Size and passivation effects in the torsion of thin metallic wires

Torsion tests are performed on microscale copper wires of diameters ranging from 25 to 50 μm, with and without passivation layers, to investigate the size and passivation effects. The copper wires coated with titanium are prepared by magnetron sputtering. An increase in the yield strength and flow stress arising from diameter reduction and the passivation layer is observed in the experiments. Compared with unpassivated wires, passivated wires exhibit more potent size effects. The normalized torque of passivated wires increases more significantly with the diameter decrease. The theoretical predictions based on the Fleck-Hutchinson strain gradient plasticity theory agree well with the experimental observations. The physical mechanism of the passivation layers affecting the torsional deformation of wires is elucidated.