Microstructure and properties of laser-cladding inconel 625-based WC/TiC gradient composite coat-ings on 65Mn steel

ABSTRACT

A Inconel 625 transition layer was deposited on the damaged surface of 65Mn steel laser cladding, followed by cladding of a Inconel 625-WC-TiC composite alloy as the working layer. The effects of reinforcement content on the microstructure, elemental distribution, phase constitution, microhardness, and tribological behavior of the composite coatings were investigated. The results indicate that the Inconel 625 transition layer forms sound metallurgical bonding with both the 65Mn substrate and the working layer. The coatings are mainly composed of (Ti,W)C, an (Fe,Ni) solid solution, and carbide phases. The microstructure exhibits a distinct gradient evolution, transitioning from columnar grains near the bottom of the transition layer to a dendrite–eutectic mixed structure in the working layer. The working layer with 30wt.% WC and 10wt.% TiC achieves an average hardness of 480 HV_0.2, more than twice that of the substrate, along with a 21.29% reduction in friction coefficient and the minimum wear-scar depth of 0.8μm. Abrasive wear is identified as the dominant wear mechanism. In contrast, the working layer with 40wt.%WC and 0wt.%TiC fails due to through-thickness cracking induced by coefficient of thermal expansion (CTE) mismatch.

Keywords:
Laser cladding; Inconel 625/WC/TiC; Gradient composite coating; Microstructure

1. INTRODUCTION

With the continuous advancement of modern agriculture, key soil-engaging components in agricultural machinery-such as rotary tiller blades, plowshares, and deep loosening shovel blades-play a pivotal role in agricultural production [¹]. 65Mn steel has emerged as the preferred material for these agricultural soil-engaging components [²], owing to its high strength, excellent wear resistance, and good toughness [³]. Although 65Mn steel has undergone performance optimization through heat treatment processes (such as quenching, tempering, and annealing) in practical applications, these components operate for extended periods in complex agricultural environments containing moist soil, sand, and gravel [⁴]. In such scenarios, the synergistic interaction of abrasive wear and fatigue wear leads to a drastic reduction in their service life by 50%~70%, rendering them unable to fully adapt to harsh service conditions. Therefore, laser cladding technology is utilized to fabricate high-hardness wear-resistant composite coatings on the surface of 65Mn steel, aiming to enhance the surface properties of 65Mn steel.

To address the wear failure of 65Mn steel soil-engaging components, surface modification technologies have been widely explored to improve the surface properties of the substrate while retaining its bulk toughness. Common surface strengthening methods include surfacing [⁵], thermal spraying [⁶], laser cladding [⁷], and plasma technology [⁸]. Among these methods, laser cladding has achieved extensive application and recognition in the engineering field, attributed to its advantages such as broad material adaptability, low dilution rate, and excellent metallurgical bonding with the substrate [⁹, ¹⁰]. Ni-based alloys exhibit good wettability and fluidity, and their coating microstructures contain many ductile phases [¹¹], which can effectively inhibit the propagation of potential cracks. Therefore, using Inconel 625 as the transition layer material not only ensures the bonding quality of the coating but also enhances the coating’s toughness. However, the low hardness and poor wear resistance of Inconel 625 coating limit their service life in wear-prone environments [¹², ¹³]. Therefore, ceramic hard phases (such as SiC, TiC, and WC) are added to Ni-based powders to achieve the reinforcement of the coating [¹⁴, ¹⁵].

Among various ceramic particles, tungsten carbide (WC) possesses a high melting point of up to 2870°C [¹⁶], extremely high hardness, and excellent wettability. Notably, during the cladding process, the weak affinity between W and C prevents their easy chemical reaction for the in-situ formation of WC phases [¹⁷]; instead, they tend to react with the base metal elements to form complex carbide phases such as M₆C and M₂₃C₆ [¹⁸]. The formation of these intermetallic compound phases significantly enhances the microhardness of the coating. Because the density of WC (15.55g/cm³) is significantly higher than that of Inconel 625(8.4g/cm³), under the combined effects of molten pool convection and gravitational field, WC exhibits obvious inhomogeneity in distribution and sedimentation to the bottom of the coating [¹⁹, ²⁰]. This results in the hardness and WC content in the upper part of the coating being significantly lower than those in the lower part and induces adverse effects such as cracking [²¹].

As another high-hardness ceramic material, titanium carbide (TiC) has a density of 4.25g/cm³ [²²], and its thermodynamic stability and hardness are both higher than those of WC. During laser cladding, it endows coatings with higher hardness and superior wear resistance. However, Ti and C elements tend to undergo a strong exothermic reaction under the high-temperature environment of laser cladding, rapidly forming TiC phases in-situ. This intense reaction causes severe spattering in the molten pool and poor coating formability; therefore, its addition content is generally controlled within 30wt.% to balance the coating quality [²³, ²⁴].

Although significant progress has been made in the field of coating reinforcement using WC and TiC as single ceramic reinforcing phases, there remains a paucity of research on laser-cladded (LC) multi-layer composite coatings. This study proposes introducing a Inconel 625 transition layer between the 65Mn steel substrate and the Inconel 625-WC-TiC composite coating, aiming to improve the distribution uniformity of WC and TiC in the coating through transition interface design, to reduce cracking in the coating. It systematically investigates the combined effects of WC and TiC on the phase composition, microstructure, and properties of the coating, thereby providing a theoretical basis for the controllable fabrication of high-performance laser-cladded composite coatings.

2. EXPERIMENTAL PROCEDURES

2.1. Experimental materials

In this study, 65Mn steel plate was used as the substrate, with the size of 80mm×50mm×15mm. 65Mn steel was selected as the substrate material due to its excellent combination of strength, wear resistance, and toughness, as well as its superior strain hardening ability and widespread engineering application in soil-engaging components compared with other steels of similar cost. In order to ensure the excellent metallurgical bonding between the coating and the substrate during the laser cladding process, the surface of the substrate was pre-treated prior to the experiments: firstly, sandblasting was used to remove the surface oxidized layer, and then acetone was used to clean the surface thoroughly to remove the oil contamination and other pollutants. Powder system to Inconel 625 as the main powder (chemical composition show Table 1), ceramic reinforcing phase by WC and TiC powder mixed at different mass ratios, the total content of the two constants. The particle (Figure 1) sizes of the powders were controlled in the range of 53~150µm to match the requirements of the laser cladding process for powder fluidity and uniformity of the cladding layer. The mass fraction of WC increased from 25wt.% to 40wt.%, and the corresponding mass fraction of TiC decreased from 15wt.% to 0wt%, forming four groups of comparison experiments as listed in Table 2. The powders were mixed in a powder mixer at 260r/min for 2h to ensure that the WC and TiC particles were uniformly dispersed in the Inconel 625 matrix powder, and the mixed powders were dried in a vacuum drying oven at 150°C for 2h to remove the adsorbed moisture completely and avoid porosity defects due to the evaporation of moisture in the fusion coating process. The technical specifications of the laser cladding system used in this study are presented in Table 3.

Table 1
Chemical composition of 65Mn and Inconel 625 powders (mass fraction%).

NAME	CARBON	SILICON	MANGANESE	NICKEL	CHROMIUM	MOLYBDENUM	NIOBIUM	TITANIUM	IRON
65Mn	0.62~0.70	0.17~0.37	0.90~1.20	≤0.30	≤0.15	–	–	–	Bal.
Inconel 625	0.04	0.16	0.04	61.52	20.9	8.56	3.31	0.18	4.46

Figure 1
Cladding materials morphology (a) Inconel 625 particles (b) WC particles, and (c) TiC particles.

Table 2
Sample numbers and chemical composition (wt./%).

SAMPLE	WC	TIC	INCONEL 625
S1	25	15	60
S2	30	10	60
S3	35	5	60
S4	40	0	60

Table 3
Specifications of the laser cladding equipment.

PARAMETER	SPECIFICATION
Laser Type	Continuous Wave Fiber Laser
Wavelength	1070 nm
Maximum Power	6 kW
Beam Delivery	Fiber Optic
Focus Spot Diameter	2.5 mm
Powder Feeding System	Coaxial Nozzle
Motion Control	KUKA-KR-60
Parameter	Specification

2.2. Coating preparation

The schematic of the laser cladding test is shown in Figure 2. With reference to the actual working requirements of the workpiece, the transition layer has a better metallurgical bonding with the substrate and reduces defects such as cracks and porosity. The laser head is self-developed by Raycham, with a focal length of 12mm, and the physical diagram is shown in Figure 2a. In this experiment, the four-channel coaxial powder feeding method is used, and the spot diameter is about 2.5mm. A summary of all the optimized process parameters is shown in Table 4, These parameters were optimized based on extensive preliminary experimental trials to ensure stable processing and desirable coating quality. In this experiment, Inconel 625 alloy was melted and coated on the substrate surface as the transition layer. Then Inconel 625-WC-TiC alloy was melted on the Inconel 625 layer as the reinforcing layer to prepare the composite coating, as shown in Figure 2b.

Figure 2
(a) Physical picture, and (b) Schematic diagram of laser cladding process.

Table 4
The laser power (W), scanning speed (mm/min) and powder feeder (r /min) for transition layer powders were 1400, 900 and 0.3, respectively, whereas the same parameters for working layer powders were 1600, 900 and 0.4.

NAME	LASER POWER/(W)	SCANNING SPEED/(mm/min)	POWDER FEEDER/ (r/min)
Transition layer powder	1400	900	0.3
Working layer powder	1600	900	0.4

2.3. Methods

• (1)

  Phase detection of the coating was performed using an X-ray diffractometer (XRD) with Cu target Kα radiation. The operating parameters were set as follows: tube voltage of 40kV, tube current of 300mA, scanning rate of 5°/min, and diffraction angle range (2θ) of 20°~90°.

• (2)

  Specimens were longitudinally sectioned perpendicular to the coating surface, and samples with dimensions of 10mm × 10mm × 15mm were obtained. They were cleaned in an ultrasonic cleaner with ethanol as the medium for 8 min to thoroughly remove surface oil contaminants. The specimens were then mounted, sequentially ground with 200# ~2000# sandpapers, and polished until the test surface was smooth and free of scratches. Subsequently, the test surface was electrolytically etched using a saturated oxalic acid solution at a voltage of 6V for 8s. Immediately after etching, the surface was rinsed with absolute ethanol to remove residual electrolyte, followed by drying with a cold-air dryer. The cross-sectional microstructure of different coating specimens was observed using a ZEISS scanning electron microscope (SEM), and the elemental composition was analyzed through its attached energy dispersive spectrometer (EDS).

• (3)

  The microhardness of the coating cross-section was measured using a digital Vickers hardness tester (Model THVS-50) with a test load of 9.8N and a dwell time of 15s. For cross-sectional hardness testing, measurements were taken every 80μm along the direction from the coating surface to the substrate. At each measurement depth, three points were tested with a horizontal spacing of 50μm between adjacent points, and the average value of these points was calculated as the final hardness value for that depth.

• (4)

  The friction and wear tests were conducted using CHMT-23 high-speed friction and wear tester. Firstly, the specimens were cut into circular samples with a diameter of 25mm for friction and wear testing. The test surfaces were ground to a flat finish using 600#~1200# sandpapers, followed by polishing and ultrasonic cleaning. The friction and wear tests were performed at room temperature using a disc-type friction pair configuration, where the friction pair was a silicon nitride (Si₃N₄) ceramic ball with a diameter of 6mm. The applied load was 50N, and the friction duration was 30min. Under the same experimental conditions, the wear tests were repeated three times, and the average value was taken as the result. Additionally, the 3D wear morphology of the specimens was observed using a 3D confocal laser scanning microscope.

3. RESULTS AND ANALYSES

3.1. Physical phase of gradient composite coating

Figure 3 shows the X-ray diffraction pattern of the Inconel 625-WC-TiC coatingshows the X-ray diffraction (XRD) patterns of the Ni-WC-TiC coatings. The working layer is mainly composed of (Ti,W)C, W₂C, and an (Fe,Ni) solid solution. After the addition of TiC, diffraction peaks corresponding to TiC and the (Ti,W)C solid solution were observed in coatings S1, S2, and S3. For these three coatings, the (Ti,W)C diffraction peaks with identical crystal plane indices exhibit a rightward shift. This phenomenon can be attributed to increased W content resulting from WC decomposition: as W content rises, the lattice parameters of the (Ti,W)C solid solution further decrease. According to Bragg’s law (2d sinθ = nλ, where d is the lattice spacing, θ is the diffraction angle, n is the diffraction order, and λ is the wavelength of Cu Kα radiation), a decrease in lattice spacing (d) leads to an increase in the diffraction angle (θ). This manifests as an enhanced rightward shift trend in the (Ti,W)C diffraction peaks. Literature [²⁵,²⁶,²⁷] has conducted research.

Figure 3
X-ray diffraction pattern of the Inconel 625-WC-TiC coating.

3.2. Macroscopic morphology of the coating

Figure 4 shows the macroscopic morphologies of coatings with different ratios. Specifically, Figure 4a exhibits relatively high surface flatness but contains a small number of pores. This is because TiC addition is excessive, leading to typical pore characteristics on the surface. Figure 4b has high surface flatness, with no pores or cracks observed. This improvement can be attributed to enhanced molten pool fluidity and more complete powder melting when the TiC content is 10 wt.% [²⁸]. The roughness of the Figure 4c surface coating increases significantly, and many fine cracks appear. When the WC content is 40wt.% (Figure 4d), large-area reticular cracks form. This phenomenon is attributed to the increased thermal stress, which causes the energy release rate of crack propagation to exceed the surface energy-consistent with the phenomenon in Griffith’s theory where the stress concentration factor exceeds the critical value.

Figure 4
Macroscopically detected morphology of different ratios of liquid penetrants, (a) S1; (b) S2; (c) S3; (d) S4

3.3. Microscopic morphology of the coating

Figure 5 shows the microstructural morphology of Coating S2. Figure 5a presents the cross-section of the S2 specimen, which is clearly divided into three layers: the upper layer is the Inconel 625-WC-TiC working layer, the lower layer is the 65Mn substrate, and the middle gray part is the Inconel 625 transition layer. The locally magnified images corresponding to different regions of the coating are shown in Figure 5(b~g). Given the characteristics of rapid melting and solidification in laser cladding (LC), combined with solidification theory, the microstructure morphology of the coating is determined by the ratio of G/R, where G is the temperature gradient and R is the growth rate. During the laser cladding process, due to the large G and small R in the bottom region of the molten pool, the grains exhibit obvious epitaxial growth characteristics at the bonding interface. As a result, columnar crystals are predominantly distributed near the metallurgical bonding zone between the transition layer and the substrate, with no obvious defects observed in the coating (Figure 5b). As shown in Figure 5(c, d) the middle and top regions of the transition layer mainly consist of equiaxed grains of very similar sizes and types. This differs from the typical single-layer LC, which is attributed to the inability to form a new molten pool on top of the Inconel 625 coating when the Inconel 625-WC-TiC alloy is cladded. Meanwhile, due to the dilution effect, a large amount of Ni elements diffuses into the working layer. As shown in Figure 5d that no obvious fusion line is observed between the Inconel 625 layer and the Inconel 625-WC-TiC layer, confirming the achievement of metallurgical bonding between them. This excellent interface bonding is attributed to the combined effect of multiple factors: the thermal accumulation during repeated LC processes reduces G at the bottom of the new molten pool, resulting in a G/R ratio lower than the threshold for planar grain growth; the similar dendritic morphology inside the Inconel 625 layer facilitates the bonding of the two different cladding layers. With the increase of R, columnar crystal structures with distinct growth orientations are formed in the interface region (Figure 5e), and rod-like and granular precipitates with large spacing are distributed between the dendrites. In the middle region of the working layer (Figure 5f), the microstructure exhibits a dispersed distribution of molten bright white particles and gray-black particles. When the solid-liquid interface of the molten pool moves to the top of the working layer (Figure 5g), the heat dissipation on the surface promotes R to reach a peak, causing the grain growth rate to be much faster than the nucleation rate. Eventually, uniform equiaxed grains and fine dendrites are formed. The microstructure is characterized by a mixture of dendrites and interdendritic eutectic structures [²⁹].

Figure 5
SEM images of S2 coating. (a) cross-sectional image; (b) zone B; (c) zone C; (d) zone D; (e) zone E; (f) zone F; (g) zone G.

Quantitative analysis of the chemical composition at the coating interface was performed using EDS. Combined with the elemental contents in Table 5 and the XRD results (Figure 3), it is confirmed that Points H and I (Figure 5f) correspond to undissolved WC and TiC phases, respectively. Further analysis of Point J (Figure 6b) indicates that it corresponds to complex carbides (W₂C, M₆C) formed by the reaction between decomposed WC and the matrix. This phenomenon originates from the dissolution and diffusion behavior of WC in the high-temperature molten pool, forming a typical reaction layer at the WC/matrix interface [³⁰]. For the phase composition of the regular rectangular particle K (Figure 6c), Table 5 shows that its main elemental components are C, W and Ti. Combined with the analysis of characteristic peaks in the XRD pattern, it is confirmed that this microstructural feature corresponds to the (Ti,W)C solid solution. This (Ti,W)C solid solution shares the same crystal structure (face-centered cubic, FCC) as TiC [³¹]. It not only retains the high hardness and excellent chemical stability of Tic but also exhibits improved wettability with the binder phase and enhanced interface bonding strength [³², ³³].

Table 5
Analysis of EDS results in Fig. 5f (Points H and I), Fig. 6 (Points J and K).

POINT	TUNGSTEN/（wt%）	CARBON/ （wt%）	FERRUM/（wt%）	NICKEL/ （wt%）	TITANIUM/（wt%）	CHROMIUM/（wt%）
H	46.87	53.13	–	–	–	–
I	3.36	50.12	0.12	0.49	35.7	0.21
J	13.43	67.12	1.83	1.64	14.15	1.83
K	12.1	45.86	9.71	16.09	–	16.24

Figure 6
Microstructure and magnification of interfacial layer of S2 specimen: (a) microstructure of the overlap region of the working layer formed by laser cladding; (b, c) enlarged views of the selected regions in (a).

To further clarify the phase constitution and microstructural evolution of the coating at the nanoscale, transmission electron microscopy (TEM) combined with selected area electron diffraction (SAED) was performed, as shown in Figure 7. The corresponding elemental mapping results in Figure 7(b–d) further elucidate the compositional distribution within the selected region. The corresponding SAED patterns (Figure 7(f–h)) provide crystallographic identification of these phases. As shown in Figure 7f, the diffraction pattern can be indexed to an FCC-structured (Ti,W)C_1-X solid solution with a zone axis of [110], where the presence of characteristic reflections indicates that W atoms have diffused into the TiC lattice, forming a substitutional solid solution. In Figure 7h, the diffraction pattern is identified as hexagonal WC with a zone axis of [100], suggesting that part of the WC particles remained undissolved during laser cladding. Meanwhile, Figure 7g exhibits a typical FCC diffraction pattern along the [100] zone axis, corresponding to TiC. This phase is primarily derived from the externally added TiC powders and undergoes partial dissolution and reprecipitation under high-temperature conditions.

Figure 7
TEM micrograph and corresponding analyses of the coating (a) TEM image of the selected region; (b–d) elemental mapping results (C, W, Ti) corresponding to the area in (a); (e) enlarged TEM image of the region marked in (a); (f–h) SAED patterns taken from regions A, B, and C in (a).

The combined TEM and SAED results indicate that a dissolution-diffusion-solid solution transformation mechanism governs microstructural evolution during laser cladding. Under high-energy laser irradiation, WC particles partially decompose, releasing W and C atoms into the molten pool. At the same time, the externally added TiC particles experience interfacial dissolution. Due to the relatively high diffusivity of W atoms in the molten matrix, they preferentially diffuse toward TiC particles and substitute Ti sites in the lattice, leading to the formation of (Ti,W)C_1-X solid solution.

In addition, the rapid solidification inherent to the laser cladding process suppresses long-range atomic diffusion, allowing part of the original WC to be retained while promoting the formation of (Ti,W)C and other carbide phases. Consequently, a multiphase composite microstructure consisting of retained WC, TiC, and (Ti,W)C solid solution is established in the working layer. This result is consistent with the XRD analysis (Figure 3), where the peak shift of (Ti,W)C further confirms the incorporation of W into the TiC lattice.

To further investigate the diffusion of elements at the interface of the S2 coating, the fused cladding layer was subjected to EDS line scanning, and the results are shown in Figure 8. As can be observed, the mutual diffusion interval of Ni and Fe (350~400μm) serves as the core region for metallurgical bonding. A compositional transition zone is formed through element diffusion, providing direct evidence of interdiffusion between the coating and substrate, which confirms metallurgical bonding. This phenomenon originates from the laser cladding process: when the substrate absorbs laser energy to form a molten pool, the melting of surface metals and powders induces annular flow within the pool, causing it to gradually expand. The fluid at the laser irradiation spot is driven by Marangoni convection, which accelerates the mutual diffusion of Ni (from Inconel 625) and Fe (from 65Mn substrate) between the substrate and coating, thereby forming a continuous compositional transition zone with a width of approximately 50μm. In addition, W and Ti are only enriched in the working layer, with extremely low contents in the transition layer. This indicates that WC and TiC particles do not settle into the transition layer, the Inconel 625 transition layer (with low density and good fluidity) inhibits the sedimentation segregation of high-density ceramic particles during molten pool solidification. This further validates the effectiveness of the transition layer in optimizing the distribution uniformity of hard phases.

Figure 8
Result of EDS cross-sectional scan of S2.

4. DISCUSSION

4.1. Effect of content of reinforcing phase on the coating

Under rapid solidification conditions, the coating microstructure may be arranged along grain boundaries or specific directions, forming reticular or dendritic structures. As shown in Figure 9a, a high proportion of TiC promotes uniform bonding between particles, controlling the average grain size within the range of 2~5μm. Notably, the cellular structures are distributed radially along the dendritic arms, forming dendrite-cellular ­composite strengthening units, which significantly improve the microstructural uniformity. However, pores are observed on the coating surface from Figure 4a; this is because in the high-temperature environment of laser cladding, the violent reaction of TiC causes severe spattering in the molten pool, resulting in poor coating formability. With the adjustment of powder composition parameters (Figure 9b), the grains are observed to exhibit a slight growth trend (with an average size of 5~8μm) [³⁴]. The smaller cellular structures disappear, and reticular structures begin to emerge in the cladding zone. Within these reticular structures, (Ti,W)C core solid solutions are uniformly distributed. The microstructures of Samples S3(Figure 9c) and S4(Figure 9d) exhibit obvious network-like carbide structures and cracks, which are characteristic of brittle structures caused by ­thermal stress accumulation or carbide coarsening.

Figure 9
SEM images of different scales of complex phase ceramic coatings (a)S1; (b) S2; (c) S3; (d) S4.

4.2. Hardness

Figure 10 shows the microhardness distribution across the cross-section of the Ni-WC-TiC composite coating. The microhardness of the 65Mn substrate is approximately 220HV₀.₂. Along the depth direction of the coating, four distinct regions are identified, namely the working layer, transition layer, heat-affected zone (HAZ), and the substrate.

Figure 10
Hardness of coating cross-section.

In the working layer region, Coating S1 exhibits a uniform and gentle microhardness variation characteristic, with an average microhardness gradient of less than 50HV₀.₂. This uniformity originates from the high mass fraction (15wt.%) of TiC addition, which promotes the uniform and dispersed distribution of the hard phase (Ti,W)C and effectively suppresses local stress concentration. By contrast, Coatings S2 and S3 exhibit a similar microhardness variation trend; however, the overall microhardness of Coating S3 is approximately 12% lower than that of Coating S2, with an average microhardness of 480HV₀.₂. This difference is attributed to the gradient adjustment of TiC addition content: when the TiC content increases from 5wt.% to 10wt.%, the mass fraction of the high hardness reinforcing phase (Ti,W)C in the upper region of the coating increases [³⁵], resulting in the surface layer microhardness rising to 500HV₀.₂.

4.3. Frictional wear performance

The coefficient of friction (COF) curves and wear morphology of the 65Mn substrate and gradient Ni-WC-TiC composite coatings under dry sliding conditions (S2 Coating) are shown in Figure 11. As presented in Figure 11a, all specimens undergo a significant running-in period during the initial wear stage. The direct contact between the silicon nitride (Si₃N₄) ceramic ball and the substrate/coating induces severe friction, resulting in a sharp increase in 0.4~0.6COF to the range within 5min. Subsequently, entering the steady-state wear stage, the friction behaviors of the substrate and coatings exhibit distinct differences. The 65Mn substrate continues to rise after 5min and finally stabilizes at 0.78COF; this is directly associated with the low hardness and poor wear resistance of 65Mn steel. By contrast, all four groups of gradient coatings (S1-S4) demonstrate superior friction stability: their average values are 0.58,0.55,0.57, and 0.52COF, respectively, all significantly lower than the substrate.

Figure 11
Friction wear results (a) dynamic friction coefficient curve; (b) wear morphology of S2 coating.

This excellent wear resistance originates from the synergistic strengthening effect of hard phases (WC, TiC, and (Ti,W)C) in the coating [³⁶]. Specifically, the coating hardness is enhanced through solid solution strengthening and precipitation strengthening mechanisms. The hard phase particles reduce the direct contact area between the Si₃N₄ ceramic ball and the coating through the “micro-cutting” effect, which inhibits dislocation movement and adhesive wear while reducing the wear rate. Although Coating S4 exhibits the highest hardness (550HV_0.2), its friction coefficient shows slight fluctuations during the steady-state stage. This indicates that high-hardness coatings must balance toughness and crack resistance to ensure stable tribological performance. In contrast, Coating S2 achieves the lowest friction coefficient (0.55) with minimal fluctuations, demonstrating the optimal tribological performance. This is attributed to the optimized matching between the TiC addition content (10wt.%) and laser processing parameters. As observed in Figure 11b, under the applied load during sliding, the micro-protrusions on the counterpart ball surface perform micro-cutting on the coating surface. The worn surface exhibits numerous shallow and continuous grooves, with a small number of fine debris in the grooves and some scattered original coating residues retained. This is a typical characteristic of abrasive wear.

As described in Section 3.3 and illustrated in Figure 4a, the high TiC content causes intense exothermic reactions during laser cladding, resulting in severe spattering of the molten pool. This spattering leads to two types of TiC inhomogeneity: (1) Local TiC enrichment, due to incomplete dispersion after spattering; (2) Pores caused by sputtering. The pores reduce the effective load-bearing area of the coating, while local TiC enrichment causes microhardness fluctuations (Section 4.2), ultimately leading to a higher wear scar depth (1.94μm, Figure 12a-1) compared to Sample S2. Coating S2 demonstrated outstanding wear resistance, characterized by a minimum maximum wear scar depth of only 0.89μm. This performance advantage is directly attributed to the hard phase particles and precipitated carbides (W₂C and (Ti,W)C) in the coating, this structure effectively inhibits dislocation motion and reduces adhesive wear. In contrast, the increased wear scar depth of Coating S3(1.61μm) and S4 (1.58μm) can be ascribed to the stress concentration effect induced by cracks.

Figure 12
Wear resistance test graphs of friction results of gradient coatings (a) S1、(b) S2、(c) S3 and (d) S4 are 3D localized wear topography of gradient coatings; (a-1) S1, (b-1) S2, (c-1) S3 and (d-1) S4 are the wear depth curves of gradient coatings.

5. CONCLUSION

• (1)

  A sound gradient architecture was successfully established by introducing a Inconel 625 transition layer between the 65Mn substrate and the Inconel 625-WC-TiC working layer. The transition layer enabled robust metallurgical bonding at both interfaces and provided a compositional/structural buffer that mitigated typical laser-cladding defects (cracking and porosity) and improved coating integrity.

• (2)

  Phase constitution and solid-solution evolution were governed by WC decomposition and TiC participation. The working layer primarily consisted of (Ti,W)C, W₂C, and an (Fe,Ni) solid solution, accompanied by additional carbide phases. The rightward shift of (Ti,W)C peaks indicate a reduction in lattice spacing, consistent with increased W incorporation into the (Ti,W)C solid solution during cladding, confirming an active reaction pathway that contributes to strengthening.

• (3)

  An optimal reinforcement balance was identified at 30wt.% WC+10wt.% TiC (S2). A dense-crack-free coating with an average working-layer hardness of 480 HV_0.2(more than twice that of the substrate), together with a 21.29% reduction in friction coefficient and the minimum wear-scar depth of 0.89μm. The improvement arises from the combined effects of hard-phase dispersion, precipitation/solid-solution strengthening, and stable interfacial bonding. The dominant wear mechanism of the optimized coating was abrasive wear, characterized by shallow continuous grooves with limited debris.

6. ACKNOWLEDGMENTS

This work was supported Anhui Province Quality Engineering Project No. 2023xjzlts075, Bengbu University Applied Projects No. 2024YYX03QD, 2024YYX04QD and 2024YYX08pj.

7. DATA AVAILABILITY

All data analyzed during this study are included in this published article.

8. BIBLIOGRAPHY

• ^[1] NDUMIA, J.N., KANG, M., BELLO, Z.A., et al, “Microstructure and wear mechanism of FeCrMoCBWNb coating deposited by arc-spraying and its application on 65Mn steel blades”, Surface and Coatings Technology, v. 492, pp. 131195, Sep. 2024. doi: https://doi.org/10.1016/j.surfcoat.2024.131195.
  » https://doi.org/10.1016/j.surfcoat.2024.131195
• ^[2] ZHANG Y, DU Q, LIN T., et al, “A study on the corrosion resistance of hydrophobic coatings on 65Mn steel”, Coatings, v. 11, n. 11, pp. 1399, Nov. 2021. doi: https://doi.org/10.3390/coatings11111399.
  » https://doi.org/10.3390/coatings11111399
• ^[3] CHEN, L., CHEN, W., LI, D., et al, “Effect of nano-La2O3 doping on the tribological behavior of laser cladded WC-12Co coating on 65Mn steel under water lubrication condition”, Tribology International, v. 169, pp. 107428, May 2022. doi: https://doi.org/10.1016/j.triboint.2022.107428.
  » https://doi.org/10.1016/j.triboint.2022.107428
• ^[4] YAN, Z., HUANG, J., LEI, S., et al, “Investigation of in-situ synthesized WC-Ni coating with TiC as heterogeneous nucleation site: first-principles calculation and experimental research”, Applied Surface Science, v. 679, pp. 161159, Jan. 2025. doi: https://doi.org/10.1016/j.apsusc.2024.161159.
  » https://doi.org/10.1016/j.apsusc.2024.161159
• ^[5] XIA, J., FENG, Y., LING, Z., et al, “Influence of WC particle content of Fe-based powder-cored wire on microstructure and properties of plasma cladding reinforced layers”, Journal of Alloys and Compounds, v. 922, pp. 166148, Nov. 2022. doi: https://doi.org/10.1016/j.jallcom.2022.166148.
  » https://doi.org/10.1016/j.jallcom.2022.166148
• ^[6] SETH, S., JONES, A.H., LEWIS, O.D., “Wear resistance performance of thermally sprayed Al3Ti alloy measured by three body micro-scale abrasive wear test”, Wear, v. 302, n. 1, pp. 972–980, Apr. 2013. doi: https://doi.org/10.1016/j.wear.2013.01.075.
  » https://doi.org/10.1016/j.wear.2013.01.075
• ^[7] LIU, M., CAI, Y., DUAN, C., et al, “Key techniques in parts repair and remanufacturing based on laser cladding: a review”, Journal of Manufacturing Processes, v. 132, pp. 994–1014, Dec. 2024. doi: https://doi.org/10.1016/j.jmapro.2024.11.039.
  » https://doi.org/10.1016/j.jmapro.2024.11.039
• ^[8] YU, J., ZHOU, H., ZHANG, L., et al, “Microstructure and properties of modified layer on the 65Mn steel surface by pulse detonation-plasma technology”, Journal of Materials Engineering and Performance, v. 31, n. 2, pp. 1562–1572, Feb. 2022. doi: https://doi.org/10.1007/s11665-021-06258-2.
  » https://doi.org/10.1007/s11665-021-06258-2
• ^[9] ZHU, L., XUE, P., LAN, Q., et al, “Recent research and development status of laser cladding: a review”, Optics & Laser Technology, v. 138, pp. 106915, Jun. 2021. doi: https://doi.org/10.1016/j.optlastec.2021.106915.
  » https://doi.org/10.1016/j.optlastec.2021.106915
• ^[10] SHANG, X., LIU, Q., GUO, Y., et al, “Nano-TiC reinforced [Cr–Fe4Co4Ni4] Cr3 high-entropy-alloy composite coating fabricated by laser cladding”, Journal of Materials Research and Technology, v. 21, pp. 2076–2088, Nov. 2022. doi: https://doi.org/10.1016/j.jmrt.2022.10.048.
  » https://doi.org/10.1016/j.jmrt.2022.10.048
• ^[11] QIU, Y., WANG, Y., SHU, L., et al, “Microstructure and properties of Ni-ZrB₂ gradient composite coating on the 27SiMn steel surface by laser cladding”, Materials Today Communications, v. 43, pp. 111657, Feb. 2025.
• ^[12] WANG, Z., FU, B., CHEN, T., et al, “The microstructure and mechanical properties analysis of H13/Ni60 functionally gradient coating fabricated by laser cladding method”, Optics & Laser Technology, v. 190, pp. 113204, Nov. 2025. doi: https://doi.org/10.1016/j.optlastec.2025.113204.
  » https://doi.org/10.1016/j.optlastec.2025.113204
• ^[13] XUE, K.N., LU, H.F., LUO, K.Y., et al, “Effects of Ni25 transitional layer on microstructural evolution and wear property of laser clad composite coating on H13 tool steel”, Surface and Coatings Technology, v. 402, pp. 126488, Nov. 2021. doi: https://doi.org/10.1016/j.surfcoat.2020.126488.
  » https://doi.org/10.1016/j.surfcoat.2020.126488
• ^[14] WU, X., FENG, S., ZHANG, W., “Microstructure, phase composition and wear properties of iron-based gradient coatings by laser cladding on 65Mn steel”, Surface and Coatings Technology, v. 477, pp. 130290, Feb. 2024. doi: https://doi.org/10.1016/j.surfcoat.2023.130290.
  » https://doi.org/10.1016/j.surfcoat.2023.130290
• ^[15] WU, X., FENG, S., ZHANG, W., “Microstructure, phase composition and wear properties of iron-based gradient coatings by laser cladding on 65Mn steel”, Surface and Coatings Technology, v. 423–426, pp. 885–889, Sep. 2013.
• ^[16] MENG, L., ZHU, B., LIU, X., et al, “Investigation on the Ni60-WC composite coatings deposited by extreme-high-speed laser-induction hybrid cladding technology: Forming characteristics, microstructure and wear behaviors”, Surface and Coatings Technology, v. 473, pp. 130033, Nov. 2023. doi: https://doi.org/10.1016/j.surfcoat.2023.130033.
  » https://doi.org/10.1016/j.surfcoat.2023.130033
• ^[17] HU, D., LIU, Y., CHEN, H., et al, “Microstructure and wear resistance of Ni-based tungsten carbide coating by laser cladding on tunnel boring machine cutter ring”, Surface and Coatings Technology, v. 404, pp. 126432, Dec. 2020. doi: https://doi.org/10.1016/j.surfcoat.2020.126432.
  » https://doi.org/10.1016/j.surfcoat.2020.126432
• ^[18] SHU, D., LI, Z., ZHANG, K., et al, “In situ synthesized high volume fraction WC reinforced Ni-based coating by laser cladding”, Materials Letters, v. 195, pp. 178–181, May. 2017. doi: https://doi.org/10.1016/j.matlet.2017.02.076.
  » https://doi.org/10.1016/j.matlet.2017.02.076
• ^[19] WANG, Q., LI, Q., ZHANG, L., et al, “Microstructure and properties of Ni-WC gradient composite coating prepared by laser cladding”, Ceramics International, v. 48, n. 6, pp. 7905–7917, Mar. 2022. doi: https://doi.org/10.1016/j.ceramint.2021.11.338.
  » https://doi.org/10.1016/j.ceramint.2021.11.338
• ^[20] SONG, B., YU, T., JIANG, X., et al, “Development mechanism and solidification morphology of molten pool generated by laser cladding”, International Journal of Thermal Sciences, v. 159, pp. 106579, Jan. 2021. doi: https://doi.org/10.1016/j.ijthermalsci.2020.106579.
  » https://doi.org/10.1016/j.ijthermalsci.2020.106579
• ^[21] LUO, X., LI, J., LI, G.J., “Effect of NiCrBSi content on microstructural evolution, cracking susceptibility and wear behaviors of laser cladding WC/Ni–NiCrBSi composite coatings”, Journal of Alloys and Compounds, v. 626, pp. 102–111, Mar. 2015. doi: https://doi.org/10.1016/j.jallcom.2014.11.161.
  » https://doi.org/10.1016/j.jallcom.2014.11.161
• ^[22] YUTAO, L., HANGUANG, F., TIEJUN, M., et al, “Microstructure and wear resistance of AlCoCrFeNi-WC/TiC composite coating by laser cladding”, Materials Characterization, v. 194, pp. 112479, Dec. 2022. doi: https://doi.org/10.1016/j.matchar.2022.112479.
  » https://doi.org/10.1016/j.matchar.2022.112479
• ^[23] FAN, Z., REN, W., ZUO, W., et al, “Comparative study on the reinforcement effects of WC and TiC in the laser cladding layer of Ti-6Al-4V alloy”, Journal of Manufacturing Processes, v. 134, pp. 589–602, Jan. 2025. doi: https://doi.org/10.1016/j.jmapro.2024.12.076.
  » https://doi.org/10.1016/j.jmapro.2024.12.076
• ^[24] POLOCZEK, T., LONT, A., JACEK, G., “Structure and properties of laser-cladded Inconel 625-based in situ composite coatings on S355JR substrate modified with Ti and C powders”, Materials Science Poland, v. 40, n. 4, pp. 14–27, Jan. 2023. doi: https://doi.org/10.2478/msp-2022-0039.
  » https://doi.org/10.2478/msp-2022-0039
• ^[25] SONG, M.S., HUANG, B., ZHANG, M.X., et al, “Study of formation behavior of TiC ceramic obtained by self-propagating high-temperature synthesis from Al–Ti–C elemental powders”, International Journal of Refractory & Hard Metals, v. 27, n. 3, pp. 584–589, May. 2009. doi: https://doi.org/10.1016/j.ijrmhm.2008.09.009.
  » https://doi.org/10.1016/j.ijrmhm.2008.09.009
• ^[26] LEMBOUB, S., BOUDEBANE, S., GOTOR, F.J., et al., “Core-rim structure formation in TiC-Ni based cermets fabricated by a combined thermal explosion/hot-pressing process”, International Journal of Refractory & Hard Metals, v. 70, pp. 84–92, Jan. 2018. doi: https://doi.org/10.1016/j.ijrmhm.2017.09.014.
  » https://doi.org/10.1016/j.ijrmhm.2017.09.014
• ^[27] AZADMEHR, A., TAHERI-NASSAJ, E., “An in situ (W,Ti)C–Ni composite fabricated by SHS method”, Journal of Non-Crystalline Solids, v. 354, n. 27, pp. 3225–3234, Jun. 2008. doi: https://doi.org/10.1016/j.jnoncrysol.2008.01.002.
  » https://doi.org/10.1016/j.jnoncrysol.2008.01.002
• ^[28] POLOCZEK, T., LONT, A., GÓRKA, J., “The structure and properties of Laser-Cladded Inconel 625/TiC composite coatings”, Materials (Basel), v. 16, n. 3, pp. 1265, Feb. 2023. doi: https://doi.org/10.3390/ma16031265. PubMed PMID: 36770270.
  » https://doi.org/10.3390/ma16031265
• ^[29] GARCÍA, A., FERNÁNDEZ, M.R., CUETOS, J.M., et al, “Study of the sliding wear and friction behavior of WC + NiCrBSi laser cladding coatings as a function of actual concentration of WC reinforcement particles in ball-on-disk test”, Tribology Letters, v. 63, n. 3, pp. 41, Aug. 2016. doi: https://doi.org/10.1007/s11249-016-0734-3.
  » https://doi.org/10.1007/s11249-016-0734-3
• ^[30] KATSICH, C., BADISCH, E., “Effect of carbide degradation in a Ni-based hardfacing under abrasive and combined impact/abrasive conditions”, Surface and Coatings Technology, v. 206, n. 6, pp. 1062–1068, Dec. 2011. doi: https://doi.org/10.1016/j.surfcoat.2011.07.064.
  » https://doi.org/10.1016/j.surfcoat.2011.07.064
• ^[31] JING, P., WANG, H., CHEN, W., et al, “Effect of Ti addition on microstructure and tribological properties of laser cladding Ni35/WC coating in an oxygen-free environment”, Surface and Coatings Technology, v. 440, pp. 128480, Jun. 2022. doi: https://doi.org/10.1016/j.surfcoat.2022.128480.
  » https://doi.org/10.1016/j.surfcoat.2022.128480
• ^[32] SRIVASTAVA, A.K., DAS, K., TOOR, S.K., et al, “Corrosion Behavior of TiC and (Ti,W)C-Reinforced Fe-17Mn and Fe-17Mn-3Al austenitic steel matrix in situ composites”, Metallography, Microstructure, and Analysis, v. 4, n. 5, pp. 371–380, Oct. 2015. doi: https://doi.org/10.1007/s13632-015-0213-5.
  » https://doi.org/10.1007/s13632-015-0213-5
• ^[33] SRIVASTAVA, A.K., DAS, K., “Microstructural and mechanical characterization of in situ TiC and (Ti,W)C-reinforced high manganese austenitic steel matrix composites”, Materials Science and Engineering A, v. 516, n. 1, pp. 1–6, Aug. 2009. doi: https://doi.org/10.1016/j.msea.2009.04.041.
  » https://doi.org/10.1016/j.msea.2009.04.041
• ^[34] LI, S., HUANG, K., ZHANG, Z., et al, “Wear mechanisms and micro-evaluation of WC + TiC particle-reinforced Ni-based composite coatings fabricated by laser cladding”, Materials Characterization, v. 197, pp. 112699, Mar. 2023. doi: https://doi.org/10.1016/j.matchar.2023.112699.
  » https://doi.org/10.1016/j.matchar.2023.112699
• ^[35] GÓRKA, J., LONT, A., POLOCZEK, T., “The microstructure and properties of laser-cladded Ni-Based Self-fluxing alloy coatings reinforced by TiC particles”, Coatings, v. 15, n. 5, pp. 527, Oct. Apr. 2025.
• ^[36] ZHANG, Y., LIU, Z., YANG, C., et al, “Microstructure and wear behavior of laser-cladded WC matrix composite coatings with different TiC/Ni contents on titanium alloys”, Ceramics International, v. 51, n. 19, pp. 27229–27242, Aug. 2025. doi: https://doi.org/10.1016/j.ceramint.2025.03.398.
  » https://doi.org/10.1016/j.ceramint.2025.03.398

Publication Dates

• Publication in this collection
  05 June 2026
• Date of issue
  2026

History

• Received
  07 Jan 2026
• Accepted
  06 May 2026