Hardening of Cr–Cu–Al–Fe–O High-Entropy Composite Films

The Cr–Cu–Al–Fe–O high-entropy oxide films were synthesized on Si (100) substrate by electron beam evaporating a four-layer metal film and consequent annealing of the film in air. X-ray diffraction, scanning electron microscope, energy-dispersive spectroscopy, transmission electron microscope, and nano-indentation methods were used to characterize the films. The as-deposited film was composed of FCC and BCC phases and had a dense surface. After annealing at 800 and 1000 °C in air, oxidative diffusion took place among the Cr–Cu–Al–Fe metal multilayer. The inter-diffusion and oxidation behaviors resulted in the formation of a three-layer composite film: spinel layer, copper-doped SiO_x layer, and Co oxide layer. The unreacted Cu grains were also found. At 1000 °C, the hardness and elastic modulus of the composite film increased to 9.5 and 147.63 GPa, respectively.