RF magnetron sputtering of Ti-Zr-Hf-Co-Ni-Cu high-entropy metallic glass thin films: Tailoring surface roughness, hardness, and strain rate sensitivity through substrate bias and temperature

Ti-Zr-Hf-Co-Ni-Cu high-entropy metallic glass thin films (HEMG-TFs) were first deposited on SiO₂/Si substrates by radio-frequency (RF) magnetron sputtering, and the effects of substrate bias and temperature on their microstructure, surface morphology, mechanical properties and strain rate sensitivities were systematically investigated. Under all deposition conditions studied, the HEMG-TFs remained fully amorphous, showed uniform elemental distribution, and maintained compositions close to that of the target, indicating that RF sputtering was effective for preparing compositionally and structurally stable Ti-Zr-Hf-Co-Ni-Cu HEMG-TFs. A major outcome of this work was that RF magnetron sputtering enabled an exceptionally smooth surface in these films. With increasing negative substrate bias, the surface roughness first decreased and then increased, reaching a minimum roughness value of 0.26 ± 0.003 nm at −100 V, which was the lowest value reported for Ti-Zr-Hf-Co-Ni-Cu HEMG-TFs. Increasing substrate temperature also reduced the surface roughness. Simultaneously, the hardness and elastic modulus increased continuously with increasing substrate bias and temperature. More importantly, the HEMG-TFs exhibited extremely high strain rate sensitivity via controlling substrate temperature and bias with values of 0.16–0.22 which were markedly higher than those reported for direct-current (DC)-sputtered Ti-Zr-Hf-Co-Ni-Cu films and most other metallic thin films. The present results showed that RF magnetron sputtering provided an effective route to concurrently achieve ultralow roughness and excellent deformation adaptability, making these Ti-Zr-Hf-Co-Ni-Cu HEMG-TFs promising for micro-/nano-scale devices and advanced surface coating applications.