Metal Matrix Composite Coatings Deposited by Laser Cladding: On the Effectiveness of WC Reinforcement for Wear Resistance and Its Synergy with the Matrix Material (Ni Versus Co Alloys)

Abstract

This work investigates the effect of the addition of tungsten carbide (WC) particles as reinforcements to Ni (Inconel 625) versus Co (Stellite 6) alloys during deposition by laser cladding to form wear-resistant metal matrix composite (MMC) coatings. While the related literature often associates the presence of WC with the enhanced wear performance of MMC coatings, this work shows that such an effect is not universal as it may critically depend on the metallic matrix employed. Thus, to demonstrate whether the reinforcement and matrix act synergically in such a scenario or not, MMC coatings formed by Inconel 625 and Stellite 6, both with WC content ranging from 10% to 40%, were deposited under the same laser cladding setup on AISI 304 stainless steel substrates, being WC-free samples produced together for comparison basis. As expected, the hardness levels increased with more WC presence in both matrices, but the wear resistance was specifically evaluated by means of the metal wheel abrasion test (ASTM B611). The results revealed that the use of WC as a reinforcement indeed affects the matrix materials differently; for Stellite 6, the wear resistance increased with up to 20% of WC (in contrast to the hardness indication), whereas for Inconel 625, the wear resistance progressively decreased with more WC content. It was observed via scanning electron microscopy (SEM) that the WC particles within the Inconel 625 alloy tended to intensive cracking, being in this way more prone to detach from the matrix and hence representing a weakening factor for the effectiveness of the coatings produced. Thus, it is concluded that the addition of WC particles, as potential reinforcements for MMC coatings, is not always effective (synergic with the matrix) in providing wear resistance, hence, opposing the prevailing consensus. This outcome and its reasons will certainly help with insights into proper design of MMC coatings, starting with the importance of matrix material selection.

1. Introduction

Laser cladding is widely adopted as a surface modification technique for metallic parts, and it involves the melting and deposition of coating/protecting materials onto a substrate by means of a focused high-energy laser beam. This process allows for a strong metallurgical bonding between coatings and substrates, precise control over dilution, and minimal heat-affected zones when compared to other techniques [1]. Basically, a typical laser cladding process involves the simultaneous feeding of metallic and ceramic materials into a molten pool, where rapid solidification governs the resulting microstructure and mechanical properties. Parameters, such as laser power, scanning (travel) speed, and powder feed rate, significantly influence the characteristics of the coatings, including dilution levels, hardness, and wear resistance [2].

A wide variety of materials can be used in laser cladding to improve the wear and corrosion resistance of components. Metallic powders and wires can be used as feedstock. Powder-based laser cladding is ideal for complex geometries, ensuring good precision. In addition, the use of powdered feedstock facilitates the mixing of different materials to improve certain properties [3]. Wire-based laser cladding is advantageous in terms of efficiency, where almost 100% of the feedstock is processed (turned into a coating); this option is widely used for large industrial production [4]. Fe, Ni, Co, Al, and Ti-based alloys are some of the main matrix materials generally processed by laser cladding to promote improvements on the surface of metallic components [5]. Given these advantages, laser cladding has been extensively used to enhance the wear resistance of parts by incorporating hard ceramic reinforcements into metallic matrices, forming composite coatings with notable mechanical performances [6,7]. Such wear-resistant metal matrix composite (MMC) coatings are widely utilized to enhance the durability of components exposed to abrasive and erosive conditions [8]. Among the strategies for surface performance improvement via laser cladding, the incorporation of tungsten carbide (WC) particles into metallic matrices, such as Ni-based and Co-based alloys, is an already established practice to increase the hardness and wear resistance of parts, as summarized in Table 1.

Concerning the use of matrices based on Ni alloys, several studies have reported the effects of WC addition to Inconel 625 for MMC coatings. Zhang et al. [6], for example, demonstrated that the addition of WC as a reinforcement significantly increases hardness and improves wear resistance in coatings based on such an alloy. Also, with Inconel 625 being used as a matrix material, Li et al. [9] analyzed the influence of WC shape and distribution in MMC coatings and showed that the addition of spherical WC particles lead to better wear performance. Janicki and Musztyfaga-Staszuk [10] investigated the laser cladding of Inconel 625 combined with WC particles and found out that the addition of such carbides enhances erosion resistance. However, Wang et al. [11] highlight that excessive WC contents may lead to material embrittlement, hence reducing the coating wear resistance.

Regarding the use of matrices based on Co alloys, Stellite 6 is also widely explored for MMC coatings, such as by Bartkowski and Bartkowska [7], who observed a 27% improvement in wear resistance with WC addition in field tests. Conversely, Xu et al. [12], after studying multi-layer laser cladding of Stellite 6 with WC, reported that the presence of such carbides modified the microstructure of the coatings but did not significantly affect their wear resistance. On the other hand, Wu et al. [13] demonstrated that the wear resistance of coatings based on Stellite 6 improved when reinforced with angular WC particles. Additionally, Wang et al. [14] analyzed high-temperature wear resistance of coatings formed by Stellite 6 and WC and found out that the reinforcement carbides altered the dominant wear mechanism, improving performance under specific conditions.

Table 1. Summary of studies on WC-reinforced Inconel 625, Stellite 6, and other matrix materials for coatings deposited by laser cladding as well as by other processes.

Process	Substrate	Coating	WC Shape	WC Size (μm)	WC Amount	Dilution	Hardness	Wear Test	Wear Performance	Ref.
HVOF 1	AISI 316L	Inconel 625	-	-	22–42%	-	250–610 HV	Pin-on-disc	Increased 92%	[15]
Laser cladding	TWZ-2 (steel)	Inconel 625	Spherical	4–105	15–30%	-	258–466 HV1	Reciprocating	Increased 65%	[6]
Laser cladding	Q235 (steel)	Inconel 625	Spherical and angular	15–45	15%	-	311–429 HV0.2	Pin-on-disc	Increased 86%	[9]
Laser cladding	AISI 304	Inconel 625	Spherical and angular	100–200	30%	28–42%	230–500 HV0.2	Erosion	Increased 43%	[10]
Laser cladding	Q235 (steel)	Inconel 625	Angular	~0.1	2–10%	-	230–500 HV0.2	Block-on-disc	Increased 45%	[11]
Laser cladding	AISI 1045	Inconel 625	Spherical	-	35–65%	13–42%	843 HV	Reciprocating	Increased 84%	[16]
Laser cladding	SM400B (steel)	Stellite 6	Spherical	>45	0–47%	-	590–800 HV0.05	-	No variation	[12]
Laser cladding	60Si2Mn (steel)	Stellite 6	Angular	65–245	30%	-	380–732 HV0.1	Pin-on-disc	Increased 60%	[13]
Laser cladding	AISI H13	Stellite 6	Spherical	45–106	30%	-	600–660 HV0.01	-	Increased 39%	[17]
Laser Cladding	S355 (steel)	Stellite 6	Angular	>100	60%	-	550–580 HV0.05	Block-on-ring	Increased 90%	[18]
PTA 2	AISI 1020	Stellite 6	Angular	35	35%	12–18%	430–650 HV0.5	Pin-on-disc	Increased	[19]
PTA 2	Q235 (steel)	NiCrMo	Angular	-	5–20%	-	500–580 HV	Ball-on-disc	Increased 42%	[20]
PTA 2	Cr5 steel	18Ni300	Spherical	45–105	5%	10–12%	350–400 HV0.3	-	Increased 56%	[21]
PTA 2	AISI 1044	AISI 420	Spherical	45–125	15–45%	-	590–620 HV0.5	Ball-on-disc	Increased	[22]
Laser cladding	AISI 304	AlCoCrFeNi	Spherical	35–135	10–40%	-	351–426 HV0.3	Ball-on-disc	Increased	[23]
Laser Cladding	AISI 304	Ni alloy	Spherical	35–145	10–60%	-	301–502 HV	Ball-on-disc	Increased 88%	[8]
HVOF 1	AISI 1018	Woka 3603	Angular	-	10–30%	-	202–1421 HV0.3	Erosion	Decreased 16%	[24]
Laser Cladding	AISI 304	FeCrNiMnAl	Angular	-	5–20%	-	-	Erosion	Decreased 28%	[25]

¹ HVOF—High Velocity Oxygen Fuel; ² PTA—Plasma Transferred Arc.

Table 1. Summary of studies on WC-reinforced Inconel 625, Stellite 6, and other matrix materials for coatings deposited by laser cladding as well as by other processes.

Process	Substrate	Coating	WC Shape	WC Size (μm)	WC Amount	Dilution	Hardness	Wear Test	Wear Performance	Ref.
HVOF 1	AISI 316L	Inconel 625	-	-	22–42%	-	250–610 HV	Pin-on-disc	Increased 92%	[15]
Laser cladding	TWZ-2 (steel)	Inconel 625	Spherical	4–105	15–30%	-	258–466 HV1	Reciprocating	Increased 65%	[6]
Laser cladding	Q235 (steel)	Inconel 625	Spherical and angular	15–45	15%	-	311–429 HV0.2	Pin-on-disc	Increased 86%	[9]
Laser cladding	AISI 304	Inconel 625	Spherical and angular	100–200	30%	28–42%	230–500 HV0.2	Erosion	Increased 43%	[10]
Laser cladding	Q235 (steel)	Inconel 625	Angular	~0.1	2–10%	-	230–500 HV0.2	Block-on-disc	Increased 45%	[11]
Laser cladding	AISI 1045	Inconel 625	Spherical	-	35–65%	13–42%	843 HV	Reciprocating	Increased 84%	[16]
Laser cladding	SM400B (steel)	Stellite 6	Spherical	>45	0–47%	-	590–800 HV0.05	-	No variation	[12]
Laser cladding	60Si2Mn (steel)	Stellite 6	Angular	65–245	30%	-	380–732 HV0.1	Pin-on-disc	Increased 60%	[13]
Laser cladding	AISI H13	Stellite 6	Spherical	45–106	30%	-	600–660 HV0.01	-	Increased 39%	[17]
Laser Cladding	S355 (steel)	Stellite 6	Angular	>100	60%	-	550–580 HV0.05	Block-on-ring	Increased 90%	[18]
PTA 2	AISI 1020	Stellite 6	Angular	35	35%	12–18%	430–650 HV0.5	Pin-on-disc	Increased	[19]
PTA 2	Q235 (steel)	NiCrMo	Angular	-	5–20%	-	500–580 HV	Ball-on-disc	Increased 42%	[20]
PTA 2	Cr5 steel	18Ni300	Spherical	45–105	5%	10–12%	350–400 HV0.3	-	Increased 56%	[21]
PTA 2	AISI 1044	AISI 420	Spherical	45–125	15–45%	-	590–620 HV0.5	Ball-on-disc	Increased	[22]
Laser cladding	AISI 304	AlCoCrFeNi	Spherical	35–135	10–40%	-	351–426 HV0.3	Ball-on-disc	Increased	[23]
Laser Cladding	AISI 304	Ni alloy	Spherical	35–145	10–60%	-	301–502 HV	Ball-on-disc	Increased 88%	[8]
HVOF 1	AISI 1018	Woka 3603	Angular	-	10–30%	-	202–1421 HV0.3	Erosion	Decreased 16%	[24]
Laser Cladding	AISI 304	FeCrNiMnAl	Angular	-	5–20%	-	-	Erosion	Decreased 28%	[25]

¹ HVOF—High Velocity Oxygen Fuel; ² PTA—Plasma Transferred Arc.

As noticed, although the related literature largely assumes that WC reinforcement enhances wear resistance in MMC coatings (Table 1), there are controversies. While many studies indicate an improvement in wear performance, a few others highlight the detrimental effect of excessive WC content, particularly in Ni-based matrices. This inconsistency suggests that the effectiveness of WC reinforcement depends not only on its amount but also on its interaction (synergy or not) with the specific matrix material. Factors, such as the matrix material itself and WC content, particle size, and shape, as well as wear conditions, seem to directly influence the WC effect on MMC coatings.

Thus, this study aims to provide a systematic investigation into the role of WC reinforcement in MMC coatings based on Inconel 625 versus on Stellite 6 deposited by laser cladding. By evaluating the microstructure, hardness, and wear behavior under identical processing conditions, this work seeks to clarify the matrix-dependent effects of WC (verifying whether reinforcement–matrix synergy exists or not) and challenge the prevailing assumption that WC addition universally enhances wear resistance. The related findings will contribute to the optimization of MMC coatings, guiding proper material selection and process parametrization for the performance improvement of metallic parts in applications characterized by intensive wear.

2. Materials and Methods

2.1. Deposition of the Coatings

The coatings were deposited by the laser cladding system shown Figure 1. It consists of a CNC table with five motion axes integrated with a diode laser source (with a maximum nominal power of 6 kW), two powder feeders, and a continuous coaxial deposition head. The laser beam generated is transmitted through an optical fiber to the deposition head, which contains an optical system responsible for focusing the laser beam onto the surface of the substrate. The powder feeders dose the desired amount of powdered feedstock materials (matrix and reinforcement), which are fluidized and transported by carrier gas flows (argon) through anti-static hoses to the deposition head, which then directs the flows with the powders in a continuous coaxial manner (hollow cone) into the molten pool (both powdered matrix and reinforcement materials are directly injected into the molten material down below). The addition of the metal matrix materials and of the WC reinforcement particles was performed in situ by independently adjusting the feed rate in the respective powder feeders (Figure 1b).

A group of AISI 304 substrates with dimensions of 76 (length) × 25.4 (width) × 15.9 (thickness) mm were coated with Inconel 625 and another one with Stellite 6, both with 10%, 20%, 30%, and 40% of WC content. For comparison purposes, coatings without any WC addition were also deposited. Gas-atomized metallic powders (Inconel 625 and Stellite 6) with a nominal particle size range of 50–150 µm were used for the matrices, while a gas-atomized ceramic powder (tungsten carbides—WC) with a nominal particle size range of 45–105 µm was used for the reinforcement. The general morphology of the powders used is shown in Figure 2, while their chemical compositions and that of the substrate are presented in

Table 2. Nominal chemical composition of the gas-atomized Inconel 625 (IN625) powder (wt.%).

Material	Ni (%)	Cr (%)	Mo (%)	Fe (%)	Mn (%)	Si (%)	C (%)
IN625	Bal.	21.1	8.9	0.7	0.4	0.4	0.03

,

Table 3. Nominal chemical composition of the gas-atomized Stellite 6 (ST6) powder (wt.%).

Material	Co (%)	Cr (%)	W (%)	Ni (%)	Si (%)	Fe (%)	C (%)
ST6	Bal.	28	4.5	1.0	1.1	0.6	1.0

,

Table 4. Nominal chemical composition of the gas-atomized WC powder (wt.%).

Material	W (%)	C (%)	Others (%)
WC	Bal.	3.8	0.7

and

Table 5. Chemical composition of the AISI 304 substrate measured by PMI FRX ¹ (wt.%).

Material	Fe (%)	Cr (%)	Ni (%)	Co (%)	Mn (%)	Cu (%)
AISI 304	70.4	18.6	8.2	0.3	1.6	0.5

¹ Positive Material Identification by X-Ray Fluorescence (PMI XRF).

. Prior to deposition, both the metallic and ceramic powders were stored in an oven to eliminate/prevent moisture, and the substrate was shot blasted to remove any surface contaminants and enhance laser beam energy absorption (by reducing reflection).

The deposition conditions used for the different coatings are summarized in

Table 6. Laser cladding deposition conditions used.

Laser power	3130 W
Laser spot diameter on substrate	5 mm
Travel speed	1720 mm/min
Powder feed rate	40 g/min
Shielding gas and carrier gas flow rate	Argon at 8 L/min
Lateral overlap	30%

. To reduce the interaction with the substrate (dilution), two layers were deposited on each substrate (Figure 3a) so that the top layer would actually be considered for evaluation later. Each layer was formed by parallel beads deposited transversely to the substrate to minimize distortion (Figure 3a). After deposition, the bottom of each sample (substrate side) was milled to correct any warping, and the coating top same surface was subsequently ground to standardize the surface finish (Figure 3b).

2.2. Characterization of the Coatings

The coatings were characterized in terms of dilution, microstructure, and hardness. The dilution index refers to the degree of mixing between the deposited material (the one to be protective) and the substrate (the one to be protected) and it is commonly used to quantify the extent to which substrate elements are incorporated into the coating (i.e., the loss of coating elements to the substrate). In this study, it was estimated based on localized chemical composition analysis, specifically by measuring the Fe content at the top surface of the coatings. Since such an element is a major constituent of the substrate but not of the feedstock materials, its presence in the upper region of the coating serves as an indicator of dilution. The chemical composition was determined by a PMI XRF (Positive Material Identification by X-Ray Fluorescence) Olympus Vanta Element S Handheld XRF analyzer (Olympus Corporation, Waltham, MA, USA).

For cross-sectional analysis, the samples underwent standard metallographic preparation steps: cutting and grinding followed by polishing with diamond paste down to 1 μm. The microstructure of each coating was analyzed in the respective cross-section by means of optical microscopy (OM) and scanning electron microscopy (SEM). All the coatings (both with Inconel 625 and Stellite 6) were etched by immersion in an aqua regia solution (3:1) for 15 s to reveal their typical phases. Vickers microhardness profiles were then extracted from the respective cross-sections with a load of 0.5 kgf (HV0.5). Additionally, the microhardness measurements were also conducted on the top surface of each coating (the surface where the abrasive wear test was to be performed in each case).

2.3. The Wear Performance of the Coatings

Initially, some wear tests were conducted using the rubber wheel test (ASTM G65), but due to the excessive wear of the rubber wheel itself, the metal wheel test (ASTM B611) was chosen instead, for which a schematic representation is shown in Figure 4. In this test, the sample was fixed vertically so that the surface of interest was pressed against a rotating metal wheel under a constant normal load, which was applied by means of a weight placed on a lever arm. As the metal wheel rotated, it was partially immersed in an alumina–water slurry, forcing alumina particles to slide/roll over the sample surface. Four radial paddles were attached to the wheel to stir the slurry and direct it to the contact region. This way, the metal wheel test mimics applications where the applied stress on the abrasive is sufficient to fragment it. Additionally, it is worth mentioning that such a wear test is particularly aggressive due to the use of alumina as abrasive body, and it is specifically designed for determining the high-stress abrasion resistance of hard materials.

The wear tests were then performed according to Procedure A of the ASTM B611-13 standard [26], more specifically with 1000 revolutions (test cycles) at 100 rpm, resulting in a test duration of 10 min. A SAE 1020 steel wheel with a hardness of 91.4 ± 0.2 HRB was used, which falls within the standard-specified hardness range (80–95 HRB). In accordance with Section 6.2 of the ASTM B611-13 standard, the wheel width was 12.7 ± 0.1 mm and its diameter varied between 165 and 169 ± 0.1 mm. By following Section 7.5 of the same standard, the abrasive slurry was prepared with alumina (30 mesh~600 μm) and water in a 2:1 ratio. Additionally, as specified in its Section 7.4, a weight load of 200 N was applied.

Table 7. Summary of the metal wheel test conditions according to the ASTM B611 standard [26].

Wheel rotation speed	100 rpm
Number of cycles	1000
Test duration	10 min
Normal load	200 N
Abrasive	Alumina (30 mesh~600 μm) with water in a 2:1 ratio (slurry)
Wheel	SAE 1020 steel with average hardness of 91.4 ± 0.2, width of 12.7 ± 0.1 mm, and diameter of 165–169 ± 0.1 mm

summarizes the wear test conditions.

Examples of samples with dimensions and surface finish (ground) compatible with the recommendations of Section 6.3 of the ASTM B611-13 standard are shown in Figure 5. According to this standard, the test surface of the test specimen should be flat and not contain errors of form (ridges, waves, bumps, etc.) greater than 2.0 µm, as indeed verified to be the case in all the samples by a surface roughness gage check. Three wear tests were performed for each condition (i.e., combination of matrix and reinforcement content), except for the Inconel 625 case without WC addition, for which only two tests were carried out due to sample (feedstock materials) limitations. The metal wheel test method evaluates a relatively large contact area and is, therefore, less susceptible to local heterogeneities. As shown in the Supplementary Materials, the variability between replicates was low, supporting the reliability and significance of the results.

Both before and after the wear tests, the samples were washed in running water, cleaned in an ultrasonic acetone bath for 5 min, dried with thermal air blowing, and then weighed by using a Shimadzu AUY 220 scale (Shimadzu Corporation, Kyoto, Japan), which has a nominal capacity of 220 g and a resolution of 0.1 mg, thus meeting the requirements of Sections 7.2 and 7.3 of the ASTM B611-13 standard. Since the density of the coatings depends on the WC particle content and these particles may not be indeed homogeneously dispersed throughout the coating section due to their high density, the results were presented in terms of mass loss rather than worn volume (as suggested in Section 8.2 of the ASTM B611-13 standard).

3. Results

3.1. General Characterization of the Coatings

The impact of the addition of WC on the chemical composition of the coatings is presented in

Table 8. Chemical composition ¹ of the coatings based on Inconel 625.

WC Addition	Ni (%)	Cr (%)	Mo (%)	Nb (%)	Fe (%)	Mn (%)	W (%)
0%	62.2	21.6	9.4	3.5	2.7	0.4	-
10%	50.8	20.3	7.8	3.4	8.4	0.5	8.60
20%	47.8	19.1	7.1	2.9	10.52	0.6	11.80
30%	43.3	18.1	6.5	3.1	11.2	0.6	17.00
40%	33.7	16.2	5.0	2.8	15.1	0.7	26.60

¹ Measured by PMI FRX on the top (ground) surface of the coatings.

and

Table 9. Chemical composition ¹ of the coatings based on Stellite 6.

WC Addition	Co (%)	Cr (%)	W (%)	Ni (%)	Fe (%)
0%	60.4	28.6	7.6	1.2	1.9
10%	54.9	27.5	13.0	1.35	3.3
20%	47.6	25.2	19.9	1.54	5.4
30%	41.0	22.8	27.1	1.49	7.23
40%	29.8	18.9	31.8	2.3	17.2

¹ Measured by PMI FRX on the top (ground) surface of the coatings.

. As graphically shown in Figure 6, the variation in Fe content indicates that the dilution index in the coatings increased with the addition of WC reinforcement particles.

The macrographs of the cross-sections of the coatings based on Inconel 625 and Stellite 6 with the varying WC additions are compiled in Figure 7, where a relative homogeneity in the distribution of the carbides can be observed. Overall, the coatings exhibited a regular geometry but with an increasing trend in the size and frequency of internal defects as the WC content was raised.

As shown in Figure 8, the microstructure of the coatings based on Inconel 625 was composed of a dendritic structure with a γ-Ni matrix and interdendritic regions enriched with Nb and Mo, where Laves phases can precipitate. In contrast, as presented in Figure 9, the coatings based on Stellite 6 exhibited a typical hypoeutectic microstructure, consisting of a Co-rich γ-Co dendritic matrix with interdendritic regions containing Cr-rich carbides (M7C3 and M23C6). In both cases, the WC particles further modified the resultant microstructure by acting as nucleation sites. For the coatings based on Inconel 625, in particular, and as exemplified in Figure 10, the WC particles tended to crack.

The hardness profiles of the coatings based on Inconel 625 and Stellite 6 with the different WC contents are presented, respectively, in Figure 11 and Figure 12, while the average hardness values on their top surfaces are shown in Figure 13. As expected, for both the alloys, the increase in the addition of WC resulted in higher hardness levels. The nearly flat hardness profiles confirm the homogeneous distribution of WC particles throughout the thickness (depth) of the coatings. The hardness levels of the cases based on Stellite 6 were higher due to the presence of carbides inherent to the composition of such a material.

3.2. Evaluation of the Abrasive Wear Resistance of the Coatings

Figure 14 presents the effect of WC addition on the abrasive wear resistance (measured in terms of mass loss) of the coatings based on Inconel 625 and Stellite 6. Contrary to expectations, the coatings made of Inconel 625 only (without WC) exhibited a mass loss significantly lower than the Stellite 6 case, i.e., resulted in a better abrasive wear resistance. Focusing on the coatings based on Stellite 6, the higher the WC addition, the lower the mass loss (the greater the wear resistance). This effect appears to be more significant for small additions of WC (up to 10%, beyond which further increases in the WC content did not significantly enhance the performance). On the other hand, the wear resistance decreased with the addition of WC particles for the coatings based on Inconel 625.

To expand the understanding of the effects of WC addition on the wear resistance of the coatings, SEM analyses were performed at the center of the wear tracks. In this case, indentations with characteristics of abrasive particle rolling were observed in the coatings based on Inconel 625 (Figure 15). In contrast, more grooves were noticed for the coatings based on Stellite 6 (Figure 16), which are characteristic of abrasive particle sliding. Additionally, in such cases, WC particles appear to have interrupted and/or hindered the formation of the grooves (Figure 16b).

4. Discussion

The WC particles added to the coatings deposited by laser cladding significantly influenced their resulting dilution, defect formation, microstructure, hardness, and wear behavior.

The addition of WC to the Ni-based and Co-based alloys accounted for an increase in the dilution of the coatings that were formed (Figure 6), which is likely a consequence of the high thermal conductivity of such a reinforcement, which in turn, affects the heat distribution during laser cladding. This characteristic prolongs the time during which the molten pool volume remains in liquid state, allowing for greater interaction/mixing between the coating and substrate, hence leading to higher dilution levels. Furthermore, the presence of partially melted WC particles in the molten pool modifies its viscosity, promoting disturbance and a more intense mixing of the molten material and consequently, favoring deeper penetration into the substrate. These effects have been, in fact, reported in the literature as key factors influencing the dilution levels in MMC coatings [27].

The coatings processed with the powder combinations containing WC particles revealed an increase in the number of defects, such as cracks and pores (Figure 7). This behavior might be attributed to the thermal expansion mismatch between the WC reinforcement and the metallic matrix, which generates residual stresses during cooling, favoring crack initiation and propagation. Additionally, the partial dissolution of WC in the molten pool might lead to the formation of secondary brittle phases, reducing the toughness of the coatings formed and making the material prone to cracking. The high degree of the hygroscopy of the WC particles mixed with the atomized metallic materials allows those particles to carry water molecules on their surface, hence inducing the formation of moisture-related pores during deposition. Besides being linked to this factor, the porosity verified might also be associated with the increased viscosity of the molten pool due to the presence of partially melted WC particles together with the fast solidification rates, which tend to hinder gas escape and hence, result in pore formation. Similar trends have also been observed in other studies on WC-reinforced MMC coatings, as per Wang et al. [28].

As expected, the coatings processed with the powder combinations containing WC particles exhibited a dispersion of partially melted carbides (Figure 8 and Figure 9) regardless of the matrix composition being based on Co or Ni. However, a critical difference referred to the cracking of WC particles, which occurred exclusively in the coatings based on Ni, i.e., formed in combination with Inconel 625 (Figure 10). This trend may be primarily attributed to thermal expansion coefficient differences between the Inconel 625 alloy and the WC particles. To assess this behavior, it is of relevance to follow step by step the phenomena occurring during deposition as WC particles are melted in the laser beam, subjected to subsequent solidification inside the molten pool, and then cooled within the deposits. The thermal history of such particles (consequence of the laser cladding parametrization) will determine if they are totally or partially melted during deposition depending on their melting temperature as well as on that of the matrix being used. The Inconel 625 alloy melts within the 1288–1349 °C range, whereas the WC particles melt at a much higher level (2870 °C). For coatings formed with such a combination of materials, after deposition, solidification starts with the formation of Ni-based dendrites at the fusion line and advances very rapidly due to the fast-cooling rates imposed by the high-density energy of the laser beam that induces a sharp temperature gradient between the molten pool and the substate. Also, the partially melted carbides in the molten pool play a small role as nuclei for solidification, although this factor is more significant in coatings based on Co alloys, as is the case with the Stellite 6 matrix (Figure 9b). The fast solidification of the Ni alloy together with the aforementioned difference in thermal expansion coefficients likely generate high residual stresses at the reinforcement–matrix interface, leading to crack formation across the WC particles. As a matter of fact, this behavior aligns with a previous investigation that examined stress evolution in laser-cladded coatings reinforced with WC [8]. Opposite to the results found for the Ni alloy matrix (Inconel 625), the coatings based on the Co alloy matrix (Stellite 6) better accommodated the WC particles. In relation to that, it has been reported that the solubility of WC in Ni is higher than in Co [29], and this behavior may explain the more significant solidification surrounding the carbides in the coatings based on Stellite 6. Also, the infiltration of Co into the WC particles results in WC-Co formation, an arrangement that preserves both the hardness and strength of the carbides.

Under the conditions applied in the present work, a superior wear resistance (lower mass loss) was, in general, verified for the coatings based on Inconel 625 in comparison with the Stellite 6 case, especially without the addition of WC particles. This outcome contrasts with the general assumption that Stellite 6 should exhibit better performance in abrasive wear conditions owing to its higher hardness level and inherent carbide content. Although intriguing at first sight, this behavior can be understood when analyzing the severe wear mechanisms induced by the metal wheel test. The SEM analysis of the worn surfaces revealed that the coatings based on Inconel 625 predominantly exhibited indentations, a characteristic of rolling abrasive particles, whereas the Stellite 6 cases displayed more grooves, indicating sliding wear occurrence. The ability of the Inconel 625 alloy to undergo plastic deformation likely contributed to its superior wear resistance (lower mass loss) as its ductile nature allows for the accommodation of stresses and, therefore, prevents localized fractures under abrasive contact. In contrast, in the Stellite 6 case, given that this material is a Co and Cr alloy with dispersed carbides, it is more prone to carbide fracture and delamination, leading to material loss due to wear [30,31,32,33].

Despite the possible expectation that WC as a reinforcement would, overall, enhance the wear resistance in MMC coatings, it was observed that an increased WC fraction reduced the wear resistance of the coatings based on Inconel 625, hence, with the reinforcement, in such a case, not acting synergically with the matrix for mechanical performance. This negative effect can be attributed to the partial dissolution of WC in the matrix, reducing its effectiveness as a reinforcing phase. The higher solubility of WC particles in such a Ni-based matrix impairs their hardness and strength as Ni infiltrates them forming a WC-Ni compound [29], and in this way, the wear resistance of the carbides and, in turn, of the coatings as a whole, is undermined. Additionally, the presence of partially melted WC particles, combined with thermal-related stresses, leads to crack formation at the reinforcement–matrix interface, making the coating, in this case, more susceptible to material removal under abrasive wear. Another factor is the transition in the wear mechanism, where brittle fracture at the reinforcement–matrix interface becomes more dominant, leading to premature material loss. Similar trends have been reported in studies on WC-reinforced Inconel coatings, where excessive WC contents led to embrittlement and increased wear rates [8,27,28].

Reinforcement–Matrix Sinergy: Design Reflections on MMC Caotings and Research Outlook

The fundamental question that this work aims to respond is the following: does the addition of WC particles always lead to improved wear resistance in MMC coatings? To begin with the response, although the incorporation of higher amounts of WC generally results in increased surface hardness, this enhancement does not necessarily correlate with improved wear resistance. As evidenced in this work, this is particularly true for coatings based on the Inconel 625 matrix, where the addition of WC resulted in reduced wear performance due to changes in the microstructural behavior and in the nature of the reinforcement–matrix interaction (lack of synergy).

The findings being presented here indeed highlight the critical influence exerted by the matrix metallic alloy on the effectiveness of WC particles as reinforcements in laser-cladded coatings. While the addition of WC significantly improved the wear resistance with Stellite 6, its incorporation into an Inconel 625 matrix resulted in particle cracking and interfacial debonding, ultimately impairing the wear performance under severe abrasive conditions, an operational situation typically found in many industrial scenarios, such as in the mining and material transformation sectors. These results, although counterintuitive to conventional expectations, are valid for the specific conditions tested and call attention to the importance of matrix-reinforcement compatibility/synergy when selecting feedstock materials for the efficient design of MMC coatings and during their thermal formation as in processing via laser cladding. Thus, from an industrial perspective, Co-based matrices may offer a more reliable platform for WC reinforcement in applications where abrasive wear is seen as critical. For Ni-based matrices, attention must be given to processing parameters and WC-matrix interactions.

Further related studies, including thermal stress simulations, detailed phase analyses, and advanced tribological characterization, are recommended to better understand the underlying mechanisms behind the formation of MMC coatings and to support their tailored design for challenging operations and environments.

5. Conclusions

This study investigated the effect of the addition of tungsten carbide (WC) particles as a reinforcement in Ni-based (Inconel 625) and Co-based (Stellite 6) alloys deposited by laser cladding to form wear-resistant metal matrix composite (MMC) coatings. The main findings demonstrate that the use of WC as a reinforcement is not universally beneficial, given that its effectiveness under severe abrasive conditions is strongly dependent on the matrix material utilized. Thus, concerning the addition of WC as a potential reinforcement in such metallic alloys, the key conclusions can be summarized as follows:

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The WC presence increases the dilution index (the mixing of the coating and substrate) with both matrix materials (Inconel 625 and Stellite 6);

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The WC particles tend to crack within an Inconel 625 matrix;

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The WC particles increase both the hardness and wear resistance in coatings based on Stellite 6, resulting in reinforcement–matrix synergy;

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The WC particles increase the hardness but reduce the wear resistance in coatings based on Inconel 625, which does not result in reinforcement–matrix synergy.