The evolution of hardness in Cu-W alloy thin films

A series of Cu-W films with the W content higher than 17.2 at% were prepared by magnetron sputtering focused co-deposition technology. The micro-structures and nanoidentation hardness of the films were investigated. Except for the fcc, amorphous and bcc phases, a novel multilayered structure has been observed at the range of about 17.2-90 at% W. Within the multilayered structure, the high-density interfaces can inhibit the motion of dislocations, which resulted in the hardness at least 2 GPa higher than that of the mixing rule. Depending on different microstructures, the hardness curve of the Cu-W films can be divided into three regions instead of the linear increase with the W content. From 17.2-40 at% W, the hardness increased slowly due to lots of amorphous phase. Within 40-65 at% W, the hardness increased linearly, which was caused by the gradually obvious multilayered structure and decreasing amorphous content. Above 65 at% W, single-phase body-centered cubic (bcc) films were observed. In this region, the upward trend of hardness slowed down due to the porous columnar structure and gradually disappeared multilayered structure.