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Boronizing treatment has been considered as a promising strategy to enhance the surface hardness, self-lubrication and wear resistance of high entropy alloy (HEA), endowing the advancements of HEA in engineering applications. However, the anti-friction, anti-wear and corrosion resistance properties of the boronized HEA in sulfuric acid and seawater environments remain unclear. Here, the self-lubrication and corrosion resistance of the boronized CoCrFeNiMn HEA in these corrosive media were studied to assess the applicable environments. Firstly, a boronized layer with thickness of approximately 25 μm on the CoCrFeNiMn HEA surface was fabricated using the powder-pack boronizing treatment. Tribological assessments demonstrated exceptional self-lubricating properties of boronized HEA in various liquid conditions, with over 30 % reduced friction coefficients and over 80 % decreased wear rates. XPS analysis confirmed the formation of a boron-containing tribofilm, enhancing self-lubrication and reducing friction and wear in liquid environments. However, electrochemical tests indicated that the corrosion resistance of the boronized HEA was significantly affected by the liquid medium. The impedance value (|Z|) of boronized HEA decreased to 4.6 % of that of the original HEA in 3.5 wt% NaCl solution, revealing poor corrosion resistance. Conversely, an enhancement of corrosion resistance was observed in 0.05 M H 2 SO 4 solutions with the increase of the |Z| value by 8 times. Therefore, for applications requiring service safety, boronizing HEAs are more suitable for acidic environments, but not in seawater environments, offering valuable insights for the industrial application of CoCrFeNiMn HEA in reducing friction and wear. • The COF of boronized HEA is over 30 % lower than original HEA in liquid conditions. • The tribofilm (composed of H 3 BO 3 and B 2 O 3) significantly reduce friction and wear. • Boronized HEAs are more suitable for acidic environments, but not in seawater. [ABSTRACT FROM AUTHOR]

Boronizing has been considered as a promising strategy to enhance the surface hardness and corrosion resistance of the new emerged alloys, like CoCrNi medium-entropy alloy (MEA). However, recent studies showed that a brittle silicide layer was generated on the surface for Ni-containing alloys boronizied with the SiC-containing powder (SiC+KBF 4 +B 4 C). In this study, Si-free powder packing (900 °C 4 h) and electrochemical boronizing (900 °C 30 min) methods were explored to obtain the boronized layer for CoCrNi MEA. The morphology, element distributions and phases of the boronized layer were characterized by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometer (EDS) and X-ray diffraction (XRD). The results show that the powder-pack boronizing method with 90 wt% B 4 C + 10 wt% NaBF 4 (B 4 C) and electrochemical (EC) methods have the thickest boronized layer (~70 μm) with hardness of about 22 GPa. The boronized layer is dominated by columnar CrB and Co 2 B with obvious preferred orientation and strong texture. In addition, CoB, Ni 2 B and Ni 3 B also exists in the boronized layer, and these two optimal boronizing processes avoid the influence of Si. This study shows that the B 4 C and EC boronizing methods can be considered as the viable alternative strategy for the preparation of strengthened layers. • The optimal boronizing methods were selected to obtain the strengthened layers. • Si-free boride layers were observed on the CoCrNi MEA surface. • The surface hardness reached to 17–22 GPa. • Borides in the boronized layer have obvious preferred orientation and strong texture. [ABSTRACT FROM AUTHOR]

Boronizing has been considered as a promising strategy to enhance the surface hardness and corrosion resistance of the new emerged alloys, like CoCrNi medium-entropy alloy (MEA). However, recent studies showed that a brittle silicide layer was generated on the surface for Ni-containing alloys boronizied with the SiC-containing powder (SiC+KBF 4 +B 4 C). In this study, Si-free powder packing (900 °C 4 h) and electrochemical boronizing (900 °C 30 min) methods were explored to obtain the boronized layer for CoCrNi MEA. The morphology, element distributions and phases of the boronized layer were characterized by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometer (EDS) and X-ray diffraction (XRD). The results show that the powder-pack boronizing method with 90 wt% B 4 C + 10 wt% NaBF 4 (B 4 C) and electrochemical (EC) methods have the thickest boronized layer (~70 μm) with hardness of about 22 GPa. The boronized layer is dominated by columnar CrB and Co 2 B with obvious preferred orientation and strong texture. In addition, CoB, Ni 2 B and Ni 3 B also exists in the boronized layer, and these two optimal boronizing processes avoid the influence of Si. This study shows that the B 4 C and EC boronizing methods can be considered as the viable alternative strategy for the preparation of strengthened layers. • The optimal boronizing methods were selected to obtain the strengthened layers. • Si-free boride layers were observed on the CoCrNi MEA surface. • The surface hardness reached to 17–22 GPa. • Borides in the boronized layer have obvious preferred orientation and strong texture. [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]

The boronizing treatment has been an effective way to strengthen and enhance the lubrication performance of alloys. However, the traditional power-pack boronizing method has been limited due to its time-consuming. Improving the boronizing efficiency is highly demanded for the strengthening and industry application of the high-entropy alloys (HEAs). In this study, the electrochemical boronizing strategy was applied to improve the surface hardness and self-lubrication performance of AlCoCrFeNi 2.1 eutectic HEA (EHEA). The microstructure, phase formation and hardness of boronized alloys were studied. Results confirm the high efficiency of electrochemical boronizing to form a boronized layer with a thickness of about 35 μm in 30 min. The surface hardness of the boronized AlCoCrFeNi 2.1 HEA reached to about 18 GPa due to the presence of NiB, FeB and Cr 3 B 4 in the boronized layer. Tribological tests reveal that the self-lubrication property of AlCoCrFeNi 2.1 HEA in water condition was significantly enhanced by the electrochemical boronizing treatment, e.g., the friction and wear reductions reach to about 25% and 80%, respectively. Lubrication mechanism investigation by X-ray photoelectron spectroscopy (XPS) analysis showed the formation of a H 3 BO 3 tribofilm on the sliding surface during the friction process, which effectively reduce friction and wear. This study provided a novel and fast approach for surface hardening and self-lubrication enhancement of HEA systems using the electrochemical boronizing strategy. • A boronized layer was obtained on the AlCoCrFeNi 2.1 alloy surface in 30 min. • The surface hardness was about 4 times higher than that of the original EHEA. • The self-lubrication performance of boronized EHEAs in the water is excellent. • The lubrication enhancement was due to the in-situ formation of H 3 BO 3 tribofilm. [ABSTRACT FROM AUTHOR]