Effects of periods on the evolution of microstructure and mechanical properties of multilayered Cu-W films during thermal annealing

Benefiting from the unique structure, the copper‑tungsten (Cu-W) multilayered film is attractive due to its high strength and fracture toughness, diffusion barrier effects and radiation tolerance. However, the influence of period on the interfacial stability and strengthening mechanism during annealing has seldom been touched. Hence, the Cu-W multilayers with the periods from 1.6 to 27 nm were synthesized by altering substrate rotation speed using co-sputtering. In contrast to the findings in literature by sequential sputtering, kinetically constrained deposited atoms contribute to the more amorphous phase near interface in the films with smaller periods and result in the reduced hardness. During annealing, Cu and W atoms are thermal-activated and up-hill diffuse towards Cu- and W-rich layers, respectively, which can not only eliminate the amorphous phases but also enlarge the concentration ratio and result in the spinodal decomposition. In turn, the films with larger periods present higher thermal resistibility against phase separation due to the stronger diffusion barrier effect of thicker W-rich layers. The mechanical properties of the annealed films generally increase with the temperature up to 400 °C due to the reduced amorphous phases and the enhanced interfacial strengthening. At 500 °C, the separation of soft Cu phase and the loss of coherency mainly lead to the reduced mechanical properties. It is striking that the Cu-W film with the moderate thickness exhibits the highest hardness at higher temperature of 500 °C, indicating the significant importance of period to control their high temperature mechanical properties.