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 Fe
28.2Ni
18.8Mn
32.9Al
14.1Cr
6 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 NH
3 or N
2 + H
2 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 mm
3/N∙m; an improvement compared to the untreated specimen wear rate of 6.38 × 10
−4 mm
3/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) cm
3. 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 (NH
3) 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:
where
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.