Mechanical strengthening, stiffening, and oxidation behavior of pentatwinned Cu nanowires at near ambient temperatures

The complex effects of near ambient temperature exposure, i.e. 20-150 °C, on the oxidation and the mechanical properties of thermal solution grown faceted Cu nanowires were investigated. The mechanical behavior was quantified with experiments on individual Cu nanowires using a MEMS-based method for nanoscale mechanical property studies. The elastic modulus of pristine Cu nanowires with diameters 300-550 nm was 117 ± 1.2 GPa which agreed very well with polycrystalline bulk Cu, while the ultimate tensile strength was more than three times higher than bulk Cu, averaging 683 ± 55 MPa. Annealing at just 50 °C resulted in marked strengthening by almost 100% while the elastic modulus remained unchanged. Heat treatment in ambient air distinguished three different regimes of oxidation, namely the (a) formation of a thin passivation oxide at temperatures up to 50 °C, (b) formation of thermal oxide obeying an Arrhenius type process for Cu⁺ migration at temperatures higher than 70 °C, which was accelerated by grain boundary diffusion resulting in activation energies of 0.17-0.23 eV, and (c) complete oxidation following the Kirkendall effect at temperatures higher than 150 °C and for prolonged exposure times, which did not obey an Arrhenius law. Notably, the formation of a weaker and more compliant thermal Cu₂O did not compromise the effective strength and elastic modulus of oxidized Cu nanowires: experiments in Ar at temperatures higher than 70 °C showed mechanical strengthening by ∼50% and ultimate stiffening to ∼190 GPa, which is near the upper limit for the elastic modulus of single crystal Cu in the <111> direction.