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

Abstract

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.

An investigation on the friction losses between cylinder liner and piston rings

“…Steel ring material has obviously shown superiority in having less friction with TWA aluminum cylinder liner compared to the two other materials, cast iron ring and ductile iron plasma molly faced ring. The surface of steel gives the overall friction among these materials, this was investigated by Viktoria Westlund et al [20] which concluded that smooth sliding surfaces were consistently and strongly related to low sliding friction. The results of our study have showed that ductile iron is still better choice for friction than cast iron.…”

Advances in friction of aluminium alloy deep drawing

“…5(a) that the surface hardness of aluminium alloys is significantly less than that of tool steel. The plastic deformation of aluminium pin tip gradually increases with the increase of sliding distance s u [17]. The scratches during the friction process cause the aluminium alloy detachment from surface, which is easy to adhere to tool (Fig.…”

“…Aluminium chips are easy to adhere to die surface. Accompanied by the oxidation reaction that might occur, a transfer layer with complex compositions will be formed [17]. Because of the significant adhesion between the die and the aluminium alloy sheet, the AADD system has unique friction characteristics.…”

Coefficient of friction of dry sliding Al 6061-T6 alloy under different loading conditions

“…Most of the researchers (Wang et al , 2010; Westlund et al , 2018) have studied the wear and friction characteristics of aluminium alloys by considering steel as a counter-face material, having a flat on flat contact configuration, during the experiments. Few researchers (Dewan and Mohammad, 2012; Pan et al , 2018; Srinivasula Reddy and Kaliveeran, 2019a) focused on the friction behaviour of aluminium-to-aluminium metal alloy during full sliding.…”