Cold spraying was used to prepare Al/Al₂O₃ composite coatings. The Al₂O₃ content was controlled to increase the mechanical property and corrosion resistance of the composite coating. The inclusion of Al₂O₃ particles results in considerable plastic deformation of Al particles and grain size refinement in the coating. Additionally, the coating’s surface roughness decreased from 24.63 μm to 9.02 μm, and the porosity decreased from 6.34% to 2.07%. The increase in microhardness of the composite coatings was attributed to the combined effect of residual compressive stress, second phase strengthening of Al₂O₃, and plastic hardening of Al particles. The electrochemical test results indicate that the mass fractions of Al₂O₃ significantly affected the corrosion resistance of the Al/Al₂O₃ composite coating. Compared to the Al coating, the composite coating exhibited improved corrosion resistance, with a reduction in corrosion current density from 1.09 × 10⁻³ A/cm² to 2.67 × 10⁻⁶ A/cm² and an increase in corrosion potential from −1.57 V to −1.14 V. However, when the alumina particle content exceeded 17.7%, it led to more Al₂O₃ particle breakage, increasing the weak bonding interfaces in the composite coating, and consequently reducing its corrosion resistance.

CoCrNiTiAl_x high-entropy alloy films with varied Al contents were fabricated on 42CrMo steel substrates via magnetron sputtering. By adjusting the sputtering power of the Al target, an investigation was systematically carried out to explore the effect of different Al contents on the microstructural evolution, mechanical properties, and corrosion resistance of the film, with the underlying synergistic mechanism governing these properties being elucidated. With increasing Al content, the film microstructure gradually transforms from an amorphous phase at low Al contents to a nanocrystalline–amorphous composite structure, until it is converted into the BCC phase, and the film’s crystallinity exhibits a trend of first increasing and then decreasing. In terms of mechanical properties, the film hardness is significantly enhanced from 7.6 ± 1.3 GPa to 18.9 ± 1.1 GPa with increasing Al content, while the toughness gradually declines. Wear tests show that the film wear rate first decreases and then increases with rising Al content, reaching a minimum of 2.06 × 10⁻⁵ mm³/N·m. The superior protective state, characterized by a corrosion potential reaching −361.2 mV and corrosion current density dropping to 1.12 μA/cm², arises from the generation of an integrated, consistently structured composite passivation barrier in 3.5 wt.% solution. This study confirms that appropriate Al doping can synergistically optimize the microstructure, mechanical properties, and corrosion resistance of CoCrNiTiAl_x films, providing experimental and theoretical support for the compositional design and engineering applications of high-performance high-entropy alloy protective films.