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CoCrNi medium‐entropy alloy (MEA) is a promising material for aviation and aerospace applications due to its excellent strength and ductility. However, its low surface hardness and poor wear resistance pose challenges for its tribological performance. To address this issue, electrochemical boronizing is explored as a potential solution, offering advantages such as a fast boronizing rate and low cost. This study demonstrates that electrochemical boronizing can significantly enhance the surface hardness and self‐lubrication performance of CoCrNi MEA, as well as retain impressive mechanical performance. Results show that a boronized layer with a thickness of about 50 μm is formed on the surface in 45 min. The boronized layer is mainly composed of Co2B, Ni2B, CrB, CoB, and Ni3B. After boronizing, the surface hardness of the sample increases by about 9 times, and the ultimate tensile strength and uniform elongation can reach 97% and 73% of the original sample, respectively. Compared with the original alloy, the friction coefficient and wear rate of boronized MEA are reduced by 38% and 85% under deionized water conditions, respectively. X‐ray photoelectron spectroscopy analysis reveals the generation of boron‐containing tribofilm on the wear scars, which significantly reduces friction and wear during the friction process. [ABSTRACT FROM AUTHOR]

The as-cast AlCoCrFeNi 2.1 eutectic high-entropy alloy (EHEA) has high fracture strength and ductility, which displays an excellent application prospect in key structural materials. However, its low surface hardness and poor wear resistance at room temperature limit the actual applications in tribology. Here we showed the enhancement of surface strengthening and self-lubrication properties of AlCoCrFeNi 2.1 EHEA by a powder-pack boronizing treatment. SEM and XRD analysis indicate that a double affected layer with a thickness of about 42 μm was formed on the surface at 900 °C for 4 h, which were mainly consisted of Ni 2 Si and FeB. The surface hardness of the alloy was significantly increased from 396 HV to 1108 HV after the treatment. Tribological results show a remarkable self-lubrication property of the boronized EHEA. The friction coefficient of boronized EHEA under the water condition was reduced by 21%, and the corresponding wear volume was decreased by 65%. EDS and XPS analysis demonstrate the formation of silicon/boron-containing tribofilm on the wear scar, which benefits in the achievement of self-lubrication properties with friction and wear reduction under the water condition. Our research provides a general strategy with the powder-pack boronizing method to strengthen AlCoCrFeNi 2.1 EHEA and enhance self-lubrication properties, which would certainly boost its industry application. • An impressive enhancement of surface hardness was achieved after boronizing. • The affected layer showed a double-layer structure, which was mainly Ni 2 Si and FeB. • Boronized EHEA showed the remarkable self-lubrication property in water. • The formed silicon/boron lubrication film benefited in self-lubrication property. [ABSTRACT FROM AUTHOR]

The CoCrNi medium-entropy alloy (MEA) exhibits an impressive ductility, but its low hardness (∼352 HV) and poor wear resistance limit its engineering applications. In this study, a significant increase of surface hardness and enhancement of self-lubrication were achieved for the CoCrNi MEA using powder-pack boronizing. Results indicate that dual reaction layers with a total thickness of about 50 μm were formed on the surface. These two strengthened layers are mainly consisted of silicide (Ni 2 Si) and borides (CoB, Cr 5 B 3). The hardness of the affected region reaches to 11–21 GPa dependence on the cross-sectional depth, being about 6 times higher than that of untreated MEA. In the deionized water condition, the friction and wear rate reductions of the boronizing treated specimen reach to about 25% and 63%, respectively. Energy Dispersive Spectrometer (EDS) and X-ray Photoelectron Spectroscopy (XPS) analysis on the wear scars showed that a silicon/boron-containing tribofilm was formed under the water environment, which reduces the friction coefficient and enhances the wear resistance. Our study demonstrates that powder-pack boronizing is a powerful way to enhance surface hardness and self-lubrication properties of CoCrNi MEA. • Double-strengthened layers were observed on the boronized CoCrNi alloy surface. • An obvious improvement of surface hardness was obtained. • The self-lubrication performance of boronized CoCrNi MEAs in the water is excellent. • The self-lubrication enhancement was due to the formation of silicon-boron based tribofilm. [ABSTRACT FROM AUTHOR]