Microstructure and Tribological Properties of WC/Ni-MoS₂ Titanium-Based Composite Coating on TC4

Abstract

To improve the mechanical properties of a TC4 surface, TC4 + Ni-MoS₂ + xWC (x = 5%, 10% and 15% wt.%) composite coatings were prepared by the coaxial feeding laser cladding technique, and the effect of the WC content on the microstructure and tribological properties of the coatings were investigated using multiple characterization methods. The results indicated that increasing the WC content negatively impacted the forming quality of the coating, but did not change the coating phase which predominantly comprised Ti₂Ni, Ti₂S, TiC, matrix β-Ti and residual WC. With the addition of WC, TiC exhibited an increase in both quantity and particle size, accompanied by a transition in growth morphology from spherical to petal-like. MoS₂ completely dissolved in all coatings and the S element provided by it effectively synthesized a strip-like phase Ti₂S which presented a morphology similar to the lubricating phase TiS in the Ti-based melt pool system. The microhardness and wear-resistance of all the coatings were higher than that of TC4 and gradually improved with the addition of WC, which indicated that raising the WC content was conducive to enhancing the mechanical properties of the coatings. The friction coefficient of TC4 was lower than that of the three WC content coatings, indicating that Ti₂S was not the lubricating phase. The wear mechanism of all coatings was abrasive wear.

1. Introduction

The TC4 alloy has been widely utilized in the aerospace industry due to its low density, high specific strength and excellent corrosion resistance, making it a crucial material for aircraft lightweight design and performance enhancement [1,2,3,4,5]. However, TC4 had a low hardness and poor wear resistance, which seriously restricts its service and application under harsh working conditions [6,7,8,9,10]. Laser cladding is an advanced alloy surface modification technology with multiple advantages such as excellent compatibility, strong bonding and low dilution rate, which has been considered an effective approach to enhancing the comprehensive performance of a TC4 surface and prolonging its safe service life, receiving widespread attention from researchers [11,12,13,14,15].

WC ceramic powders—characterized by high hardness, elevated melting point, and stable chemical properties—were commonly used in the design and preparation of wear-resistant, laser-clad layers, which could improve the mechanical properties of coatings [16,17]. Qi et al. [18] prepared TC4 + WC wear-resistance coatings on TC4 through laser cladding, which indicated that a significant increase in both the average microhardness and wear resistance of the coatings compared to the substrate. Li et al. [19] investigated the effect of the WC content on the properties of WC-Ni60 coatings, which revealed that moderate WC content could effectively improve the microhardness and wear resistance of the coating. However, an excessive WC content resulted in a deterioration in the performance of the coating. Ortiz et al. [20] fabricated Ni base metal matrix coatings reinforced by spherical-shaped WC particles, which showed a better wear performance during the dry sliding contacts.

MoS₂ exhibited a low melting point and possessed self-lubricating properties, making it widely used in the preparation of coatings on metal alloys [21,22]. Qu et al. [23] fabricated a new self-lubricating TiS₂ laser-clad layer on TC4 using NiCrBSi + MoS₂ cladding materials, which formed a dense and complete layer of TiS₂ on the coating surface that significantly reduced the friction coefficient of the coating during sliding. Zhang et al. [24] employed Ni-MoS₂ particles to prepare a Ti₂S/Ti₂Ni reinforced Ti-based composite coating on TC4 through laser cladding, and found that the microhardness and wear resistance of the coating greatly improved compared with that of the TC4. Zheng et al. [25] prepared Ni60/WC-Cu/MoS₂ composite coatings using the laser cladding technique, which revealed that the undissolved MoS₂ could effectively reduce the friction coefficient of the coatings.

In summary, this study designed and prepared TC4 + Ni-MoS₂ + WC Ti-based composite coatings with varying WC contents (5 wt.%, 10 wt.%, and 15 wt.%) on TC4 alloy by utilizing coaxial powder feeding laser cladding technology. The macrostructure, phase composition, microstructure, microhardness, and tribological properties of the coatings were analyzed using X-ray diffractometry (XRD), Scanning electron microscopy (SEM), an Energy dispersive spectrometer (EDS), Transmission electron microscopy (TEM), Vickers hardness tester and friction and wear testing equipment. The goal was to provide technical guidance and theoretical insights into the surface modification of the TC4 alloy.

2. Experimental Procedures

2.1. Materials

TC4 with dimensions of 100 mm × 50 mm × 10 mm was used for the experimental substrates, and its chemical compositions are shown in

Table 1. Chemical composition of the TC4 substrate (wt.%).

Elements	Al	V	Fe	C	N	O	Ti
Content	6.04	4.01	0.15	0.014	0.009	0.165	Bal

. Before the laser cladding, the specimens were cleaned in absolute alcohol using an ultrasonic cleaner for 15 min, and the working surfaces were sandblasted to remove dirt and oxide layers.

A design scheme for the optimized cladding material system is shown in

Table 2. Design scheme for the optimized cladding materials system.

No.	Coating	TC4/wt.%	WC/wt.%	Ni-MoS2/wt.%
1	5 wt.% WC	60	5	35
2	10 wt.% WC	55	10	35
3	15 wt.% WC	50	15	35

and the SEM morphology and particle size distributions of TC4, WC and Ni-MoS₂ powders (the mass ratio of Ni to MoS₂ is 3:1 and the encapsulation rate of Ni-coated MoS₂ is 100%) is shown in Figure 1.

The chemical compositions of TC4 and WC powders are shown in

Table 3. Chemical composition of the TC4 powder (wt.%).

Elements	Al	V	Fe	C	N	O	Ti
Content	6.36	4.06	0.05	0.12	0.011	0.077	Bal

and

Table 4. Chemical composition of the WC powder (wt.%).

Elements	WC	Mo	Fe	Ca	K	Mg	Al
Content	≥99.8	0.010	0.020	0.0020	0.0015	0.0020	0.020

. The three powders with different proportions (Table 1) were stirred for 8 h in a mechanical vibration mixer and placed in a vacuum chamber at 80 °C for use.

2.2. Laser Cladding Experiments

The German TruDisk4002 coaxial powder feeding laser (TrumPF Group, Ditzingen, Germany, Figure 2) was selected in the experiments, in which helium gas (He) was used to feed powders and argon gas (Ar) prevented oxidation of the molten pool.

The optimized manufacturing plan for the laser cladding was based on an analysis of the literature and other experiments [24,26], and is presented in

Table 5. The optimized manufacturing plan.

Parameters	Value
Laser power (W)	700
Scan speed (mm/min)	360
Spot diameter (mm)	3
Overlap ratio (%)	50
Powder feed rate (g/min)	3
Powder gas flow rate (He) (L/min)	5
Shield gas flow rate (Ar) (L/min)	18

.

The test samples, measuring 10 mm × 10 mm × 11 mm, were cut from the middle area of the coatings using the wire EDM machine. Their detecting surfaces were ground and polished, then etched for 12 s in a corrosion solution (HF:HNO₃:H₂O = 1:1:20) before the SEM and EDS detections of the microstructure.

2.3. The Methods of Microstructure Characterizations and Properties Tests

The purposes and equipment of experimental characterizations are listed in

Table 6. The purposes and equipment of experimental characterizations.

No.	Device (Material) Name	Device Model	Experimental Purpose	Remark
1	XRD	X’Pert-Pro MPD (Nalytica, Almelo, The Netherlands)	Initial determination of phases	Scanning range: 30~90°, Scanning speed: 6°/min
2	SEM	Hitachi S-3000N (Hitachi, Tokyo, Japan)	Macro-Microstructure analysis
			Wear topography analysis
3	EDS	Oxford INCAPentaFET-x3 (Oxford Instruments, Oxford, UK)	Element composition of phases
4	TEM	FEI-Tecnai G2 F30 (Field Electron and Ion Company, Hillsboro, OR, USA)	Phases confirmation
5	Microhardness tester	KB 30SR-FA (KB Pruftechnik, Munich, Germany)	Microhardness analysis	Applied load: 5 N, Test time: 12 s
6	Tribology Tester (Dry)	RTEC MFT-5000 (RTEC-instruments, San Jose, CA, USA)	Evaluation of tribological properties	Normal load: 5 N, Test time: 60 min Grinding ball: Al2O3 (1.5 mm)
7	Non-contact white light interferometer profiler	RTEC UP-Lambda (US)	Wear profile analysis

.

3. Results and Discussion

3.1. Macrostructure Morphology Analysis

Figure 3a–c show the cross-sectional low magnification SEM morphology of the coatings with different WC content. As can be seen from the figure, the subsurface of the 5 wt.% WC coating contained a small number of pores but no cracks were found. The 10 wt.% and 15 wt.% WC coatings both exhibited crack defects and their depth increased with the addition of WC. There were white spherical particles in all coatings and their quantities increased gradually with the rise in the WC content. Local enlargement SEM (Figure 3d) and EDS detection (Figure 3e) of the white particles showed that they were residual WC, which indicated that the WC powders had not completely melted and their morphology had gradually evolved from irregular lumps to spherical shapes during the process of cyclicly stirring the molten pool.

3.2. XRD Results Analysis

Figure 4 shows the XRD detection results of the 5 wt.% WC, 10 wt.% WC and 15 wt.% WC coatings, respectively, which indicates that the precipitated phases in all three coatings were identical, including β-Ti, TiC, Ti₂Ni and Ti₂S. In addition, the diffraction peaks of WC were detected, which proved that the residual WC particles which existed in the coatings were consistent with the SEM and EDS analysis. In terms of the types of precipitated phases, β-Ti belonging to the matrix phase was a typical product under the rapid cooling environment of the melt pool [27]; TiC and Ti₂Ni were coating-reinforcing phases, which contributed to the enhancement of the coating hardness and wear resistance [28]; and Ti₂S was a synthetic phase of the Ti-S system, but its frictional properties were unknown.

In the laser cladding process, the molten pool formed by the melting of cladding powders and TC4 substrate primarily contained elements such as Ti, C, Ni and S. Due to the non-equilibrium solidification state during the cooling process of the molten pool, the elemental distribution was uneven. Therefore, the possible in situ synthesis reactions of TiC, Ti₂Ni and Ti₂S phases were as follows [29,30,31,32]:

$$\mathit{Ti}+C=\mathit{TiC}$$

(1)

$$\mathit{Ti}+S=\mathit{TiS}\to \mathit{Ti}+\mathit{TiS}=S$$

(2)

$$2\mathit{Ti}+S={\mathit{Ti}}_{2}S$$

(3)

$$Ti+\mathit{Ni}=\mathit{TiNi}\to \mathit{Ti}+\mathit{TiNi}={\mathit{Ti}}_2\mathit{Ni}$$

(4)

$$2\mathit{Ti}+\mathit{Ni}={\mathit{Ti}}_2\mathit{Ni}$$

(5)

From the Gibbs free energy change (ΔG) in the above reactions, it can be seen that the ΔG of the Ti + TiS reaction was positive at temperatures above 791 K, which indicated that the synthesis of Ti₂S as described in reaction (2) did not easily occur in the molten pool. In addition, the ΔG of Ti + TiNi = Ti₂Ni was close to a zero value and higher than that in reaction (5). Therefore, the in situ synthesis of Ti₂S and Ti₂Ni in the coating was mainly based on reactions (3) and (5) (Figure 5).

3.3. Microstructure Analysis

Figure 6a1–c1 present the SEM results of the central region microstructures in the 5 wt.%, 10 wt.% and 15 wt.% WC coatings. Figure 6a2,a3,b2,b3,c2,c3 depict the corresponding local regions at progressively higher magnifications, showing that the coatings mainly consisted of spherical phases A1, petal-like phases A2, strip-like phases A3, irregular block-like phases A4 and matrix phases A5. It could also be observed that the strip-like phases in the 5 wt.% and 10 wt.% WC coatings grew in clusters, while their distribution uniformity in 15 wt.% WC coatings was improved. Furthermore, the number of the spheroid phases gradually decreased; the petal-like phases gradually increased; and the strip-like phases and the irregular block-like phases basically remained basically unchanged with the increase in the WC content.

Table 7. EDS results of each phase in the coatings.

Phase	Proportion	Ti	W	C	Al	V	Ni	S	Mo
A1 (TiC)	wt.%	63.23	—	36.77	—	—	—	—
	at.%	50.36	—	49.64	—	—	—	—
A2 (TiC)	wt.%	66.59	—	33.41	—	—	—	—
	at.%	52.66	—	47.34	—	—	—	—
A3 (Ti2S)	wt.%	58.66	2.50	2.42	2.68	3.29	7.90	21.63	2.18
	at.%	57.35	2.52	3.18	2.45	1.43	5.25	25.69	2.03
A4 (Ti2Ni)	wt.%	44.66	5.89	4.37	1.99	2.65	25.94	7.96	6.54
	at.%	40.36	3.35	12.65	4.21	2.03	19.35	8.59	9.46
A5 (β-Ti)	wt.%	74.24	6.61	1.54	4.11	3.03	3.07	1.35	6.05
	at.%	71.45	5.33	6.02	3.25	1.77	2.44	2.63	7.11

shows the EDS results of different phases in the coatings, in which the atomic ratio of Ti and C elements was basically similar (1:1) in spherical phases A1 and petal-like phases A2. The atomic ratio of Ti and S elements in the strip-like phases A3 was approaching 2:1. Ti and Ni elements existed in the irregular block-like phases A4 and their atomic ratio was also approximately 2:1. The Ti element mass fraction in the matrix phases A5 exceeded 75%. Based on the XRD detection results, it could be confirmed that A1 and A2 were TiC and A3, A4 and A5 were Ti₂S, Ti₂Ni and β-Ti, respectively. According to the above analysis, increase in the WC content led to an increase in the amount of C element in the molten pool, which caused the TiC synthesis quantity and particle size to be augmented.

3.4. TEM Results Analysis

In order to confirm the precipitated phases of the coatings from the perspective of crystal structure, TEM detection was used to analyze the central microregion in the 10 wt.% WC coating, as shown in Figure 6. In Figure 7a1–d1, it can be observed that the coating was mainly composed of spherical phase B1, strip-like phase B2, irregular block-like phase B3 and matrix phase B4, which were similar to the SEM detection results. Figure 7a2–d2 present the analysis and calculation results of the selected area’s electron diffraction pattern in the above precipitated phases, determining that the spherical phase B1 was TiC (PDF#71-0298, Fm-3m) with a face-centred cubic structure; the strip-like phase B2 was Ti₂S (PDF#72-0013, Pnnm) with a simple orthorhombic structure; the irregular block-like phase B3 was Ti₂Ni (PDF#72-0442, Fd-3ms) with a face-centered cubic structure; and the granular phase B4 was β-Ti (PDF#44-1288, Im-3m) with a body-centred cubic structure, consistent with the XRD and EDS detection results.

It is worth noting that the lubricating phases of TiS and TiS₂ existed in the Ti-based cladded layers as strip-like phases in the studies of other scholars [23,33] and their morphology was similar to that of Ti₂S. However, whether Ti₂S was the lubricating phase still needs to be verified by measuring the friction coefficient of the coatings [23,26].

3.5. Microhardness

Figure 8 shows the microhardness distributions and average values of the coatings, indicating that the average microhardness of the 5 wt.%, 10 wt.% and 15 wt.% WC coatings was 637.7 HV₀.₅, 699.6 HV₀.₅ and 746.1 HV₀.₅, respectively, which was about 1.92, 2.11 and 2.24 times higher than that of the substrate (the average hardness of TC4 in Figure 8b was about 332.1 HV₀.₅). It could also be observed that the microhardness of the coatings gradually improved with the rise in the WC content, which is mainly related to an increase in the content of TiC and WC particles, as well as the solid solution strengthening of the W element. Notably, the fluctuations in the coating’s microhardness increased significantly with the increase in the WC content, which may be due to the fact that the microhardness values were influenced by the large amount of distributed residual WC. The average microhardness of all coatings in this study was higher than that of TC4 + WC wear-resistance coatings [18], which provided the theoretical basis and experimental guidance for the TC4 surface coating preparation.

3.6. Tribological Properties

Figure 9 shows the friction coefficient detection results of the TC4 and coatings. As seen in the figure, the average friction coefficients of the three coatings were 0.4875, 0.5440 and 0.5807, respectively, which were higher than that of the substrate (0.4184). The friction coefficients of the TiS and TiS₂ self-lubricating coatings on TC4 were lower than that of the substrate according to literature research [23,33], which demonstrated that the TC4 + Ni-MoS₂ + xWC (x = 5, 10 and 15 wt.%) Ti-based material system designed in this study did not synthesize a self-lubricating phase in the coatings, illustrating that Ti₂S was not an in-situ lubricating phase. The coating friction coefficient increased gradually with the rise in the WC content, which was mainly because the grinding ball of Al₂O₃ was blocked by the increasing number of residual WC during the loading sliding process, resulting in elevated frictional drag. In addition, residual WC particles were crushed by the grinding ball, then spread across the coating surface because the microhardness of WC was lower than that of Al₂O₃, causing an increase in the friction coefficient fluctuations in the coatings [16,17,18].

Figure 10 and Figure 11 show the three-dimensional worn surface profiles and the wear rate calculation results of the 5 wt.%, 10 wt.% and 15 wt.% WC coatings, respectively. In Figure 10, the higher WC content correlated with increased roughness values in accordance with the roughness variation law of worn surface, which indicated that the quality of the wear surface decreased with the increase in WC content. As can be seen from Figure 11, the wear rates of the 5 wt.%, 10 wt.% and 15 wt.% WC coatings decreased by 37.78%, 56.9% and 72.97% compared to the TC4, respectively, explaining that raising the WC content can effectively improve the wear resistance of coatings [34].

Figure 12 shows the wear cross-section white light interferometric detection results for the TC4 and coatings with different WC additions, indicating that the maximum wear depths of TC4 with 5 wt.%, 10 wt.% and 15 wt.% WC coatings were 128.36 μm, 89.37 μm, 76.05 μm and 56.01 μm, respectively, which was consistent with the rule of change in the wear rate. Combined with the changing pattern of the wear profiles, the degree of wear curve protrusion and depression gradually intensified with the addition of WC; this was mainly due to a large amount of crushed WC debris being inlaid on the wear surface, which was the reason for the increase in the roughness and friction coefficients.

Figure 13 shows the wear surface SEM morphology of TC4 with 5 wt.%, 10 wt.% and 15 wt.% WC coatings. Combined with the EDS analysis of the worn surface (

Table 8. EDS results of worn surface.

Phase	Proportion	Ti	W	C	Al	V	Ni	S	Mo	O
C1	wt.%	78.69	—	—	11.58	5.55	—	—	—	4.18
	at.%	75.96	—	—	12.59	8.46	—	—	—	12.81
C2	wt.%	5.28	75.87	8.73	0.96	1.05	2.08	1.33	0.67	4.03
	at.%	7.07	37.86	40.63	1.07	1.33	2.28	0.88	0.72	8.16
C3	wt.%	15.34	46.08	4.47	1.52	1.44	14.01	8.14	2.32	6.58
	at.%	18.55	20.98	21.31	2.24	13.91	13.91	6.04	3.06	12.85
C4	wt.%	7.82	72.61	7.33	1.75	1.63	3.21	0.65	1.07	3.93
	at.%	10.01	35.23	37.07	2.36	2.01	4.05	0.32	1.29	7.66
C5	wt.%	18.18	42.16	5.33	0.93	0.91	15.98	9.04	3.32	4.15
	at.%	23.66	22.81	20.16	1.03	0.64	12.19	8.33	4.07	7.11
C6	wt.%	8.71	68.35	9.01	2.35	0.98	3.09	1.53	0.97	5.01
	at.%	9.03	33.65	39.11	2.22	1.62	3.17	1.03	0.66	9.51
C7	wt.%	14.33	45.19	5.23	3.04	0.98	18.34	7.11	1.96	3.82
	at.%	18.78	22.61	20.99	4.01	0.78	15.23	6.23	2.04	9.33

), the TC4 worn surface had a lower oxygen content and significant spalling and plastic deformation (Figure 13a), which indicated that its wear mechanism was mainly adhesive wear [4]. The EDS analysis results of the coatings indicate that the spherical particles were WC and the worn surface of the coatings was primarily composed of Ti, W, C, Al, V, Ni, S, Mo and O, which was consistent with the EDS results of the coating elements (Table 7), except O element. As seen in Figure 13, the quantity and diameter of the residual WC particles gradually increased with the addition of WC, which was consistent with the change rule of the cross-sectional morphology of the coatings (Figure 3). For the 5 wt.% WC coating (Figure 13b), the phenomenon of deformation and spalling completely disappeared and a large number of wear furrows appeared on the wear surface which showed typical characteristics of an abrasive wear mechanism [24,28]. With the crushing and breaking of more residual WC, the residual abrasive debris was significantly increased and the features of plough furrows were more obvious on the wear surfaces of the 10% and 15 wt.% WC coatings (Figure 13c,d), indicating that their wear mechanisms were all abrasive wear. It should be noted that the WC particles were high-hardness brittle ceramics, which not only increased the frictional resistance and friction coefficient of the coatings but also led to stress concentration during the service life of the coating. Therefore, when WC powder is added to the cladding material system to prepare a TC4 surface wear-resistant coating, its dosage should be reasonably controlled.

4. Conclusions

(1)

Ti-based composite coatings were fabricated on TC4 using TC4 + Ni-MoS₂ + xWC (x = 5, 10 and 15 wt.%) via laser cladding, in which the 5 wt.%-Cu coating showed the optimal forming quality. The phase composition of the coatings with different WC content remained unchanged, which consisted of TiC, Ti₂Ni, Ti₂S, β-Ti and residual WC.

(2)

The quantity and particle size of in-situ TiC increased as the WC content was raised. Ti₂S was a specific product of the S element in the Ti-rich melt pool environment and was characterized by a strip-like distribution in the coatings. Its growth morphology was also highly similar to lubricating phases TiS and TiS₂.

(3)

The average microhardness and wear-resistance of the coatings were higher than TC4 and they gradually improved with an increase in the WC content. The increasing residual WC particles caused continuous deterioration of the wear surface quality, resulting in higher friction coefficients and larger performance fluctuations of the coatings. The 5 wt.%-Cu coating exhibited the least worn surface and lower tribological property fluctuations. The three WC coatings did not have anti-friction properties and their wear mechanism was all abrasive wear.