Tribology of plane strain compression tests on aluminium strip using ToF-SIMS analysis of transfer films

An experimental investigation of the tribology during plane strain compression of aluminium strip using steel tools is described. A formulated lubricant containing ester, and hexadecane containing 0, 0.05 and 0.5% stearic acid, are used as lubricants. The friction factor is measured for a wide variety of process conditions and correlated with surface analysis techniques including ToF-SIMS (time-of-flight secondary ion mass spectroscopy). The roughness of the tool is clearly seen imprinted on the strip, indicating that any hydrodynamic or hydrostatic film separating the tool and strip is very small. With clean tools and longitudinal strip roughness, the friction stress equals the shear yield stress of the metal. For transverse strip roughness, hydrodynamic or hydrostatic effects lead to a reduced friction factor. With these cleaned tools there is no effect of additive concentration. After a small number of indents a transfer film builds up on the tool, leading to a fall in friction factor. The strip takes on a smeared appearance with black lines running in the direction of relative slip between strip and tool. This transfer layer is of a ‘low friction slurry’ type, which is easily wiped-off using tissue paper. The effectiveness of this transfer layer in reducing friction increases with increasing indentation speed and additive concentration. ToF-SIMS analysis shows that this transfer layer contains aluminium and aluminium stearate—the latter being formed specifically by reaction of stearic acid with fresh metal surface formed during deformation.