot be characterised with a specific coefficient of friction? The present paper sets out to add understanding to this area, by conduction and analysing an experimental series involving sliding between a needle-like aluminium tip against tool steel flats. The load is high enough to cause substantial plastic deformation of the aluminium needle; its tip becomes formed by the contact against the tool steel. These small-scale, low sliding distance tests facilitate detailed studies of the initial stages of various friction trends, and the effects of initial surface roughness and shifts of this roughness caused by material transfer between the sliding surfaces. Specifically, the effects on the transfer and friction behaviour from presence or absence of a boundary lubricant film and atmospheric oxygen were studied. It was found that very smooth sliding surfaces can offer low-friction conditions for these metal types. However, the smooth sliding interface is very fragile. In all unlubricated cases tested, it very rapidly (in less than a few mm sliding) became ruined due to transfer, and the friction level correspondingly increased. The boundary lubricant could only offer low friction in cases where the flat steel surface was very smooth. The lubricant also facilitated smoothening of transferred aluminium. As long has been well known, boundary lubrication films typically do not totally hinder direct metallic contact in solid to solid contact. The present results strengthen this view and further suggests that in these direct contacts one of the major friction reducing effects of the lubricant is to efficiently limit transfer, which otherwise acts to make the sliding surface rough