In Situ Ceramic Phase Reinforcement via Short-Pulsed Laser Cladding for Enhanced Tribo-Mechanical Behavior of Metal Matrix Composite FeNiCr-B₄C (5 and 7 wt.%) Coatings

Abstract

This study elucidates the dynamic tribo-mechanical response of laser-cladded FeNiCr-B₄C metal matrix composite (MMC) coatings on AISI 1040 steel substrate, unraveling the intricate interplay between microstructural features and phase transformations. A multi-faceted approach, employing high-resolution scanning electron microscopy (SEM) and advanced X-ray diffraction/Raman spectroscopy techniques, provided a comprehensive characterization of the coatings’ behavior under mechanical and scratch testing, shedding light on the mechanisms governing their wear resistance. Specifically, microstructural analysis revealed uniform coatings with a columnar structure and controlled defect density, showcasing an average thickness of 250 ± 20 μm and a transition zone of 80 ± 10 μm. X-ray diffraction and Raman spectroscopy confirmed the presence of α-Fe (Im-3m), γ-FeNiCr (Fm-3m), Fe₂B (I-42m), and B₄C (R-3m) phases, highlighting the successful incorporation of B₄C reinforcement. The addition of 5 and 7 wt.% B₄C significantly increased microhardness, showing enhancements up to 201% compared to the B₄C-free FeNiCr coating and up to 351% relative to the AISI 1040 steel substrate, respectively. Boron carbide addition promoted a synergistic strengthening effect between the in situ formed Fe₂B and the retained B₄C phases. Furthermore, scratch test analysis clarified improved wear resistance, excellent adhesion, and a tailored hardness gradient. These findings demonstrated that optimized short-pulsed laser cladding, combined with moderate B₄C reinforcement, is a promising route for creating robust, high-strength FeNiCr-B₄C MMC coatings suitable for demanding engineering applications.

1. Introduction

The advanced design of metal matrix composite (MMC) coatings is a critical scientific and technological endeavor [1,2,3]. This is primarily due to the growing demands for increased service life and improved reliability of industrial equipment in modern economic conditions. Austenitic stainless steels (ASSs), based on the FeNiCr system, stand as a distinguished and extensively employed category of materials for durable coatings [4,5]. They are widely used in various industries, including automotive engineering, maritime applications, petroleum processing, water resource management, environmental remediation, and transportation infrastructures [6]. ASSs also play a vital role in demanding applications such as in-core components for nuclear light water reactors, as well as turbine discs and gas compressors [7,8,9,10], underscoring the technological significance of these alloys.

ASSs are predominantly composed of iron (Fe), leveraging nickel (Ni), and chromium (Cr) as key alloying additions to tailor structural, phase, and mechanical properties [4,5,6]. Nickel enhances impact toughness, ductility, and high-temperature strength while simultaneously boosting resistance to corrosive and oxidative degradation. While chromium contributes to increased hardenability, acting as the primary agent for improved corrosion resistance, particularly in oxidizing environments. Such a carefully optimized chemical composition results in a unique set of properties. These properties include exceptional formability, ductility, weldability, non-ferromagnetic behavior, and superior impact toughness at cryogenic temperatures, making ASSs suitable for extreme operational conditions [11]. The face-centered cubic (FCC) crystal structure, which is prevalent in ASSs, dictates fundamental physical, thermal, and mechanical responses [4,12]. Influencing thermal expansion, heat capacity, and plastic deformation behavior, the FCC phase thereby impacts the overall performance in demanding engineering applications.

A promising strategy for enhancing the tribo-mechanical parameters of conventional ASS coatings involves strengthening them by other reinforcing materials. This is achieved through the incorporation of various carbide, boride, and intermetallic phases within their microstructure [13,14,15,16,17,18]. Specifically, the introduction of ceramic reinforcements into ASSs promotes the in situ formation of novel reinforcing primary and secondary phases. This leads to demonstrable enhancements in both their structural integrity and mechanical performance [14,15]. Among the various ceramic materials, boron carbide (B₄C) is frequently favored. This is due to its superior hardness (47 GPa) compared to a majority of other carbides and borides [19]. The advantageous properties of B₄C include thermodynamic stability and resistance to wear and corrosion [19,20]. Alongside this, its lower relative cost compared to various carbides based on Ti, Zr, Hf, Nb, and W elements [21] contributes to its viability as a cost-effective reinforcement for MCC coatings.

The FeNiCr alloy system, especially in its equiatomic configuration, has garnered significant research interest [16,18,22,23,24,25,26]. As reported in [16,18], equiatomic FeNiCr coatings (3.2 GPa) exhibit a 50% higher microhardness than AISI 1040 steel (2.2 GPa), thereby highlighting their potential for various industrial applications. The addition of 1 and 3 wt.% B₄C to the FeNiCr matrix [16] failed to induce the anticipated in situ formation of iron boride intermetallic compounds (FeB/Fe₂B) and the associated synergistic strengthening effect from the B₄C/FeB(Fe₂B) composite. The Fe₂B phase is preferable to FeB due to the latter’s greater brittleness and tendency to form continuous, embrittling networks along grain boundaries, particularly at higher B₄C concentrations. This can significantly reduce ductility and toughness. However, B₄C additions must be carefully controlled, as excessive concentrations can be detrimental to material properties. The increased volume fraction of residual B₄C particles in the matrix, while enhancing hardness, can also act as crack nucleation sites, compromising the overall strength of the composite. Such MMC coatings exhibit increased brittleness, limited load-bearing capacity, and a propensity for premature failure under stress. In this study, building upon the authors’ prior investigations, the B₄C concentration was systematically increased to 5 and 7 wt.% to promote the in situ formation of FeB or Fe₂B intermetallic phases while mitigating any deleterious effects on the coating microstructure and tribo-mechanical behavior.

Achieving optimal microstructure and mechanical performance in MMC coatings requires careful consideration. Effective outcomes depend on both the selection of reinforcement elements, as detailed above, and the chosen deposition technology [27]. Among the established coating techniques, electroplating [28], thermal spraying [29], and laser cladding [30,31,32,33,34,35,36,37] are widely recognized and commercially utilized. Electroplating, known for its cost-effectiveness, suffers from inherent limitations in achievable coating thickness and interfacial bond strength [38]. Thermal spraying, while offering increased operational efficiency, often results in coating-substrate interfaces plagued by defects like porosity and microcracking, potentially leading to premature failure [39]. Laser cladding, conversely, presents benefits such as reduced dilution, a minimized heat-affected zone, and strong metallurgical bonding between the coating and substrate [33], improving overall performance. However, addressing process-inherent defects, particularly solidification cracking [40,41], remains crucial. Ultimately, realizing the full potential of laser cladding for advanced MMC coatings depends on a thorough understanding of the process-structure-property relationships.

This research expands upon the current understanding of laser-cladded MMC FeNiCr-based coatings by examining the synergistic effects of B₄C reinforcement. In this regard, a systematic comparative analysis on the impact of B₄C (5 and 7 wt.%) on microstructure, chemical and phase compositions, and tribo-mechanical behavior was performed. This provided a comprehensive assessment of the advantages and limitations associated with each composition. Informed material selection and optimized coating designs are crucial for specific applications; consequently, this knowledge significantly contributes to extended service life and improved performance in industrial machinery.

2. Materials and Methods

2.1. Powder Materials

Mechanical mixtures of FeNiCr-B₄C powders were prepared using custom-synthesized spherical equiatomic FeNiCr powder (PJSC “Ashinsky Metallurgical Plant”, Asha, Russia) as the matrix material and either 5 or 7 wt.% of commercial irregular B₄C powder (LLC IPK “Umex”, Ufa, Russia) as the reinforcement phase. The FeNiCr powder exhibited a particle size range of 50–150 µm, while the B₄C powder was characterized by a finer particle size range of 3–5 µm. The chemical compositions (wt.%) of the FeNiCr and B₄C powders are given in

Table 1. The chemical compositions of the FeNiCr, B₄C powders, and AISI 1040 steel (wt.%).

Material	Fe	B	Ni	Cr	Mn	C	S	P	Si
FeNiCr	Base	-	35.6	29.8	-	0.37	<0.001	0.008	1.62
AISI 1040 steel		-	-	-	0.6–0.9	0.37–0.44	≤0.05	≤0.04	0.15–0.35
B4C	-	78.3	-	-	-	21.7	-	-	-

.

To achieve a homogeneous distribution of the B₄C reinforcement within the FeNiCr matrix, the powder mixtures were subjected to vibratory mixing using a custom-built laboratory setup (UdSU, Izhevsk, Russia). Pre-weighed quantities of the FeNiCr and either 5 or 7 wt.% B₄C powders were combined in a mortar and vibrated continuously for 10 min at a frequency of 50 Hz and an amplitude of 5 mm. These parameters were selected to optimize mixing efficiency while minimizing potential comminution of the powder constituents, thereby yielding a uniformly dispersed composite powder.

2.2. Laser Cladding Process Parameters

Short-pulsed laser cladding was performed using an ytterbium fiber laser system (UdSU, Izhevsk, Russia) to deposit MMC FeNiCr-B₄C coatings (5 and 7 wt.% B₄C) onto an AISI 1040 steel substrate (Table 1, LCC “ANEP-Metal”, Ekaterinburg, Russia).

Table 2. Laser cladding parameters.

Parameter	Value
Material delivery method	Pre-placed powder bed
Shielding gas (Ar), L/min	10
Laser mode	Pulsed
Laser power, W	50
Laser wavelength, µm	1.065
Scanning speed, mm/s	5
Pulse frequency, Hz	20
Pulse duration, ms	3.5
Overlap rate, %	~25
Pulse energy, J	8.3
Pulse energy density, J/cm2	1057
Laser spot area, cm2	0.00785
Spot size, mm	1

provides detailed information on the laser cladding parameters.

The optimized laser cladding parameters, carefully selected for this study, promoted a desirable microstructure in the resulting coatings. The pulsed laser mode with a power of 50 W, wavelength of 1.065 µm, pulse frequency of 20 Hz, and pulse duration of 3.5 ms, combined with a scanning speed of 5 mm/s and an overlap rate of ~25%, ensured a controlled energy input. This precise control minimized excessive heat input and promoted localized melting and solidification, leading to a refined grain structure and reduced residual stresses, ultimately enhancing the coating’s tribo-mechanical properties and overall performance. The calculated pulse energy of 8.3 J and resulting energy density of 1057 J/cm² over a 1 mm spot size provided sufficient energy for adequate powder consolidation and bonding to the substrate without causing significant thermal distortion or unwanted phase transformations. The Ar shielding further minimized oxidation during the process, preserving the desired chemical composition and preventing the formation of undesirable phases. A double-pass cladding strategy was implemented to reduce substrate dilution of the coatings. The resulting coatings exhibited a consistent thickness of 250 ± 20 μm on specimens with overall dimensions of 5 × 5 × 3 mm and a roughness (R_a) of ~1 μm measured using a profilometer model 250 (JSC “Caliber”, Moscow, Russia).

2.3. Scanning Electron Microscopy

The microstructure and chemical composition of the FeNiCr-B₄C samples were investigated through a scanning electron microscope (SEM), Tescan MIRA LMS (Tescan Brno S.R.O., Brno, Czech Republic), coupled with energy-dispersive X-ray spectroscopy (EDS) (Oxford Instruments, Abingdon, England) to examine the interfacial characteristics and element distribution.

2.4. X-Ray Diffraction

The XRD analysis of the FeNiCr + 5 wt.% B₄C and FeNiCr + 7 wt.% B₄C coatings was carried out by a Shimadzu XRD-7000 diffractometer (Shimadzu Corporation, Tokyo, Japan) and coupled with a graphite monochromator using CuK_α radiation. The diffraction spectrum was recorded in the angular range 2Θ = 30–120° with the scanning step ∆Θ = 0.03° and pulse accumulation for 2 s. The X-ray reflections were identified using the X’Pert HighScorePlus 3.0.5 (Malvern Panalytical, Malvern, UK).

2.5. Raman Spectroscopy

Raman spectroscopy was performed using a Confotec^® MR200 confocal microscope (SOL Instruments, Augsburg, Germany) with a 532 nm excitation laser. A laser power of 76 mW was delivered to the sample surface. Spectra were acquired with an integration time of 20 s, averaging 5–10 accumulations per spectral segment to improve the signal-to-noise ratio. Local heating effects were deemed negligible under these conditions. A 40× objective lens (Olympus Corporation, Tokyo, Japan) with a numerical aperture (NA) of 0.75 and a confocal pinhole diameter of 100 μm was employed to achieve high spatial resolution. Scattered light was dispersed using a 1200 lines/mm diffraction grating and detected with an electrically cooled charge-coupled device (CCD).

2.6. Tribo-Mechanical Characterization

Micromechanical measurements and scratch testing (ISO 14577-1:2015 [42]) of laser-cladded FeNiCr-B₄C coatings were performed using a NanoTest-600 system (Micro Materials Ltd., Wrexham, UK) equipped with a Berkovich and a 25 µm radius Rockwell diamond indenters.

(1)

Microindentation tests were performed with a loading/holding/unloading time of 20 s each and a maximum load of 250 mN. The vertical and horizontal measurement steps were 50 µm, with a total number of indenter prints of 200 for each coating. The measurement error was determined using standard deviation with a confidence probability of 0.95.

(2)

The scratch testing procedure involved creating three scratches per specimen under a constant load of 250 mN and a velocity of 2 μm/s, with scratch lengths of 300–340 μm. A pre-test scratch (1 mN load, 10 μm length) was performed before each test. Surface topography scans (1 mN load) were conducted before and after scratching to evaluate the initial surface condition and measure residual scratch depth. Prior to scratch testing, cross-sections of the specimens were polished to minimize surface roughness effects. The resulting scratches were then analyzed via SEM to identify the dominant failure mechanisms.

3. Results and Discussion

3.1. Microstructure and Chemical Composition

Figure 1 shows the schematic illustration of a short-pulsed laser cladding process and the phase formation of the FeNiCr + B₄C powder mixture after laser exposure (according to X-ray diffraction/Raman spectroscopy-based analyses provided below).

As shown in Figure 1b, the laser cladding process involves a complex interplay of physical phenomena within the process zone, including thermal conduction, thermocapillary flow (Marangoni effect), powder and shield gas forces on melt pool, mass transport, diffusion, laser/powder, melt pool/powder, and laser/substrate interactions [33]. According to the X-ray diffraction/Raman spectroscopy-based analyses, this led to the formation of the corresponding α-Fe, γ-FeNiCr, Fe₂B and B₄C phases. The SEM analysis of the custom spherical FeNiCr powder confirmed the fraction of 50–150 μm and the chemical composition close to the equiatomic one, according to the manufacturer’s test certificate (Figure 1c). The purity (99.6%) and fraction (3–5 µm) of the commercial irregular-shaped B₄C powder used in this study also met the manufacturer’s specifications (Figure 1d). A general view of the manufactured FeNiCr + 5 wt.% B₄C and FeNiCr + 7 wt.% B₄C samples is shown in Figure 1e.

The SEM-based microstructural characterization of both synthesized FeNiCr-B₄C (5 and 7 wt.% B₄C) coatings revealed that their cross-sections, etched with HNO₃, possess a homogeneous surface texture with a typical column structure (Figure 2e,j) and rare defects such as elongated (Figure 2h) and keyhole pores (Figure 2c,h), as well as cracks (Figure 2h). A significant decrease in the size of indentation marks is observed in Figure 2b,g. This indicates a noticeable increase in the microhardness values of these areas.

The elemental mapping of the FeNiCr + 5 wt.% B₄C and FeNiCr + 7 wt.% B₄C cross-sections (Figure 2a,f) clearly demonstrates the transition (remelting) zones between the substrate and coating materials. The transition zones are especially visible on the Fe mappings, as indicated by horizontal arrows on the inserts. The average thickness of both coatings is 250 ± 20 μm.

Table 3. The chemical composition of laser-cladded FeNiCr-B₄C (5 and 7 wt.% B₄C) coatings (wt.%).

Sample	Fe	Ni	Cr	B	C
FeNiCr + 5 wt.% B4C	33.45	31.26	29.04	4.65	1.60
FeNiCr + 7 wt.% B4C	31.56	29.52	30.23	6.75	1.94

shows the chemical composition of the selected coating areas (Figure 2d,i).

The data obtained confirm that the coatings retain a near-equiatomic composition, closely mirroring that of the original custom-synthesized FeNiCr powder. Furthermore, the implementation of a short-pulsed laser cladding process facilitates the creation of coatings with uniform thickness and a homogeneous microstructure. This suggests that the laser cladding parameters were optimized to minimize elemental segregation and promote consistent solidification, leading to a high-quality coating with predictable and reliable properties.

3.2. X-Ray Diffraction Analysis

According to the X-ray diffraction (XRD) analysis presented in Figure 3, the phase composition of both FeNiCr-B₄C (5 and 7 wt.% B₄C) coatings was determined to consist of α-Fe (space group Im-3m) [43], γ-FeNiCr (space group Fm-3m) [22,23,24,25,26], Fe₂B (space group I-42m) [44,45], and B₄C (space group R-3m) phases [19,46].

The observed intensity distributions of the XRD spectra were noticeably influenced by the preferred crystallographic orientation, or texture, of the crystallites within each sample. This texturing effect is evident, as the integral intensities of the Bragg peaks, particularly those associated with the Fe₂B and B₄C phases, were more pronounced in the FeNiCr + 7 wt.% B₄C coating compared to the FeNiCr + 5 wt.% B₄C coating. Such variations in peak intensities suggest differences in the volume fractions and/or crystallographic orientations of these phases between the coatings. Furthermore, the diffraction lines corresponding to all observed phases exhibited significant broadening. This broadening is indicative of a highly defective crystal structure, incorporating, for example, internal stresses, and the presence of numerous dislocations and/or small crystallite sizes, which are all known to contribute to peak broadening in XRD patterns. The observed broadening provides direct evidence of the non-equilibrium solidification kinetics inherent in the laser cladding process, and, consequently, a refinement of the grain size and a high density of lattice defects. These structural features played a dominant role in shaping the tribo-mechanical performance of the coatings.

The phase composition of laser-cladded FeNiCr-B₄C (5 and 7 wt.% B₄C) coatings with calculated unit cell parameters is presented in

Table 4. The phase composition and unit cell parameters of laser-cladded FeNiCr-B₄C (5 and 7 wt.% B₄C) coatings.

Sample	Phase	Unit Cell Parameters, Å
		a	c	V
FeNiCr-B4C (5 and 7 wt.% B4C)	α-Fe, Im-3m	2.87	-	23.64
	γ-FeNiCr, Fm-3m	3.591	-	46.31
	Fe2B, I-42m	5.078	4.223	108.89
	B4C, R-3m	5.60	12.12 (120°)	329.16

.

Rietveld refinement quantified the phase compositions of both coatings. The FeNiCr + 5 wt.% B₄C coating consisted of 3.1% α-Fe, 80.3% γ-FeNiCr, 7% B₄C, and 9.6% Fe₂B, while the FeNiCr + 7 wt.% B₄C coating comprised 4.6% α-Fe, 72% γ-FeNiCr, 10.4% B₄C, and 13% Fe₂B. These phase ratios, notably the increased presence of B₄C and Fe₂B, were achieved through the controlled addition of B₄C and optimization of the laser thermal profile. Increasing the B₄C concentration to 5 and 7 wt.% directly impacts the available boron, driving the formation of Fe₂B intermetallic phases, as evidenced by the observed XRD patterns. Concurrently, the laser thermal profile, determined by parameters such as power, scanning speed, and pulse duration, governs the local temperature distribution and cooling rates during the laser cladding process. These thermal conditions dictate the kinetics of phase transformations, affecting the size, morphology, and distribution of both B₄C and Fe₂B phases within the FeNiCr matrix. For example, a higher energy input promotes more extensive melting and mixing, potentially leading to more uniform phase distributions, while faster cooling rates may favor the formation of finer microstructures. By carefully controlling both the B₄C content and the laser parameters, we successfully tailored the phase composition and microstructure, thereby achieving optimized tribo-mechanical properties of the FeNiCr-B₄C coatings.

Summarizing the above findings, the addition of 5 and 7 wt.% B₄C to the FeNiCr matrix resulted in reinforcement by in situ formed Fe₂B and retained B₄C phases, which directly contributed to the observed superior microhardness. In addition to the bulk phase identification by XRD, Raman spectroscopy was employed to investigate the distribution and bonding characteristics of B₄C and the newly formed Fe₂B within the FeNiCr-B₄C coatings.

3.3. Raman Spectroscopy Analysis

The characteristic surface areas named spots 1 (grey areas), 2 (white areas), 3 (dark areas), and microdrops were chosen on the surface of the FeNiCr-B₄C (5 and 7 wt.% B₄C) coatings for Raman spectra detection. Optical images of the above detection spots are shown using FeNiCr + 5 wt.% B₄C coating as an example (Figure 4). The Raman spectra measured on the present FeNiCr + 5 wt.% B₄C and FeNiCr + 7 wt.% B₄C coatings, as well as the AISI 1040 steel substrate and the previously reported FeNiCr-B₄C (0, 1 and 3 wt.% B₄C) coatings [16], are shown in Figure 4a–d. Figure 4 demonstrates the difference in the intensity of the Raman bands between different detection spots. The Raman peaks, averaged over 5–7 spectra, were identified at each detection spot for all the samples. The interpretation of the Raman spectra is presented in

Table 5. The average peak positions of the Raman spectra detected for different surface spots: AISI 1040 steel substrate and FeNiCr-B₄C (0, 1, 3 [16], 5, and 7 [this study] wt.% B₄C) coatings.

AISI 1040 Steel	FeNiCr-B4C Coatings					Interpretation
	0 wt.% B4C	1 wt.% B4C	3 wt.% B4C	5 wt.% B4C	7 wt.% B4C
207	196	194	209	205	208	α-Fe2O3
267	293	279	273	267	271	α-Fe2O3
-	-	-	-	-	320	Stretching C-B-C chains in B4C
380	-	377	383	376	378	γ-Fe2O3
-	-	469	478	472	462	Stretching C-B-C chains in B4C
480	477	-	-	-	-	NiO
-	518	-	-	-	-	Fe3O4
-	-	533	532	523	-	Vibrational mode of B11C icosahedra
-	527	528	543	530	-	Cr2O3
-	-	-	-	-	556	Vibrational mode of B11C icosahedra
578	-	-	-	-	-	γ-Fe2O3
645	648	-	-	-	-	γ-Fe2O3
-	672	665	657	654	667	NiFe2O4/NiCr2O4
-	-	-	-	-	897	Intraicosahedral B-B bonds
-	-	-	-	974	-	Rotating mode of C-B-C chain in B4C
-	-	-	-	-	1098	Breathing vibrations of B11C icosahedra
1280	-	-	-	-	-	α-Fe2O3
-	1330	1328	1330	1330	1332	Amorphous carbon (D peak)
-	-	1560	1554	1572	1562	Amorphous carbon (G peak)

.

As previously reported in [16], the Raman spectra detected on the FeNiCr-B₄C (0, 1, and 3 wt.% B₄C) coatings sufficiently differed from the spectra of the AISI steel substrate, which also corresponded to the present FeNiCr + 5 wt.% B₄C and FeNiCr + 7 wt.% B₄C coatings presented in this study. The most prominent Raman peaks of the AISI steel substrate corresponded to α-Fe₂O₃ phase [47], whereas the FeNiCr-B₄C (0, 1, 3, 5, and 7 wt.% B₄C) coatings were characterized by the lower-intensity iron oxide (α-Fe₂O₃, γ-Fe₂O₃) peaks, and the intense peaks at 527–543 cm⁻¹ and 654–672 cm⁻¹ corresponding to the Cr₂O₃ and NiCr₂O₄ phases [48], respectively.

The appearance of Raman peaks at 320, 462–478 cm⁻¹ corresponding to the stretching of C-B-C chains in B₄C and the 974 cm⁻¹ peak attributed to the rotating mode of C-B-C chain in B₄C [49,50,51] were found only for the FeNiCr-B₄C (1, 3, 5, and 7 wt.% B₄C) coatings. It should be noted that the intensity of peaks corresponding to the characteristic oscillations of the C-B-C chains in B₄C was higher for the coating reinforced with 7 wt.% B₄C, which confirmed a higher concentration of B₄C carbides inside the FeNiCr + 7 wt.% B₄C coating among the other ones. The 462–478 cm⁻¹ peak maxima were shifted to lower frequencies for coatings with 5 and 7 wt.% B₄C, which indicated a higher in-plane strain in the FeNiCr-B₄C coating with higher B₄C content. Additional Raman peaks at 523–533, 556 cm⁻¹, attributed to the vibrational mode of B₁₁C icosahedra [49], appeared in the FeNiCr-B₄C (1, 3, 5 and 7 wt.% B₄C) coatings. The Raman peaks at 897 and 1098 cm⁻¹, related to intraicosahedral B–B bonds and breathing vibrations of B₁₁C icosahedra [49], respectively, were found only for the FeNiCr + 7 wt.% B₄C coating, which was associated with a higher concentration of B₄C in this sample. The high-intensity peaks at 1328–1332 cm⁻¹ corresponding to amorphous carbon (D peaks) were detected for all the FeNiCr-B₄C (0, 1, 3, 5, and 7 wt.% B₄C) coatings, while the 1554–1572 cm⁻¹ peaks (G peaks) attributed to the presence of free carbon in B₄C were found only for the FeNiCr-B₄C (1, 3, 5, and 7 wt.% B₄C) ones, which additionally confirmed the presence of B₄C carbides in these samples.

By means of different surface spots taken for measuring Raman spectra (Figure 4), it was found that peaks corresponding to α-Fe₂O₃ (206, 270 cm⁻¹) were detected only for spot 1. The broader peak at 668 cm⁻¹ for spot 1, corresponding to NiCr₂O₄ phase, indicated a more stressed state in the FeNiCr + 7 wt.% B₄C coating compared to the FeNiCr-B₄C (1, 3 and 5 wt.% B₄C) ones. Spot 2 could be characterized by a lower content of the NiCr₂O₄ phase inside the FeNiCr-B₄C (3, 5, and 7 wt.% B₄C) coatings, while spot 1 and microdrops were associated with more intense peaks of the NiCr₂O₄ phase for all the FeNiCr-B₄C samples. The dark areas (spots 3) of all the FeNiCr-B₄C coatings were characterized by high-intensity peaks corresponding to amorphous carbon, which indicated a higher content of B₄C carbides in them. The Raman peaks related to the rotating mode of the C-B-C chain in B₄C and breathing vibrations of B₁₁C icosahedra were detected only for spots 2 (white areas).

3.4. Mechanical Characterization

As detailed in Figure 5 and

Table 6. Mechanical characteristics of the AISI 1040 steel substrate and FeNiCr-B₄C (0, 1, 3 [16], 5 and 7 [this study] wt.% B₄C) coatings.

Material	Work, nJ			HIT, GPa	E*, GPa	HIT/E*	HIT3/E*2, GPa	Re, %	δA
	Total	Plastic	Elastic
AISI 1040 steel	206.81	188.79	18.02	2.154	184.68	0.0117	0.0003	3.85	0.90
B4C-free FeNiCr	186.14	163.84	22.30	3.228	172.11	0.0188	0.0011	5.90	0.86
FeNiCr + 1 wt.% B4C	167.04	143.15	23.89	3.825	178.34	0.0214	0.0018	6.60	0.83
FeNiCr + 3 wt.% B4C	147.50	121.22	26.28	5.167	190.64	0.0271	0.0038	8.30	0.78
FeNiCr + 5 wt.% B4C	115.95	82.16	33.79	8.091	181.01	0.0447	0.0162	13.70	0.59
FeNiCr + 7 wt.% B4C	106.23	72.79	33.44	9.711	201.17	0.0483	0.0226	14.54	0.54
Comparative analysis (change (↓↑) in %)
7 wt.% B4C vs. 5 wt.% B4C	8 ↓	11 ↓	1 ↓	20 ↑	11 ↑	8 ↑	40 ↑	6 ↑	8 ↓
5 wt.% B4C vs. B4C-free	38 ↓	50 ↓	52 ↑	151 ↑	5 ↑	138 ↑	1373 ↑	132 ↑	31 ↓
7 wt.% B4C vs. B4C-free	43 ↓	56 ↓	50 ↑	201 ↑	17 ↑	157 ↑	1955 ↑	146 ↑	37 ↓
5 wt.% B4C vs. AISI 1040 steel	44 ↓	56 ↓	88 ↑	276 ↑	2↓	282 ↑	5300 ↑	256 ↑	34 ↓
7 wt.% B4C vs. AISI 1040 steel	49 ↓	61 ↓	86 ↑	351 ↑	9 ↑	313 ↑	7433 ↑	278 ↑	40 ↓

, the average instrumented microhardness (H_IT) values for the laser-cladded FeNiCr + 5 wt.% B₄C and FeNiCr + 7 wt.% B₄C coatings were approximately 8.1 GPa and 9.7 GPa, respectively. This represents a substantial 20% increase in microhardness with the higher B₄C content. Furthermore, the obtained results clearly demonstrate a significant enhancement in microhardness resulting from the 5 and 7 wt.% B₄C reinforcement, compared to previously reported values for equiatomic FeNiCr coatings without B₄C [16,18] and the AISI 1040 steel substrate. Specifically, the microhardness of the FeNiCr + 5 wt.% B₄C and FeNiCr + 7 wt.% B₄C coatings exhibited a 151% and 201% improvement, respectively, relative to the B₄C-free FeNiCr coating, and a 276% and 351% improvement, respectively, compared to the AISI 1040 steel substrate. This significant increase in hardness can be attributed to a synergistic combination of strengthening mechanisms. These include solid solution strengthening from the alloying elements, grain refinement induced by the rapid solidification inherent in laser cladding, and, most notably, in situ precipitation strengthening from the formation of hard Fe₂B phases and the presence of retained B₄C phases. Increased B₄C content in the 7 wt.% B₄C coating further facilitates the formation and distribution of these strengthening phases, thereby accounting for the observed higher microhardness compared to the 5 wt.% B₄C coating.

The microhardness profiles (Figure 5a) effectively differentiate the coating layers, which exhibit negligible mixing with the AISI 1040 steel substrate, from the compositional transition zones where significant intermixing is evident. The average coating thickness, validated by these profiles, is ~250 μm, with the compositional transition zones measuring ~80 μm. Moreover, subtle deviations in microhardness values are observed within both the coating and transition zones. These localized variations likely stem from the heterogeneous redistribution of B₄C and Fe₂B phases during the rapid solidification associated with the laser cladding process. This uneven distribution results in regions enriched in these hard phases, leading to localized increases in microhardness, while regions depleted of these phases exhibit correspondingly lower values. Therefore, the microhardness profiles provide valuable insights into the microstructural homogeneity and phase distribution within the composite coatings.

The effective elastic modulus (E*), as depicted in Figure 5b, reveals that the FeNiCr + 7 wt.% B₄C coating exhibits a superior stiffness, with E* values exceeding those of the FeNiCr + 5 wt.% B₄C coating, AISI 1040 steel substrate, and B₄C-free FeNiCr coating by 11%, 9%, and 17%, respectively. Conversely, the E* value of the FeNiCr + 5 wt.% B₄C coating is closely aligned (within ~2%) with that of the AISI 1040 steel substrate and demonstrates a 5% increase compared to the B₄C-free FeNiCr coating. The elevated E* value observed for the FeNiCr + 7 wt.% B₄C coating signifies its enhanced resistance to elastic deformation [52,53], suggesting improved dimensional stability and load-bearing capacity relative to the other materials under consideration.

The loading/unloading curves, illustrated in Figure 5c, effectively demonstrate the impact of B₄C particle reinforcement on the mechanical response of the equiatomic FeNiCr coating. These curves, representing the average behavior across multiple indentations, reveal a marked reduction in the maximum indentation depth (h_max) for both the FeNiCr + 5 wt.% B₄C (1206.81 nm) and FeNiCr + 7 wt.% B₄C (1114.67 nm) coatings compared to the B₄C-free FeNiCr coating (1877.43 nm) and the AISI 1040 steel substrate (2146.02 nm). Furthermore, the residual depth (h_r), which represents the point of intersection between the tangent and the unloading curve, provides insight into the elastic recovery behavior of the materials. A lower h_r value signifies a greater degree of elastic recovery [54,55,56], and the FeNiCr + 7 wt.% B₄C coating exhibits the lowest h_r value (952.3 nm), indicating the most pronounced elastic recovery among the materials tested (Figure 5c). This suggests that the FeNiCr + 7 wt.% B₄C coating possesses a superior ability to resist permanent deformation under load.

The elastic, plastic, and total work values were determined from the areas under the loading and unloading segments of the nanoindentation curves (Figure 5d). An analysis reveals that the FeNiCr + 5 wt.% B₄C and FeNiCr + 7 wt.% B₄C coatings exhibit contrasting behavior compared to both the AISI 1040 steel substrate and the B₄C-free FeNiCr coating: a decrease in both plastic and total work, accompanied by an increase in elastic work (Figure 5d, Table 6). Furthermore, the 7 wt.% B₄C coating demonstrates superior mechanical properties compared to the 5 wt.% B₄C coating. This is evidenced by the 8% higher H_IT/E* ratio, a 40% higher H_IT³/E*² ratio, and a 6% higher R_e value, collectively signifying a greater resistance to plastic deformation and improved wear resistance. Compared to the B₄C-free FeNiCr coating, the FeNiCr + 5 wt.% B₄C coating displays a significant increase in the H_IT/E, H_IT³/E*², and R_e parameters, rising by 138%, 1373%, and 132%, respectively. The FeNiCr + 7 wt.% B₄C coating also exhibits improvements, with increases of 157%, 1955%, and 146% in the same parameters. These substantial increases in H_IT/E and H_IT³/E*² underscore the enhanced stiffness and improved resistance to plastic deformation and wear, as per the established literature [52,53]. The observed elevation in the R_e parameter also confirms the enhanced capacity of the B₄C-reinforced coatings to elastically withstand mechanical stress up to the point of plastic deformation [55,56,57].

Conversely, the plasticity index (δ_A) exhibits a reduction in the B₄C-reinforced coatings. Specifically, the δ_A values for the FeNiCr + 5 wt.% B₄C and FeNiCr + 7 wt.% B₄C coatings are reduced by 31% and 37%, respectively, compared to the B₄C-free FeNiCr coating, and by 34% and 40%, respectively, compared to the AISI 1040 steel substrate material. This reduction in δ_A confirms the increased hardening of the coatings resulting from the incorporation of B₄C particles, with a more pronounced effect observed in the FeNiCr + 7 wt.% B₄C coating. The lower plasticity index signifies a reduced tendency for plastic deformation and an increased propensity for elastic behavior under applied stress.

3.5. Scratch Test Assessment

Scratch test analysis of the FeNiCr-B₄C (5 and 7 wt.% B₄C) cross-sections is shown in Figure 6. This provided a comprehensive assessment by collecting quantitative data (average scratch depth and width) and conducting qualitative analysis (visual assessment of deformation morphology) for both the coatings and the AISI 1040 steel substrate. Topographical profiles on the FeNiCr-B₄C cross-sections (Figure 6a,g) revealed a gradual increase in penetration depth as the indenter moved from the coating to the substrate. The observation suggests both good adhesion and hardness gradient, attributed to B₄C reinforcement of the FeNiCr matrix. This behavior shows that the coatings effectively support the load, resisting penetration better than the AISI 1040 steel substrate.

The penetration depth/distance graphs (Figure 6a,g) show a distinct transition at the coating/substrate interface, marked by an abrupt change in depth as the indenter passes from the coating to the substrate. While the relatively uniform penetration depth profile across the coating suggests a constant resistance to deformation, implying a controlled level of ductility, the substrate exhibits significant fluctuations in the scratch curve. The SEM analysis (Figure 6b,h) confirmed this differential behavior. A quantitative assessment of the scratch profiles clarified an average scratch thickness within the substrate region of 22.29 nm for the FeNiCr + 5 wt.% B₄C sample and 23.43 nm for the FeNiCr + 7 wt.% B₄C sample. In contrast, the average scratch thickness within the coating layer was markedly reduced, measuring 13.28 nm and 12.35 nm for the FeNiCr + 5 wt.% B₄C and FeNiCr + 7 wt.% B₄C samples, respectively. Both the FeNiCr-B₄C coatings show uniform, narrow scratches without delamination or defects, indicating a predominantly elastic deformation regime with limited plastic flow under the applied load. This suggests that the coatings possess a higher yield strength and are more prone to elastic recovery, resulting in a shallower, more uniform groove. In contrast, the substrate shows non-uniform scratch widening accompanied by delamination at the edges. The latter observations indicate greater ductility; a lower yield strength results in greater plastic deformation and possibly lower fracture toughness, resulting in the observed delamination. The inconsistent penetration depth and pronounced scratch widening in the substrate indicate a non-uniform material response and a greater propensity to plastic deformation under the applied stress compared to the more elastically resistant and homogeneous coating.

EDS mapping of the cross-sections (Figure 6c–f,i,j) further elucidated the spatial variation in elemental composition within the FeNiCr-B₄C (5 and 7 wt.% B₄C) samples. This compositional information, combined with the visual assessment of scratch depth (Figure 6a,g) and thickness (Figure 6b,h) variations, provides a reliable method for precisely defining the coating-substrate interface. The correlation of EDS data with mechanical scratch behavior allows for a comprehensive understanding of the coating’s structure and adherence to the substrate.

In summary, the comprehensive scratch test analysis, incorporating topographical profiling, quantitative measurements, and morphological assessment, confirmed the enhanced wear resistance and superior mechanical properties of the FeNiCr-B₄C coatings. These coatings exhibit excellent adhesion, a beneficial hardness gradient, high cohesive strength, a degree of plasticity, and homogenous B₄C dispersion, all contributing to their overall durability and making them suitable for demanding applications.

4. Conclusions

This research aimed to evaluate the potential of laser-cladded FeNiCr-based MMC coatings reinforced with moderate concentrations of B₄C (5 and 7 wt.%). The coatings were systematically studied by a comparative analysis of their microstructure, phase composition, and tribo-mechanical properties. The following key results were obtained:

(1)

The FeNiCr-B₄C (5 and 7 wt.% B₄C) cross-sections are characterized by an average thickness of 250 ± 20 μm and a transition zone of 80 ± 10 μm. Both coatings possess a homogeneous surface texture with a typical column structure and rare defects, such as elongated and keyhole pores, as well as cracks.

(2)

The XRD analysis confirmed that both FeNiCr-B₄C (5 and 7 wt.% B₄C) coatings consist of the following phases: α-Fe, space group Im-3m; γ-FeNiCr, space group Fm-3m; Fe₂B, space group I-42m; and B₄C, space group R-3m.

(3)

Raman spectroscopy also revealed the presence of B₄C phases inside the FeNiCr + 5 wt.% B₄C and FeNiCr + 7 wt.% B₄C coatings through detected peaks corresponding to amorphous carbon, stretching C-B-C chains in B₄C and vibrational modes of B₁₁C icosahedra. The additional peaks related to intraicosahedral B-B bonds, rotating mode of the C-B-C chain in B₄C, and breathing vibrations of B₁₁C icosahedra were identified for both presented coatings. This was an extra indicator of the B₄C presence.

(4)

In situ reinforcement with 5 and 7 wt.% B₄C significantly increased microhardness, showing enhancements up to 201% compared to the B₄C-free FeNiCr coating, and up to 351% relative to the AISI 1040 steel substrate, respectively. This was promoted by the synergistic strengthening effect between the in situ formed Fe₂B and the retained B₄C phases.

(5)

The comprehensive scratch test analysis demonstrated the enhanced wear resistance and robust mechanical properties of the FeNiCr-B₄C coatings. These coatings exhibit excellent adhesion, a beneficial hardness gradient, high cohesive strength, a degree of plasticity, and homogenous B₄C dispersion, all contributing to their overall durability.

Thus, optimized short-pulsed laser cladding, coupled with moderate B₄C (5 and 7 wt.%) reinforcement, presents a viable and promising approach to fabricate high-strength FeNiCr-B₄C MMC coatings for challenging engineering applications. Future studies should focus on analyzing the high-temperature behavior and application-specific performance (cavitation, abrasion, and corrosion resistance) of the FeNiCr-B₄C coatings. This comprehensive investigation represents a critical avenue for advancing our understanding of these coatings’ potential in demanding environments and guiding future materials development efforts.