Tuning microstructure and enhancing mechanical properties of Co-Ni-V-Al medium entropy alloy thin films via deposition power

Abstract

Recently, medium-entropy alloys (MEAs) combining high hardness with excellent ductility have attracted numerous attentions. Here we synthesize a series of Co-Ni-V-Al MEA thin films by magnetron sputtering at room temperature with deposition power from 60 W to 300 W. The film microstructure, morphology, and mechanical properties depended remarkably on atomic fluence, proportional to deposition power. With increasing atomic fluence, the amorphous phase fraction experienced a process of first decreasing and then increasing, and fully amorphous structure was obtained at 300 W. Surface diffusion is dominated in low incident atomic energy range, while deposition rate effect is dominant over surface diffusion effect in high incident atomic energy range, resulting in the crossover in phase selection. The nanocolumn size increased with atomic fluence from 60 W-films to 150 W-films, accompanied by roughness rise, and remained constant with further increasing atomic fluence, along with roughness drop caused by fully amorphous structure. Excellent mechanical properties including higher hardness, tensile fracture strength, lateral Young’s modulus, better scratch-resistance, lower coefficient of friction, were observed for higher incident atomic energy films. In low atomic fluence range from 60 W-films to 150 W-films, the improved mechanical response mainly come from the reduced fraction of interfacial region between adjacent nanocolumns. Further increasing atomic fluence, it is the bombardment-induced dense nanocolumn boundaries, rather than enhanced adatom diffusion at high atomic fluence, that cause further improved mechanical response. Our current work could pave a way for a controlled synthesis of high-performance MEA thin films via tuning deposition power.