The friction size effect in thin-sheet microforming constrains the attainable forming quality of microscale sheet components. In this study, T2 copper foils with thicknesses of 0.04, 0.08, 0.16, and 0.32 mm were investigated by comparative tensile testing, pin-on-disk testing, sliding-friction experiments, surface characterization, and reduced-order analysis under dry friction and three liquid-lubrication conditions. The results showed that, as the thickness decreased from 0.32 mm to 0.04 mm, elongation and tensile strength decreased by nearly 60% and 40%, respectively, whereas the direct contribution of the mechanical size effect to the friction coefficient remained limited. Under dry friction, the friction coefficient changed little with specimen size. Under soybean oil, castor oil, and Vaseline lubrication, however, the friction coefficient increased markedly as specimen size decreased and gradually approached the dry-friction value; the lowest-viscosity lubricant exhibited the greatest loss of effectiveness at small scales. This behavior was associated with the expansion of the edge non-lubricated region and the loss of closed lubricant pockets, both of which increased the real contact area. On this basis, a size-dependent friction model was established for the present material and surface conditions, and its prediction for the castor oil case was consistent with the experimental trend.
Sun et al. (Sat,) studied this question.