Effect of additives on friction during plane strain compression of aluminium strip

The paper describes an experimental investigation of the tribology during plane strain compression testing (PSCT) of aluminium strip with steel tools, lubricated by mineral base oils containing either stearic acid or aluminium stearate as additive in concentrations up to 0.5%. The present study is a follow-up to the work of Sutcliffe et al. [Boundary lubrication of aluminium strip during plane strain compression, Wear 254 (2003) 65–79]. The close conformance of the strip and tool indicate that boundary lubrication should play a significant role in the tribology. The new work supports the conclusions of the previous study, that for the first indent hydrodynamic influences are important, while additives do not affect friction. After a number of indentations a transfer layer builds up on the tool. Depending on the additive concentration and tool roughness, this may either lead to a rise or fall in friction factor. Scanning electron microscope and optical observations show how the transfer layer can either form a smooth mottled appearance which is easily removed using tissue paper for higher additive concentrations, or takes the form of strongly attached particles for lower additive concentrations or in the absence of additives. Similar behaviour is observed for the two types of additive, indicating that the stearate chain in the soap is able to react with the Fe-based tool as effectively as when it is part of the acid—and probably also with the Al strip.