Preparation and performance analysis of micro-nano composite coatings reinforced with WC particles

In order to research the effect of Tungsten carbide (WC) particles with different particle sizes additions in the reinforced coating, the strengthening coatings were produced by the compound process of high frequency induction cladding. The microstructure, phases and basic properties of the composite strengthening coatings were analyzed by means of metallographic microscope and FESEM. It was found that the addition of WC micro-nano particles further refined the grain phase in the strengthened coatings. The average hardness of the coating significantly increased with the increase of nanoparticle composition. The coating hardness of 30% na-WC involvement was the highest, 17% higher than that of 30% micron WC involvement. In the frictional wear experiments, the minimum amount of wear was the coating with the involvement of 20% micron WC and 10% na-WC, 20% less compared to that of 30%na-WC. The experiments showed that the introduction of an appropriate proportion of WC nanoparticles further improves the hardness of the reinforced coating. But due to the agglomeration phenomenon of nanoparticles, the hardness will be reduced instead as the proportion of WC nanoparticles further increased. The appropriate proportion of nanoparticles can effectively improve the wear resistance, but excessive proportion of nanoparticle addition can lead to a decrease in wear resistance. The involvement of 20% micron WC and 10%na-WC is the best addition ratio considering all factors.


Introduction
The preparation of high-performance reinforced coatings on the surface of critical parts for the mechanical operation to improve the service life of machines has been one of the hot research topics in surface strengthening work [1][2][3][4].Nickel-based alloys are widely used to prepare coatings because of their good wear resistance, corrosion resistance, and high-temperature resistance [5][6][7].Tungsten carbide (WC) has the property of high hardness, wear resistance, and good high-temperature chemical stability.This Material is often used as particlereinforced phases of nickel-based self-soluble alloy powders to prepare high-performance wear-resistant coatings on the surfaces of many critical components [8][9][10][11], and have been widely used in many fields such as automotive, metallurgy, mining, and aerospace [12][13][14][15].The most common method for preparing nickel-based WC particle-reinforced composite coatings is thermal spraying.Dong et al used the dry ice impact technique to prepare thicker composite coatings [16].Acker et al focused on the effect of WC content and particle size on the performance of composite coatings [17].However, the high preparation cost due to the complex equipment and low powder utilization of the thermal spray process affects the application of high-performance composite coatings.In order to improve the utilization of alloy powder and the speed of coating preparation, researchers have tried to use induction cladding technology for coating preparation.For example, Du et al prepared a highperformance Ni-based WC coating using high-frequency induction melting supplemented with mechanical vibration technology [18], and Zhang et al further improved the performance of Ni-based WC coating using induction remelting combined with heat treatment [19].He et al tested the wear resistance of the coating after preparing an induction melting Ni-based WC coating and proved that the hardness and wear resistance of the coating increased with the WC.The hardness and wear resistance of the coating increased with the increase of WC content [20].The relative wear resistance increased by 2-6 times with the addition of WC particles to enhance the phase compared with no addition.However, all the above studies were conducted using micronsized WC as the reinforcing phase for coating preparation.Wang et al added a nano-hard phase to the micronlevel hard phase to improve the corrosion and wear resistance of the material, and successfully prepared microand nano-WC composite coatings on the steel substrate surface [21].But they did not conduct in-depth research on it.
Most of the current studies on WC as a reinforcing phase to obtain Ni-based composite reinforced coatings have been done around micron-scale WC.From the above studies it can be seen that less research involves nanoscale WC and micro-nanoscale WC as a particle reinforcing phase for reinforced layers.Nickel-based alloy powders have good self-melting properties, good surface wettability for heterogeneous materials, and a wide range of compatibility with different materials.In this paper, Ni-based reinforced coatings were prepared using different ratios of micro-and nano-WC particles as the reinforcing phase using high-frequency induction melting technique.The microstructure morphological characteristics and composition of the obtained composite coatings were also analyzed, and the properties of the coatings with different composition ratios were tested.

Experimental materials and preparation
Quenched and tempered 40Cr alloy steel was used as the elementary stuff with dimensions of 40 mm × 40 mm × 10 mm.In the experiments, the surface of quenched and tempered 40Cr alloy steel was polished with anangle grinder.After removing the oxide layer with contaminants, the surface was rinsed with anhydrous ethanol to remove oil and residues.Finally, to improve the bonding strength between the coating and the substrate, the substrate was sandblasted.Ni-based alloy powder is Ni60 high hardness alloy powder for thermal spraying, which has good self-solubility and can quickly wet the substrate and particle surface, with a particle size of about 350 mesh.The average particle size of a micron WC is 75 μm, and the purity is 99.5%.Nanometer WC was prepared by high-energy ball milling method, and the micron WC particles were placed in the grinding tank of BXQM-12L planetary ball mill for high-energy ball milling (argon gas protection in the tank during ball milling, ball milling medium was 10 mm diameter carbide balls, the ball to material ratio was 10:1, speed was 350 r min −1 , time was 50 h), the obtained powder particle size was 46 ∼ 573 nm.The specific composition can be seen in table 1.
Firstly, the Ni-based alloy powder was mixed with different proportions of WC micro-nano powder (as shown in table 2).Rosin was added to make a paste after mixing uniformly in the ball mill, and then the paste mixture was evenly coated on the surface of 40Cr substrate.Finally the pretreated specimens were inductively coated in a JQ25KW high-frequency induction furnace after they were dried in a drying oven at 120 °C.The selection of experimental parameters should allow for a shorter test time, ensuring that the pre coating is stable and does not peel off, so that subsequent tests can achieve better coating effects.After multiple experiments, the reasonable experimental parameters have been determined : oscillation current 1200A, oscillation frequency 100 KHz, experiment time 24S, the coil 4 turns, and the distance between specimen and coil 1-2.5 mm.The thickness of the prepared coating was 0.2-0.4mm.The coatings with added WC content of 30% micron WC, 20% micron WC +10%na-WC, 10% micron WC +20%na-WC, and 30%na-WC were named as WC30m, WC20m-10n, WC2m-20n, and WC30n coatings, respectively.The microstructure morphology of the coating cross-section and the wear surface morphology were observed and analyzed using a VHX-1000 super depth-of-field microscope and an S-4800 field emission scanning electron microscope (FESEM).To analyze the physical phase of the coating, an x-ray diffractometer (XRD) of type D/max-2550PC was used for the corresponding analysis, and a wear test was conducted on an MJP-20 friction wear machine with a contact pressure of 20 MPa, an average relative motion speed of 1 m s −1 and a time of 4 h.Each specimen was tested 3 times to obtain its value.

Experimental results and analysis 2.2.1. Micromorphology
The microstructure morphology of the coating cross-section with different proportions of WC micro-nano powder is shown in figure 1. FESEM diagram of the coating cross-section with different proportions of WC micro-nano powder is shown in figure 2. From figure 1(A), it can be seen that there is a very obvious bright white diffusion transition zone between the coating and the substrate, and this transition zone is called the diffusion transfer belt (DTB) [22].The presence of the bright white zone is a unique sign of the HF induction cladding process and indicates that a metallurgical bond has been formed between the coating and the substrate.DTB also can be seen in the figures 1(B), (C) and D. By comparing the metallographic diagrams of the coating crosssection with different proportions of WC micro-nano powder, it was found that as the number of nanoparticles increased, the agglomeration phenomenon also seemed to be increasing.This phenomenon was further validated through FESEM analysis.
From figure 2(A), The disordered distribution of coating grains is mainly due to the process limitations of prefabricated cold coating, which results in uneven powder particle size, gaps between metal particles.In addition, factors such as the high melting point of WC can also lead to a disordered distribution of coating grains.WC powder cannot be completely uniformly dispersed on the surface of the matrix, resulting in uneven phenomena.When heated at high temperatures, the molten Ni based alloy cannot be quickly filled, and there  will be some small bubbles remaining in the coating.This will inevitably cause pores and defects in the coating during rapid cooling after fusion.It can also be seen that the distribution of particles is more uniform, and this distribution pattern is relatively balanced for wear.
From figure 2(B), the small white bright lumpy micron WC particles are relatively uniformly distributed, but some dendritic grains have appeared to be irregularly distributed within the coatings.This phenomenon occurs due to the thermal agglomeration of na-WC particles during induction heating, which is specific to nanoparticles.Due to the relatively low nano content, the proportion of agglomerated grains in the coating is not high.Due to the significant size difference between relatively large micron WC particles and na-WC particles, there are relatively few agglomerated grains.During the coating preparation process, the heat obtained by nano particles is less than micrometer particle.Nano particles cannot be well wetted by Ni based alloys, which results in a loose grain structure at the agglomeration site.
From figure 2(C), it can be found that pores also appear within the grains, which is due to the fact that at high temperatures, the residual gas in the coating is hindered by the large agglomerated nano-WC during the uplifting process, and the uplifting rate of the gas is less than the cooling rate of the liquid alloy, making more pores in the coating [23].
From figure 2(D), The WC content in the coating has not changed, but all of them have been replaced by na-WC particles, and it can be seen that the agglomeration phenomenon has been very obvious, and the WC particles that should be distributed diffusely are agglomerated inside the coating.Because the particles are small, the heat obtained by the heat nanoparticles after agglomeration is faster, and at the same time, with the stirring effect of electromagnetic effect, it can still make the matrix and particles solidify quickly, but because the agglomeration phenomenon is more serious, there are still holes inside the grains of the agglomerates.

Hardness analysis
In the work on the determination of mechanical properties of reinforced coatings, hardness is one of the important indicators, which can be used to measure both the softness and hardness of the coating and can also be understood as the ability of the coating to resist elastic and plastic deformation and damage [24].Figure 3 shows the hardness distribution of the cross-section of the reinforced coating with respect to the substrate for different ratios.It can be seen from the figure that the trend of hardness distribution between the coating and the substrate is basically similar for different specimens, and the hardness of the substrate is basically the same for different ratios of specimens, starting from the bright white zone, where the hardness is significantly enhanced, as a result of the mutual integration of the elements within the coating and within the substrate in the transition region.The hardness distribution slowly becomes stable again as it tends to the surface of the coating [5].The average hardness of the coating is gradually increased from WC30m to WC30n coating, with the highest average hardness of 842 HV for WC30n coating, which was 17% higher than the average hardness of WC30m coating.This is because the ability of metallic materials to resist surface plastic deformation is actually closely related to the grain structure, and the addition of nano-WC allows the grains inside the coating to be further refined, the specific area of the grain surface increases, and the length of phase boundaries and grain boundaries increases accordingly, which can hinder the movement of dislocations when the surface is damaged [25].However, the occurrence of the nanomaterial agglomeration phenomenon makes the distribution more dispersed inside the coating, and the pores and looseness increase, which also affects the hardness of the coating [26].
However, it can also be seen from the figure that the hardness variation in the coating with the addition of nanoparticles is uneven due to the agglomeration phenomenon, and this unevenness increases with the addition of nanoparticles, which also indirectly indicates the increase of the agglomeration phenomenon.

XRD analysis
As demonstrated in figure 4, the coating mainly contains Ni 3 Fe, Co 6 W 6 C, W 2 C, Fe 6 W 6 C, and Fe 5 C 2 phases.The generation of these phases is due to the decomposition of W and C elements at high temperatures and the combination of W and C with other elements in the coating.The Ni 3 Fe phase in the coating is mainly used to bind and fix the hard particle phases such as WC.The appearance of Co 6 W 6 C and Fe 6 W 6 C phases indicates the phenomenon of the decomposition of WC particles and their rapid combination with other elements in the powder do exist during the high-frequency induction coating process, and the generation and rapid dispersion of these hard phases are beneficial to improve the wear resistance of the coating [26].

Wear resistance analysis
According to the relevant literature [24], the wear resistance of reinforced coatings is not only related to their hardness but also to the content, distance, and structure of the hard particle phases in the coating.A good bond between the hard phase and the substrate in the reinforced coating can effectively inhibit the fatigue crack generation in the coating.The hard grains precipitated in the Ni-based alloy have a diffuse strengthening effect, and this diffuse strengthening can effectively stop the dislocation and fatigue crack expansion during the wear process, which is the main reason for the increased hardness and wear resistance of the reinforced coating.As can be seen from figure 5, there is a high contact load on the contact surface of the specimen during the experiment, and the joint repeated action of the cutting force and compressive stress caused by the contact load easily makes the surface of the specimen reach the yield limit of shear and produce plastic deformation, and as the experiment continues, the plastic deformation of the surface will produce fatigue micro-cracks, and when the cracks continue to develop, it will lead to the appearance of surface particles/chip flaking The surface shape of the material is changed [27].The spalled abrasive particles are relatively hard, and some of them are not  excluded in time and will be embedded in the surface of the specimen under compressive stress.As the experiment continues, the fatigue flaking and embedded abrasive particles under the joint action of the fatigue flaking, so that it quickly enters the stage of severe wear, if the continuation of the experiment will make the specimen surface severe wear, the experimental machine will also occur serious vibration, resulting in the experiment cannot continue.
Figure 6 shows the average value of mass loss of different samples after frictional wear experiments.From the figure 6, it can be seen that the wear amount of WC20m-10n coating is only 17.2 mg, which is 20% less compared to the wear amount of 21.6 mg of WC30n coating.From the figure 5(B), due to the high hardness, extremely small size, and high surface activity of nanoparticles, they can further enhance their dispersion and hardness in coatings.The combination of nanoparticles and micrometers can further amplify the advantages of the two materials in the strengthening coating.During the wear process, even if some materials may peel off from the reinforced coating, they will quickly fill the small gaps on the friction surface, which increases the actual contact area and reduces the load per unit area.At the same time, the nanoparticles can also play a kind of 'microrollerball' in the middle of the friction surface to reduce wear [28].Therefore, the addition of hard nanoparticles to the reinforced coating can contribute to the improvement of the wear resistance and the extension of the stable wear phase.
From figures 5 and 5(C), it can be seen that the denseness and hardness inside the coating of WC10m-20n, were not significantly improved.Theoretically, the nanograins in the coating will make the hard grain phase with solid solution strengthening less prone to loosening and spalling in wear, but the excessive nanograins makes the chance of agglomeration increase greatly.This leads to a decrease in the ability of the reinforced coating to resist contact fatigue cracks and peeling during the wear process, rather than further improving.
From figure 5(D) , further confirm that the agglomeration phenomenon has an effect on the reinforced coating.Loose nanocrystals are easily squeezed and peeled off from the substrate during the experimental process.Sharp peeling edges and holes in the Ni-based alloy are prone to stress concentration.During repeated wear, the stress concentration causes cracks to sprout and expand and then peel off, forming large peeling pits, which cause a larger amount of wear.

Conclusion
Micro-and nano-WC particle-reinforced composite reinforced coatings were prepared on the 40Cr surface by induction melting technique with different proportions of WC micro-nano powder, and the total content of WC particles accounted for 30%.The microstructure and wear resistance of the composite reinforced coatings were analyzed.It was found that the addition of nanoparticles led to further refinement of the grain phase inside the reinforced coating.The average hardness of the coating is gradually increased from WC30m to WC30n, The coating hardness of WC30n was the highest, 17% higher than that of WC30m coating.Meanwhile the agglomeration became more apparent.In the frictional wear experiments, the minimum amount of wear on the coating was WC20m-10n, 20% less compared to WC30n.The experiments showed that the introduction of an appropriate proportion of WC nanoparticles further improves the hardness of the reinforced coating.But due to the agglomeration phenomenon of nanoparticles, the hardness will be reduced instead as the proportion of WC nanoparticles further increased.The wear resistance of the reinforced coating is also related to the amount of WC nanoparticles added.The appropriate proportion of nanoparticles can effectively improve the wear resistance, but excessive proportion of nanoparticle addition can lead to a decrease in wear resistance.The involvement of 20% micron WC and 10%na-WC is the best addition ratio considering all factors.

Figure 3 .
Figure 3.The hardness distribution of cross section and substrate of reinforced coating with different proportions of WC micro-nano powder.

Figure 4 .
Figure 4. XRD diffraction patterns of the coatings with different proportions of WC micro-nano powder.

Figure 6 .
Figure 6.The weight loss of the coating with different proportions of WC micro-nano powder.

Table 1 .
Chemical composition (wt%) and grain size of the original powder.

Table 2 .
Compositon of the original powder.