In order to improve the wear resistance of soil-contacting parts of agricultural machinery, FeCoCrNiMn high entropy alloy, Fe90 alloy and Ni60A alloy powders were selected for comparative study. The wear-resistant coating was prepared by laser cladding technology with 65Mn steel as the substrate, and its wear performance was tested by friction and wear testing machine. The results showed that the FeCoCrNiMn high entropy alloy coating had the densest structure, relatively simple grains, and no complex intermetallic compounds were formed; the microstructure grain distribution of Ni60A and Fe90 alloy coatings was relatively disordered. The wear losses of 65Mn steel substrate, Ni60A alloy, Fe90 alloy and FeCoCrNiMn high entropy alloy coating were 9, 4, 5 and 2 mg respectively, and the wear loss of the substrate was much greater than that of the coating. The Vickers hardness of Fe90 and Ni60A alloy coatings is 683.87 and 663.62 HV, and the hardness of Fe-CoCrNiMn high entropy alloy coating is 635.81 HV, which is slightly lower than other coatings, but its wear resistance is good.

With the rapid development of agricultural machinery and equipment, the soil-contacting parts of agricultural machinery are affected by the impact wear and friction wear of abrasives such as soil and sand for a long time, which puts higher requirements on the wear resistance of traditional soil-contacting parts. Among various anti-wear measures, laser cladding and surfacing treatment of the failed surface of the soil-contacting parts are two commonly used treatment methods. They both use different fillers to melt or heat the coating material into a semi-molten state and cover it on the surface of the substrate, thereby improving the wear resistance of the substrate. The two most common coating materials for soil-contacting parts are iron-based alloys and nickel-based alloys. Both coating materials are based on an alloy element and improve the coating performance by adding other appropriate elements. At present, the research and application of improving the wear resistance of traditional metal materials has been close to saturation, and the space for research is getting smaller and smaller.

High entropy alloys are composed of a variety of alloying elements with similar atomic ratios, with more uniform and simple solid solution phases, showing high strength, high wear resistance and good corrosion resistance. Using high entropy alloy powder to prepare wear-resistant coatings on agricultural machinery soil-contacting parts, the parts have high wear resistance and can further extend their service life.

Laser cladding technology is used to prepare coatings, which has the advantages of heat concentration and small heat-affected zone. The organizational structure produced in the casting area is also different from other cladding methods, such as electrospark deposition, magnetron sputtering and plasma cladding. At the same time, laser cladding technology is used to prepare coatings, and amorphous organizational structures are formed in the coating organization. At present, there are few studies on the application of high entropy alloy coating materials in the preparation of wear-resistant coatings for agricultural machinery soil-contacting parts. In this paper, Fe90 alloy, Ni60A alloy and FeCoCrNiMn high entropy alloy wear-resistant coatings were prepared on the surface of 65Mn steel using laser cladding technology. The friction and wear properties of high entropy alloy coatings were compared and studied, and their tribological laws were explored to provide a reference for the application expansion of high entropy alloys.

1 Experimental materials and methods

1. 1 Coating preparation
The sample used 65Mn high carbon spring steel as the base material, and was cut into samples with a size of 200 mm × 400 mm × 4 mm using a metallographic cutting machine. The sample was ground and polished before cladding to prevent the oxide layer, oil and other impurities on the surface of the sample from affecting the bonding strength between the coating and the sample. 80, 120, 220, 800, 1 000, 1 500 and 2 000 grit sandpaper were used for grinding in turn. The polished sample was ultrasonically cleaned in ethanol for 5 min, placed in a drying oven at 105 ℃ for 10 min, and sealed and stored after drying. Fe90 alloy, Ni60A alloy and FeCoCrNiMn high entropy alloy powder (particle size of 45 to 105 μm) were selected as the cladding layer materials. The chemical composition of the test materials and powders are shown in Table 1. The maximum output power of the CW-C-B-W-8000G-91-20L laser cladding equipment is 25,000 W. The test adopts the side-axis broadband powder feeding method, argon protective gas, and the cladding coating thickness is 1 mm. The cladding process parameters are shown in Table 2.

1.2 Test characterization
65Mn steel is sample S1, Ni60A alloy coating is sample S2, Fe90 alloy coating is sample S3, and FeCoCrNiMn high entropy alloy coating is sample S4. The metallographic etching solution of sample S1 is 4% nitric acid solution (concentrated nitric acid and anhydrous ethanol, volume ratio is 4: 100); the metallographic etching solution of sample S2 is copper sulfate pentahydrate solution (hydrochloric acid, water and copper sulfate, volume ratio is 10: 10: 1); the metallographic etching solution of samples S3 and S4 is 5% aqua regia (concentrated hydrochloric acid and concentrated nitric acid, volume ratio is 3: 1).

The metallographic microstructure of the sample was observed by Leica DM4000M metallographic microscope; the surface and cross-section hardness of the sample was measured by Jinan Times TMVS-1 digital display Vickers hardness tester; the friction and wear performance of the material was detected by MMU-10 microcomputer-controlled end face friction and wear tester; the pin-disc friction pair was used for the test, and the grinding ball was a ZrO2 grinding ball with a diameter of 6 mm. The test parameters were load 50 N, speed 80 r/min, and friction time 120 min; the wear scar morphology after the friction and wear test of the sample was observed by an optical microscope.

2 Test results and analysis

2.1 Metallographic structure of coating
Figure 1 shows the surface metallographic structure diagram of samples S1, S2, S3 and S4. As shown in Figure 1a, the structure of sample S1 is mainly composed of ferrite and pearlite distributed in a grid shape. It can be clearly seen from Figure 1b that the microstructure of the coating of sample S2 is dendrites and reticular eutectics, the organizational phase is relatively fine, and the dendrites are relatively messy, and the long strip and blocky organizations are generated irregularly. As shown in Figure 1c, the microstructure of the cross section of the coating of sample S3 is coarse and uniform dendrites, interlaced dendrite organizations, and a large number of light-colored shiny granular precipitations. As shown in Figure 1d, the cross-sectional organization of the coating of sample S4 is the most dense, mainly composed of equiaxed crystals evenly distributed, and irregular holes are precipitated. Comparing the four organizations, the surface grain size of the S4 coating is the smallest, the grains are dense and uniform, the grains are relatively simple, and there is no complex intermetallic compound formation.

2. 2 Microhardness of coating
Figure 2 is a comparison of the surface microhardness of the samples. The Vickers hardness of samples S1, S2, S3 and S4 is about 234.02 HV, 683.87 HV, 663.62 HV and 635.51 HV respectively. Figure 3 is a comparison of the cross-sectional microhardness of the samples. It can be seen from Figure 3 that the average Vickers hardness of the coatings of samples S2 and S3 is 3 to 4 times higher than that of sample S1, indicating that the hardness of the coatings of S2 and S3 is higher and the cladding metallurgy crystallization effect is better. The average Vickers hardness of the coating surface of sample S4 is slightly lower than that of samples S2 and S3. This is because when the FeCoCrNiMn high entropy alloy powder is rapidly solidified, the lattice distortion is small, and the FCC crystal structure is precipitated and dispersed in the amorphous of the cladding layer, which can reflect to a certain extent that the FeCoCrNiMn high entropy alloy coating has good toughness and low hardness.

2.3 Friction and wear properties
2.3.1 Average friction coefficient
Figure 4 is the average friction coefficient curve of samples S1, S2, S3 and S4. It can be seen that at room temperature, the average friction coefficient of the surface of sample S1 is about 0.53, and the average friction coefficient fluctuates the most in the first 20 minutes, rising to about 0.6; as time goes on, the average friction coefficient tends to be stable. This is because in the early stage of friction between sample S1 and ZrO2 grinding ball, there are a lot of wear debris between the wear mark and the grinding ball, which produces a large shear stress, resulting in a sharp fluctuation of the friction coefficient. The average friction coefficients of samples S2, S3 and S4 are about 0.38, 0.32 and 0.25. The complex distribution of hard phase particles in sample S2 causes the average friction coefficient curve to fluctuate more violently. The hardness of samples S3 and S4 is much smaller than that of ZrO2 grinding ball. The coating alloy material with lower hardness also has lower shear strength, which is conducive to reducing the average friction coefficient during friction. The average friction coefficient curves of samples S3 and S4 have basically the same trend, maintaining a relatively stable dynamic balance. Among them, the average friction coefficient of sample S4 is the lowest, the friction force under the same force is the smallest, and the wear degree is the lowest. This is because when sample S4 is rapidly cooled, there are fewer impurity phase particles, the coating surface is smoother and has fewer defects, and the contact with the ZrO2 grinding ball is smoother, without obvious and drastic fluctuations.

2. 3. 2 Wear weight loss
The wear weight loss data of the samples are shown in Figure 5. The maximum wear loss of sample S1 is 9 mg, and the wear losses of samples S2 and S3 are 4 mg and 5 mg respectively. Among them, the wear loss of sample S4 is the lowest, which is 2 mg. This is because the FeCoCrNiMn high entropy alloy coating has a single FCC phase, high plasticity and good toughness. Under the friction side effect of a load of 50 N, the FeCoCrNiMn high entropy alloy material can absorb a large amount of energy, is not easy to form fatigue peeling, and has good wear resistance.

2.3.3 Wear morphology analysis
Figure 6 shows the wear scar morphology of the four samples observed under the same test conditions after 120 minutes of wear. As can be seen from Figure 6a, S1 has severe plastic deformation due to its low overall hardness, the concave surface of the wear scar is rough, there is a large area of ​​bonding layer, and delamination occurs. As can be seen from Figure 6b, the coating surface of sample S2 is irregularly distributed with elliptical dot-shaped white compounds, which enhances the hardness of the coating, accompanied by obvious wear scars and unidirectional furrows. The coating surface hardness of sample S3 is the highest, as shown in Figure 6c, the wear scar width is narrow, and the grooves on the coating surface are shallow. In contrast, in Figure 6d, the grooves of the coating of sample S4 are very smooth, which is due to the uniform structure of the cladding layer, fine grains, and good wear resistance; there are obvious irregular pores in the grooves, which may be caused by the high-entropy alloy powder being mixed with gas in the molten state under the high temperature of the laser beam, and the gas gushing out when the sample is cooled to form pores.

Under the same test conditions, the larger the width of the test wear scar, the greater the wear weight loss. By comparing the weight loss of different samples in Figure 5, it can be seen that the relationship between the size of the sample wear scar is S1> S3> S2> S4. This is consistent with the test results of wear weight loss shown in Figure 5.

Conclusion

1) The FeCoCrNiMn high entropy alloy coating has the densest structure and the smallest grain size, while the microstructure grain distribution of the Ni60A and Fe90 alloy coatings is more chaotic. The FeCoCrNiMn high entropy alloy coating has a relatively simple grain structure and no complex intermetallic compounds are formed.

2) The Vickers hardness of the Ni60A alloy, Fe90 alloy and FeCoCrNiMn high entropy alloy coatings is approximately 683.87, 663.62 and 635.51 HV, which is significantly higher than the Vickers hardness of the substrate (234.02 HV). The hardness value of the Fe-CoCrNiMn high entropy alloy coating is slightly lower than that of the Ni60A alloy and Fe90 alloy coatings, which does not affect its wear resistance.

3) The wear losses of 65Mn steel substrate, Ni60A alloy, Fe90 alloy and Fe-CoCrNiMn high entropy alloy coating are 9, 4, 5 and 2 mg respectively. The wear scar of FeCoCrNiMn high entropy alloy coating is the smoothest, with shallow wear scar depth, small material loss and the highest wear resistance.