In this work, CoCrFeNiSi-xTiB2 (x = 2. 5%, 5. 0%, 7. 5%, 10. 0%, mass fraction) high entropy alloy (HEA) composite coating was laser clad on 40Cr surface. The phase, microstructure, hardness, friction and wear and electrochemical corrosion properties of the coating were analyzed, and the effect of nano-TiB2 ceramic particles on HEA coating was discussed. The results show that when x = 2. 5%, 5. 0%, 7. 5%, the coating phase is composed of two-phase FCC and BCC; when x = 10. 0%, boride CrB is generated on the basis of the two phases, and the coating microstructure changes from equiaxed crystals to typical columnar dendrites. The microhardness of the coating increases with the increase of nano-TiB2 particles, and when x = 10. When the content of TiB2 is 0%, the average hardness of the coating reaches the highest, which is HV547.11, which is about 2.72 times that of the substrate. The main reasons for the increase in its hardness are solid solution strengthening and dispersion strengthening. With the increase of TiB2 content, the wear loss of the composite coating is significantly reduced. When x = 10.0%, the wear loss weight is only 0.13 mg. In general, the increase of TiB2 content changes the main wear mechanism of the composite coating from severe abrasive wear and oxidation wear to slight abrasive wear and oxidation wear, and the wear resistance is significantly improved. In 3.5% NaCl solution, the corrosion resistance of the composite coating is the best when x = 7.5%. Keywords: high entropy alloy (HEA), laser cladding, nano-ceramics, wear resistance, corrosion resistance

40Cr alloy structural steel is one of the most commonly used steels for coal mining machine picks. It has good comprehensive mechanical properties, low temperature impact toughness, low notch sensitivity, and a reasonable alloy element ratio, so it is widely used in the machinery manufacturing industry. However, due to the harsh working environment, 40Cr is often squeezed and sheared when cutting coal seams, which is prone to surface wear, corrosion, tooth deformation and other defects, seriously affecting the service life. Laser cladding technology is one of the most commonly used means in the field of surface repair in recent years. This technology uses a high-energy laser beam to melt and solidify the coating powder and the substrate surface to form a good metallurgical bond. At the same time, laser cladding also has the advantages of fast cooling, fast solidification, small heat-affected zone, and dense coating structure. It can improve the hardness, wear resistance and corrosion resistance of the coating surface. Its unique advantages and huge application prospects make it widely concerned. Unlike traditional alloys, high-entropy alloys (HEAs) are alloys composed of five or more elements in equal or nearly equal molar ratios. In the traditional alloy concept, when multiple main elements are mixed, intermetallic compounds are easily produced, resulting in a significant decrease in the comprehensive performance of the material. HEAs are different. They have unique “four effects” that can inhibit the formation of intermetallic compounds, making them have high strength, high hardness, excellent high temperature performance and wear and corrosion resistance. Their internal structure is usually divided into FCC, BCC, and HCP solid solution phases. HEAs improve the mechanical properties of alloys through solid solution strengthening and second phase strengthening. Studies have shown that laser cladding technology has the effect of refining grains. Therefore, preparing HEAs by laser cladding is the best choice.

In order to further improve the hardness and wear resistance of HEA coatings, researchers are paying more and more attention to the effects of various ceramic particles (such as TiC, NbC, B4C and TiB2) on the performance of HEAs. Shang et al. prepared a nano-TiC particle-reinforced (Cr-Fe4Co4Ni4)Cr3HEA coating on the surface of 904L stainless steel. With the addition of TiC, the hardness, wear resistance and corrosion resistance of the composite coating gradually improved. When 15% (volume fraction) TiC was added, the microhardness of the composite coating was about twice that of the substrate. Dong et al. prepared an Al-CrCoFeNi2.1-xTiB2HEAs composite coating on the surface of 304 stainless steel by ultra-high-speed laser cladding. The results showed that the addition of TiB2 improved the hardness and wear resistance of the coating, and the wear rate decreased with the increase of TiB2 particles. Zhao et al.  prepared B4C and SiC ceramic particles reinforced CoCrFeNiTi HEA coatings by laser cladding technology. The results showed that B4C had the best performance on alloy performance. The coating hardness increased from HV0.5666.2 to HV0.5886.9. At the same time, the room temperature wear resistance was enhanced, and the friction coefficient and wear rate decreased significantly. Among the ceramic particles, TiB2 particles are one of the commonly used hard phases. They have high hardness, low thermal expansion coefficient, good thermal stability, and excellent wear and corrosion resistance. These characteristics can better improve the mechanical properties of HEA coatings.

There are few studies on the effect of nano-TiB2 metal ceramic particles on the performance of CoCrFeNiSi HEA. Therefore, this work prepared CoCrFeNiSi-xTiB2 (x = 2.5%, 5.0%, 7.5%, 10.0%, mass fraction, the same below) HEA composite coating on the surface of 40Cr steel by laser cladding, and analyzed the effect of nano-TiB2 on the microhardness, friction and wear and corrosion properties of HEA coating from the perspective of cladding coating structure and phase.

1. Experiment

The substrate is 40Cr steel with a size of 150 mm×60 mm×8 mm. The cladding materials are 40-70 μm high-purity Co, Cr, Fe, Ni, Si single powders and 650-800 nm TiB2 powders. 2.5%, 5.0%, 7.5%, and 10.0% TiB2 metal ceramic powders are added to CoCrFeNiSiHEA powders respectively, and the powders are mixed in a KQM-ZB planetary ball mill for 3 h. YLS-2000 fiber laser is used to perform laser cladding in the form of pre-setting powders. Before cladding, the mixed powders are evenly stirred with the binder polyvinyl alcohol (2%) and pre-set on the substrate with a thickness of 1.45 mm. According to the preliminary experimental research of the research group, the optimal process parameters for cladding are laser power of 900 W, scanning rate of 4 mm·s-1, and spot diameter of 2. 0 mm, overlap rate 30%. To ensure the quality of the cladding layer, argon was used as the protective gas during the cladding process.

After the cladding process, the sample was processed by wire cutting with an electrospark, and its size was 8 mm×8 mm×8 mm, and the overlap sample size was 25 mm×8 mm×8 mm, and then polished with sandpaper. The phase structure of the alloy coating was detected by D8-Advance X-ray diffractometer, the target material was copper target, and the scanning range was 20-100°. Aqua regia (volume ratio of concentrated hydrochloric acid to concentrated nitric acid 3:1) was selected as the corrosion solution, and the microstructure of the sample was observed by 4XB inverted metallographic microscope and SUPRA55VP field emission electron microscope. Energy dispersive spectrometer (EDS) was used to analyze the element distribution, and the hardness of the coating was measured by a microhardness tester. The applied load was 1 000 N, and the loading time was 15 s. At the cross section of the sample, the test was conducted from the top of the coating to the top of the substrate. Each sample was measured three times and the average value was taken. The wear test was carried out using an M-5000 friction and wear machine. Dry sliding reciprocating friction was selected. The grinding pair Si3N4 was used. The normal load was 20 N, the frequency was 2 Hz, the wear time was 30 min, and the reciprocating distance was 20 mm. The sliding velocity and total sliding distance were calculated to be 4 cm/s and 72 m, respectively.

2 Results and discussion

2.1 Phase analysis

Figure 1 shows the XRD spectrum of the CoCrFeNiSi-xTiB2HEA composite coating. XRD spectra show that the CoCrFeNiSi-xTiB2 (x = 2.5%, 5.0%, 7.5%) HEA composite coating is composed of BCC phase and FCC phase. When x = 10.0%, the composite coating generates intermetallic compound CrB on the basis of the original two phases. This may be due to the negative mixing enthalpy of non-metallic B element and Cr element. With the increase of nano-TiB2, the content of FCC phase gradually decreases and the content of BCC phase gradually increases. The reason is that part of Ti and B elements decomposed by nano-TiB2 in laser cladding promote the formation of BCC phase, which is consistent with the results of some previous studies [23-26], indicating that Ti and B elements added to HEA can play the role of BCC stabilizer and solid solution enhancer. In addition, no TiB2 diffraction peak was observed in the spectra of all composite coatings, indicating that the nano-TiB2 particles were completely decomposed or their number was below the XRD detection range. Observing the local enlarged image in the upper right corner of Figure 1, it can be clearly seen that the (110) diffraction peak of the BCC phase moves to a larger angle, indicating that the lattice constant of BCC decreases. This may be due to the decomposition of TiB2 particles during laser cladding, which causes the B atoms with smaller element radius to dissolve and replace other elements in the BCC phase structure. According to the Bragg law, the main peak of the BCC diffraction of the alloy coating shifts to the right. Origin was used to quickly fit the XRD spectrum, and the grain size (D) of the CoCrFeNiSi-xTiB2 HEA composite coating was calculated by formula (1), as shown in Table 1. The average grain size (D*) of CoCrFeNiSi-xTiB2 (x = 2.5%, 5.0%, 7.5%, 10.0%) HEA composite coatings is 15.89, 15.30, 14.97, 14.12 nm, respectively, indicating that the addition of nano-TiB2 particles to CoCrFeNiSi HEA coatings can effectively reduce the average grain size of the coatings and refine the grain structure of the composite coatings. This is because TiB2 particles are a common heterogeneous nucleating agent that can increase the nucleation rate and thus refine the coating grains.
See formula (1) in the figure, where: k is a constant (0.89), λ is the X-ray wavelength (0.15405 nm), β is the half-height width of the diffraction peak (FWHM), and θ is the diffraction angle.

2. 2 Microstructure

The microstructure of CoCrFeNiSi-xTiB2 HEA composite coating is shown in Figure 2. The corresponding chemical element contents of the marked areas in Figure 2 are listed in Table 2. From the SEM photos, it can be seen that the structure of CoCrFeNiSi-xTiB2 HEA composite coating presents two different regions: dark gray dendrite (DR) and light gray interdendritic (ID) regions. DR and ID regions are typical solid solution structures in HEA coatings prepared by laser cladding. From the EDS analysis of the coating test points, it can be seen that the DR region mainly includes Fe and Ni elements, while the ID region mainly includes Fe, Cr and Ti elements. Therefore, the DR region corresponds to the FCC solid solution structure rich in Fe and Ni, and the ID region corresponds to the BCC solid solution structure rich in Fe and Cr, which is consistent with the results of the previous XRD analysis. Further combined with Figure 2 and Table 2, it can be seen that when x = 2.5%, the microstructure of the coating is a uniform equiaxed crystal structure. When x = 5.0%, the coating microstructure changes from equiaxed crystal to columnar dendrite structure. When x = 7.5%, 10.0%, the coating microstructure has completely changed into columnar dendrite. Figure 3 shows the EDS surface scan and element distribution of the composite coating when x = 5.0%, 10.0%. Combined with the data in Table 2, it can be seen that when a small amount of nano-TiB2 particles is added (x = 2.5%), the composite coating shows obvious Si element segregation, and with the increase of TiB2 content, the Si element segregation gradually weakens and finally tends to be uniform. This is because the addition of TiB2 promotes the redistribution of solutes, and the Ti element has a strong tendency to segregate to the grain boundary. This local inhomogeneity causes the Ti element to segregate to the grain boundary preferentially than the Si element, which inhibits the segregation of the Si element to a certain extent. In addition, when x = 2.5%, 5.0%, the B element is relatively uniformly distributed in the composite coating due to its relatively small content. With the increase of TiB2 content, the B element mainly exists in the form of segregation at the grain boundary, which further reduces the grain size.

2.3 Microhardness

The distribution curve of the microhardness of the composite coating along the depth direction is shown in Figure 4. The hardness curve is divided into three regions: cladding zone, heat-affected zone and substrate zone. Each region is separated by a vertical dotted line in the figure. The microhardness of the heat-affected zone is relatively high because laser cladding has the characteristics of rapid cooling and rapid solidification. The rapid cooling process from a high temperature state is equivalent to quenching, which can improve the hardness. The average microhardness of the CoCrFeNiSi-xTiB2 (x = 2.5%, 5.0%, 7.5%, 10.0%) HEA composite coating and 40Cr substrate is HV342.98, HV404.13, HV460.51, HV547.11 and HV201.23, respectively, that is, the hardness of each composite coating is 1.7 times, 2.0 times, 2.0 times, and 2.5 times that of the substrate, respectively. 29 times and 2.72 times. It can be clearly seen from Figure 4 that with the increase of the content of nano-TiB2 particles, the microhardness of the composite coating gradually increases. When x = 2.5%, 5.0%, the microhardness of the coating is relatively low. The reasons may include: ① The content of TiB2 added is small, the probability of Ti atoms dissolving into the solid solution and replacing other atoms or B atoms entering the lattice interstitial vacancies is small, the lattice distortion is small, and the solid solution strengthening caused is not obvious; ② When x = 2.5%, 5.0%, the content of FCC phase is larger than that of BCC phase, the ductility of FCC phase is higher, but its strength is lower than that of BCC phase. When x = 7.5%, 10.0%, the microhardness of the coating is relatively low. The microhardness of the coating is relatively low. When x = 8.5%, 11.0%, 12.0%, 14.0%, 16.0%, 18.0%, 19.0%, 20.0%, 21.0%, 23.0%, 24.0%, 26.0%, 27.0%, 28.0%, 29.0%, 24.0%, 26.0%, 27.0%, 28 … When x = 0%, the microhardness of the composite coating is relatively high. This is because with the increase of nano-TiB2 content, the FCC phase transforms to the BCC phase structure, and the BCC phase structure content in the composite coating is relatively high; with the increase of Ti and B elements, the Ti atoms with larger radius dissolve into the solid solution and replace other atoms and occupy the lattice position, while the B atoms occupy the interstitial vacancies in the lattice as interstitial atoms. The combined effect of the two leads to severe lattice distortion, which enhances the degree of solid solution strengthening. When x = 10.0%, the intermetallic compound CrB is generated in the composite coating, resulting in dispersion strengthening of the coating. The rapid solidification during laser cladding also helps to improve the solubility and enhance the solid solution strengthening effect. In addition, the introduction of B element controls the grain size of the coating, refines the grains, increases the number of grain boundaries, and the grain boundaries play a role in hindering the movement of dislocations, so the coating exhibits high microhardness. According to the Hall-Petch equation Hg = H0 + kd1/2, the hardness of the coating is inversely proportional to the grain size.

2.4 Friction and wear properties

2.4.1 Friction coefficient and wear weight loss

Figure 5 shows the friction coefficient (COF) curve of the CoCrFeNiSi-xTiB2 HEA composite coating, which shows two different stages: the running-in stage and the stable wear stage. In the running-in stage, the friction pair Si3N4 ceramic ball first contacts the surface of the cladding layer. During friction and wear, wear debris will be generated on the surface of the cladding layer, and point contact friction will occur, resulting in an unstable and significantly increased friction coefficient. With the increase of wear time, the friction contact area gradually increases and becomes surface contact friction, making the friction system tend to be stable and enter the stable wear stage. At this stage, the COF of the CoCrFeNiSi-xTiB2HEA composite coating is between 0.67 and 0.72, indicating that the content of nano-TiB2 particles has little effect on the COF of the composite coating in the stable wear stage. Similar phenomena have been found in previous reports. Figure 6a shows the average friction coefficient between the substrate and the composite coating. It can be found that the COF of the composite coating is lower than that of the 40Cr substrate. Figure 6b shows the wear loss weight of the substrate and the composite coating. It can be seen that the composite coating with the addition of nano-TiB2 particles greatly improves the friction and wear properties of the substrate. When x = 10.0%, the wear loss weight of the composite coating is reduced by 88% compared with the substrate.

2.4.2 Wear volume and wear

In order to further explore the wear resistance of the substrate and the composite coating, the three-dimensional profile analysis of the wear marks of the samples was carried out, and the 3D profile map and wear profile curve of the substrate and coating were extracted, as shown in Figure 7. Compared with the substrate, the wear width and depth of the coating decreased to varying degrees with the increase of nano-TiB2 content. The cross-sectional areas of the wear scars of the 40Cr substrate and the CoCrFeNiSi-xTiB2 (x = 2.5%, 5.0%, 7.5%, 10.0%) coatings were 5 696.85, 1 250.10, 1 233.45, 1 122.02, and 770.74 μm2, respectively, indicating that the cross-sectional area of ​​the wear scars in the coatings gradually decreased with the increase of TiB2 content, and the corresponding wear volume continued to decrease. Using formula (2) to calculate the wear rate of the coating, Figure 8 shows the wear volume and wear rate of the 40Cr substrate and the CoCrFeNiSi-xTiB2 (x = 2.5%, 5.0%, 7.5%, 10.0%) coating. Their wear volumes are 0.056 97, 0.012 50, 0.012 33, 0.011 22, and 0.007 71 mm3, respectively, and the wear rates are 39.561 5×10-6, 8.681 3×10-6, 8.565 6×10-6, 7.791 8×10-6, and 5.352 4×10-6 mm3·N-1·m-1, respectively. The trend of the friction and wear performance of the coating is consistent with the trend of the microhardness, indicating that high hardness is generally accompanied by excellent wear resistance. When x = 10.0%, the coating has the smallest wear depth, wear volume and wear rate, indicating that the coating has the best wear resistance when x = 10.0%.
See formula (2) in the figure, where: W is the wear rate, Vloss is the wear volume, FN is the load, and H is the total sliding distance.

2.4.3 Wear surface morphology

The wear surface morphology of the sample is shown in Figure 9, which further shows the potential wear process related to the substrate and the composite coating. Figure 9a is the wear scar morphology of the 40Cr substrate. Obvious plastic deformation is shown on the substrate surface. A large number of peeling pits and adhesion layers can be observed along the sliding direction. The generated wear debris adheres to the wear surface. At the same time, with the action of the loading force, an adhesion layer is formed on the wear scar surface. Due to the low microhardness of the substrate, when sliding relative to the friction pair, the loading force causes shear plastic deformation on the sample surface along the sliding direction. The plastic deformation causes microcracks to appear under the wear scar of the substrate. The microcracks expand and break, resulting in peeling pits and delamination on the surface of the substrate. At the same time, there are a small number of furrows on the substrate surface, indicating that the substrate undergoes adhesive wear accompanied by a small amount of abrasive wear.

Figure 9b-e shows the wear scar morphology of composite coatings with different nano-TiB2 contents. It can be seen that the wear scar is divided into two parts: the dark gray area and the light gray area. Figure 10 shows the energy spectrum analysis of the CoCrFeNiSi-10.0%TiB2 sample (starting position on the left side of the wear scar). It can be found that the distribution trend of Si and O elements in the dark gray area is consistent, and it can be seen that the dark gray area is an oxide formed by the combination of Si and O elements. When the friction pair reciprocates, oxide wear debris is continuously generated on the surface of the composite coating. These wear debris are discharged along the reciprocating friction direction, but some debris cannot be completely discharged and are continuously compacted at the furrows or on both sides of the wear marks, forming an oxide layer on the surface of the composite coating. As shown in Figure 9b and c, when x = 2.5% and 5.0%, wide furrows and a large number of oxide layers appear on the surface of the coating, accompanied by a small number of spalling pits, and different degrees of plastic deformation can be found, indicating that the wear mechanism is severe abrasive wear and oxidation wear, accompanied by adhesive wear. When x = 7. When x = 5%, it can be found from Figure 9f that the coating surface has furrows of varying depths, which is a typical abrasive wear phenomenon. The number of oxide layers in the dark gray area has also decreased significantly, indicating that the degree of oxidation wear has weakened. At the same time, it is also observed that due to the reciprocating motion of the friction pair, a large amount of friction heat is generated to weld the surface, and the micro-connection formed is torn off to produce flaky adhesion and peeling pits, indicating that the coating also has adhesive wear. It is worth noting that during the friction and wear process, the friction pair squeezes part of the oxide layer, causing the oxide layer to break and produce a large amount of wear debris. Due to the high microhardness of the coating, a large amount of wear debris causes the relative movement of the wear surface to change from sliding friction to rolling friction, thereby reducing the friction coefficient of the coating to a certain extent. It can be seen from Figure 9e that when x = 10.0%, the number of oxide layers continues to decrease, the coating surface is smoother, and shallow and narrow furrows appear, accompanied by a small amount of peeling pits, indicating that the coating has slight abrasive wear and oxidation wear, accompanied by a small amount of adhesive wear. From the enlarged view on the right side of Figure 9e, it can be seen that the wear debris shed from the wear surface is small in size and small in number, so the relative motion of the wear surface is transformed into sliding friction, which increases the friction coefficient of the coating, which is consistent with the trend of the average friction coefficient of the coating in Figure 6a. In summary, with the increase of nano-TiB2 content, the main wear mechanism of the composite coating changes from severe abrasive wear and oxidation wear to slight abrasive wear and oxidation wear, indicating that the addition of nano-TiB2 significantly improves the wear resistance of the composite coating.

2.5 Electrochemical corrosion

2.5.1 Dynamic potential polarization curve

Figure 11 shows the dynamic potential polarization curves of 40Cr matrix and CoCrFeNiSi-xTiB2 (x = 2.5%, 5.0%, 7.5%, 10.0%) HEA composite coating in 3.5% NaCl solution. The substrate and the composite coating show similar states in the cathode region, indicating that the change in the content of nano-TiB2 ceramic particles has no effect on the cathode part of the polarization curve. A typical passivation platform appears in the anode region. The slope of the passivation zone curve is related to the degree of protection of the passivation film. When x = 7.5%, the slope of the coating passivation zone curve is the largest, and secondary passivation occurs at the same time, indicating that the coating produces a denser passivation film during the corrosion process, which improves the corrosion resistance of the passivation film.

The self-corrosion potential (Ecorr) and corrosion current density (Icorr) of the CoCrFeNiSi-xTiB2 (x = 2.5%, 5.0%, 7.5%, 10.0%) HEA composite coating were obtained by using the Tafeel polarization curve extrapolation method, as shown in Table 3. Generally speaking, the thermodynamic parameter Ecorr can reflect the corrosion trend and possibility of the material, while the kinetic parameter Icorr can characterize the corrosion rate of the material [46-47]. When x = 7.5%, the Icorr (1.252×10-4 A/cm2) of the composite coating is lower than that of other coatings, and the Ecorr (-0.816 V) is the largest, higher than the substrate and other coatings, indicating that the CoCrFeNiSi-7.5%TiB2 coating has the best corrosion resistance. By comparing the Icorr and Ecorr of other coatings, it can be found that the corrosion resistance of the CoCrFeNiSi-10.0%TiB2 coating is better than that of the CoCrFeNiSi-2.5%TiB2 and the CoCrFeNiSi-5.0%TiB2. Among the latter two, the Ecorr value of CoCrFeNiSi-2.5%TiB2 coating is larger than that of CoCrFeNiSi-5.0%TiB2, while their Icorrr values ​​are of the same order of magnitude, so the corrosion resistance of CoCrFeNiSi-2.5%TiB2 coating is slightly better. It is worth noting that the corrosion resistance of all CoCrFeNiSi-xTiB2 coatings is better than that of 40Cr, indicating that CoCrFeNiSi-xTiB2 coatings have improved the ability to resist Cl- intrusion and have good corrosion resistance. The corrosion resistance of the substrate and each coating is ranked as CoCrFeNiSi-7.5%TiB2 > CoCrFeNiSi-10.0%. 0%TiB2>CoCrFeNiSi-2. 5%TiB2>CoCrFeNiSi-5. 0%TiB2>40Cr.

2.5.2 Electrochemical impedance analysis

Electrochemical impedance spectroscopy (EIS) is an effective tool for studying corrosion performance and corrosion mechanism. It reflects the structural composition characteristics of the electrode interface by comparing its kinetic information and polarization curve. Figures 12a and 12b are the Nyquist and Board diagrams of the substrate and the CoCrFeNiSi-xTiB2 HEAs composite coating. As can be seen from Figure 12a, the Nyquist curve of the sample is semicircular, which is due to the charge transfer on the heterogeneous surface. Studies have shown that the larger the semicircle diameter, the better the corrosion resistance. The semicircle diameters of the Nyquist diagram are x = 7.5%, x = 10.0%, x = 2.5%, x = 5. 0%, 40Cr, indicating that the appropriate amount of TiB2 particles can effectively improve the corrosion resistance of the coating in 3.5% NaCl solution. In the Bode diagram, the impedance modulus Z can indicate the degree of Cl- invasion. The larger the Z value, the smaller the degree of Cl- invasion. In the mid-frequency region (1-103 Hz) of Figure 12, the logarithm of the impedance modulus and the frequency show a linear relationship with a slope less than -1. At a fixed frequency of 0.1 Hz, the value of Z is approximately equal to the value of polarization resistance (RP). The larger the RP, the more difficult the sample is to be corroded [51]. As shown in Figure 12b, at f = 10-2 Hz, the Z value of the coating with x = 7.5% is the largest, which indicates that the corrosion resistance of the passive film on the coating surface is the highest when x = 7.5%. In the high frequency region of 104-105 Hz, the phase angle is close to zero, indicating that the solution resistance is low. In the medium frequency range, the phase angle of all coatings does not reach 90°, indicating that the coating has a “semi-adaptive” feature, indicating that the passivation film on the coating surface gradually transforms into a pure capacitance layer with good insulation performance, which has a good protective effect on the coating. In order to evaluate the corrosion process of the coating, the EIS spectrum is analyzed using an equivalent circuit. As shown in Figure 12c, Rs is the solution resistance, Rf is the corrosion product film resistance, Rct is the charge transfer resistance of the electrode, and a constant phase element (CPE) is used to compensate for the non-uniformity of the system (surface roughness and surface defects), which are CPE1 and CPE2 respectively. It can be clearly seen from Table 4 that when x = 7.5%, Rct and Rf are the largest, which also shows that the coating with x = 7.5% has the best corrosion resistance. In summary, the substrate and each coating have good corrosion resistance at 3.5%. The corrosion resistance in 5% NaCl solution is ranked as CoCrFeNiSi-7.5% TiB2 > CoCrFeNiSi-10.0% TiB2 > CoCrFeNiSi-2.5% TiB2 > CoCrFeNiSi-5.0% TiB2 > 40Cr, which is consistent with the results of potentiodynamic polarization curve analysis.

2.5.3 Corrosion surface morphology

Figure 13 shows the electrochemical corrosion morphology of 40Cr substrate and composite coating. It can be clearly seen that the surface of 40Cr is rough, the corrosion degree is the most serious, and pitting pits appear. This is because there are more Cr elements on the surface of 40Cr, the passivation film formed is uneven, Cl- touches the weak part of the passivation film, and forms soluble chlorides through the film surface, resulting in the appearance of pitting pits. x = 2.5%, 5.0%. When x = 0.0% and 10.0%, the surface of the composite coating is smoother than that of the 40Cr substrate, and a small amount of corrosion pits of varying degrees appear on both surfaces. When x = 7.5%, the coating surface is smooth and no corrosion pits appear, which indicates that the CoCrFeNiSi-7.5%TiB2 coating has good corrosion resistance. It is worth noting that when the TiB2 content increases to 10.0%, the corrosion resistance of the coating decreases instead. This is because the excessive addition of TiB2 leads to more B elements in the coating. From the XRD analysis in Figure 1, it can be seen that the intermetallic compound CrB is generated in the coating with x = 10.0%, which increases the non-uniformity of the passivation film and reduces the corrosion resistance of the passivation film in NaCl solution; the intermetallic compound CrB will form a microbattery in the coating, causing galvanic corrosion. Therefore, the corrosion resistance of the coating with x = 10.0% is better than that of the coating with x = 7.5%. 5% coating is reduced.

3 Conclusions

(1) Adding nano-TiB2 particles to CoCrFeNiSi HEA coating can effectively reduce the average grain size of the coating and refine the grain structure of the composite coating. The phase composition of CoCrFeNiSi-xTiB2 HEA coating is FCC phase, BCC phase and CrB boride. Ti and B atoms dissolve in the solid solution, and the combined action of the two leads to severe lattice distortion. From the microstructure, it can be seen that with the increase of TiB2 content, the coating structure transitions from equiaxed crystals to columnar dendrites. At the same time, the addition of TiB2 inhibits the segregation of Si elements at grain boundaries.

(2) The microhardness of the coating is positively correlated with the TiB2 content. When x = 10. 0%, the average microhardness of the coating reaches the maximum value of HV547. 11, which is about 2. 72 times. The improvement of microhardness is the result of the combined effect of solid solution strengthening, dispersion strengthening and fine grain strengthening. The wear resistance of the coating increases with the increase of TiB2 content. When x = 10.0%, the wear loss weight is the smallest, reaching 0.13 mg, which is 88% less than that of the substrate. The wear rate of the coating also decreases with the increase of TiB2. The increase of TiB2 changes the main wear mechanism of the coating from severe abrasive wear and oxidation wear to slight abrasive wear and oxidation wear.

(3) According to the polarization curve and EIS fitting results, the increase of TiB2 particle content can effectively improve the corrosion resistance of the coating. The improvement of the corrosion resistance of the coating is mainly due to the secondary passivation of the coating during the corrosion process, which produces a denser passivation film and improves the ability to resist Cl- invasion. Among them, the CoCrFeNiSi-7.5% TiB2 coating has the best corrosion resistance.