Nanostructured nitride films of multi-element high-entropy alloys by reactive DC sputtering

Multi-element high-entropy alloys are alloy systems with n (5≤n≤13) principal elements each having an atomic percentage no more than 35%. Using the alloys of Fe-Co-Ni-Cr-Cu-Al-Mn and Fe-Co-Ni-Cr-Cu-Al0.5 as target material in reactive sputtering, nitride films were deposited. The hysteresis curves of the two high-entropy alloys in reactive sputtering are quite different in comparison to those of elements or simple alloys. The film deposition rate decreased with increasing nitrogen gas flow and the highest film thickness was in excess of 2.5 μm. The alloy films are crystalline with structures of a mixed FCC and BCC or simple FCC solid solution, while the crystallinity of the nitride films decreased and approached amorphous with increasing nitrogen gas flow. The composition of the alloy films was similar to their original targets, and the nitrogen content of the nitride films increased with increasing nitrogen flow, to a maximum of 41.1 at.% nitrogen. The values of resistivity of the two alloy films were 108 and 135 μΩ cm, respectively, and those of their nitride films increased with nitrogen flow, to a factor of 3 of the alloy film. The rms surface roughness measured by AFM decreased significantly from 9 to 13 nm for the alloy films to only 1–3 nm for the nitride films. Values of hardness are about 4 GPa for alloy films and about 11 GPa for nitride films. The growth rate, the resistivity, and the hardness of the resulting nitride films were not affected too much by substrate bias due to the amorphous nature of the nitride films.