Effect of Nb on Laves Phase Formation and Wear Resistance in Laser-Cladding CrFeNi Medium-Entropy Alloy Coatings

Abstract

In this study, 20 wt.% of Nb was incorporated into a CrFeNi medium-entropy alloy (MEA) powder system to prepare CrFeNi-Nb composite coatings on a Q235B mild steel substrate by laser cladding technology. The effects of Nb addition on the phase composition, microstructure, and wear resistance of CrFeNi coatings were systematically investigated. Microstructural characterization revealed that the CrFeNi coating exhibited a single face-centered cubic (FCC) phase structure, while the CrFeNi-Nb composite coating demonstrated a dual-phase structure comprising FCC phase and Laves phase. The Laves phase significantly enhanced the microhardness and wear resistance of the coating. The average microhardness of the CrFeNi-Nb coating increased by 259.62% compared to the substrate and 190.58% compared to the Nb-free CrFeNi coating. The average coefficient of friction (COF) of the coating decreased from 0.74 to 0.68; the wear rate reduced from 5.77 × 10⁻⁴ mm³ N⁻¹ m⁻¹ to 2.26 × 10⁻⁴ mm³ N⁻¹ m⁻¹; and the weight loss decreased from 10.77 mg to 4.3 mg. The experimental results demonstrated that the addition of Nb promoted the formation of the Laves phase in the CrFeNi MEA, which effectively improved the wear resistance of the coating.

1. Introduction

In recent years, high entropy alloys (HEAs) have become a hot research topic in the field of surface engineering by virtue of the “cocktail effect” and “high entropy effect” [1], which breaks through the design framework of traditional alloys centered on a single major element. However, its tetrameric and more equimolar multi-component composition is prone to phase structure instability, and the high preparation cost limits the large-scale application. In contrast, the optimization of medium entropy alloys (MEAs) into ternary equimolar systems, while retaining the core properties of high strength, excellent plasticity, and corrosion resistance [2], significantly reduces the difficulty of component regulation and process costs and provides a more universal solution for the practical needs in the fields of industrial protection, structural materials, and so on [3].

In recent decades, MEAs have attracted the attention of more and more researchers due to their simpler crystal structure and superior combination of mechanical properties [1,2]. CrFeNi exhibits a single FCC phase structure in the as-cast or solid solution state [4] and has a Cr content of up to 33 at%. This means that CrFeNi MEA has outstanding corrosion resistance and is positioning it as an ideal candidate for marine environments and aerospace [5,6]. Despite the good corrosion resistance of CrFeNi MEA, a single FCC phase usually results in a coating with low microhardness and relatively poor wear resistance [7]. Such poor mechanical properties have largely limited the application and development of CrFeNi MEA. Enhancing the coating properties by adding other elements is a common approach [8,9,10]. Wang et al. [11] successfully achieved simultaneous enhancement of tribological and anti-corrosion performance in CrFeNi MEA through B/Si alloying; Cheng et al. [12] examined the dual-element synergistic effects of Co and Cu double doping on the mechanical and tribological properties of CrFeNi. They found that the addition of Co can refine the grain size of CrFeNi alloys, reduce stress concentration, and improve ductility and tensile strength. However, the simultaneous doping of Co and Cu accelerates the phase transformation and reduces the plastic deformation ability of the alloy; Duan et al. [13] promoted the generation of coating σ-phase by adding Mo with a large atomic radius to CrFeNi MEA to improve the mechanical properties of the coating; Gao et al. [14] improved the mechanical properties of the coatings by adding Ti elements with a larger atomic radius in CoCrNi meso-entropic alloys, which induced crystal distortion and promoted the generation of BCC phases in the coatings.

Nb, as a refractory metal, is widely used in rocket systems due to its high thermal strength and excellent machining strength [15]. The relatively superior mechanical properties make Nb ideal for enhancing the performance of MEA. Interestingly, the elemental density of Nb is very similar to that of Fe, Cr, and Ni, but with a larger atomic radius, which means that the choice of adding Nb elements to enhance the FeCrNi MEA properties is highly advantageous [16].

With the advancements of laser systems and processing methods, this technology has gradually emerged in many fields. As one of the most advanced surface modification techniques, the distinctive benefits of laser cladding are low dilution rate, high density, and good metallurgical bonding between the cladding layer and the substrate [17,18,19]. This makes the process particularly effective and very suitable for the application of MEA surface engineering applications.

Laser cladding technology provides a new solution to overcome the difficult-to-melt elements by virtue of its highly concentrated energy and precise focusing characteristics. Therefore, in this study, the effect of Nb on the phase composition, microstructure, and mechanical properties of FeCrNi MEA is investigated by using the cladding technology as the preparation process, with emphasis on the analysis of the enhancement mechanism of FeCrNi MEA by Nb elements. This study aims to provide unique insights and technical validation for the optimization of the properties of FeCrNi series alloys, to promote the wide application of MEA in more fields, and to promote its application value.

2. Experimental Procedures

2.1. Experimental Material Preparation

Q235B mild steel (size: 90 × 50 × 3 mm³) was used as the experimental substrate. In order to remove the impurities and oxide film on the surface of the substrate, it was sequentially ground with 80–1000 mesh sandpaper, then ultrasonic ethanol, and dried using a drying oven.

Table 1. Chemical composition of experimental materials.

Material	Composition (Mass.%)
	Fe	Cr	Ni	Mn	O	N	C	S	P
Q235B	Bal.	−	0.15	0.25	−	−	0.15	0.11	0.12
CrFeNi	Bal.	30.98	35.27	−	0.0025	0.0029	0.01	0.0029	−

shows the chemical compositions of Q235B mild steel and CrFeNi MEA powder (CrFeNi, Sichuan Quanyue Matel Materials Co. Ltd., Mianyang, China), and the experiments were conducted using both Nb powder of 99.97% purity supplied by the company. Figure 1a,b illustrates the particulate features of CrFeNi and Nb powders.

2.2. Mixing and Coating Preparation of CrFeNi-Nb Powder

The blended powder mixture (20 wt.% Nb powder combined with a CrFeNi matrix) was subjected to planetary ball milling at 350 rpm for 3 h, followed by a 24 h drying process. As shown in Figure 2, the powder preset methodology was employed this experiment, and the mixed powder was uniformly applied to the surface of the substrate using a standard mold with a thickness of (2 ± 0.1) mm. The coating is then prepared by laser processing equipment (XL-F2000W). The parameters of the laser equipment are as follows: Laser power: 1200 W, scanning rate: 500 mm/min, spot diameter: 2.5 mm, defocus rate: +5 mm, overlap rate: 50%, number of melting channels: 17, melting length: 45 mm. Argon shielding at 6 L/min flow rate was maintained throughout the deposition process coating preparation. After the coating was prepared, the samples were divided by wire cutting, and finally the surface of the samples was mechanically ground and polished, respectively, for subsequent microstructure, mechanical properties, and corrosion resistance tests.

3. Characterization

3.1. Microstructure and Elemental Distribution

The phase identification of the coatings was obtained by scanning the coatings using high-resolution X-ray diffraction (SmartLab 9 KW, Rigaku, Tokyo, Japan) at 40 kV and 40 mA with a scanning speed of 2°/min and scanning angles of 20–90°. The micro-morphology and elemental distribution of the coatings were characterized by a field emission scanning electron microscope (SEM, Zeiss, Baden-Württemberg, Germany, ZEISS Sigma) and an energy spectrometer (EDS, Oxford 30Xplore, Oxford, UK).

3.2. Mechanical Properties

Cross-sectional hardness measurements were performed on the MHVD-1000AT microhardness system (Yizong Precision Instruments, Shanghai, China) using a 200 gf diamond indenter with 10 s dwell cycles. Tribological evaluations utilized the SFT-2M tribological analyzer (Lanzhou Zhongkehua, Lanzhou, China) configured with a 4 mm GGr15 spherical counterface. Testing protocols included a 25 N applied load, a 2 mm orbital radius, a 200 rpm rotational frequency, and a 30 min sliding contact duration.

4. Result and Discussion

4.1. Phase Composition

X-ray diffraction analysis (Figure 3) reveals distinct phase evolution patterns. The CrFeNi coating mainly exhibits the FCC phase. The rapid thermal cycling characteristic of laser deposition enhances elemental solubility thresholds, facilitating extensive solid solution formation as documented in reference [20]. Upon comparison, the intensity of the FCC peak of the CrFeNi-Nb coating was significantly reduced with the addition of Nb; the Laves phase appeared in the CrFeNi-Nb coating, which transformed the MEA from an initial single-phase FCC configuration into a duplex system comprising an FCC phase and a Laves phase. Upon fitting the peaks, the Laves phase mainly exhibits Fe2Nb (#PDF00-0908) and Nb2Ni (#PDF21-0587). This is due to the more negative enthalpy ΔHmix (kJ mol⁻¹) between Nb and Fe, Ni in laser fusion cladding (

Table 2. Enthalpy of mixing of elements.

Element	ΔHmix(KJ·mol−1)				r/Å
	Cr	Fe	Ni	Nb
Cr	−	−1	−7	−7	1.249
Fe	−	−	−2	−16	1.241
Ni	−	−	−	−30	1.246
Nb	−	−	−	−	1.429

), which leads to its preferential generation [21]. At the same time, Nb reacts with Fe, Cr, or Ni in solid solution [22] to form the intermetallic compound Laves phase, which exhibits higher structural entropy, reducing the content of the elements generating the FCC phase, and consequently decreasing the intensity of the FCC peak.

4.2. Microscopic Morphology

Figure 4 shows the low-magnification SEM image of the coating cross-section. Figure 4a shows the CrFeNi coating with an overall uniform coating; as in Figure 4b for CrFeNi-Nb, a small number of incompletely melted white Nb pellets are present at the bottom, middle, and top of the coating.

Microstructural evolution across coating cross-sections reveals distinct zonal characteristics (Figure 5). From the bottom to the middle and then to the upper of the CrFeNi coating, the crystals grow from planar to dendritic crystals. The microstructure of the coatings shows significant changes from the bottom to the center to the upper. For the CrFeNi coating from the bottom to the upper, the crystals grow from planar to equiaxed crystals; after the addition of Nb, the coating grows from planar to dendritic crystals. Proximity to the thermally conductive substrate during phase transition creates steep thermal gradients (G) near the interface coupled with reduced solidification velocities (R). Maximum thermal gradients occur near the substrate interface, while enhanced solidification kinetics dominate upper regions. The G/R ratio governs morphological selection in solidified microstructures. (planar crystals, dendritic crystals, or isometric crystals) [23]. Classical solidification principles [24] suggest nucleation grains preferentially extend against the direction of maximum heat dissipation, along the direction perpendicular to the melt pool, which is consistent with the crystal growth direction of CrFeNi coatings. Progressive G reduction coupled with R enhancement drives G/R diminution, inducing planar-to-columnar structural transitions. Such transitional behavior is evident in the Nb-containing coating shown in Figure 5. Figure 6 shows the results of the surface scanning of the coating cross-section. For the CrFeNi coating, the elements are uniformly distributed on the CrFeNi coating; after the addition of Nb, the coating generates many irregular white crystals. Combined with the EDS analysis, the white crystals are Nb-rich compounds, and combined with the XRD analysis in the previous section, it can be inferred that they are Laves phase crystals. Combined with the XRD analysis, it can be inferred that they are Laves phase crystals. Combined with the distribution of white crystals in the CrFeNi-Nb coating in Figure 5, it can be concluded that the Laves phase associated with Nb is uniformly distributed in the coating after the addition of Nb.

Figure 7 presents the EDS elemental mapping results of the white particles in the CrFeNi-Nb coating. The analysis clearly identifies these particles as incompletely melted Nb aggregates, attributed to the elevated Nb concentration in the localized regions. Correlating with previous XRD characterization data, the adjacent white crystalline phases surrounding the Nb aggregates are confirmed as Laves phases. Figure 7a illustrates spherical Nb aggregates retaining their original morphology, while Figure 7b shows partially reacted hemispherical Nb particles interacting with Cr/Fe/Ni matrix elements. This interfacial evolution supports the proposed mechanism of Laves phase formation in Nb-modified coatings, depicted schematically in Figure 8. The metallurgical process involves reactive diffusion at the Nb-CrFeNi interfaces during laser cladding, where dissolved Nb from partially melted aggregates chemically combines with matrix elements to nucleate Laves phases through solid-state precipitation [25].

4.3. Microhardness and Nanoindentation

Figure 9 presents the mechanical characterization of coating systems through microindentation analysis. Figure 9a represents the microhardness profile of the coatings. The Nb-modified system exhibited superior microhardness characteristics in both deposited layers and HAZ compared to the baseline CrFeNi coating, demonstrating niobium’s hardening efficacy. Figure 9b represents the average microhardness of the coatings, which were all enhanced compared to the base material, by 23.76% for the CrFeNi coating and 259.62% for the CrFeNi-Nb coating. Figure 10 shows the indentation imprints of the coatings. Figure 10a CrFeNi as coating, after Nb doping, the indentation imprint of the CrFeNi-Nb coating is obviously much smaller (Figure 10b), which is consistent with the analysis of the results in Figure 9. This dramatic enhancement stems from Nb-induced phase evolution, where the original FCC structure transitions into a dual-phase system incorporating FCC and hard Laves phases.

Figure 11a shows the COF curve of the coating. The COF of the sample fluctuates up and down within a certain range after a sharp increase. Comparative analysis in Figure 11b reveals that Nb alloying effectively diminishes the mean COF values. Figure 11c shows the wear profile of the coatings, and after measurement, the wear profile width of the CrFeNi coating is 1.71 mm and the depth is 132.62 μm; the wear profile width of the CrFeNi-Nb coating is 1.03 mm, and the depth is 74.66 μm. The test results show that the addition of Nb makes the wear profile width of the coatings narrower and the depth shallower. Volumetric wear quantification via Archard’s formulation [26] (Equation (1)) yields critical performance metrics. The comprehensive wear assessment in Figure 11d integrates specific wear rates with mass loss data. The wear rate of the CrFeNi coating is 5.77 × 10⁻⁴ mm³ N⁻¹ m⁻¹ and the wear rate of the CrFeNi-Nb coating is 2.26 × 10⁻⁴ mm³ N⁻¹ m⁻¹, which is a reduction of 60.83% in the amount of wear and a reduction in the wear loss of 60.07%. These synergistic improvements in tribological performance verify Nb-induced wear resistance enhancement through hard phase reinforcement mechanisms.

$$L={V·\left(N·d\right)}^{-1}$$

(1)

The Archard’s model defines volumetric loss (V, mm³) as a function of normal force (N, N) and cumulative sliding distance (d, m).

Figure 12 and Figure 13 present the surface wear profiles of the CrFeNi coating and CrFeNi-Nb coating, respectively. It can be seen that the wear profile of the CrFeNi-Nb coating is narrower and shallower (Figure 12(a1) and Figure 13(b1)), consistent with the results of the previous analysis in Figure 11c, exhibiting good abrasion resistance. As a single-phase FCC structure, the CrFeNi coating has low microhardness and poor resistance to micro-cutting, likely to generate more wear debris during the wear process [16] (Figure 12(a4)). Following the introduction of Nb, the formation of a high-microhardness Laves phase contributes to grain refinement. This process effectively hinders dislocation movement, thereby enhancing the coating’s resistance to micro-cutting and reducing the amount of debris produced during wear. Consequently, there is a decrease in the coefficient of friction (COF) curve for the coating [27], leading to an overall enhancement in its wear resistance capabilities. More microcracks and obvious delamination spalling were found in the CrFeNi coatings (Figure 12(a2,a4)), which showed typical adhesive wear [28]. The CrFeNi-Nb coatings showed less delamination spalling and wear debris and mainly exhibited abrasive wear.

Figure 14 schematically illustrates the wear protection architecture inherent to the Nb-alloyed coating system. The interface between the coating and sphere experiences plastic deformation and fracture under varying tangential stresses during wear, leading to the formation of wear debris. Laser processing induces localized energy accumulation and transient heat transfer dynamics, driving instantaneous melting and subsequent solidification of feedstock material. This results in spatially heterogeneous thermal profiles across the coating, accompanied by steep thermal gradients and residual thermomechanical stresses. Tribological interactions at the coating-counterface interface induce crack nucleation due to localized shear stress intensification and compressive loading during sliding contact. When the yield limit is exceeded, the coating in the vicinity of the crack is spalled to form a crater [29] (Figure 13(b3)).

5. Conclusions

In this investigation, CrFeNi-Nb composite coatings were fabricated on Q235B steel substrates using laser cladding. Microstructural analysis revealed distinct phase transformations between the two coatings: The CrFeNi coating predominantly consists of FCC structures with planar and equiaxed crystal formations [3], while Nb-modified coatings display dendritic growth patterns along with dispersed irregular Laves phase precipitates. Mechanical property evaluations demonstrated that the single-phase FCC structure in CrFeNi coatings exhibits relatively low microhardness. However, Nb incorporation induces the formation of hard Laves phases, substantially enhancing the coating’s microhardness through dispersion strengthening. Tribological testing further indicated that the FCC-dominated CrFeNi coating shows limited resistance to abrasive wear mechanisms. Conversely, the Nb-containing coating demonstrates superior wear resistance due to the dense stacking architecture of Laves phases, which effectively hinders dislocation motion and mitigate microcutting damage during frictional contact. These findings highlight the critical role of Nb-induced phase modifications in optimizing coating performance. The following conclusions were obtained:

(1)

The addition of Nb can promote the generation of high-hardness Laves phase in CrFeNi meso-entropic alloy coatings.

(2)

The microhardness of the CrFeNi-Nb coating was increased by 259.62% over the substrate and 190.58% over the CrFeNi coating.

(3)

The addition of 20% Nb significantly reduced the wear rate by 60.83% and wear loss by 60.07%.