Effects of Y₂O₃ Content on the Microstructure and Tribological Properties of WC-Reinforced Ti-Based Coatings on TC4 Surfaces

Abstract

To extend the safety service life of aviation TC4 alloy, the composite coatings of TC4 + Ni-MoS₂ + WC + xY₂O₃ (x = 0, 1, 2, 3, 4 wt.%) were prepared on TC4 by coaxial powder feeding laser cladding technology. The results showed that all the coatings had the same generated phases which mainly consisted of TiC, Ti₂Ni, Ti₂S, matrix β-Ti, and unfused residual WC. Y₂O₃ formed co-dependent growth relationships with TiC, Ti₂S, and Ti₂Ni. Meanwhile, TiC-Ti₂S, TiC-Ti₂Ni, and Ti₂S-Ti₂Ni coherent composite structure phases were effectively synthesized in all the coatings. With the increase in the Y₂O₃ content, the exposed area of the matrix increased and other phases refined progressively. When the Y₂O₃ content in the coatings were 3 and 4 wt.%, the degree of phase refinement in the coatings was consistent and the phases grew along grain boundaries, but microstructure segregated in the 4 wt.% Y₂O₃ coating. The microhardness of all the coatings was higher than that of TC4 and decreased with the increase in the Y₂O₃ content. Higher friction coefficients and lower wear rates both appeared in all the coatings than in the substrate, and they presented a trend of decreased first and then increased with the addition of Y₂O₃, in which the 3 wt.% Y₂O₃ coating had the lowest friction coefficient and optimal wear resistance. The research found that the Y₂O₃ could not change the types of phases in the coatings and could serve as a heterogeneous nucleation center for the refinement of the TiC-Ti₂S-Ti₂Ni coherent structure phase. Meanwhile, except for the matrix phase, Y₂O₃ could attract other phases to pinning on the grain boundaries of the coatings. The content of Y₂O₃ was negatively correlated with the hardness and wear resistance of the coating and it had the optimal tribological properties with the moderate amount of Y₂O₃. The wear mechanism of all coatings was abrasive wear.

1. Introduction

TC4 (Ti-6Al-4V), characterized by its low density, superior corrosion resistance, and high specific strength, is a crucial raw material for manufacturing key components in civil aircraft, such as engine blades, flaps, and fuselage fasteners, which makes it play a critical role in enhancing the transient response capabilities of civil aircraft in flight, garnering widespread attention from aerospace manufacturing companies [1,2]. However, TC4 has poor wear resistance, making it difficult to meet the safety and service requirements under severe friction conditions, which significantly limits its promotion and application range [3,4,5,6].

The coatings prepared by laser cladding technology has dense tissue, stable properties, and form metallurgical bonds with substrate; they are widely used in key areas such as aerospace, petrochemical, and other fields. Li et al. [7] indicated that TC4/ SnAgCu/Al₂O₃ self-lubricating laser-cladded layers on aero titanium alloy could meet the safe service conditions under harsh working conditions. Gu et al. [8] manufactured CrMnFeCoNi high entropy alloy coatings on oil storage and transportation steel by laser cladding technology, which could improve the bearing and anti-corrosion capacity of the transportation pipeline during the long-term service process.

Rare earth oxides (Y₂O₃, CeO₂, La₂O₃, and so on) played an important role in improving the forming quality, enhancing the mechanical properties, and addressing heterogeneous defects for the laser-cladded layers, which received considerable attention from researchers [9,10,11,12]. Zhang et al. [13] studied the effect of Y₂O₃ addition on the TC4 + Ni45 + Co-WC-cladded layers on TC4, which demonstrated that a large number of submicron ceramic phases were uniformly precipitated in the appropriate amount of Y₂O₃ coating. Zhang et al. [14] proved that CeO₂ effectively improved the forming quality, suppressed crack initiation, and enhanced the wear resistance of the cladded layers. Zhao et al. [15] found that the La₂O₃ addition could refine the microstructure and reduce the wear rates of WC-Ni60 (Ni-coated WC) coatings on AISI 304 steel. Shao et al. [16] prepared Cu10Al/MoS₂/CeO₂ coatings on Q235 steel, finding that a moderate addition of CeO₂ could effectively improve the uniformity of tissue distribution and reduce the fluctuation of coating properties. The aforementioned studies indicated that the addition of earth oxides could effectively refine the coating microstructure and enhance mechanical properties, but the refining mechanisms of coatings needed to be further clarified.

Therefore, the material system of TC4 + 35 wt.% Ni-MoS₂ + 5 wt.% WC + xY₂O₃ (x = 0, 1, 2, 3, 4 wt.%) was designed in this study, and the coatings were prepared on TC4 by coaxial powder-feeding laser cladding technology. Characterization and testing techniques such as X-ray diffraction (XRD, Nalytical, Almelo, The Netherlands), Scanning Electron Microscope (SEM, Hitachi, Tokyo, Japan), Energy Dispersive Spectrometer (EDS, Oxford Instruments, Oxford, UK), Transmission Electron Microscope (TEM, Field Electron and Ion Company, OR, USA), microhardness testing, and friction and wear testing machines were employed to investigate the effects of Y₂O₃ addition on the microstructure and tribological properties of the coatings. The two-dimensional lattice misfit theory was used to reveal the behaviors of Y₂O₃ in refining the in situ phase of the coating. It laid a theoretical and experimental foundation for the further development and application of TC4 alloy.

2. Experimental Materials and Procedure

2.1. Substrate Material

The chemical composition of the TC4 substrate for the experiments is shown in

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

Element	Al	V	Fe	O	N	C	Ti
Content	5.95	4.11	0.23	0.16	0.07	0.14	Bal.

. The substrate was cut into pending blocks with dimensions of 100 mm × 40 mm × 10 mm, which underwent ultrasonic cleaning and surface sandblasting and then placed in a vacuum drying oven.

2.2. Cladding Material

The WC possessing high hardness is a common cladding powder, which can effectively enhance the wear resistance of the coatings [17]. MoS₂ is a frequently used self-coating lubricant and the S element provided by its dissolution can form a variety of Ti-S compounds in the Ti-based molten pool, which can improve the comprehensive mechanical properties of cladded layers [18]. Therefore, this study took WC + Ni-MoS₂ (Ni-coated MoS₂) powders as the coating functional material and designed the cladding material systems with various Y₂O₃ content, which are shown in

Table 2. Cladding material systems (wt.%).

Sequence Number Material	TC4	WC	Ni-MoS2	Y2O3
1	60	5	35	0
2	59	5	35	1
3	58	5	35	2
4	57	5	35	3
5	56	5	35	4

.

The morphologies of the TC4, WC, Ni-MoS₂, and Y₂O₃ powders are depicted in Figure 1, with particle sizes ranging from 60 to 160 µm, 80 to 200 µm, 10 to 30 µm, and 10 to 25 µm, respectively. The EDS analysis result for the TC4 powders is presented in

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

Element	Al	V	N	Fe	O	C	Ti
Content	5.94	4.36	0.13	0.08	0.077	0.22	Bal.

. The mass ratio of Ni to MoS₂ in Ni-MoS₂ was 3:1 and the parcel rate of Ni to MoS₂ was 100%. Both the WC and Y₂O₃ powders had a purity of over 99%.

2.3. Experimental Procedure

The Trumpf 4002 disk laser equipment (Figure 2) was adopted in preparing the coatings, in which the He gas was used to feed powder and the Ar gas was used to prevent the oxidation of the molten pool in the cladding process.

The optimized cladding process parameters were detailed in

Table 4. Process parameters of laser cladding.

Laser Cladding Process Parameters	Numerical Value
Laser power/(W)	900
Laser scanning speed/(mm/min)	400
Spot diameter/(mm)	3.0
Powder feed rate/(g/min)	3
Powder gas flow rate (He)/(L/min)	7.0
Protective gas flow rate (Ar)/(L/min)	11
Laser focal length/(mm)	16
Multi-track overlap rate/(%)	50

through literature research [19,20,21,22] and exploratory experiments.

Figure 3 is the schematic diagram of the test sample preparation, in which Figure 3a shows the 10 mm × 10 mm × 11 mm square sample that was cut out in the central area of coating by wire cutting machine, and Figure 3b marks the detection surfaces of the samples, which was designated for the XRD, SEM, EDS, microhardness test, and tribological characterization.

Table 5. Experimental test equipment and purpose.

No.	Experiment Purpose	Device Name	Device Model and Parameters
1	Preliminary judgment of phases	XRD	X’Pert-Pro MPD (Nalytical, Almelo, The Netherlands) Scanning range: 20°–80° Scanning speed: 6°/min
2	Microstructure analysis Wear topography analysis	SEM	Hitachi S-3000N (Hitachi, Tokyo, Japan)
3	Elemental analysis of phases	EDS	Oxford INCAPentaFET-x3 (Oxford Instruments, Oxford, UK)
4	Conformed phases	TEM	FEI-Tecnai G2 F30 (Field Electron and Ion Company, Hillsboro, OR, USA)
5	Microhardness analysis	Microhardness tester	KB 30SR-FA (KB Pruftechnik, Munich, Germany) Applied load: 5 N Test time: 12 s
6	Evaluation of tribological properties	Tribology Tester (Dry)	RTEC MFT-5000 (Rtec-instruments, San Jose, CA, USA) Normal load: 5 N Test time: 60 min

lists the purpose, device name, model, and parameters of experimental characterizations.

3. Microstructure Analysis of Coatings

3.1. XRD Analysis of Coatings

The XRD test results of the coatings with varying Y₂O₃ additions are shown in Figure 4, indicating that all of the coatings had the same precipitated phases which contained TiC, Ti₂Ni, Ti₂S, the matrix phase β-Ti, and WC. As also can be seen from Figure 4, the intensity of TiC, Ti₂Ni, Ti₂S, and WC diffraction peaks gradually decreased, and that of β-Ti increased with the Y₂O₃ content increase, which is attributed to the increase in the dilution rate of the coatings caused by the Y₂O₃ addition, leading to a decrease in the mass fraction of TiC, Ti₂Ni, Ti₂S, and WC, except for the matrix phase. Furthermore, WC was preserved in the molten pool because of its high melting point [17] and MoS₂ disappearing in the spectrum indicated that it had completely dissolved.

3.2. SEM and EDS Analysis of Coatings

Figure 5 shows the cross-sectional macroscopic morphology of the coatings with varying Y₂O₃ contents. It was evident that the other coatings exhibit significant defects except for the 3 wt.% Y₂O₃ coating. The measurements showed that the average thicknesses of the coatings with 0, 1, 2, 3, and 4 wt.% Y₂O₃ were 0.968, 1.165, 1.192, 1.231, and 1.447 mm, respectively, indicating that the thickness of the cladded layers increased with the addition of Y₂O₃, which could be explained that the Y₂O₃ promoted the laser absorption rate of the molten pool, resulting in enhancing the dilution rate of the coatings [23,24]. Furthermore, white spherical particles were found in all the coatings, and their composition needed to be further detected.

Figure 6 shows the SEM morphology of the white spherical particle (Figure 6a) in the 0 wt.% Y₂O₃ coating and that in its neighboring area (Figure 6b). The results of the EDS analysis indicated that the white spherical particle was unmelted WC powder, which was consistent with XRD results. It can be seen from Figure 6c that the coating included strip-shaped phase A, irregular block-shaped phase B, spherical-like phase C, and matrix phase D, and their results of EDS analysis are detailed in

Table 6. EDS results of phases in the coatings (at.%).

Phase	Ti	Ni	C	Al	V	S	Mo	Y	O
A (Ti2S)	54.18	6.38	9.84	1.97	3.24	24.39	–	–	–
B (Ti2Ni)	59.72	27.04	2.09	8.57	2.58	–	–	–	–
C (TiC)	42.61	10.64	36.06	7.04	3.65	–	–	–	–
D (β-Ti)	62.21	6.97	13.45	9.16	3.89	0.37	3.95	–	–
E (Y2O3)	22.66	0.65	4.54	–	–	7.71	–	39.41	25.03

. It could be concluded that phase A was Ti₂S, phase B was identified as the metal intermetallic compound Ti₂Ni, phase C represents the reinforcing phase TiC, and phase D was characterized as β-Ti combined with the EDS and XRD analysis results. Furthermore, Table 6 manifests that the Mo element completely exists in the matrix β-Ti, which has the effect of solid solution strengthening on the coatings.

Figure 7 shows the progressively magnified microstructures of the 0 to 4 wt.% Y₂O₃ coatings in areas distant from the WC particles, which show bright white micron-sized spherical phases E pinned on the grain boundaries of 3 to 4 wt.% Y₂O₃ coatings (Figure 7(d3,e3)), and the EDS analysis (Table 6) confirm that they are Y₂O₃. Additionally, only a minimal amount of TiC was observed in the coatings (Figure 7(a3)), indicating that the regions distant from the WC particles were carbon-depleted zones.

The particle size of Ti₂S and Ti₂Ni in the 1 wt.% Y₂O₃ coating was significantly reduced compared to that in the 0 wt.% Y₂O₃ coating and the exposed area of the matrix increased combined with Figure 7(b3). In the 2 wt.% Y₂O₃ coating, the in situ phases excepted for the matrix were further refined and experienced a phenomenon of clustering, which had been unable to distinguish the shape properties (such as strip-shaped or irregular block-shaped and so on) of each phase. When the content of Y₂O₃ reached 3 and 4 wt.% in the coatings, the morphological characteristics of all the synthetic phases disappeared completely and the network structures of the strip phases were formed on the grain boundary of the coatings, which noted that the reinforce phases of the coatings were gradually refined with the increase of Y₂O₃ content.

The analysis of the above experimental principles was as follows: The microstructure of the 1 to 4 wt.% Y₂O₃ coatings was refined because Y₂O₃ could raise the supercooling of the molten pool and act as the heterogeneous nucleation center of synthetic phases [25,26]. Moreover, the refined phases in the 3 and 4 wt.% Y₂O₃ coatings grew along the grain boundaries for the reason that Y₂O₃ pinning on grain boundaries easily attracted them to triggering the above behavior [27,28,29,30].

It was worth noting that although the degree of microstructure refinement was similar in the 3 and 4 wt.% Y₂O₃ coatings, local segregation occurred in the 4 wt.%Y₂O₃ coating (Figure 7(e2)), which would make the cladded layer cracked (Figure 5e), suggesting that the Y₂O₃ content needed to be carefully controlled in the coating material system.

3.3. TEM Analysis of Coatings

In order to confirm the precipitated phases of the coatings from the perspective of crystal structure, the TEM detection was used to analyze the central microregion in the 3 wt.% Y₂O₃ coating, as shown in Figure 8. It could be observed that the coating was mainly composed of strip-like phase A, irregular block-like phase B, petaloid phase C, matrix phase D, and spherical phase E combined in Figure 8(a1–e1), which is similar to the SEM detecting results. Figure 8(a2–e2) presents the analysis and calculation results of the selected area electron diffraction pattern of all the phases in the coating, which determined that phase A was Ti₂S (pdf # 72-0013, space group Pnnm), phase B was Ti₂Ni (pdf # 72-0442, space group Fd-3ms), phase C was TiC (pdf # 71-0298, space group Fm-3m), D was β-Ti (pdf # 89-4913, space group Im-3m), and phase E was Y₂O₃ (pdf # 72-0927, space group La-3), which were consistent with the XRD and EDS detecting results.

3.4. Heterogeneous Nucleation Rule Analysis of Y₂O₃ and Reinforcements

In order to further clarify the dependent growth rules between Ti₂S, Ti₂Ni, TiC, and Y₂O₃, their nucleation relationships were calculated by Bramfitt’s two-dimensional lattice mismatch theory of which the applied formula is shown in Equation (1) [31].

$${\mathsf{\delta }}_{{\left(\mathrm{hkl}\right)}_{\mathrm{n}}}^{{\left(\mathrm{hkl}\right)}_{\mathrm{s}}}={\displaystyle \frac{1}{3}}{\sum }_{\mathrm{i}=1}^3\left[{\displaystyle \frac{\left|{\mathrm{d}}_{{\left[\mathrm{uvw}\right]}_{\mathrm{s}}^{\mathrm{i}}}\cos \mathsf{\theta }-{\mathrm{d}}_{{\left[\mathrm{uvw}\right]}_{\mathrm{n}}^{\mathrm{i}}}\right|}{{\mathrm{d}}_{{\left[\mathrm{uvw}\right]}_{\mathrm{n}}^{\mathrm{i}}}}}\right]\times 100\%$$

(1)

Equation (1) is explained in

Table 7. Variable definitions of Bramfitt’s two-dimensional lattice misfit.

Variable	Definition
(hkl)s	Nucleated basal low exponential crystal plane
(hkl)n	Nucleation phase low exponential crystal plane
${\left[\mathrm{uvw}\right]}_{\mathrm{s}}^{\mathrm{i}}$	Low exponential orientation of (hkl)s
${\left[\mathrm{uvw}\right]}_{\mathrm{n}}^{\mathrm{i}}$	Low exponential orientation of (hkl)n
${\mathrm{d}}_{{\left[\mathrm{uvw}\right]}_{\mathrm{s}}^{\mathrm{i}}}$	$\mathrm{Atomic}\mathrm{spacing}\mathrm{along}\mathrm{the}{\left[\mathrm{uvw}\right]}_{\mathrm{s}}^{\mathrm{i}}$ orientation
${\mathrm{d}}_{{\left[\mathrm{uvw}\right]}_{\mathrm{n}}^{\mathrm{i}}}$	$\mathrm{Atomic}\mathrm{spacing}\mathrm{along}\mathrm{the}{\left[\mathrm{uvw}\right]}_{\mathrm{n}}^{\mathrm{i}}$ orientation
θ	$\mathrm{The}\mathrm{angle}\mathrm{between}\mathrm{the}\mathrm{orientation}\mathrm{between}{\left[\mathrm{uvw}\right]}_{\mathrm{s}}^{\mathrm{i}}$$\mathrm{and}{\left[\mathrm{uvw}\right]}_{\mathrm{n}}^{\mathrm{i}}$

.

The lattice parameters of Ti₂S, Ti₂Ni, TiC, and Y₂O₃ are listed in

Table 8. Lattice constants of Ti₂S, Ti₂Ni, TiC, and Y₂O₃.

Crystal	Space Group	Crystal System	Lattice Constant/Å
			a	b	c
Ti2S	Pnnm	Orthorhombic	11.350	14.060	3.320
Ti2Ni	Fd-3ms	Cubic	11.278	11.278	11.278
TiC	Fm-3m	Cubic	4.328	4.328	4.328
Y2O3	La-3	Cubic	10.607	10.607	10.607

. The calculation results of the two-dimensional lattice mismatch and the low-index crystal plane mismatch relationships are presented in

Table 9. Bramfitt’s two-dimensional matching results.

Corresponding Planes	(001)Y2O3//(100)Ti2Ni			(100)Y2O3//(010)Ti2S			(001)Y2O3//(100)TiC
${\left[\mathrm{uvw}\right]}_{{\mathrm{Y}}_2{\mathrm{O}}_3}^{\mathrm{i}}$	[100]	[010]	[011]	[100]	[101]	[001]	[100]	[101]	[001]
${\left[\mathrm{uvw}\right]}_{\mathrm{n}}^{\mathrm{i}}$	[$00\overline{1}$]	[$01\overline{1}$]	[010]	[$2\overline{1}0$]	[310]	[010]	[100]	[101]	[001]
θ (°)	0	3.472	0	0	11.000	18.000	0	0	0
${\mathrm{d}}_{{\left[\mathrm{uvw}\right]}_{{\mathrm{Y}}_2{\mathrm{O}}_3}^{\mathrm{i}}}$ (Å)	10.607	8.000	10.607	5.303	7.500	5.303	10.606	10.606	10.606
${\mathrm{d}}_{{\left[\mathrm{uvw}\right]}_{\mathrm{n}}^{\mathrm{i}}}$ (Å)	11.319	8.004	11.319	5.291	6.925	5.277	10.820	10.820	10.820
δ (%)	4.273			3.671			1.974

and Figure 9, respectively.

Through the calculation results shown in Table 9, it was found that coherent interface relationships were formed between Y₂O₃ and TiC, Ti₂Ni, and Ti₂S, meaning that they could be refined by Y₂O₃ [32]. However, the stripy reinforcements of network structure growth in the 3 and 4wt.% Y₂O₃ coatings might be related to the adherent growth relationship between Ti₂Ni, Ti₂S, and TiC.

3.5. Dependent Growth Relationship Analysis between Reinforcements

The nucleating basal phases were determined according to the melting point of TiC, Ti₂S, and Ti₂Ni, and their mismatch relationships were judged by Equation (1), indicating TiC, Ti₂S, and Ti₂Ni formed a coherent growth structure (

Table 10. The two-dimensional matching results.

Corresponding Planes	(001)TiC//(100)Ti2S			(001)TiC//(110)Ti2Ni			(001)TiS//(100)Ti2Ni
${\left[\mathrm{uvw}\right]}_{\mathrm{S}}^{\mathrm{i}}$	[$100$]	[$010$]	[$310$]	[$100$]	[$010$]	[$110$]	[$2\stackrel{\mathrm{-}}{1}0$]	[$310$]	[$010$]
${\left[\mathrm{uvw}\right]}_{\mathrm{n}}^{\mathrm{i}}$	[$100$]	[$010$]	[$101$]	[$101$]	[$010$]	[$011$]	[$100$]	[$101$]	[$001$]
θ (°)	0	1.114	11.000	0	0	1.762	0	11.000	18.000
${\mathrm{d}}_{{\left[\mathrm{uvw}\right]}_{\mathrm{s}}^{\mathrm{i}}}$ (Å)	4.328	10.82	6.925	4.328	8.656	9.180	5.291	6.925	5.277
${\mathrm{d}}_{{\left[\mathrm{uvw}\right]}_{\mathrm{n}}^{\mathrm{i}}}$ (Å)	4.260	10.034	11.929	4.279	8.004	8.763	4.834	6.837	4.834
δ (%)	3.991			4.665			4.597

), which also explains their continuous growth behaviors in the 3 and 4 wt.% Y₂O₃ coatings.

4. Mechanical Properties Analysis of Coatings

4.1. Microhardness Analysis of Coatings

Figure 10 presents the microhardness test results of TC4 and the coatings with varying Y₂O₃ additions, revealing that the fluctuation amplitude of microhardness is the smallest in the 3% Y₂O₃ coating (Figure 10a), which might be related to its optimal uniformity of microstructure distribution. Figure 10b shows that the average microhardness of the coatings containing Y₂O₃ is much higher than that of TC4, and exhibits a negative correlation with the Y₂O₃ content for the reason that the addition of Y₂O₃ increased the dilution rate of the coating, which gradually raised the exposed area of the lower hardness β-Ti matrix.

4.2. Friction-Reducing Properties Analysis of Coatings

The time-dependent curves of the friction coefficient between TC4 and the coatings are shown in Figure 11, of which the 3 wt.% Y₂O₃ coating exhibited relatively stability, suggesting that friction resistance and stress concentration were comparatively small during the grinding process because the microstructure refinement degree and distribution uniformity of the coatings were optimal. The average friction coefficients of the TC4 and coatings are illustrated in Figure 12, showing that they were higher than TC4, which manifest that they did not possess self-lubricating properties in the coatings.

4.3. Wear-Resistant Properties Analysis of Coatings

The calculation formula of the coating wear rate is shown in Equation (2) of which the variables are explained in

Table 11. Interpretations of wear rate variables.

Variable	Unit	Definition
W	mm3·N−1·m−1	Wear rate
V	mm3	Ware volume
F	N	Normal load
S	m	Total sliding distance

. The calculation results (Figure 13) indicated that the wear rates of all the coatings were lower than that of TC4, and presented a trend of initially decreasing and then increasing with the addition of Y₂O₃, in which the 3 wt.% Y₂O₃ coating had the lowest wear rate. The wear resistance of the 4 wt.% coating was less than that of the 3 wt.% Y₂O₃ coating due to the segregation of the tissue, suggesting that the Y₂O₃ should not be added excessively.

$$\mathrm{W}={\displaystyle \frac{\mathrm{V}}{\mathrm{F}·\mathrm{S}}}$$

(2)

4.4. Worn Mechanism Analysis of Coatings

The SEM morphology of worn surfaces for TC4 and the coatings with varying Y₂O₃ additions are presented in Figure 14, in which Figure 14a displays noticeable flake peeling, plastic deformation, and localized cracking, manifesting that the ground substrate presented typical characteristics of adhesive wear mechanisms. The furrow characteristic was remarkable and the plastic tearing and deformation phenomena had completely disappeared on the worn surfaces of the 0 to 4 wt.% Y₂O₃ coatings, indicating they belong to abrasive wear [33,34].

Notably, the quantity and size of the wear debris appeared a transformation law of reducing first and then rising on the 0 to 4 wt.% Y₂O₃ coatings in which that on the 3 wt.% Y₂O₃ coating was few (Figure 14e), illuminating that the quality of the worn surface exhibits variation rules which are consistent with the principles of wear rate and microhardness, which demonstrated that the appropriate Y₂O₃ content in the cladded layer could enhance the comprehensive tribological properties of the coating.

5. Application and Conclusions

From the application level, TC4 + Ni-MoS₂ + WC + Y₂O₃ composite coatings on the TC4 surface by laser cladding could enhance the surface mechanical properties of critical components including turbine disks, low-pressure compressor blades, and fan blades of aircraft engines, which will provide the theoretical and experimental basis for the aviation maintenance and manufacturing industry. The conclusions of this study are as follows:

• These laser-cladded layers all contained Ti₂S, Ti₂Ni, TiC, β-Ti matrix, and residual unmelted WC powder. With the increase in Y₂O₃ content, the diffraction peaks of the other phases in the coating gradually decrease except the matrix. The 3 wt.% Y₂O₃ coating had the best distribution uniformity of microstructure.
• The calculation results of the two-dimensional lattice mismatch theory indicated that TiC, Ti₂S, and Ti₂Ni could be refined by Y₂O₃ through heterogeneous nucleation and the TiC-Ti₂S-Ti₂Ni composite structural phases formed coherent structures. All the refined reinforcements formed a network structure on the grain boundary of the coatings because Y₂O₃ was pinned on the grain boundary.
• The coating microhardness decreases and the dilution rate increases with the addition of Y₂O₃. The friction coefficients of all the coatings were higher than that of the TC4, indicating that the coatings did not possess self-lubricating properties. All the coatings exhibited higher wear resistance than the TC4, with abrasive particle wear mechanisms observed. The coating had the optimal tribological property under the condition of proper Y₂O₃ addition (3 wt.%).