Effect of aluminum content on the microstructure and properties of plasma electrolytic oxidation coatings on AlxCoCrFeNi high-entropy alloys

Because high-entropy alloys (HEAs) contain multiple major elements, the growth behavior of their plasma electrolytic oxidation (PEO) coatings differs significantly from that of traditional valve metals. Introducing oxide coatings can effectively compensate for insufficient wear resistance caused by elemental segregation, but the regulatory mechanism of valve metal content on the structure and properties of the HEAs-PEO coatings still needs further clarification. Therefore, this study used PEO technology to treat AlxCoCrFeNi high-entropy alloys (x = 0, 0.25, 0.5, and 1) and systematically investigated the effects of Al content on the microstructure, tribological properties, and electrochemical corrosion behavior of the PEO coating. The results show that with increasing Al content, the surface roughness of the PEO coating increases from approximately 3.3 μm (Al0) to 4.8 μm (Al1). Meanwhile, the surface porosity showed a trend of first decreasing and then increasing, with the Al0.25 sample exhibiting the lowest porosity of only 7.03%, significantly lower than the 11.59% of the Al0 sample, and its Vickers hardness reached 500 HV0.5. Tribological tests showed that under the three friction pair conditions of GCR15, ZrO2, and 304 stainless steel, the Al0.25 sample exhibited the lowest average coefficient of friction, approximately 0.41, 0.45, and 0.6, respectively, demonstrating excellent and stable wear resistance. Wear track morphology analysis indicated that it had the narrowest wear track width, and the iron oxide layer and CoAl2O4 lubricating phase formed during wear effectively reduced frictional shear stress. Electrochemical testing results showed that the corrosion resistance of the PEO coating significantly increased with increasing Al content. Among the samples, the Al1 sample exhibited the lowest corrosion current density of only 2.22 × 10−10 A cm−2, nearly 3 orders of magnitude lower than the Al0 sample (2.14 × 10−7 A cm−2). Simultaneously, its charge transfer resistance reached a high value of 1.14 × 108 Ω cm2, indicating that the thicker and denser PEO coating significantly improved its barrier ability against corrosive media. However, excessively high Al content led to increased coating porosity and the formation of CoAl2O4 impurity phases, negatively impacting wear resistance. In summary, Al content significantly modulates the structure and service performance of the AlxCoCrFeNi high-entropy alloy PEO coatings: the Al0.25 sample showed the best wear resistance, while the Al1 sample exhibited the best corrosion resistance. This study provides experimental evidence and theoretical reference for the compositional design and synergistic optimization of wear and corrosion resistance in high-entropy alloy PEO coatings.