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Article

The Effect of La on the Surface Properties of Plasma Nitrided CoCrCuFeNi High-Entropy Alloys at 440 Degrees Celsius

by
Yifan Wang
1,
Yuan You
1,*,
Mufu Yan
2,
Yanxiang Zhang
2,
Wenping Sheng
1,
Yan Wang
1,
Shimiao Xu
1,
Feng Gu
1,
Chaohui Wang
1 and
Weidong Cheng
1
1
Heilongjiang Provincial Key Laboratory of Polymeric Composition Materials, School of Materials Science and Engineering, Qiqihar University, Qiqihar 161006, China
2
National Key Laboratory for Precision Hot Processing of Metals, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
*
Author to whom correspondence should be addressed.
Coatings 2024, 14(3), 303; https://doi.org/10.3390/coatings14030303
Submission received: 29 December 2023 / Revised: 20 February 2024 / Accepted: 28 February 2024 / Published: 29 February 2024
(This article belongs to the Section Plasma Coatings, Surfaces & Interfaces)
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Abstract

:
This paper investigates the effect of the element La on plasma nitriding of the CoCrCuFeNi high-entropy alloy (HEA) at 440 °C for 8, 16, and 24 h. The phase composition, morphology, and hardness distribution of the nitrided layer are characterized using XRD, optical microscopy, and a microhardness tester. Furthermore, the corrosion resistance is tested using an electrochemical workstation. The study evaluated the friction and wear performance using a testing machine and scanning electron microscope. The thickness of the effective hardening layer after 16 h of treatment with La was similar to that after 24 h of treatment without La. The addition of La significantly increased the growth rate constant of the effective hardening layer from 0.53 × 10−14 m2/s to 0.72 × 10−14 m2/s. In addition, an expanded FCC phase with greater interplanar spacing can be formed on the surface of the sample by introducing La into the plasma nitriding process. This indicates that the expanded FCC phase, with a higher concentration of interstitial nitrogen atoms, can effectively improve the corrosion resistance of the specimen surface. The corrosion rate of the specimen surface was reduced by 27.5% and the wear rate was reduced by 41.7% after 16 h of treatment with the addition of La compared to 24 h of nitriding without the addition of La. It has been shown that the addition of La to the plasma nitriding process results in a higher quality nitrided layer in a shorter time and also demonstrates that La has the potential to optimize the surface properties of plasma nitrided HEAs.

1. Introduction

High-entropy alloys (HEAs), as a novel class of metallic materials, diverge fundamentally from traditional alloy systems in their design concept [1,2,3]. In conventional alloys, typically, one metal element serves as the matrix, supplemented by a small amount of other elements (Co, Cr, Ni, Al, etc.) to adjust the microstructure and enhance mechanical properties [4]. In contrast, high-entropy alloys consist of five or more principal elements, with each element’s molar fraction ranging between 5% and 35% [5,6], collectively forming simple solid solution phases. This unique compositional strategy endows high-entropy alloys with numerous superior physical and chemical properties, such as high hardness, excellent wear resistance [7,8], outstanding fatigue resistance [9,10], strength retention at high temperatures [11,12,13], and thermal stability [14,15].
In specific application scenarios, researchers have employed traditional surface treatment technologies, including gas nitriding, to improve the surface performance of high-entropy alloys. However, studies on gas nitriding are relatively scarce. For instance, Meng et al. [16] treated an Fe28.2Ni18.8Mn32.9Al14.1Cr6 high-entropy alloy with pure nitrogen gas nitriding at 1000 °C and 800 °C for 100 h. This treatment formed an aluminum-rich nitrided zone on the sample surface, where the hardness significantly increased to 344 ± 18 HV (1000 °C) and 364 ± 8 HV (800 °C). After annealing at 1000 °C for 100 h, the maximum thickness of the nitrided zone could reach 1000 μm. Moreover, this research indicates that the performance enhancement after gas nitriding may be closely related to the spacing between the nitrided zones.
Meanwhile, plasma nitriding, as a surface modification technique, has been more widely applied in high-entropy alloys. Since 2009, scholars have begun to conduct plasma nitriding experiments on high-entropy alloys to enhance the performance of the material’s surface layer and expand its application fields. Previous studies have focused on nitriding temperatures ranging from 490 °C [17] to 550 °C, using NH3 or N2 + H2 as the nitriding atmospheres, with nitriding times between 9 and 45 h. After nitriding treatment, the maximum thickness of the modified layer could reach 92.2 μm [18], with a significant increase in hardness and the formation of a uniform nitrided layer. The nitrided layer on the high-entropy alloy improved wear resistance by 49 to 80 times [18] compared to its untreated high-entropy alloy counterpart. On the other hand, when comparing the nitrided high-entropy alloy to nitrided traditional steel, the high-entropy alloy demonstrated a 25 to 54 times [19] increase in wear resistance over the nitrided traditional steel material.
Tang et al. [20] conducted a study on the nitriding of high-entropy alloys. They found that the presence of aluminum (Al) significantly impacts nitrogen diffusion within the sample. The study indicates that the absence of aluminum results in the most effective nitrogen diffusion depth, leading to a nitrided layer that can reach a thickness of up to 33 μm. On the other hand, an increase in aluminum content reduces the thickness of the nitrided layer. Precisely, at an aluminum content of 0.5, the wear rate after nitriding is measured at 3.69 × 10−5 mm3/N∙m; an improvement compared to the untreated specimen wear rate of 6.38 × 10−4 mm3/N∙m. Although the study demonstrates that a nitrided layer of up to 33 μm enhances wear resistance, the 45 h nitriding duration may be excessively long from an industrial efficiency perspective.
Based on current research [18,19,20,21,22,23], plasma nitriding technology is an effective method for modifying surface properties. However, the efficiency of plasma nitriding is hindered by the sluggish diffusion effective in complex alloy matrices. To overcome the limitation of sluggish diffusion, previous studies have attempted to accelerate the nitriding process by adjusting process parameters, such as temperature and time, but these attempts have yet to resolve the diffusion bottleneck fully. Therefore, this study aims to explore a new method to improve nitriding efficiency.
Scholars have confirmed that rare earth elements can accelerate the nitriding process significantly. Research by Peng et al. [24] demonstrated that rare earth elements acted as catalysts in the plasma nitriding of 38CrMoAl steel, increasing the thickness of the modified layer and significantly enhancing the wear resistance of the co-nitrided layer. Bell et al. [25], in their review, elaborated on the role of rare earth elements in surface engineering, proposing mechanisms of rare earth chemical heat treatment, including promoting the generation of active atoms (such as carbon and nitrogen), accelerating the mass transfer process of active atoms, and the microalloying effect. Additionally, rare earth elements in the thermal spraying process of nickel-based coatings not only refine the microstructure of the coating and enhance the bonding strength between the coating and the substrate but also improve wear resistance and load-bearing capacity while purifying the interface. You et al. [26] have found, through first-principles calculations, that the element lanthanum (La) interacts with nitrogen atoms near the surface of the specimen, accelerating the movement of nitrogen atoms toward the specimen. Additionally, La atoms create reverse trapping zones within the specimen, facilitating the inward diffusion of nitrogen atoms. Furthermore, other research has experimentally shown that the element La can expedite the diffusion of nitrogen atoms in plasma nitriding [27,28].
Therefore, this study proposes introducing rare earth elements (lanthanium) to accelerate nitrogen diffusion during plasma nitriding of CoCrCuFeNi high-entropy alloys. The innovation of this method lies in the untapped potential of La in the surface modification of high-entropy alloys. The integration of La aims to decrease the activation energy required for nitrogen diffusion. This results in an acceleration of the nitriding kinetic process and the production of thicker nitrided layer with improved surface properties without requiring higher temperatures or a longer treatment time.

2. Materials and Methods

High-entropy alloy ingots with a nominal composition of CoCrCuFeNi (at. %) (chemical composition listed in Table 1) were prepared by arc-melting. The ingots were remelted two times under a vacuum environment to ensure chemical homogeneity. The specimens were cut to 10 mm × 10 mm × 2 mm and then mechanically polished to 2000 grade with silicon carbide paper. The polished specimens were ultrasonically cleaned in alcohol before subsequent processing.
The plasma nitriding treatment was performed using a LDMC-30AFZ plasma nitriding furnace (Wuhan Shoufa Surface Engineering Co., Ltd., Wuhan, China) after the chamber was pumped to below 10 Pa by a rotary pump. The rare earth (RE) element used in the experiments was obtained from La (lanthanium) blocks with a volume of (0.5 × 0.5 × 1) cm3. The samples were uniformly tied to the sample holder with wire, and the La block was placed at the center of the sample holder. Nitriding experiments were performed in ammonia (NH3) at 440 °C with flow rate of 100 mL/min, a gas pressure of 260 Pa during nitriding, and nitriding times of 8, 16, and 24 h (specific processes are listed in Table 2). Following plasma nitriding, the specimens were slowly cooled in a vacuum furnace. For comparative studies, plasma nitriding without La was also performed.
Microhardness (HV-1000IS, Shanghai Jvjing, Shanghai, China) was determined with a microhardness tester under an indentation load of 10 g for 15 s. According to GB/T 11354-2005 (determination of nitrided case depth and metallographic microstructure examination for steel iron parts) from the surface of the specimen to the Vickers hardness value of 50HV higher than the base of the vertical distance for the depth of the nitrided layer (termed the effective hardening layer). At least three microhardness indentations at each position were tested, and average values were used to ensure the accuracy of the microhardness profiles. To identify the phases of the compound layer, X-ray diffraction (XRD, Bruker, Billerica, MA, USA) with Cu-Kα radiation was carried out in a range of glancing degrees 20–100° at a voltage of 45 kV.
In order to evaluate the wear resistance of the nitrided layer, the HSR-2M friction and wear test machine was employed. In the wear test, the specimens were rubbed against a WC ball with a diameter of 5 mm under a load of 5 N at a speed of 200 r/min for 60 min and a rotation radius of 4 mm. The wear rate, η, was calculated according to the following equation:
η = Δ m L N
where Δ m is the mass difference before and after wear. L (m) is the total sliding distance. N is the normal contact load.
The potentiodynamic polarization tests were carried out to estimate the corrosion resistance of the specimens in a 3.5 wt.% NaCl solution, the reference electrode was Ag/AgCl, and a platinum column was used as the auxiliary electrode.

3. Results

3.1. Microstructure and Microhardness of Modified Layer

The cross-sectional microstructures of the PN440) and PN440RE specimens are depicted in Figure 1. Figure 1a–c show the microstructures of PN440-8, PN440-16, and PN440-24 with thicknesses of 10.2, 15.1, and 19.9 μm, respectively. Figure 1d–f show the microstructures of PN440RE-8, PN440RE-16, and PN440RE-24 with the thicknesses of 12.8, 17.9, and 21.6 μm, respectively. Compared to PN440, the nitrided layer thickened by 25.49%, 18.54%, and 8.54% for PN440RE, suggesting that the presence of La is likely to form a thicker nitrided layer within the same nitriding time.
Figure 2a,b show the hardness distribution from the surface to the core of CoCrCuFeNi HEA after nitriding at 440 °C. In Figure 2a, the hardness profiles of PN440-8, PN440-16, and PN440-24 are shown, with average surface hardness values of 1243.6, 1290, and 1307 HV0.01, respectively. The average hardnesses of the nitrided layer are 860, 1233, and 1096.8 HV0.01. In Figure 2b, the hardness profiles of PN440RE-8, PN440RE-16, and PN440RE-24 are presented with average surface hardness values of 1393, 1379, and 1466 HV0.01, respectively. The average hardness of the nitrided layer is 1132.3, 1170.6, and 1001 HV0.01. According to Figure 2, the hardness method (GB/T 11354-2005) can determine the effective hardening layer thickness of high-entropy alloys after nitriding. The effective hardening layer thicknesses of PN440 increase with time to 11.7 μm, 17.3 μm, and 21.7 μm, respectively, while for PN440RE, they increase to 14.7 μm, 21.6 μm, and 24.3 μm, respectively.
Compared with the rapid decrease in hardness of PN440, La can adjust the organization of the nitrided layer of PN440RE after entering the specimen to make the hardness of PN440RE decrease more slowly. This suggests that the nitrided layer of PN440RE has a more evenly distributed nitrogen element [29,30].

3.2. Growth Kinetic of the Nitrided Layer

Figure 3a illustrates the relationship between treatment time and effective hardening layer thickness for PN440 and PN440RE treatments. The growth of the effective hardening layer in both treatments follows a parabolic law, increasing with treatment time. Notably, the effective hardening layer thickness is significantly more significant in the PN440RE treatment than in the PN440 treatment under equal treatment time. Based on hardness measurements (Section 3.1), the effective hardening layer of PN440RE-treated samples exhibited an increase of 25.64%, 24.85%, and 11.98% compared to PN440 at the corresponding treatment time. It is implied that adding La can accelerate the growth of the effective hardening layer.
In order to compare more clearly the accelerating effect of the element La on the diffusion of N atoms, the diffusion coefficient is calculated by Equation (2) [31,32]:
d2 = kt
where d is the effective hardening layer thickness, t is the treating time (s), and k is the growth rate constant. The linear relationship between the square of the effective hardening layer thickness and time can be observed, as depicted in Figure 3. Therefore, the growth rate constant k can be determined by the slope of each line for the different processes. Table 3 shows the growth rate constant k of HEA after treatment with PN440 and PN440RE. It is evident that the growth rate constant of PN440RE treatment surpasses that of PN440 treatment at the same temperature, implying that the addition of La into PN440RE can accelerate N atom diffusion and achieve higher diffusion efficiency (the diffusion efficiency of N atoms can attain the level observed in AISI 1045 steel during the plasma oxynitriding process at 783 k).
The mechanism by which La accelerate the diffusion of nitrogen atoms can be explained as follows. On the one hand, the La, once added to the treatment medium (NH3), can act as a catalyst for a variety of chemical reactions in an nitriding atmosphere, by changing the reaction paths [31]. On the other hand, because La forms many anti-trapping regions [26] after entering the crystal, these anti-trapping regions repel nitrogen atoms and reduce the diffusion activation energy.
After incorporating La, plasma nitriding can be carried out at lower temperatures, thus reducing the nitriding time to achieve a similar effective hardening layer thickness. After 16 h of treatment with La, the increase in effective hardening layer thickness becomes limited (shown in Figure 1).
This phenomenon can be attributed to the limited activity of La atoms at lower temperatures [33]. Therefore, the introduction of La atoms into the lattice interior accelerates the diffusion of nitrogen atoms, which becomes the main factor for the increase in the thickness of the nitrided layer. The slower growth rate of the nitrided layer after being treated for 16 h may be attributed to the limited number of La atoms added during the PN440RE treatment in this study. These atoms (La atoms) were unable to diffuse to deeper locations.
Table 3. Growth rate constant in PN440 and PN440RE.
Table 3. Growth rate constant in PN440 and PN440RE.
ProcessGrowth Rate Constant k (10−14 m2/s)
PN4400.53
PN440RE0.72
Plasma oxynitriding (AISI 1045)0.821 (783k)[34]
Plasma nitriding (AISI 1045)0.28 (783k)[34]

3.3. Phase Composition of the Modified Layer

Figure 4a,b show the X-ray diffraction patterns of PN440 and PN440RE at different treatment times. The nitrided layer of both PN440 and PN440RE is composed mainly of FCC, expanded FCC (FCCN), Fe4N, and CrN. The relative content of Fe4N in the nitrided layer increases gradually with nitriding time. However, the diffraction intensity of the XRD diffraction peak of CrN (111) remains low and does not change significantly with the treatment time. Some scholars have demonstrated that CrN (expanded austenite will not decompose) is not formed below 450 °C [35,36,37,38]. Therefore, it can be assumed that trace amounts of CrN are formed on the surface of the specimen due to sputter deposition. The FCCN is similar to the γN that occurs after nitriding austenitic stainless steels. The FCC can also be considered γ-Fe. This assumption has been shown to be the main component of the permeation layer after nitriding high- or medium-entropy alloys [39,40].
Table 4 presents the diffraction peak intensities and diffraction angles of FCCN and Fe4N. The FCCN (200) of PN440 shifts towards a smaller angle with time, which is related to the lattice expansion caused by the interstitial solid solution of nitrogen with the increase in time [41]. The diffraction peaks of FCCN (200) shift to a smaller angle after PN440RE treatment, in contrast to PN440. Additionally, the diffraction peaks of FCCN (200) shift to a larger angle with increasing treatment time. Furthermore, the diffraction peak angle of PN440RE (200) is larger than that of the FCCN (200) of PN440, indicating that the interplanar distance of PN440RE is smaller. The cause of this phenomenon may be the acceleration of nitrogen atoms by La atoms, as explained in Section 3.2. In simpler terms, La reduces the diffusion activation energy of nitrogen atoms, which results in the easier migration of nitrogen atoms to the next interstitial position, resulting in fewer nitrogen atoms inside the FCC lattice (due to the lower concentration of nitrogen atoms capable of triggering their diffusion). The X-ray diffraction peak angle of Fe4N after PN440 and PN440RE treatments remained almost unchanged, only the diffraction intensity changed.

3.4. Corrosion Properties of the Nitrided Layer

The polarization curve data of PN440 in Figure 5a were used for fitting, and the fitting results are shown in Table 5. From the data in Table 5, it can be seen that the passivation zone of the corrosion resistance curve of PN440 becomes wider, the corrosion current density and corrosion rate decrease, and the corrosion potential increases with the extension of the treatment time. Among the samples, PN440-24 has the best corrosion resistance. Experimentally, it has been proven that under the PN440 process, the longer the treatment time, the better the corrosion resistance. The change in corrosion resistance can be related to the concentration of interstitial nitrogen atoms in FCCN [36,42]. When an austenitic stainless steel containing interstitial nitrogen is subjected to pitting, interstitial nitrogen atoms are released. The released nitrogen atoms can react with H+ to form NH4+ [43]. Table 4 shows the change in the angle of the FCCN (200) diffraction peak of PN440, and the rule of change is that the angle of the diffraction peak is shifted to a small angle with the increase in the treatment time. It shows that the interplanar distance of FCCN (200) becomes larger, which means that the concentration of interstitial nitrogen atoms inside the FCCN lattice increases with treatment time. Therefore, the corrosion resistance of PN440 becomes stronger as the treatment time increases.
By analyzing the polarization curve data in Figure 5b and the fitting results in Table 6, it can be seen that the corrosion rate and corrosion current density of PN440RE-8 to PN440RE-16 decrease significantly while the passivation zone expands. On the contrary, for PN440RE-16 to PN440RE-24, the corrosion rate and current density increase slightly while the passivation zone shrinks significantly. Table 4 shows that the diffraction peak angle of the FCCN of PN440RE-24 is smaller than that of PN440RE-16, indicating that the FCCN of PN440RE-24 has a smaller interplanar distance. This also means that there are fewer interstitial nitrogen atoms in the FCCN after treatment of PN440RE-24, resulting in a decrease in the corrosion resistance of PN440RE-24.

3.5. Friction Performance Analysis

Figure 6a displays the surface friction coefficient curves of the CoCrCuFeNi high-entropy alloy after nitriding. Figure 6b shows the friction coefficient curves of the CoCrCuFeNi high-entropy alloy after nitriding. By combining this with Figure 7a, it is possible to obtain the stable friction coefficients at 8, 16, and 24 h of nitriding, which are 0.71, 0.56, and 0.62, respectively. The stabilized friction coefficients after nitriding for 8, 16, and 24 h are 0.79, 0.59, and 0.48, respectively, as shown in Figure 7b.
The friction coefficient curves of the PN440-treated specimens exhibited more fluctuations than those of PN440RE, as shown in Figure 6a. Additionally, a stick-slip behavior characterized by a sudden drop in the curve was observed for the PN440-treated specimens. This phenomenon can be explained by the “oxidation-scrape-reoxidation” theory [44]. According to this theory, wear debris alternately undergo oxidation, compaction and delamination processes on the friction surface [45] during the wear process, leading to oscillation of the friction coefficient.
The wear rates of the PN440 treatment after 8, 16, and 24 h are 1.34 × 10−7 g/N∙m, 3.45 × 10−7 g/N∙m, and 7.29 × 10−7 g/N∙m, respectively, as shown in Figure 7a. Zhang et al. [46] also observed this phenomenon. The change in wear rate is attributed to oxidative wear at lower sliding velocities, which destroys the oxide film during the wear process.
The EDS analysis in this study indicates that oxidative wear occurred during the wear process. Figure 8a,c shows traces of adhesive wear [47,48] and fragmented oxide particles on the wear track surface, indicating a high degree of oxide film breakage. These changes affect the nitrided wear rate without the addition of La.
Figure 7b displays the wear rates of PN440RE after 8, 16, and 24 h, which were 4.65 × 10−7 g/N∙m, 4.25 × 10−7 g/N∙m, and 1.02 × 10−7 g/N∙m, respectively. The wear rate decreases with nitriding time after adding La. This is likely due to the increase in relative Fe4N content with the longer nitriding time and the fact that the oxide film is not severely damaged during wear, as shown in Figure 8a (with many fewer adhesive wear traces compared to PN440-24).
Figure 8 shows the surface wear morphology of the CoCrCuFeNi high-entropy alloy after two nitriding processes. In Figure 8a, friction abrasion marks are visible after the wear test of PN440-24. The width of the abrasion marks is 0.54 mm, and a higher number of traces of adhesive wear (dark-colored flakes) can be observed. Therefore, the wear mechanism of PN440-24 is more severe adhesive wear. Figure 8c shows the wear marks of PN440RE-24 after the wear test. The width of the wear marks is 0.72 mm. Compared to PN440, there are fewer traces of adhesive wear on the wear marks of PN440RE. Therefore, the wear mechanism of PN440RE-24 is slight adhesive wear.
Table 7 shows the chemical compositions of points A, B, C, and D in Figure 8b,d. Each point A, B, C, and D exhibits a higher concentration of elemental oxygen, indicating that the wear mechanism of PN440-24 and PN440RE-24 involves both adhesive and oxidative wear.

4. Conclusions

The study successfully employed the method of the addition of La to the plasma nitriding process to increase the growth rate of the effective hardening layer and overcome the sluggish diffusion effect of high-entropy alloys. This resulted in a better quality nitrided layer in a shorter nitriding time. From the experimental data, it is possible to draw the following conclusions:
  • The thickness of the nitrided layer of PN440RE increased by 25.49%, 18.54%, and 8.54% compared to PN440, suggesting that La may contribute to a thicker nitrided layer for the same nitriding time. The hardness curve indicates a more homogeneous distribution of nitrogen atoms in the nitrided layer of PN440RE.
  • The effective hardening layer versus time curve shows that the growth of the effective hardening layer of PN440 and PN440RE follows a parabolic law. This means that the plasma nitriding of CoCrCuFeNi high-entropy alloys is diffusion-controlled, and the effective hardening layer thickness growth is limited after 16 h of nitriding treatment with La. The results indicate that the addition of La can produce a higher quality effective hardening layer in a shorter time.
  • The phase composition of the nitrided layer is affected by the nitriding time. Specifically, as the nitriding time increases, the diffraction peak angle of FCCN changes differently in PN440 and PN440RE. After the PN440 treatment, the diffraction peak angle of FCCN shifts toward a smaller angle, while the opposite is true for PN440RE.
  • Corrosion resistance is determined by the concentration of interstitial nitrogen atoms in the FCCN lattice of the nitrided layer. The angle of the diffraction peak of FCCN can determine the nitrogen concentration. A smaller diffraction peak angle indicates a larger interplanar distance, higher nitrogen concentration, and better corrosion resistance.
  • Both PN440-24 and PN440RE-24 experience oxidative wear accompanied by adhesive wear. However, due to the higher level of adhesive wear in PN440-24, the wear rate of PN440RE-24 is 86.0% lower than that of PN440-24.

Author Contributions

Conceptualization, Y.W. (Yifan Wang) and Y.Y.; methodology, Y.W. (Yifan Wang) and Y.Y.; formal analysis, Y.Y.; investigation, Y.W. (Yifan Wang), W.S., Y.W. (Yan Wang), and S.X.; resources, Y.Y. and M.Y.; data curation, Y.Y., F.G., C.W., W.C. and Y.Z.; writing—review and editing, Y.W. (Yifan Wang) and Y.Y.; visualization, Y.W. (Yifan Wang); supervision, M.Y.; project administration, Y.Y. and M.Y.; funding acquisition, Y.Y and M.Y. All authors have read and agreed to the published version of the manuscript.

Funding

Supported by the National Natural Science Foundation of China (51401113) and Natural Science Foundation of Heilongjiang Province of China (E2016069), Key research and development guidance projects in Heilongjiang Province (GZ20210034); The Opening Foundation of Heilongjiang Provincial Key Laboratory of Polymeric Composition Materials (CLKFKT2021B7).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare that they have no conflict of interest.

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Coatings 14 00303 g001
Figure 1. Microstructure of modified layer of CoCrCuFeNi HEA under different nitriding conditions: (a) PN440-8 h; (b) PN440-16 h; (c) PN440-24 h; (d) PN440RE-8 h; (e) PN440RE-16 h; (f) PN440RE-24 h.
Figure 1. Microstructure of modified layer of CoCrCuFeNi HEA under different nitriding conditions: (a) PN440-8 h; (b) PN440-16 h; (c) PN440-24 h; (d) PN440RE-8 h; (e) PN440RE-16 h; (f) PN440RE-24 h.
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Figure 2. Microhardness profiles of modified layer cross-section under nitriding conditions for (a) PN440, (b) PN440RE.
Figure 2. Microhardness profiles of modified layer cross-section under nitriding conditions for (a) PN440, (b) PN440RE.
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Figure 3. Layer growth kinetics of PN440 and PN440RE for different times: (a) effective hardening layer thickness vs. treating time, (b) the square of effective hardening layer thickness vs. treating time.
Figure 3. Layer growth kinetics of PN440 and PN440RE for different times: (a) effective hardening layer thickness vs. treating time, (b) the square of effective hardening layer thickness vs. treating time.
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Figure 4. X-ray diffraction patterns of CoCrCuFeNi HEA without and with RE. (a) PN440, (b) PN440RE.
Figure 4. X-ray diffraction patterns of CoCrCuFeNi HEA without and with RE. (a) PN440, (b) PN440RE.
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Figure 5. Corrosion resistance curve of modified layer of HEA after plasma nitriding at different times. (a) PN440 (b) PN440RE.
Figure 5. Corrosion resistance curve of modified layer of HEA after plasma nitriding at different times. (a) PN440 (b) PN440RE.
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Figure 6. Friction coefficient curve of CoCrCuFeNi high-entropy alloy after nitriding at 440 °C. (a) PN440, (b) PN440RE.
Figure 6. Friction coefficient curve of CoCrCuFeNi high-entropy alloy after nitriding at 440 °C. (a) PN440, (b) PN440RE.
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Figure 7. Wear rates and friction coefficient of CoCrCuFeNi high-entropy alloy treated by NH3 at 440 °C. (a) PN440; (b) PN440RE.
Figure 7. Wear rates and friction coefficient of CoCrCuFeNi high-entropy alloy treated by NH3 at 440 °C. (a) PN440; (b) PN440RE.
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Figure 8. Wear morphologies of CoCrCuFeNi high-entropy alloy after wear test. (a) PN440-24; (c) PN440RE-24; Enlarged Image of a localized wear track, (b) PN440-24; (d) PN440RE-24.
Figure 8. Wear morphologies of CoCrCuFeNi high-entropy alloy after wear test. (a) PN440-24; (c) PN440RE-24; Enlarged Image of a localized wear track, (b) PN440-24; (d) PN440RE-24.
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Table 1. Chemical compositions of as-cast CoCrCuFeNi HEA (at.%).
Table 1. Chemical compositions of as-cast CoCrCuFeNi HEA (at.%).
CompositionCoCrCuFeNi
Nominally2020202020
Actual18.8821.7518.8220.9319.62
Table 2. Details of plasma nitriding of CoCrCuFeNi.
Table 2. Details of plasma nitriding of CoCrCuFeNi.
Identified NameTemperature, °CTime, hAtmosphereGas Pressure, Pa
PN440-84408NH3260
PN440-1644016NH3260
PN440-2444024NH3260
PN440RE-84408NH3260
PN440RE-1644016NH3260
PN440RE-2444024NH3260
Table 4. List of the phases identified in X-ray diffraction patterns with their crystallographic data.
Table 4. List of the phases identified in X-ray diffraction patterns with their crystallographic data.
2θ (°)Intensity (a.u.)PhasePlane (hkl)
untreated43.90828.30FCC111
51.02156.20FCC200
75.0876.30FCC220
PN440-840.8663.80FCCN111
46.4646.60FCCN200
41.2667.70Fe4N111
48.0627.00Fe4N200
PN440-1640.9040.50FCCN111
46.3627.50FCCN200
41.2644.60Fe4N111
48.0612.80Fe4N200
PN440-2440.9458.20FCCN111
46.2836.50FCCN200
41.4049.70Fe4N111
48.1236.00Fe4N200
PN440RE-840.8641.70FCCN111
46.0463.10FCCN200
41.3231.40Fe4N111
48.2226.90Fe4N200
PN440RE-1641.0232.40FCCN111
46.18120.10FCCN200
41.3831.70Fe4N111
48.1254.20Fe4N200
PN440RE-2441.0425.60FCCN111
46.7035.80FCCN200
41.5232.20Fe4N111
48.1430.30Fe4N200
Table 5. Fitting data of the polarization curve of the modified layer of PN440.
Table 5. Fitting data of the polarization curve of the modified layer of PN440.
Untreated8 h16 h24 h
Corrosion rate (×10−2 mm/a)6.896.054.923.45
Io (×10−6 A/cm2)5.925.204.242.96
Eo/V−0.88−0.89−0.87−0.79
Passivation zone width/V1.171.271.301.36
Table 6. Fitting data of the polarization curve of the modified layer of PN440RE.
Table 6. Fitting data of the polarization curve of the modified layer of PN440RE.
Untreated8 h16 h24 h
Corrosion rate (×10−2 mm/a)6.892.832.505.21
Io (×10−6 A/cm2)5.922.432.134.48
Eo/V−0.88−0.91−0.87−0.84
Passivation zone width/V1.171.361.391.16
Table 7. Semi-quantitative EDS analysis for the signed regions (wt.%) shown in Figure 8b,d.
Table 7. Semi-quantitative EDS analysis for the signed regions (wt.%) shown in Figure 8b,d.
ElementsABCD
N0000.91
O25.2625.1930.0215.25
Cr2.682.7523.6414.34
Fe67.3567.2664.1836.70
Co1.311.330.9313.53
Ni1.141.130.6110.57
Cu2.252.341.027.97
W000.610.71
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MDPI and ACS Style

Wang, Y.; You, Y.; Yan, M.; Zhang, Y.; Sheng, W.; Wang, Y.; Xu, S.; Gu, F.; Wang, C.; Cheng, W. The Effect of La on the Surface Properties of Plasma Nitrided CoCrCuFeNi High-Entropy Alloys at 440 Degrees Celsius. Coatings 2024, 14, 303. https://doi.org/10.3390/coatings14030303

AMA Style

Wang Y, You Y, Yan M, Zhang Y, Sheng W, Wang Y, Xu S, Gu F, Wang C, Cheng W. The Effect of La on the Surface Properties of Plasma Nitrided CoCrCuFeNi High-Entropy Alloys at 440 Degrees Celsius. Coatings. 2024; 14(3):303. https://doi.org/10.3390/coatings14030303

Chicago/Turabian Style

Wang, Yifan, Yuan You, Mufu Yan, Yanxiang Zhang, Wenping Sheng, Yan Wang, Shimiao Xu, Feng Gu, Chaohui Wang, and Weidong Cheng. 2024. "The Effect of La on the Surface Properties of Plasma Nitrided CoCrCuFeNi High-Entropy Alloys at 440 Degrees Celsius" Coatings 14, no. 3: 303. https://doi.org/10.3390/coatings14030303

APA Style

Wang, Y., You, Y., Yan, M., Zhang, Y., Sheng, W., Wang, Y., Xu, S., Gu, F., Wang, C., & Cheng, W. (2024). The Effect of La on the Surface Properties of Plasma Nitrided CoCrCuFeNi High-Entropy Alloys at 440 Degrees Celsius. Coatings, 14(3), 303. https://doi.org/10.3390/coatings14030303

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