Effects of WC Addition on Microstructure and Properties of Plasma-Cladded AlCoCrFeNi High-Entropy Alloy Coatings

Abstract

In order to enhance the performance of 20# steel, this study successfully fabricated AlCoCrFeNi high-entropy alloy coatings with different WC contents (x = 0, 10, 20, 30 wt%) on its surface using plasma cladding technology. The effects of WC content on the microstructure, mechanical properties, and corrosion resistance of the coatings were systematically investigated. The results indicate that without WC addition, the coating consists of a dual-phase structure comprising BCC and FCC phases. With the incorporation of WC, the FCC phase disappears, and the coating evolves into a composite structure based on the BCC matrix, embedded with multiple carbide phases such as W₂C, M₇C₃, M_xC_γ, and Co₆W₆C. These carbides are predominantly distributed along grain boundaries. As the WC content increases, significant grain refinement occurs and the volume fraction of carbides rises. The coating exhibits a mixed microstructure of equiaxed and columnar crystals, with excellent metallurgical bonding to the substrate. The microhardness of the coating increases markedly with higher WC content; however, the rate of enhancement slows when WC exceeds 20 wt%. The hardness of 1066.36 HV is achieved at 30 wt% WC. Wear test results show that both the friction coefficient and wear rate first decrease and then increase with increasing WC content. The optimal wear resistance is observed at 20 wt% WC, with a friction coefficient of 0.549 and a wear mass loss of only 0.25 mg, representing an approximately 40% reduction compared to the WC-free coating. Electrochemical tests demonstrate that the coating with 20 wt% WC facilitates the formation of a dense and stable passive film in NaCl solution, effectively inhibiting Cl⁻ ion penetration. This coating exhibits the best corrosion resistance, characterized by the lowest corrosion current density of 1.349 × 10⁻⁶ A·cm⁻² and the highest passive film resistance of 2764 Ω·cm².

1. Introduction

High-entropy alloy (HEA) is a novel type of multi-component alloy material composed of five or more elements in equal or nearly equal atomic percentages [1,2]. It exhibits unique properties such as lattice distortion effect, sluggish diffusion effect, cocktail effect, and high-entropy effect. These properties endow HEAs with excellent characteristics including wear resistance, corrosion resistance, high-temperature resistance, and radiation resistance, making them have huge application potential in fields such as marine engineering, nuclear power, and new energy [3,4].

In the research system of high-entropy alloys (HEAs), the Al-Co-Cr-Fe-Ni series has been the most systematically and comprehensively studied, with typical compositions including the FCC-structured Al_0.3CoCrFeNi alloy [5], the BCC-structured AlCoCrFeNi alloy, and the dual-phase structured AlCoCrFeNi_2.1 eutectic alloy. The AlCoCrFeNi HEA has received extensive attention in the field of surface modification due to its typical properties of high hardness, wear resistance, high-temperature oxidation resistance, and corrosion resistance [6]. Much research has focused on exploring its mechanical and corrosion resistance properties through the addition of different elements and preparation techniques [7,8]. Parakh investigated the influence of grain size, dislocation density, and microstructure on the corrosion resistance of AlCoCrFeNi HEA. The research showed that corrosion potential and corrosion current density are important parameters for evaluating the corrosion resistance of HEAs, and changes in the alloy’s crystal structure, such as the transition from body-centered cubic (BCC) to face-centered cubic (FCC), can significantly affect its corrosion resistance [9]. Shockner studied the influence of Cr content on the microstructure of AlCoCr_xFeNi alloys. As the Cr content increases, changes occur in the alloy’s microstructure and phase composition, which have important effects on the alloy’s mechanical properties. In addition, Cr can form carbides and nitrides of Cr, which can act as strengthening phases to improve the alloy’s high-temperature strength and creep resistance [10].

However, as operating environments impose increasingly demanding requirements on materials’ high-temperature resistance, oxidation resistance, and wear resistance, it is evident that AlCoCrFeNi high-entropy alloys (HEAs) cannot fully satisfy service conditions, requiring further enhancement of their performance [11]. Studies indicate that incorporating specific elements, cermet phases, and rare earth elements into HEA coatings can significantly improve specific properties of the coatings [12,13]. Currently, commonly used cermet phases include WC, TiC, SiC, NbC, among others. WC has a low coefficient of thermal expansion, good wettability, and exceptionally high hardness (2600 HV) and melting point (2870 °C), making it an ideal material for reinforcing cemented carbides. At present, research on doping WC ceramic particles in nickel-based, cobalt-based, or iron-based alloys is relatively mature [14]. However, due to the poor retention and wettability of traditional metal matrices towards reinforcing phases, severe interface reactions occur between the matrix and reinforcement phase. This leads to issues such as uneven particle distribution, microdefects, and low bonding strength, significantly degrading coating performance [15]. Zhang et al. found that HEAs exhibit good retention and wettability towards reinforcing phases, largely solving the aforementioned problems and providing the possibility of adding high proportions of reinforcing phases in wear-resistant coatings [16].

In addition, studies have also found that during laser cladding of WC/FeCoCrNi HEA composite coatings, as the WC content increases, the interfacial bonding force of WC particles decreases, which can easily lead to the detachment of WC particles and a reduction in the mechanical properties of the coating [17]. Therefore, it is necessary to clarify the influence of WC on the phase composition, microstructure, microhardness, and wear resistance of HEA coatings.

While extensive research has focused on HEA preparation techniques such as laser cladding, electrical discharge deposition, and magnetron sputtering, these methods typically require expensive equipment, complex operations, and low preparation efficiency, leading to relatively high preparation costs [18]. As a novel surface coating technology, plasma cladding can achieve metallurgical bonding between the coating and the substrate. It has advantages such as high processing efficiency, low dilution rate, small heat-affected zone, dense coating, low investment and operating costs (30 wt% lower than laser cladding and 1/12 of the cost of magnetron sputtering), and simple operation. Plasma cladding plays a crucial role in alloy surface modification and has emerged as a prominent research focus within the HEA field internationally [19,20,21]. Based on this, this paper systematically explores the influence of WC particle addition on the phase composition, microstructure, microhardness, wear resistance, and corrosion resistance of AlCoCrFeNi HEA coatings using plasma cladding technology, aiming to provide a basis for the industrial application of high-performance HEA coatings.

2. Materials and Methods

2.1. Materials

The experimental substrate used was normalized 20# steel (chemical composition shown in

Table 1. Chemical composition of the base material (wt%).

Element	C	Mn	Si	S	P	Ni	Cr	Cu	Fe
20#	0.22	0.54	0.28	≤0.035	≤0.035	≤0.30	≤0.25	≤0.25	Bal.

), with dimensions of 100 mm × 50 mm × 10 mm. The surface of the substrate material was pre-treated by grinding and polishing to completely remove the oil, rust, and oxide film on the surface of the steel plate, bringing it to a clean and smooth state. Subsequently, the surface was wiped with anhydrous ethanol and then placed in a drying oven at 60 °C for over 6 h to ensure that the surface was dry, free from contamination and oxidation. Prior to cladding, the substrate was rapidly heated to 500 °C to reduce the thermal stress generated during the cladding process, and then the cladding experiment was carried out.

The cladding material used was AlCoCrFeNi high-entropy alloy powder prepared by vacuum gas atomization (Beijing Huake Putian Technology Co., Beijing, China). The elements in the powder are in equimolar ratios, with the main element mass fractions (wt%) being: Al 10.67 wt%, Co 23.32 wt%, Cr 20.55 wt%, Fe 22.13 wt%, and Ni 23.32 wt%. Subsequently, according to the powder ratios shown in

Table 2. Compositional design of different sprayed layers (wt%).

Sample	AlCoCrFeNi Powder	WC Powder
W0	100	0
W1	90	10
W2	80	20
W3	70	30

, the AlCoCrFeNi alloy powder was mixed with WC powder of 99.9% purity by mass ratio for later use.

Figure 1 displays the original morphology and mixing effect of the powders used in the experiment. Specifically, Figure 1a shows spherical AlCoCrFeNi high-entropy alloy powder with a particle size distribution of 30~60 μm, and Figure 1b displays spherical WC ceramic particles with a particle size distribution of 10~20 μm. Both types of powders exhibit high sphericity, smooth surfaces, and no obvious defects. The selection of larger-sized AlCoCrFeNi alloy powder paired with smaller WC particles aims to ensure good fluidity of the mixed powder, thereby achieving continuous and stable powder feeding during the cladding process, which is one of the key prerequisites for successfully preparing the cladding coating. The AlCoCrFeNi and WC in different proportions were mixed in an XQM-6 planetary ball mill produced by Changsha Tianchuang Powder Technology Co., Ltd (Changsha, China). The ball milling parameters were set at 360 r/min for 2 h to ensure uniform mixing of the powders. Figure 1c shows the morphology at a WC mass fraction of 20 wt%. Microscopic observation reveals that the WC particles are well-dispersed among the spherical AlCoCrFeNi particles after ball milling.

2.2. Experimental Process

The coating was prepared using a PDA-400 D1-ST plasma cladding machine (Shanghai Duomu Industry Co., Ltd., Shanghai, China). This equipment integrates a high-performance plasma power supply, a high-precision powder feeding system, and an automated integrated workstation, enabling precise three-dimensional control of the coating during the plasma cladding process and ensuring the stability and uniformity of the coating quality. In the early stage of the experiment, key process parameters such as cladding current, scanning speed, and powder feeding rate were optimized to ensure the quality of the coating. Under the experimental conditions, the cladding current was set at 95 A. Industrial-grade high-purity argon gas (purity ≥99.99%) was used for all gases, including the plasma gas, powder carrier gas, and shielding gas. The flow rates were controlled as follows: plasma gas at 5 L/min, shielding gas at 25 L/min, and the powder feeding rate was maintained at 0.8 g/s. The cladding speed was 6 mm/s, with a width of 20 mm and a length of 50 mm. The distance between the cladding torch and the substrate was maintained at 10 mm, resulting in an average coating thickness of 2.8 mm. After the cladding process is completed, to eliminate the residual stresses within the coating, the specimen is placed in a muffle furnace at 600 °C and allowed to cool with the furnace. This process effectively promotes the release of residual stresses, thereby ensuring the quality and stability of the coating.

Samples were cut along the cross-section of the substrate and coating to prepare metallographic samples of the coating’s side and surface. After standard mechanical grinding and polishing, the substrate and coating were subjected to electrochemical etching. The etching solution was a 4 wt% hydrofluoboric acid aqueous solution (provided by Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China). The electrochemical temperature was controlled at 25 °C, the etching voltage was 10 V, the electrode distance was 20 mm, and the electrochemical etching time was controlled between 15–20 s. The phase composition of the coating surface was analyzed using an XRD-6100 X-ray diffractometer (Shimadzu Corporation, Kyoto, Japan) with Cu Kα radiation at 25 °C and a scanning speed of 4°/min, with a 2θ scan range of 20 ° to 90°.

Metallographic observations were conducted using a an Axio Vert. A1 optical microscope (Carl Zeiss AG, Oberkochen, Germany). The microstructure of the coating was examined with a GeminiSEM 300 field-emission scanning electron microscope (Carl Zeiss AG, Germany), and the chemical composition of the cladding layer was determined with the aid of an energy dispersive spectrometer (EDS). The microhardness of the coating’s cross-section was tested using a LECO AMH43 semi-automatic microhardness tester (Laboratory Equipment Corporation, Hayward, CA, USA). The experimental load was 0.3 N, with a dwell time of 15 s for the indenter and an additional 15 s for pressure holding. Testing began on the side of the 20# steel plate, with measurements taken every 200 μm. At each distance, three measurements were taken and averaged.

The wear resistance of the coating was tested using an M-2000 ring-block friction and wear tester (Jinan Fangyuan Testing Instrument Co., Ltd., Jinan, China) under room temperature dry friction conditions. The friction pair material was quenched 45 steel, with a load of 100 N, a rotational speed of 180 r/min, and a wear time of 30 min. A MicroXAM optical profiler (KLA Corporation, Milpitas, CA, USA) was used to comprehensively observe and analyze the wear morphology of the cladding layer. The corrosion resistance mechanism of the coating was tested using a CHI-660E electrochemical workstation (CH Instruments, Inc., Austin, TX, USA). Measurements were carried out in a 3.5 wt% NaCl aqueous solution under a standard three-electrode configuration, with a platinum electrode as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode.

3. Results and Discussion

3.1. Phase Analysis of Coating

Figure 2 displays the X-ray diffraction patterns of AlCoCrFeNi high-entropy alloy coatings with different WC contents. As shown in the figure, the diffraction peak 2θ values of the coating without WC addition are 30.8°, 42.94°, 44.16°, 50.18°, 64.54°, 73.82°, and 81.62°, respectively, and its phases mainly consist of body-centered cubic (BCC) and face-centered cubic (FCC) phases. Upon WC addition, significant alterations occur in the diffraction pattern: the characteristic FCC phase peaks at 42.94°, 50.18°, and 73.82° vanish. Simultaneously, the diffraction peak intensity of the BCC phase near 44.16° gradually increases. In addition, new diffraction peaks of W₂C, M₇C₃, M_xC_y, and Co6W6C emerge. Studies have suggested that WC decomposes during the high-temperature process of cladding: 2WC → W2C + C, and the released carbon reacts with strong carbide-forming elements in the coating to form new M₇C₃, M_xC_y, and Co₆W₆C phases [22]. In AlCoCrFeNi high-entropy alloy coatings, the formation of the FCC phase depends on the mixed entropy stabilization effect between multiple metal atoms. The addition of WC provides new atomic interaction sites, increasing the formation free energy of the FCC phase. Simultaneously, the addition of WC affects the atomic diffusion process, hindering the diffusion of certain metal atoms towards the FCC phase region. The combined effect of these factors leads to the gradual disappearance of the FCC phase [23]. The gradual enhancement of the diffraction intensity of the BCC phase at 44.16° in the XRD pattern is mainly due to the large atomic radius of tungsten atoms originating from WC. After addition, the average atomic radius and related parameters of the atoms in the alloy system become more favorable for the formation of BCC phase, M₇C₃, M_xC_y, and other phases. The BCC phase matrix itself is also the main precipitation parent phase of carbides such as M₇C₃ and M_xC_y. Consequently, the addition of WC not only stabilizes/promotes the BCC phase but also provides conditions for the precipitation of various hard carbides [24].

From the locally enlarged image in Figure 2, it can be observed that as the WC content increases, the diffraction peak near the 2θ value of 44° shifts to the left. The peak values when the WC content is 0 wt%, 10 wt%, 20 wt% and 30 wt% are 44.16°, 44.08°, 44.02°, and 43.98°, respectively. The analysis suggests that the larger atomic radius of W atoms in WC induces lattice distortion in the BCC phase, increasing the lattice constant. According to Bragg’s law [25]:

$$D={\displaystyle \frac{n\lambda }{2\sin \theta }}$$

In the equation: D represents the interplanar spacing, λ represents the wavelength of the X-rays, and θ represents the diffraction angle.

When the lattice expands, the lattice constant increases, and the interplanar spacing enlarges. Consequently, the sinθ value decreases, causing the diffraction peak to shift to the left.

3.2. Microstructure of Coating

Figure 3 exhibits the microstructural characteristics of the coatings with varying WC contents. Significant differences can be observed between the bottom and the middle to upper regions of the coatings. The dark areas appearing at the bottom result from preferential corrosion due to the difference in corrosion resistance between the substrate and the coating.

There is an obvious “white-bright band” at the interface between the coating and the substrate (Figure 3(a1)), which originates from the planar crystals formed by the rapid solidification of the mixture of molten powder and semi-molten substrate, indicating effective metallurgical bonding between the coating and the substrate [26]. In addition, the microstructural and elemental distribution analyses at the interface between the AlCoCrFeNi high-entropy alloy coating and substrate are presented in Figure 4. No defects such as pores or microcracks were observed in the interfacial region. Eutectic structures were found to directionally extend from the substrate toward the coating along the thermal gradient. A mechanically mixed zone comprising eutectic structures and ferrite from the substrate was identified in the substrate region. This morphology indicates the coexistence of planar and columnar crystals at the interface. The mechanically mixed characteristics confirm the diffusion migration of high-entropy alloy elements toward the substrate side during high-temperature cladding. Elemental distribution analysis via EDS revealed continuous concentration gradients of Al, Co, Cr, and Ni from the coating to the substrate. The elemental interdiffusion behavior and interfacial mechanically mixed structure collectively demonstrate the formation of a dense metallurgical bond between the coating and substrate.

Figure 3(a1) shows the microstructure at the bottom of the fusion zone without WC addition, mainly consisting of columnar crystals growing in a specific direction. These columnar crystals exhibit obvious growth directionality, roughly perpendicular to the substrate surface and extending towards the center of the molten pool. Figure 3(a2) displays the microstructure within the mid-region of the coating, composed of white and gray equiaxed and columnar crystals, with the white regions dominating. Combining the EDS element analysis in Figure 5 and the XRD results in Figure 2, it can be determined that the white regions correspond to BCC phases, while the gray regions correspond to FCC phases. The analysis suggests that the liquid close to the substrate has a high temperature gradient (ΔT) and a low solidification rate (R). According to solidification theory, when the ratio of the temperature gradient to the solidification rate (ΔT/R) is maximized, the nucleation rate is much higher than the crystal growth rate, and the microstructure tends to epitaxially grow in a planar crystal manner. As the distance of the coating from the substrate increases, ΔT gradually decreases. while R gradually increases, resulting in a gradual transformation of the microstructure into columnar and equiaxed crystals [26].

After adding WC particles, as shown in Figure 3(b1,c1,d1), the “white-bright band” still exists at the coatings interface, and the fusion zone is still dominated by columnar crystals, indicating that the metallurgical bonding state between the coating and the substrate is maintained. However, there is a significant evolution in the microstructure of the middle part of the coating, as shown in Figure 3(b2,c2,d2). It mainly consists of a white matrix with gray-black phases distributed between the crystals. Combining the XRD results in Figure 2, the analysis shows that the white regions mainly correspond to the BCC phase matrix, and the gray-black regions are mainly composed of multiple carbide phases of M₇C₃, M_xC_y, W₂C, and Co₆W₆C phases. The analysis suggests that at high temperatures, part of the WC decomposes to release carbon. These dissolved carbon atoms strengthen and stabilize the BCC phase through interstitial solid solution, consuming some of the key stabilizing elements Fe and Cr related to the FCC phase, leading to the disappearance of the FCC phase [27]. Simultaneously, W and C react in situ with the matrix elements to form multiple carbide phases (M₇C₃, M_xC_y, W₂C, Co₆W₆C). With increasing WC content, the decomposed C elements increase, leading to enhanced formation of multiple carbide phases through chemical reactions. Consequently, the proportion of multiple carbide phases progressively rises. The in situ formed carbides manifest as fine-scale, high-melting-point, hard particulate phases. These particulates effectively serve as heterogeneous nucleation sites and grain boundary pinning particles thereby inducing microstructural refinement. The average grain size decreases from approximately 12.5 μm (Figure 3(a2)) without WC addition to 8.6 μm (Figure 3(d2)) with higher WC content.

Figure 6 presents the SEM images of high-entropy alloy coatings with different WC additions. At a lower WC content (10 wt%), the multiple carbide phases are rare and the structure is coarse in the coating (Figure 6a), resulting in insufficient dislocation glide resistance and limited enhancement of mechanical properties (lower hardness and higher wear rate) [28,29]. As the WC content increases (20 wt%), the multiple carbide phases gradually increase in number and accompanied by refinement in size and closer stacking, forming a honeycomb-like structure. This is beneficial for improving the density and mechanical properties of the coating (Figure 6b). When the WC content reaches 30 wt%, the multiple carbide phases coarsen and aggregate, with closer stacking between the interlocking the multiple carbide phases, forming a large block-like structure (Figure 6c). This tends to reduce the toughness of the cladding layer [30].

Elemental distribution mapping obtained by EDS analysis of the high-entropy alloy coating with 20 wt% WC content (Figure 7) revealed that Ni and Co were uniformly dispersed throughout the coating, whereas W, Al, Cr, and Fe exhibited significant enrichment within the white network-like phase. This elemental partitioning further confirms that carbon atoms originating from the decomposition of WC contributed to the formation of multiple carbide phases, including M₇C₃, M_xC_γ, W₂C, and Co₆W₆C [31].

3.3. Hardness and Wear Resistance of Cladding Layers with Different WC Contents

Hardness is a key indicator of the mechanical properties of a cladding layer, as it can intuitively reflect the hardness distribution and variation of the coating. Figure 8a presents the microhardness distribution curves of the substrate and AlCoCrFeNi high-entropy alloy coatings with varying WC contents. It can be observed that the substrate exhibits significantly lower microhardness than the HEA coatings. The addition of WC further enhances the hardness of the coatings. Analysis indicates that WC dissolution increases the carbon content in the cladding layer, forming high-hardness carbide phases that strengthen the coating [32]. In addition, the addition of WC refines the grains, generates a small amount of secondary phases (intermetallic compounds and carbides), increases the grain boundary density, and inhibits dislocation movement, thereby further improving the hardness.

The variation pattern of coating microhardness with increasing WC addition is shown in Figure 8b. The hardness of the coating exhibits an initial significant increase followed by a gradual saturation trend as the WC content increases. The hardness of the coating without WC addition is 778 HV, and when the WC addition increases to 30 wt% WC, the hardness increases to 1066.36 HV. Notably, when the WC content exceeds 20 wt%, the increase in hardness significantly weakens and tends to stabilize. The analysis suggests that the strengthening effects of carbide increment and grain refinement reach critical values near 20 wt% WC. Excessive WC tends to reduce the cooling rate of the molten pool, leading to the coarsening of carbide particles, which limits further enhancement of coating hardness [32].

Figure 9a exhibits the variation of the friction coefficient of the coatings and the substrate with wear time at different WC additions. In the initial stage, the friction coefficient rapidly increases due to brittle spallation of the coating caused by rough cutting of the wear ball. Subsequently, the friction coefficient tends to stabilize but oscillates in a zigzag manner, which is attributed to the alternating action of adhesion and slip caused by the inhomogeneous microstructure of the coating (differences in carbide distribution and grain orientation). Specifically, when the WC content is 0 wt%, 10 wt%, 20 wt%, and 30 wt%, the average friction coefficients of the AlCoCrFeNi high-entropy alloy coatings during the stable wear stage are 0.662, 0.598, 0.549, and 0.566, respectively. This indicates that as the WC content increases, the friction coefficient of the coating generally shows a downward trend. However, when the WC content reaches 30 wt%, the friction coefficient increases slightly compared to that observed at 20 wt% WC. Combining the analysis of Figure 6c, it can be inferred that this may be due to the agglomeration of hard phases and inhomogeneous microstructure caused by excessive WC, which reduces the stability of the coating during friction. Therefore, the increase in WC content does not unlimitedly decrease the friction coefficient; in other words, an excessively high WC content may actually lead to an increase in the friction coefficient.

When analyzing the wear resistance of the coating, mass loss constitutes a critical parameter. As shown in Figure 9b, the mass loss of coatings with WC addition is generally lower than that without WC addition, which is consistent with the trend of hardness improvement, indicating that increased hardness contributes to wear resistance. Specifically, when the WC content is 0 wt%, 10 wt%, 20 wt%, and 30 wt%, the average wear mass losses of the AlCoCrFeNi high-entropy alloy coatings are 0.42 mg, 0.28 mg, 0.25 mg and 0.26 mg, respectively. Among them, the coating with 20 wt% WC demonstrates the minimal mass loss and optimal wear resistance, which corresponds to the hardness test results, indicating a certain proportional relationship between hardness and wear resistance. However, wear resistance is not completely positively correlated with hardness. The specimen with the highest hardness, 30 wt% WC, exhibits decreased wear resistance. The analysis suggests that the increase in the brittle Co₆W₆C phase reduces the coating’s toughness, leading to increased mass loss [33,34].

Figure 10 presents 3D scanning images of the worn surfaces of cladding layers with different WC contents. It is evident from the figures that the wear tracks appear as deep grooves. The cladding layer without WC addition (Figure 10a) exhibits deeper wear tracks, while the coatings with 10 wt% and 20 wt% WC particles have relatively shallower wear tracks. A large number of pits, regular furrows, and a small amount of grinding debris attached to the furrows are present on the wear tracks of the coating surface (Figure 11). This indicates that adhesive wear and abrasive wear interact during the friction process. This wear morphology is beneficial in practical production because it means that the overall wear is uniform and controllable [35]. However, increasing the WC content to 30 wt% alters this behavior. The reduction in toughness accelerates wear progression, resulting in deeper wear tracks and localized brittle fracture. This is also related to the increase in the brittle Co₆W₆C phase and the coarsening of the multiple carbide phases [36]. Therefore, when optimizing the performance of cladding layers, the addition amount of WC needs to be carefully considered to find the optimal balance between hardness and wear resistance.

3.4. Analysis of Corrosion Resistance of Cladding Layers with Different WC Content

Figure 12a shows the open circuit potentials (OCP) of AlCoCrFeNi HEA coatings with different WC contents when immersed in a 3.5 wt% NaCl solution. During the test, all samples reached a quasi-steady state within 1600 s. The OCPs of the coatings are listed in

Table 3. Electrochemical performance of AlCoCrFeNi high-entropy alloy coatings with different contents WC.

Coatings	OCP (V vs. SCE)	Ecorr (V)	Icorr (A·cm−2)
W0	−0.362	−0.455	1.259 × 10−5
W1	−0.187	−0.401	3.548 × 10−6
W2	−0.126	−0.314	1.349 × 10−6
W3	−0.168	−0.355	2.398 × 10−6

. Before WC addition, the OCP of the coating was −0.362 V. After WC addition, the OCP decreased to −0.126 V (W2). A higher OCP is considered to indicate better corrosion resistance of the coating. Since the increase in OCP value is related to the density and integrity of the passivation film, it can be inferred that WC can promote the formation of a passivation film on AlCoCrFeNi high-entropy alloys [37].

Figure 12b shows the polarization curves of coatings W0 to W3 obtained by Tafel extrapolation method, from which the corrosion potential (Ecorr) and corrosion current density (Icorr) were derived (Table 3). These parameters serve as key indicators for evaluating the corrosion resistance of the materials. The study found that the addition of WC increased the corrosion potential and reduced the corrosion current density by an order of magnitude. The passivation region of the polarization curve was more pronounced, indicating that a denser passivation film was formed on the surface coating during the corrosion process, which corresponds to the OCP results. Combining Figure 12b and the data in Table 3, the following trend can be summarized: the corrosion potential of the coatings follows the order Ecorr (W2) > Ecorr (W3) > Ecorr (W1) > Ecorr (W0), while the corrosion current density exhibits the sequence Icorr (W2) < Icorr (W3) < Icorr (W1) < Icorr (W0). These results demonstrate that the addition of WC significantly enhances the corrosion resistance of the coatings, with the coating containing 20 wt% WC (W2) exhibiting the most superior performance. It is analyzed that the addition of WC promotes the enrichment of Cr and Al elements at the grain boundaries (as shown in Figure 7). Al and Cr elements have strong affinity for oxygen. In NaCl solution, the surface of the high-entropy alloy (HEA) coating can quickly form a stable and dense composite passive film of Al₂O₃ and Cr₂O₃. This passive film can effectively block the ingress of corrosive ions, thereby significantly enhancing the corrosion resistance of the coating. In addition, the passive film preferentially nucleates and grows in the interdendritic regions. The addition of WC refines the grains and increases the grain boundary area, providing more nucleation sites for the uniform formation of the passive film, making it more dense and continuous, and further reducing the corrosion rate. However, when an excessive amount of WC is added, the particles are prone to agglomeration in the coating, causing uneven distribution and introducing micro-defects such as pores and micro-cracks. These defects will become the preferred channels for the penetration of corrosive media, thereby leading to a decrease in the overall corrosion resistance of the coating [38,39].

To further investigate the corrosion resistance of the coating, electrochemical impedance spectroscopy (EIS) analysis was employed. The coatings were immersed in a 3.5 wt% NaCl solution for 2 h, and the collected data were fitted using ZView 2 software to produce Figure 13. In the Nyquist plot of Figure 13a, each coating exhibits a different capacitive reactance arc, with a larger radius indicating better corrosion resistance. It is clearly visible that the radii of the W1–W3 coatings are much larger than that of the W0 coating, especially for the coating with 20 wt% WC addition, indicating more difficult charge transfer at the interface between the electrolyte and the electrode. In the Bode plots of Figure 13b,c peak width, peak height, and modulus can all be used to express the corrosion resistance of the coatings. While the peak widths remain largely consistent across coatings W0 to W3, elevated peak heights and higher moduli correlate with enhanced corrosion resistance. In both figures, the coatings with WC particle-reinforced phases exhibit higher corrosion resistance, with coating W2 exhibiting the most favorable characteristics The EIS results were fitted using the equivalent circuit shown in Figure 13d to simulate the corrosion behavior of the coatings. The simplified equivalent circuit consists of three components: Rs, Rp, and a CPE, representing the solution resistance, passive film resistance, and a constant phase element, respectively.

Table 4. Fitted results of the EIS experiment.

Coatings	Rs (Ω·cm2)	CPE-T (Ω−1·sn)	CPE-P	Rp (Ω·cm2)
W0	6.378	8.326 × 10−4	0.786	2126
W1	6.652	3.766 × 10−4	0.896	2532
W2	6.876	3.532 × 10−4	0.924	2764
W3	6.674	3.985 × 10−4	0.877	2578

presents the fitted data obtained from EIS tests, where the polarization resistance (Rp) value can be directly used to evaluate the corrosion resistance of the coating. As shown in the table, the lowest and highest Rp values correspond to the W0 and W2 coatings, respectively. The results further confirm that the addition of WC to the AlCoCrFeNi high-entropy alloy coating improves corrosion resistance, reaching its peak performance at a 20 wt% WC content. This corresponds to the polarization curvers presented earlier.

4. Conclusions

Plasma cladding technology was employed to fabricate AlCoCrFeNi-xWC (x = 0, 10, 20, 30 wt%) high-entropy alloy coatings on 20# steel substrates. The effects of WC content on the microstructure, hardness, wear resistance, and electrochemical corrosion properties of the coatings were systematically investigated. The main conclusions are as follows:

(1)

The AlCoCrFeNi high-entropy alloy coating consists of both BCC (body-centered cubic) and FCC (face-centered cubic) phases. With the addition of WC, the FCC phase gradually disappears, and the coating transforms into a multiphase structure with BCC as the matrix, accompanied by the precipitation of various carbide phases such as W₂C, M₇C₃, M_xC_γ, and Co₆W₆C. The carbide phases are primarily distributed at grain boundaries. As the WC content increases, the coating grains are significantly refined, the number of carbides increases, and the coating structure evolves into a combination of equiaxed and columnar grains. Moreover, the coating exhibits excellent metallurgical bonding with the substrate.

(2)

The hardness of the coating significantly improves with increasing WC content. However, when the WC content exceeds 20 wt%, the strengthening effect tends to saturate, and the rate of increase slows down noticeably. At 30 wt% WC, the hardness reaches 1066.36 HV. The wear resistance of the coating initially decreases and then increases with higher WC content, reaching its optimal value at 20 wt% WC. At this composition, the friction coefficient is 0.549, and the wear mass loss is only 0.25 mg, representing a reduction of approximately 40% compared to the coating without WC.

(3)

During electrochemical corrosion in a 3.5 wt% NaCl solution, the appropriate addition of WC promotes the enrichment of Al and Cr elements at grain boundaries, facilitating the formation of a dense and stable passive film that effectively blocks Cl⁻ ion penetration. However, excessive WC addition increases the density of microstructural defects in the coating, reducing the continuity and protective capability of the passive film and consequently degrading the corrosion resistance. The coating with 20 wt% WC exhibits the best corrosion resistance, with the lowest corrosion current density of 1.349 × 10⁻⁶ A·cm⁻² and a passive film resistance of 2764 Ω·cm².