Tuning substrate temperature for improved adhesion and mechanical properties of magnetron sputtered high entropy alloy thin-films

This work reports on the deposition of non-equiatomic CoCrFeNi high entropy alloy (HEA) thin films at various substrate temperatures (Room Temperature (RT), 200 °C, 300 °C, and 400 °C) on EN-24 steel substrates. The deposited films exhibited a preferred {111} crystallographic orientation and possessed a single-phase face centred cubic (FCC) crystal structure. The roughness of the film (R_rms) gradually increased from ∼ 1 nm to ∼ 4 nm as the particle size grew from ∼ 20 nm to ∼ 37 nm simultaneously as the substrate temperature increased from 200 °C to 400 °C, indicating an enhancement in atomic mobility across intergranular interfaces. The hardness of the film reached to a maximum of ⁓ 17 GPa for the film fabricated at 400 °C. This increase is attributed to improved crystallinity, preferential growth orientation and higher columnar density. Notice that this hardness significantly exceeded that of steel substrate, nearly fourfold. The films deposited at 200 °C and 300 °C exhibited the exceptional adhesion in the incremental load scratch tests, with no signs of delamination. On the other hand, the films deposited at room temperature and 400 °C delaminated during the scratch test. The diffusion bond established between the film and the substrate contributed significantly towards the outstanding adhesion of the film, as evidenced by the cross-sectional Transmission Electron Microscopy (TEM) analysis.