Wear Resistance of Electrodeposited Ni–Mn–SiC Composite Coatings

: To improve the wear resistance of type 45 steel surfaces, Ni–Mn alloy coatings are prepared through electrodeposition under different sodium citrate concentrations based on which SiC particles of varying concentrations are added to prepare Ni–Mn–SiC composite coatings. The coatings are characterized by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, microhardness testing, surface roughness meter, composite material surface performance testing, and laser scanning confocal microscopy. The results show that adding an appropriate concentration of sodium citrate into the electrolyte can signiﬁcantly improve the Mn content in the coatings; however, an excessively high concentration increases the residual stress of the coatings and induces cracks on the surface. When the sodium citrate concentration is 40 g/L, the microhardness and wear resistance of the coatings are optimum. The average microhardness of the Ni–Mn alloy coatings is 522.8 HV 0.05 , and the minimum scratch area of the wear mark is 9526.26 µ m 2 . The addition of SiC particles improves the surface integrity of the composite coatings and further improves the microhardness and wear resistance of the coatings. The composite coating has a maximum average microhardness value of 648.7 HV 0.05 for SiC particle concentration of 4 g/L; this value is nearly 25% higher than that of pure Ni–Mn alloy coatings; the minimum scratch area of the wear mark is reduced to 7160.46 µ m 2 .


Introduction
In recent years, with the development of aviation, aerospace, electronics, and transportation industries, higher requirements are placed on the strength, hardness, and wear resistance of engineering materials. For example, engine cylinders operate at high-temperatures and in high-pressure environments for a long time, often resulting in wear failure due to insufficient lubrication, which affects the overall service life of the engine [1,2]. An insufficient surface protection performance of metal parts decreases the overall service life of mechanical products. Electrodeposition is a common way to prepare composite coatings that can protect material surfaces and help repair worn surfaces; this method is cost effective and has an excellent performance [3][4][5].
In the process of preparing composite coatings, Ni, Cu, and Zn have been used as common composite coating substrates, playing a decisive role in the performance of composite coatings [6][7][8]. Among them, a pure Ni substrate has been widely studied and applied owing to its good mechanical properties and chemical stability. However, in some complex environments, a pure Ni substrate cannot meet the requirements [9]. Therefore, combining two or more metallic or nonmetallic materials into a new composite material through a certain process can help improve the performance [10][11][12]. For example, adding transition metal elements to pure Ni coatings, such as Mn, can help form Ni-Mn alloy coatings and improve the surface morphology and mechanical and chemical properties of coatings [13][14][15]. The excellent properties of Ni-Mn alloys have attracted wide attention In the electrodeposition experiment, a graphite electrode plate with dimensions of 200 mm × 45 mm × 5 mm was selected for the anode and placed parallel to the cathode. Type 45 steel was used as the base metal with a sample size of 30 mm × 8 mm × 7 mm, and was ground using no. 320, no. 800 and no. 1500 wet-and-dry sandpapers successively. The sharp edges and corners in the sample were removed to prevent tip discharge from affecting the coating quality during the experiment. The sample was preprocessed before electrodeposition: degreasing → pickling → activation.
First, an experiment was conducted to explore the effect of sodium citrate concentration on the Ni-Mn alloy coatings and to determine the optimal sodium citrate concentration through a comprehensive performance comparison. Based on the results, we explored the effect of SiC particle concentration on the Ni-Mn-SiC composite coatings. Table 1 lists the electrolyte composition, where the reagents used were all analytically pure and prepared with deionized water. The average particle size of the SiC particles was 20 nm. Before disposing the electrolyte, SiC particles were added to an appropriate amount of deionized water, stirred with ultrasonic waves for 30 min and left to stand for 24 h to make the particles fully wet. After the particles were fully wetted, they were placed in a prepared basic electrolyte and stirred again under ultrasonic waves for 30 min to make the particles evenly dispersed. The process parameters for preparing the coatings were as follows: The current density was 1.35 A/dm 2 for the Ni-Mn alloy coatings and 0.9 A/dm 2 for the Ni-Mn-SiC composite coatings. The electrolyte temperature was 60 • C, and the electrodeposition time was 60 min. Electrodeposition was carried out in a 500 mL beaker, which required a constant-temperature magnetic stirrer to make the electrolyte flow at a constant speed and ensure the temperature. Finally, the samples were cleaned using an ultrasonic machine, dried, and finally preserved for testing.

Characterization of Coating Performance
The surface and cross-sectional morphology, composition and phase structure of the coatings were characterized by scanning electron microscopy (SEM, FEI Instruments Inc., Hillsboro, OR, USA), energy dispersive spectroscopy (EDS, XFlash Detector 5030, Bruker AXS, Inc., Berlin, Germany) and X-ray diffraction (XRD, PANalytical X'pert, PANalytical, Inc., Almelo, Netherlands). The HighScore Plus software (version 3.0e) was used to analyze the XRD results. A microhardness tester (Duramin-40, Struers, Cleveland, OH, USA) was used to measure the microhardness of the coating surface, with a load of 50 g and a holding time of 10 s. Each sample was tested at seven different locations. The JB-4C surface roughness meter (Shanghai Optical Instrument, Shanghai, China) was used to measure the surface roughness of the coatings. The sampling length was 0.8 cm. Data were collected for each sample thrice, and the average value was finally obtained. The dry sliding friction and wear performance of the composite coatings were tested using a material surface performance comprehensive tester (CFT-1, Zhongke Kaihua, Lanzhou, China). The grinding parts were GCr15 alloy steel balls with diameters of 4 mm. The load was 330 g, the reciprocating sliding stroke was 5 mm, the reciprocating speed was 500 r/min, the wear time was 20 min and the sampling frequency was 1 Hz. A laser scanning confocal microscope (LSCM) (OLS4100, Olympus Corporation, Tokyo, Japan) was used to characterize the 3D morphology of the coatings after wear and to measure the volume and depth of the wear scar.  Figure 1 shows the surface and cross-sectional morphologies of the Ni-Mn alloy coatings prepared with different sodium citrate concentrations. Figure 1a shows that the grains on the coating surface prepared in the electrolyte without adding sodium citrate are coarse and have evident pores. Figure 1a-e show that, with an increase in the sodium citrate concentration, the size of the cell particles increases, the pores disappear and the flatness of the coating surface is improved. When the sodium citrate concentration reaches 40 g/L, tiny microcracks appear on the coating surface, as shown in Figure 1e, and the cracks gradually increase or widen with increased sodium citrate concentration, as shown in Figure 1g,i. holding time of 10 s. Each sample was tested at seven different locations. The JB-4C surface roughness meter (Shanghai Optical Instrument, Shanghai, China) was used to measure the surface roughness of the coatings. The sampling length was 0.8 cm. Data were collected for each sample thrice, and the average value was finally obtained. The dry sliding friction and wear performance of the composite coatings were tested using a material surface performance comprehensive tester (CFT-1, Zhongke Kaihua, Lanzhou, China). The grinding parts were GCr15 alloy steel balls with diameters of 4 mm. The load was 330 g, the reciprocating sliding stroke was 5 mm, the reciprocating speed was 500 r/min, the wear time was 20 min and the sampling frequency was 1 Hz. A laser scanning confocal microscope (LSCM) (OLS4100, Olympus Corporation, Tokyo, Japan) was used to characterize the 3D morphology of the coatings after wear and to measure the volume and depth of the wear scar.

Effect of Sodium Citrate Concentration on Performance of Ni-Mn Alloy Coatings
3.1.1. Effect of Sodium Citrate Concentration on Morphology Figure 1 shows the surface and cross-sectional morphologies of the Ni-Mn alloy coatings prepared with different sodium citrate concentrations. Figure 1a shows that the grains on the coating surface prepared in the electrolyte without adding sodium citrate are coarse and have evident pores. Figure 1a-e show that, with an increase in the sodium citrate concentration, the size of the cell particles increases, the pores disappear and the flatness of the coating surface is improved. When the sodium citrate concentration reaches 40 g/L, tiny microcracks appear on the coating surface, as shown in Figure 1e, and the cracks gradually increase or widen with increased sodium citrate concentration, as shown in Figure 1g,i. The characterization results of the cross-sectional morphology and coating thickness show that the coating thickness gradually decreases with increasing sodium citrate concentration. In the electrodeposition process, citrate and metal ions in the bath form relatively stable complex ion groups. A high complexation degree can make it more difficult to reduce the ions, as reflected in the low deposition rate of the coatings. Therefore, the coating thickness gradually decreases.
In the electrodeposition process, citrate and metal ions in the electrolyte form relatively stable complex ion groups, which reduce the free metal cations in the electrolyte. The activation energy required for the electrodeposition process increases, thereby enhancing the cathode polarization and promoting the increase in cathode overpotential, resulting in a high deposition amount of Mn with a more negative reduction potential, which is conducive to the codeposition and increases the coating density [24,25]. However, with the increase in the sodium citrate concentration, the phenomenon of hydrogen evolution becomes more serious, and microcracks appear on the coating surface because of the higher residual stress in the coatings due to hydrogen embrittlement.

Effect of Sodium Citrate Concentration on Composition and Phase Structure
The elemental composition of Ni-Mn alloy coatings prepared under different sodium citrate concentration conditions is shown in Table 2. The Mn content in the coatings shows a trend of increasing first and then decreasing as the sodium citrate concentration increases, reaching a maximum value at a sodium citrate concentration of 60 g/L. The analysis suggests that when the sodium citrate concentration is low, the pH of the Ni-Mn alloy coating electrolyte is low, the H + concentration is high, the side reaction of the hydrogen evolution is violent, and the overpotential of Mn deposition is negative, resulting in little Mn 2+ discharge deposition, so the Mn content in the coatings is low [26,27]. There are very few citrate ions complexing with Ni 2+ , and most of the Ni 2+ ions are in the free state, which has no evident effect on decreasing the Ni 2+ reduction potential. With the increase in the sodium citrate concentration, the pH of the electrolyte increases, and more Ni 2+ complexes with citrate root to form stable metal complexation ions, thus improving the cathode overpotential, which is conducive to the standard negative electrode potential of Mn 2+ deposition on the workpiece surface. However, when the sodium citrate concentration reaches 80 g/L, the Mn content in the coatings decreases. This is because, in the electrodeposition The characterization results of the cross-sectional morphology and coating thickness show that the coating thickness gradually decreases with increasing sodium citrate concentration. In the electrodeposition process, citrate and metal ions in the bath form relatively stable complex ion groups. A high complexation degree can make it more difficult to reduce the ions, as reflected in the low deposition rate of the coatings. Therefore, the coating thickness gradually decreases.
In the electrodeposition process, citrate and metal ions in the electrolyte form relatively stable complex ion groups, which reduce the free metal cations in the electrolyte. The activation energy required for the electrodeposition process increases, thereby enhancing the cathode polarization and promoting the increase in cathode overpotential, resulting in a high deposition amount of Mn with a more negative reduction potential, which is conducive to the codeposition and increases the coating density [24,25]. However, with the increase in the sodium citrate concentration, the phenomenon of hydrogen evolution becomes more serious, and microcracks appear on the coating surface because of the higher residual stress in the coatings due to hydrogen embrittlement.

Effect of Sodium Citrate Concentration on Composition and Phase Structure
The elemental composition of Ni-Mn alloy coatings prepared under different sodium citrate concentration conditions is shown in Table 2. The Mn content in the coatings shows a trend of increasing first and then decreasing as the sodium citrate concentration increases, reaching a maximum value at a sodium citrate concentration of 60 g/L. The analysis suggests that when the sodium citrate concentration is low, the pH of the Ni-Mn alloy coating electrolyte is low, the H + concentration is high, the side reaction of the hydrogen evolution is violent, and the overpotential of Mn deposition is negative, resulting in little Mn 2+ discharge deposition, so the Mn content in the coatings is low [26,27]. There are very few citrate ions complexing with Ni 2+ , and most of the Ni 2+ ions are in the free state, which has no evident effect on decreasing the Ni 2+ reduction potential. With the increase in the sodium citrate concentration, the pH of the electrolyte increases, and more Ni 2+ complexes with citrate root to form stable metal complexation ions, thus improving the cathode overpotential, which is conducive to the standard negative electrode potential of Mn 2+ deposition on the workpiece surface. However, when the sodium citrate concentration reaches 80 g/L, the Mn content in the coatings decreases. This is because, in the electrodeposition process, Mn 2+ is easily oxidized in the anode to form MnO 2 precipitation with the increase in the pH of the plating solution and adsorbs onto the surface of the anode or falls off and disperses in the plating solution. This reduces the Mn content in the electrolyte, which can be confirmed by the change in the electrolyte color [28].  Figure 2 shows the XRD spectrum of the Ni-Mn alloy coatings prepared with different sodium citrate concentrations. Since the Mn content is relatively low and exists in the form of a solid solution solute, no evident diffraction peak related to the Mn element is observed in the XRD spectrum. With the increase in the sodium citrate concentration, the diffraction peak of the coatings gradually widens, and when the concentration reaches 60 g/L, a bun-shaped diffraction peak appears, and the structure becomes amorphous. The atomic radii of Mn and Ni are 132 and 124 pm, respectively. The atomic radii of Ni and Mn are similar, and during the electrocrystallization process, Mn atoms replace Ni atoms in the lattice to form a solid solution. With the increase in the deposition amount of Mn, the Ni lattice is distorted. When the Mn content is high enough, the lattice distortion is too high, thus breaking the original crystal structure and forming an amorphous structure with long-range disorder and short-range order [29,30]. The broadening of the diffraction peak is mainly due to the addition of sodium citrate, which increases the cathode overpotential and promotes the formation of crystal nuclei, thereby refining the grains. disperses in the plating solution. This reduces the Mn content in the electrolyte, which can be confirmed by the change in the electrolyte color [28].  Figure 2 shows the XRD spectrum of the Ni-Mn alloy coatings prepared with different sodium citrate concentrations. Since the Mn content is relatively low and exists in the form of a solid solution solute, no evident diffraction peak related to the Mn element is observed in the XRD spectrum. With the increase in the sodium citrate concentration, the diffraction peak of the coatings gradually widens, and when the concentration reaches 60 g/L, a bun-shaped diffraction peak appears, and the structure becomes amorphous. The atomic radii of Mn and Ni are 132 and 124 pm, respectively. The atomic radii of Ni and Mn are similar, and during the electrocrystallization process, Mn atoms replace Ni atoms in the lattice to form a solid solution. With the increase in the deposition amount of Mn, the Ni lattice is distorted. When the Mn content is high enough, the lattice distortion is too high, thus breaking the original crystal structure and forming an amorphous structure with long-range disorder and short-range order [29,30]. The broadening of the diffraction peak is mainly due to the addition of sodium citrate, which increases the cathode overpotential and promotes the formation of crystal nuclei, thereby refining the grains.
Combining the XRD spectrum and the Scherrer formula, the average grain sizes of the alloy coating were calculated as 69.1 (0 g/L), 23.4 (20 g/L), and 19.0 nm (40 g/L), and the residual stress values are 0.203 (0 g/L), 1.026 (20 g/L), and 1.914 MPa (40 g/L), respectively. This shows that, when the trisodium citrate concentration is lower than 60 g/L, the average grain size of the coating decreases with the increase in the sodium citrate concentration, and the addition of sodium citrate refines the grains in the coatings. The residual stress is proportional to the sodium citrate concentration. When the sodium citrate concentration is higher than 60 g/L, because of the amorphous structure of the coating, the required data cannot be calculated.  Combining the XRD spectrum and the Scherrer formula, the average grain sizes of the alloy coating were calculated as 69.1 (0 g/L), 23.4 (20 g/L), and 19.0 nm (40 g/L), and the residual stress values are 0.203 (0 g/L), 1.026 (20 g/L), and 1.914 MPa (40 g/L), respectively. This shows that, when the trisodium citrate concentration is lower than 60 g/L, the average grain size of the coating decreases with the increase in the sodium citrate concentration, and the addition of sodium citrate refines the grains in the coatings. The residual stress is proportional to the sodium citrate concentration. When the sodium citrate concentration is higher than 60 g/L, because of the amorphous structure of the coating, the required data cannot be calculated.
Based on the relative intensity of each peak obtained from the spectral curve, the degree of crystal plane preference is characterized by calculating the texture coefficient  Figure 3 shows the results. When sodium citrate is not added, the coatings mainly show the preferred orientation of the Ni (200) crystal planes. With the increase in the sodium citrate concentration, the preference of Ni (200) crystal plane decreases, and the preference of densely packed Ni (111) crystal plane increases, which is conducive to the microhardness and wear resistance of the coatings. Based on the relative intensity of each peak obtained from the spectral curve, the degree of crystal plane preference is characterized by calculating the texture coefficient (TC). Figure 3 shows the results. When sodium citrate is not added, the coatings mainly show the preferred orientation of the Ni (200) crystal planes. With the increase in the sodium citrate concentration, the preference of Ni (200) crystal plane decreases, and the preference of densely packed Ni (111) crystal plane increases, which is conducive to the microhardness and wear resistance of the coatings.   Based on the relative intensity of each peak obtained from the spectral curve, the degree of crystal plane preference is characterized by calculating the texture coefficient (TC). Figure 3 shows the results. When sodium citrate is not added, the coatings mainly show the preferred orientation of the Ni (200) crystal planes. With the increase in the sodium citrate concentration, the preference of Ni (200) crystal plane decreases, and the preference of densely packed Ni (111) crystal plane increases, which is conducive to the microhardness and wear resistance of the coatings.   grain size decreases, the strength and hardness of the material increase [31]. According to the Hall-Petch relationship, the material strength is directly related to the grain size: where σ represents the material strength, σ 0 and k are both material constants related to the inherent properties of the material and d is the average grain size. According to the grain boundary strengthening theory, in the process of low-temperature deformation, the grain boundary hinders the internal error movement, so the strength at the grain boundary is higher than that inside the grain [32]. A smaller grain size corresponds to a higher grain boundary density, which can effectively improve the microhardness of coatings. As the sodium citrate concentration increases, the cathodic polarization becomes stronger, and the cathodic overpotential increases, which favors Mn deposition with a lower standard electrode potential. From the crystallization kinetics, we find that a higher cathode overpotential implies a higher probability of nucleation and finer electrocrystallization. Therefore, the grain size of the coating layer will be significantly refined, and the density will be improved, thereby improving the surface hardness of the coatings. However, when the sodium citrate concentration is 60 g/L, because of the strong cathodic polarization, the hydrogen evolution reaction is serious, several pinholes are formed in the coatings, and brittleness increases [33]. An appropriate amount of sodium citrate can help form stable chelates with Ni 2+ . The cathode overpotential is increased, which is conducive to the deposition of Mn with a low standard electrode potential and the formation of Ni-Mn replacement solid solution; owing to its solid solution strengthening effect, it restricts the dislocation movement of the grains [34]. However, an excessively high Mn content will destroy the original crystal structure, and a noncrystal structure will tend to deteriorate the mechanical properties of the coatings.

Effect of Sodium Citrate Concentration on Wear Resistance
The average surface roughness (Ra) of the Ni-Mn alloy coatings under different sodium citrate concentrations is shown in Table 3. When the sodium citrate concentration increases, the surface roughness value first decreases, which is consistent with the surface morphology results given in Section 3.1.1. As the sodium citrate concentration increases, the formed microcracks increase in number and grow wider on the surface, thereby increasing the surface roughness.  Figure 5 and Table 4 show the effects of sodium citrate concentration on the wear mark morphology and section parameter of the Ni-Mn alloy coatings. Evidently, the parameter of the wear marks is significantly changed. With the increase in the sodium citrate concentration from 0 to 40 g/L, the wear width, depth and scratch area all decrease. When the sodium citrate concentration is increased to 80 g/L, the wear mark width, depth and scratch area increases. When the concentration of sodium citrate in the bath reaches 60 g/L, the width, depth and scratch area of the wear marks are minimum. The wear resistance of the coatings is closely related to the surface morphology, microhardness, and surface roughness. The higher the microhardness and the lower the surface roughness of the coatings, the better the wear resistance of the coating surface [35]. Combined with Figure 5 and Table 4, the effect of sodium citrate concentration on the wear resistance of the coatings is analyzed as follows. When the Mn content is low, the strength of the coatings is insufficient, the resistance to adhesion wear is low and evident adhesion of the wear marks can be seen by observing the appearance of the wear marks. In a certain range, with the increase in the sodium citrate concentration, the Mn content in the coatings increases, the solid solution strengthening is enhanced and the compactness and mechanical properties of the coatings are improved, which influences the wear resistance of the coatings. When the sodium citrate concentration is 40 g/L, the microcracks that appear in the coatings can accommodate the abrasive particles generated in the wear process of the coatings to prevent the abrasive particles from participating in the wear process [36]. However, as the sodium citrate concentration continues to increase, the surface quality of the coatings decreases, and the microcracks gradually increase in number and widen, characterized by an increase in the grinding width and wear volume.
Coatings 2021, 11, x FOR PEER REVIEW 9 of 18 wear marks can be seen by observing the appearance of the wear marks. In a certain range, with the increase in the sodium citrate concentration, the Mn content in the coatings increases, the solid solution strengthening is enhanced and the compactness and mechanical properties of the coatings are improved, which influences the wear resistance of the coatings. When the sodium citrate concentration is 40 g/L, the microcracks that appear in the coatings can accommodate the abrasive particles generated in the wear process of the coatings to prevent the abrasive particles from participating in the wear process [36]. However, as the sodium citrate concentration continues to increase, the surface quality of the coatings decreases, and the microcracks gradually increase in number and widen, characterized by an increase in the grinding width and wear volume.     Figure 6 shows the surface and cross-sectional morphologies of the composite coatings prepared by adding SiC particles of different concentrations when the sodium citrate concentration is 40 g/L. The addition of SiC particles into the plating solution helped improve the surface integrity of the Ni-Mn-SiC composite coatings, and there is a typical cell structure without any evident cracks or holes. However, compared with the Ni-Mn alloy coatings, the surface cell of the composite coatings is more prominent, and the flatness decreases. In addition, when the SiC particle concentration is 4 g/L, the SiC particles on the coated surface are most distributed and are more uniform, as shown in Figure 6c. The addition of SiC particles has little effect on the deposition rate, and there is no evident change in the thickness of the composite coatings with increasing SiC particle concentration.
SiC particles of an appropriate amount are more evenly dispersed between the Ni and Mn grains and rely on the strong chemical adsorption to preferentially adsorb onto defects such as vacancies and pores, which can effectively fill the intergranular pores. SiC particles act as heterogeneous units in promoting the rapid formation and growth of new crystal nuclei, while hindering the coarsening of the generated crystal nuclei, so that the grains on the coatings can be refined and the coating density is improved. However, it has little influence on the electrochemical reaction rate in the deposition process, so the deposition rate changes little [37][38][39]. When the SiC particle concentration is high, the cell structure on the composite coating surface is unevenly distributed, and there are evident agglomeration phenomena. A part of the agglomerated SiC particles settle under the action of gravity. Moreover, because of the large volume and mass of the agglomerated particles, the mass transfer process of the liquid phase is hindered, and the adsorption deposition probability of the SiC particles is reduced, resulting in a low deposition amount and decreased uniformity [40,41].  Figure 6 shows the surface and cross-sectional morphologies of the composite coatings prepared by adding SiC particles of different concentrations when the sodium citrate concentration is 40 g/L. The addition of SiC particles into the plating solution helped improve the surface integrity of the Ni-Mn-SiC composite coatings, and there is a typical cell structure without any evident cracks or holes. However, compared with the Ni-Mn alloy coatings, the surface cell of the composite coatings is more prominent, and the flatness decreases. In addition, when the SiC particle concentration is 4 g/L, the SiC particles on the coated surface are most distributed and are more uniform, as shown in Figure 6c. The addition of SiC particles has little effect on the deposition rate, and there is no evident change in the thickness of the composite coatings with increasing SiC particle concentration. SiC particles of an appropriate amount are more evenly dispersed between the Ni and Mn grains and rely on the strong chemical adsorption to preferentially adsorb onto defects such as vacancies and pores, which can effectively fill the intergranular pores. SiC particles act as heterogeneous units in promoting the rapid formation and growth of new crystal nuclei, while hindering the coarsening of the generated crystal nuclei, so that the grains on the coatings can be refined and the coating density is improved. However, it has little influence on the electrochemical reaction rate in the deposition process, so the deposition rate changes little [37][38][39]. When the SiC particle concentration is high, the cell structure on the composite coating surface is unevenly distributed, and there are evident agglomeration phenomena. A part of the agglomerated SiC particles settle under the action of gravity. Moreover, because of the large volume and mass of the agglomerated particles, the mass transfer process of the liquid phase is hindered, and the adsorption deposition probability of the SiC particles is reduced, resulting in a low deposition amount and decreased uniformity [40,41]. Figure 7 and Table 5 show the effects of SiC particle concentration on the XRD spectrum, grain size, and residual stress, respectively. The effect of SiC particle concentration on the grain size of the coatings is not evident; however, the residual stress is significantly reduced in comparison to the case where no SiC particles were added. Therefore, the addition of SiC particles helps reduce the probability of cracks in the coatings.  Table 5 show the effects of SiC particle concentration on the XRD spectrum, grain size, and residual stress, respectively. The effect of SiC particle concentration on the grain size of the coatings is not evident; however, the residual stress is significantly reduced in comparison to the case where no SiC particles were added. Therefore, the addition of SiC particles helps reduce the probability of cracks in the coatings. Due to the uneven distribution of the particles on the coating surface, there are large random factors in the testing element content using the energy spectrometer. When the SiC particle concentration is 4 g/L, the contents of each element and SiC on the composite coating surface are used merely as a reference, and the results are as follows: Ni-95.1 wt.%; Mn-4.1 wt.%; SiC-0.8 wt.%.  Due to the uneven distribution of the particles on the coating surface, there are large random factors in the testing element content using the energy spectrometer. When the SiC particle concentration is 4 g/L, the contents of each element and SiC on the composite coating surface are used merely as a reference, and the results are as follows: Ni-95.1 wt.%; Mn-4.1 wt.%; SiC-0.8 wt.%. Figure 8 shows the effect of SiC particle concentration on the microhardness of the coatings. As shown, adding SiC particles on the basis of Ni-Mn alloy coatings can effectively improve the microhardness of the coatings. As the SiC particle concentration increases, the microhardness of the composite coatings tends to increase first and then decrease. When the SiC particle concentration is 4 g/L, the average microhardness reaches a maximum of 648.7 HV 0.05 , which is nearly 24% higher than that of the pure Ni-Mn alloy coatings. The microhardness of Ni-based composite coatings is related to particle strengthening, dispersion strengthening and grain refining. When the concentration of the secondphase ceramic particles is very low, the dispersion strengthening is the main effect. In this case, the matrix metal is subjected to a load, and the addition of second-phase ceramic particles inhibits dislocation motion. When the concentration of the second-phase ceramic particles is higher than a certain amount, particle strengthening plays the dominant role. The particles and the matrix metal are subjected to the load together, and too many particles are deposited on the coating surface, thus inhibiting the deformation of the substrate layer and resulting in particle strengthening [42]. According to the Orowan mechanism, as a type of high-hardness ceramic material, SiC particles exist in the composite coatings in the form of grain boundary and grain interior inlays, playing the role of dispersion strengtheners. SiC particles fill the defects between the Ni-Mn grains, improve the dislocation resistance of the grains, reduce the local deformation, enhance the cohesion of the coating, and improve the microhardness of the coatings [43]. As the SiC particle concentration increases, particle strengthening occurs, and SiC particles bear a part of the load of the matrix metal and reduce the deformation of the matrix layer and strengthening. When the SiC particle concentration in the plating solution exceeds a certain amount, the number of SiC particles adsorbed on the workpiece surface exceeds the containment capacity of the matrix metal. This is not conducive to the deposition of SiC particles [44], and the probability of particle collisions increases. Moreover, the agglomeration phenomenon is The microhardness of Ni-based composite coatings is related to particle strengthening, dispersion strengthening and grain refining. When the concentration of the second-phase ceramic particles is very low, the dispersion strengthening is the main effect. In this case, the matrix metal is subjected to a load, and the addition of second-phase ceramic particles inhibits dislocation motion. When the concentration of the second-phase ceramic particles is higher than a certain amount, particle strengthening plays the dominant role. The particles and the matrix metal are subjected to the load together, and too many particles are deposited on the coating surface, thus inhibiting the deformation of the substrate layer and resulting in particle strengthening [42]. According to the Orowan mechanism, as a type of high-hardness ceramic material, SiC particles exist in the composite coatings in the form of grain boundary and grain interior inlays, playing the role of dispersion strengtheners. SiC particles fill the defects between the Ni-Mn grains, improve the dislocation resistance of the grains, reduce the local deformation, enhance the cohesion of the coating, and improve the microhardness of the coatings [43]. As the SiC particle concentration increases, particle strengthening occurs, and SiC particles bear a part of the load of the matrix metal and reduce the deformation of the matrix layer and strengthening. When the SiC particle concentration in the plating solution exceeds a certain amount, the number of SiC particles adsorbed on the workpiece surface exceeds the containment capacity of the matrix metal. This is not conducive to the deposition of SiC particles [44], and the probability of particle collisions increases. Moreover, the agglomeration phenomenon is strengthened, which not only reduces the dispersibility of the plating solution and the SiC nanoparticle content in the coatings but also reduces the uniformity of the nanoparticles, thereby decreasing the coating strength.

Effects of SiC Particle Concentration on Wear Resistance
The average surface roughness (Ra) of the Ni-Mn-SiC composite coatings under different SiC particle concentrations is shown in Table 6. As the SiC particle concentration increases, the surface roughness value of the composite coatings drops first and then rises. With the decrease in the surface roughness, the coating surface is flatter and more compact, and the structure is finer and even. When the composite coatings and abrasive material are in contact, the composite coating surface is flatter, the actual contact area between the two is larger, and the pressure acting on a unit area is lower. Therefore, the volume loss of the coatings after wear is lower-i.e., the wear resistance of the coatings can be improved.  Figure 9 shows the effects of SiC particle concentration on the wear mark morphologies of the Ni-Mn-SiC composite coatings. Combined with the results shown in Table 7, we find that Ni-Mn-SiC composite coatings have significantly better wear resistances than Ni-Mn alloy coatings. With the increase in SiC particle concentration, the width, depth and scratch area of the composite coatings decreased first and then increased. When the concentration of SiC particle in the bath reaches 4 g/L, the width, depth and scratch area of the wear marks are minimum. The minimum width of the wear value for the composite coatings is 540.3 µm corresponding to an SiC particle concentration of 4 g/L; this value is nearly 20% higher than that of pure Ni-Mn alloy coatings. The minimum wear scratch area of the composite coatings, i.e., 7160.46 µm 2 , is nearly 25% higher than that of the pure Ni-Mn alloy coatings.  Figure 9 shows the effects of SiC particle concentration on the wear mark morphologies of the Ni-Mn-SiC composite coatings. Combined with the results shown in Table 7, we find that Ni-Mn-SiC composite coatings have significantly better wear resistances than Ni-Mn alloy coatings. With the increase in SiC particle concentration, the width, depth and scratch area of the composite coatings decreased first and then increased. When the concentration of SiC particle in the bath reaches 4 g/L, the width, depth and scratch area of the wear marks are minimum. The minimum width of the wear value for the composite coatings is 540.3 μm corresponding to an SiC particle concentration of 4 g/L; this value is nearly 20% higher than that of pure Ni-Mn alloy coatings. The minimum wear scratch area of the composite coatings, i.e., 7160.46 μm 2 , is nearly 25% higher than that of the pure Ni-Mn alloy coatings.
After adding an appropriate amount of SiC particles, the phenomena of tearing spalling and plastic deformation of the coating are reduced, the wear form gradually changes from adhesive wear to abrasive wear and the wear condition is improved. The dispersed SiC particles disrupt the continuous growth of the coatings, so the surface morphology of the composite coatings becomes more fine, dense and uniform. The fine-grain strengthening effect of the nanoparticles makes the SiC particles play a pinning role in the coatings, restricting the movement of dislocations between the grains, enhancing the mechanical properties of the composite coatings and reducing the deformation during the wear process [45,46]. In the wear process, as the coating wears the SiC particles embedded in the composite coating are gradually exposed on the surface and play the role of load and wear lubricators, so as to reduce the wear on the substrate and enhance the wear resistance of the composite coatings [47,48]. The agglomeration phenomena of the composite coatings prepared under too high particle concentrations are more evident. Therefore, the coatings lose their excellent properties and show poor wear resistance.

Conclusions
In this study, Ni-Mn alloy coatings and Ni-Mn-SiC composite coatings were electrodeposited on the surface of type 45 steel substrate under varying sodium citrate and SiC particle concentrations. The changes in the concentration of sodium citrate and SiC particles significantly influenced the surface morphology and mechanical properties of the deposited coatings, including: Sodium citrate promoted the grain refinement of the Ni-Mn alloy coatings and improved the coating density; however, it induced cracks due to the high residual stress. When the sodium citrate concentration was 40 g/L, the mechanical properties of the coatings were the best. When the sodium citrate concentration reached 60 g/L and above, the Ni-Mn coatings exhibited amorphous structures;  After adding an appropriate amount of SiC particles, the phenomena of tearing spalling and plastic deformation of the coating are reduced, the wear form gradually changes from adhesive wear to abrasive wear and the wear condition is improved. The dispersed SiC particles disrupt the continuous growth of the coatings, so the surface morphology of the composite coatings becomes more fine, dense and uniform. The finegrain strengthening effect of the nanoparticles makes the SiC particles play a pinning role in the coatings, restricting the movement of dislocations between the grains, enhancing the mechanical properties of the composite coatings and reducing the deformation during the wear process [45,46]. In the wear process, as the coating wears the SiC particles embedded in the composite coating are gradually exposed on the surface and play the role of load and wear lubricators, so as to reduce the wear on the substrate and enhance the wear resistance of the composite coatings [47,48]. The agglomeration phenomena of the composite coatings prepared under too high particle concentrations are more evident. Therefore, the coatings lose their excellent properties and show poor wear resistance.

Conclusions
In this study, Ni-Mn alloy coatings and Ni-Mn-SiC composite coatings were electrodeposited on the surface of type 45 steel substrate under varying sodium citrate and SiC particle concentrations. The changes in the concentration of sodium citrate and SiC particles significantly influenced the surface morphology and mechanical properties of the deposited coatings, including: Sodium citrate promoted the grain refinement of the Ni-Mn alloy coatings and improved the coating density; however, it induced cracks due to the high residual stress. When the sodium citrate concentration was 40 g/L, the mechanical properties of the coatings were the best. When the sodium citrate concentration reached 60 g/L and above, the Ni-Mn coatings exhibited amorphous structures; Based on the Ni-Mn alloy coatings, the addition of SiC particles could effectively reduce the residual stress, improve the surface integrity and further improve the microhardness and wear resistance of the composite coatings. The effect was best when the SiC particle concentration was 4 g/L.

Conflicts of Interest:
The authors declare no conflict of interest.