Effect of substrate bias on the microstructure and mechanical and tribological properties of ZrNbTiMo refractory high entropy alloy film

ZrNbTiMo refractory high entropy alloy (RHEA) films were deposited via direct current magnetron sputtering under different substrate biases (0 to-200 V). The phase structure, microstructure, surface roughness, me-chanical, and tribological properties of the ZrNbTiMo RHEA films were investigated. Results show that the ZrNbTiMo RHEA films exhibit a single body-centered-cubic structure. As the substrate bias increases, the preferred orientation of the ZrNbTiMo RHEA films changes from (110) to (200). The surface roughness of the films decreases from 3.9 nm to 2.0 nm then increases to 4.1 nm. The growth mechanism can be understood as the result of the interaction of ion incident direction, ion bombardment effect, and atom diffusion ability which are controlled by the substrate biases. The hardness of the films increases from 6.8 GPa (0 V) to 21.3 GPa (-200 V). The maximum fracture toughness (0.968 MPa x m1/2) is obtained under-200 V. The wear resistance does not exhibit a monotonically increasing trend with the increase of substrate bias. The films deposited under-50 and -100 V have lower values of H/E and H3/E2, but these films still possess better wear resistance. The wear mechanism of the ZrNbTiMo RHEA films is investigated in detail from the microstructure perspective.