In order to study the effect of Nb element on the microstructure and properties of FeCoCrNiMn high entropy alloy, FeCoCrNiMnNb high entropy alloy coating was prepared on the surface of Q235 steel by laser cladding technology. The phase composition, microstructure, element distribution, nanohardness and wear behavior of the coating were studied. The results show that the FeCoCrNiMnNb high entropy alloy coating is composed of FCC phase and Laves phase. Among them, the matrix phase is FCC phase and the rod-like structure is Laves phase. Due to the solid solution strengthening and second phase precipitation strengthening effect, the nanohardness of FeCoCrNiMnNb high entropy alloy coating is significantly improved to about 9.193 GPa, which is about twice that of FeCoCrNiMn coating. Due to the significant improvement of nanohardness, the FeCoCrNiMnNb coating has excellent wear resistance, and its wear rate is 2.549×10-5 mm3/N·m, which is about 0.33 times that of the FeCoCrNiMn coating. The wear mechanism of the FeCoCrNiMnNb coating is single abrasive wear. In summary, the FeCoCrNiMnNb high entropy alloy coating has extremely high nanohardness and excellent wear resistance. (Laser cladding; FeCoCrNiMnNb high entropy alloy coating; Nb; wear resistance; nanohardness) In traditional alloy systems, alloys are usually based on one or two main elements, and their properties are improved by adding a small amount of alloying elements [1-3]. In recent years, a new alloy design idea – high entropy alloy, which uses multiple components in equal or nearly equal molar ratios to prepare alloy materials, has attracted widespread attention from many scholars [4-7]. Among them, FeCr-
CoNiMn high entropy alloys have excellent corrosion resistance and ductility. For example, Cantor et al. [8] studied the microstructure and mechanical properties of FeCoCrNiMn alloy. Due to its single-phase FCC structure, the alloy has extremely high ductility and corrosion resistance, but its strength and wear resistance are poor. In order to improve the mechanical properties of high entropy alloys, preparing dual-phase or multi-phase high entropy alloys is an excellent design idea. Liu et al. [9] used laser cladding technology to prepare FeCoCrNiMnTi high entropy alloy coating on the surface of 40CrNiMoA. The coating is composed of FCC phase and Laves phase. Due to the extremely high strength and hardness of Laves phase, the microhardness of the cladding layer can reach 700HV, which is significantly better than the base material. Wu et al. [10] prepared FeCoCrNiMox coating on the surface of 316SS stainless steel. The results showed that when the molar fraction of Mo element was 0.25%, the microhardness of the coating was increased by 90% compared with the substrate, and the wear rate was reduced by nearly 40%. Nb element is a refractory element. Its introduction into the alloy as an alloying element can significantly improve the hardness and wear resistance of the alloy [11-12]. In addition, Nb element has a large atomic radius and will form Nb-rich Laves phase with alloying elements in Fe-CoCrNiMn high entropy alloy, which can significantly improve the mechanical properties of the material [13-15]. He et al. [16] studied the effect of Nb element on the microstructure and mechanical properties of FeCoCrNi high entropy alloy. The results showed that FeCoCrNiNb0.8 has extremely high microhardness and tensile strength. Wu et al. [17] used laser cladding technology to prepare FeCoCrNiNbx high entropy alloy coatings on the surface of 316SS stainless steel, and studied the structure, phase composition, microhardness and wear resistance of FeCoCrNiNbx. The results showed that when x = 0.3, a Nb-rich Laves phase was generated in the coating. Due to the second phase precipitation strengthening effect, the microhardness and wear resistance of the coating were significantly improved. However, at present, there are relatively few studies on the preparation of FeCoCrNiMnNb equimolar high entropy alloy coatings by laser cladding technology. In this experiment, FeCoCrNiMnNb equimolar high entropy alloy coatings were prepared by laser cladding technology, and the phase composition, microstructure, nanohardness and wear behavior of the coating were studied to prepare a high entropy alloy coating with multiphase structure, high hardness and excellent wear resistance, so as to expand the application range of FeCoCrNiMnNb equimolar high entropy alloy coatings. 1 Experiment
1.1 Experimental materials
Q235 steel plate was selected as the experimental substrate material with a size of 100 mm×100 mm×10 mm. Before laser cladding, the substrate was cleaned with an angle grinder to remove the surface oxide scale. The experiment used FeCoCrNiMn high entropy alloy powder with a particle size of 53~150 μm and niobium powder with a purity of 99.9% and a particle size of 40~60 μm as raw materials to prepare FeCoCrNiMnNb high entropy alloy coating.

1.2 Experimental equipment
The laser cladding equipment uses a fiber laser, and the FeCoCrNiMnNb and FeCoCrNiMn high entropy alloy coatings are prepared by coaxial powder feeding. After preliminary process tests, the process parameters obtained are: power 2 400 W, scanning rate 16.67 mm/s, powder feeding speed 20 g/min, overlap rate 30%, Ar gas protection during the cladding process, and the protective gas flow rate is 18 L/min.

1.3 Characterization
The metallographic specimens were prepared by the standard metallographic specimen preparation method and corroded with aqua regia for 20 s. The phase composition of the high entropy alloy coating was characterized by an X-ray diffractometer. The microstructure and element distribution of the coating were characterized by a thermal field emission scanning electron microscope and a matching energy spectrometer.

1.4 Nanohardness and wear resistance
To analyze the hardness of the coating surface, the nanohardness of the high entropy alloy coating surface was tested by a nanoindenter. The test load was 15 mN, the displacement speed of the indenter to the surface was 10 nm/s, and 5 experiments were performed on each coating. The average value was taken to reduce the experimental error. The friction factor of the high entropy alloy coating was tested by a plane reciprocating friction and wear tester. The friction pair used a GCr15 steel ball with a hardness of 60 HRC, the load was 15 N, the frequency was 60 Hz, the wear scar length was 7 mm, and the wear time was 30 min. After the wear test, the wear morphology of the high entropy alloy coating was observed by scanning electron microscopy, and the wear mechanism was analyzed.

2 Results and discussion
2.1 Phase composition
The phase composition of FeCoCrNiMnNb and FeCoCrNiMn high entropy alloy coatings is shown in Figure 1. The results show that the FeCoCrNiMn coating is a single FCC structure. The FeCoCrNiMnNb coating not only contains FCC structure, but also has diffraction peaks of other phase structures at other locations. The diffraction peaks at these locations are Laves phases, which are analyzed by Jade 6 software. Since the atomic radius of Nb is much larger than that of Fe, Cr, Co, Ni, and Mn, it is difficult for Nb element to be infinitely dissolved in FCC solid solution [18]. In addition, Nb element will cause FCC phase instability and induce severe lattice distortion, thereby inducing the formation of Laves phase rich in Nb element.

2.2 Microstructure morphology The cross-sectional morphology of FeCoCrNiMnNb and FeCoCrNiMn high entropy alloy coatings is shown in Figure 2. The microstructure of FeCoCrNiMn high entropy alloy coating is dendrite and interdendritic structure. The dendrite structure in the coating presents a certain angle with the cladding direction, which has the characteristics of directional solidification structure. EDS point scanning was performed on points 1 and 2 in the figure. The composition difference between the dendrite and the interdendritic region is small, and the Fe element content is high. This is because the Fe element in the matrix is ​​diluted into the high entropy alloy coating, resulting in an increase in the Fe element content in the coating [19]. The microstructure of the FeCoCrNiMnNb coating is shown in Figure 2 (b). Due to the introduction of Nb element, the FeCoCrNiMnNb coating generates many slender rod-shaped structures. EDS point scanning was performed on the rod-shaped structure and the matrix structure, and the results are shown in Table 1. The Nb element content in the rod-shaped structure is higher than that in the matrix structure, while the Cr, Ni, and Mn element contents are reduced, indicating that the rod-shaped structure is a Laves phase rich in Nb element, while the matrix structure is a solid solution phase with FCC structure. The EDS surface distribution results of the FeCoCrNiMnNb coating are shown in Figure 3. Similar to the results of EDS point analysis, Nb is mainly enriched in the rod-shaped structure, while Mn, Ni and other elements are mainly enriched in the matrix elements. It can be seen that Mn and Ni elements are more inclined to form FCC solid solution structure, while Nb elements and Fe and Co elements are more inclined to form HCP structure with higher hardness. Combined with element distribution and XRD phase structure analysis, it is shown that the rod-shaped structure is a Laves phase with HCP structure, while the matrix phase is a solid solution phase with FCC structure.

2.3 Microhardness The displacement load curves of FeCoCrNiMn coating and FeCoCrNiMnNb coating are shown in Figure 4. The nanohardness (G), elastic modulus (E), and maximum indentation depth (dmax) values ​​of the two coatings are listed in Table 2.
The results show that the maximum indentation depth of the FeCoCrNiMnNb coating is much smaller than that of the FeCoCrNiMn coating, indicating that the FeCoCrNiMnNb coating has a higher hardness. The nanohardness of the FeCoCrNiMnNb coating and the FeCoCrNiMn coating are 9.193 and 4.832, respectively. The nanohardness of the FeCoCrNiMnNb coating is increased by about 2 times. Through the characterization of XRD and SEM of the two coatings, the improvement of the hardness of the FeCoCrNiMnNb coating can be attributed to the second phase strengthening mechanism caused by the generation of Laves phase rich in Nb element. In addition, through EDS element detection, it was found that Nb element was also detected in the matrix phase, indicating that Nb element was dissolved in the FCC solid solution phase, thus causing the solid solution strengthening effect of the FCC phase. Therefore, the high nanohardness of the FeCoCrNiMnNb coating can be attributed to the synergistic strengthening effect of solid solution strengthening and second phase precipitation strengthening.

2.4 Wear behavior
The wear volume and wear rate bar graphs of FeCoCrNiMn coating and FeCoCrNiMnNb coating are shown in Figure 5. The wear rate (μ) of the two high entropy alloy coatings can be calculated by formula (1) [20]: See formula (1) in the figure

Wherein, V is the wear volume of the coating, F is the vertical load applied during wear, and L is the wear scar length. The wear volume and wear rate of FeCoCrNiMn coating are calculated to be 8.581×106 μm3 and 6.386×10-5 mm3/N·m, respectively, and the wear volume and wear rate of FeCoCrNiMnNb coating are 3.574×106μm3 and 2.549×10-5 mm3/N·m, respectively, indicating that FeCoCrNiMnNb coating has better wear resistance.

Figure 6 shows the change curve of friction coefficient of FeCoCrNiMn coating and FeCoCrNiMnNb coating over time. The results show that the introduction of Nb element can significantly reduce the friction coefficient of the coating. The average friction coefficient of FeCoCrNiMnNb coating is about 0.57, which is about 0.83 times that of FeCoCrNiMn coating (0.68).

Combined with Figure 6, it can be seen that the friction coefficient of FeCoCrNiMn coating and FeCoCr NiMnNb coating has large fluctuations during the wear process, and the fluctuation of FeCoCrNiMn coating is more drastic. This is because the hardness of FeCoCrNiMnNb coating is higher and the coating has stronger ability to resist deformation, so the friction coefficient fluctuation is smaller [21]. The hardness of FeCoCrNiMn coating is lower and the ability to resist deformation is poor, so the friction coefficient fluctuates more drastically. In order to study the wear mechanism of FeCoCrNiMnNb coating and FeCoCrNiMn coating, the wear morphology of the two coatings was observed by scanning electron microscopy. Figure 7 (a) shows the wear morphology of the FeCoCrNiMn coating. Obvious adhesion marks and plowing grooves can be observed in the figure, indicating that the wear mechanism of the coating is adhesion wear and abrasive wear. Figure 7 (b) shows the wear morphology of the FeCoCrNiMnNb coating. Compared with the FeCoCrNiMn coating, the wear scar depth of the FeCoCrNiMnNb coating is shallower, and the wear scar width is also reduced. In addition, no adhesion marks are found on the wear scar surface, and only plowing grooves are formed on the wear scar surface, indicating that the wear mechanism of the FeCoCrNiMnNb coating is single abrasive wear. Combined with the nanoindentation results, the FeCoCrNiMnNb coating has high hardness and deformation resistance. In addition, the Laves phase rich in Nb elements generated in the FeCoCrNiMnNb coating has extremely high hardness, which also improves the deformation resistance of the coating and reduces the risk of the coating falling off due to wear [22]. Combined with the wear volume, wear rate, friction coefficient and wear morphology, the FeCoCrNiMnNb coating has better wear resistance than the FeCoCrNiMn coating.

Conclusion

(1) The phase composition of the FeCoCrNiMnNb coating is FCC phase and Nb-rich Laves phase. Since Nb has a larger atomic radius, the FCC phase is unstable and the Nb-rich Laves phase is formed. The matrix phase is FCC phase and the rod-like structure is Nb-rich Laves phase.

(2) The nanohardness of the FeCoCrNiMnNb coating is 9.193GPa, which is about twice that of the FeCoCrNiMn coating. The improvement of the nanohardness of the Fe-
CoCrNiMnNb coating can be attributed to the synergistic effect of the second phase precipitation strengthening effect of the Nb Laves phase and the solid solution strengthening effect of the Nb element.

(3) The FeCoCrNiMnNb coating has excellent wear resistance, and its wear rate is about 2.549×10-5 mm3/N·m, which is about 0.33 times that of the FeCoCrNiMn coating. The average friction coefficient of the FeCoCrNiMnNb coating is 0.57, which is about 0.83 times that of the FeCoCrNiMn coating. The higher nanohardness of FeCoCrNiMnNb improves the wear resistance of the coating, and its wear mechanism is single abrasive wear.