Objective To increase the safe service life of aviation TC4 alloy. Methods TC4+Ni-MoS2+WC+xY2O3 (x=0%, 1%, 2%, 3%, 4%, mass fraction) wear-resistant composite coatings were prepared on the surface by coaxial powder feeding laser cladding technology. The effects of Y2O3 addition on the microstructure and tribological properties of the coatings were analyzed by various detection methods and equipment such as color penetrant flaw detection, XRD, SEM, EDS, EBSD and TEM. Results The forming quality of 3%Y2O3 coating was the best. The generated phases of all coatings were the same, mainly composed of TiC, Ti2Ni, Ti2S, matrix β-Ti and unmelted WC. Y2O3 formed a coherent dependent growth relationship with TiC, Ti2S and Ti2Ni. TiC-Ti2S, TiC-Ti2Ni and Ti2S-Ti2Ni coherent composite structural phases were effectively synthesized in all coatings. As the Y2O3 content increases, the exposed area of ​​the matrix increases, and the remaining phases gradually refine, agglomerate and grow continuously along the grain boundaries. The microhardness of all coatings is higher than that of the TC4 substrate, and the size is negatively correlated with the Y2O3 content. The friction coefficient of all coatings is higher than that of the substrate, and the wear rate is lower than that of the substrate. The friction coefficient and wear rate show a change pattern of first decreasing and then increasing with the increase of Y2O3 content. Among them, the friction coefficient of the 3% Y2O3 coating is the lowest (μ=0.481 3), and the wear resistance (3.55×104 mm3/(N·m)) and wear surface quality (Ra=21.24 μm) are the best. Conclusion The appropriate amount of Y2O3 helps to improve the forming quality of the coating. The addition of Y2O3 does not affect the type of coating phases, and can serve as a heterogeneous nucleation substrate to refine the coherent TiC-Ti2S-Ti2Ni phase that grows interdependently. In addition to the matrix phase, Y2O3 easily attracts the remaining phases to pin at the grain boundaries of the coating; the increase in Y2O3 content cannot increase the microhardness of the coating. All coatings do not have friction reduction properties, but the wear resistance is better than that of the TC4 substrate. The addition of an appropriate amount of Y2O3 can optimize the comprehensive tribological properties of the coating.

TC4 alloy has low density, good corrosion resistance and high specific strength. It is one of the important raw materials for manufacturing core parts such as civil aircraft engine blades, flaps and fuselage fasteners [1-4]. It plays a key role in improving the transient response capability of civil aircraft in the air and has received widespread attention from aviation manufacturing companies. Laser cladding technology has been widely used in the surface modification of TC4 alloy, effectively improving its shortcomings such as low hardness and poor wear resistance [5-9], meeting the safety service requirements under various harsh friction pair working conditions, and effectively increasing the use scenarios and application scope.

Among the many research results, the rare earth modified laser cladding layer has a uniform and refined structure and stable performance improvement, which has become one of the hot topics for domestic and foreign experts to explore [10-13]. Zhang Tiangang et al. [14] studied the effect of Y2O3 addition on the TC4+Ni45+Co-WC cladding layer on the TC4 surface. The study showed that the addition of Y2O3 can increase the fluidity and nucleation rate of the molten pool and enhance the tribological properties of the coating. Wang Ran et al. [15] prepared Al2O3+ZrO2 ceramic coatings with different CeO2 contents on the TC4 surface by laser cladding technology. The study pointed out that an appropriate amount of CeO2 can effectively improve the coating quality, inhibit the initiation of coating cracks, refine the grains and improve the wear resistance of the coating. Debta et al. [16] believed that Y2O3 can refine the microstructure of the coating, improve the fracture toughness and ductility of the coating, and reduce the wear rate of the coating. Li et al. [17] pointed out that Y2O3 can refine the microstructure of the CoCrFeNiTiNb high entropy alloy coating, increase the uniformity of the coating microstructure distribution, and play a role in fine grain strengthening and dispersion strengthening of the coating. The above literature pointed out that the addition of rare earth oxides can effectively refine the coating microstructure and improve the mechanical properties, but no in-depth research on the coating refinement mechanism was carried out. In addition, WC ceramic powder is often used to improve the mechanical properties of the cladding layer due to its high hardness and stable physical and chemical properties [18]; Ni-MoS2 (nickel-coated molybdenum disulfide) is a common solid lubricant. After melting, it can provide Ni and S elements to the Ti-based molten pool and form a variety of Ti-Ni and Ti-S compounds, which can effectively improve the tribological properties of the coating [19].

In summary, under the premise of taking into account the compatibility and performance of the coating, after preliminary exploratory experiments, TC4+35%Ni-MoS2+ 5%WC+xY2O3 (x=0%, 1%, 2%, 3%, 4%, mass fraction) cladding layers with 5 kinds of Y2O3 contents were prepared on the surface of TC4 by coaxial powder feeding laser cladding technology, which can be used in aerospace and intelligent equipment manufacturing fields under harsh working environment and severe friction pair conditions [20-21]. Non-destructive testing, XRD, SEM, EDS, EBSD, TEM, microhardness tester and friction and wear testing machine were used to explore the influence of Y2O3 addition on the coating structure and tribological properties. The two-dimensional lattice mismatch theory was used to reveal the refinement mechanism of Y2O3 on the in-situ phase of the coating, laying a theoretical and experimental foundation for the further expansion and application of TC4 alloy.

1 Experiment

1.1 Base material
The composition of the TC4 substrate used in the experiment is shown in Table 1. It is cut into 100 mm×40 mm×10 mm test blocks to be clad. Before the experiment, the surface is sandblasted and ultrasonically cleaned, and placed in a vacuum drying oven for use.

1.2 Cladding material
The cladding material system ratio scheme is shown in Table 2. The morphology of TC4, WC, Ni-MoS2 and Y2O3 powders is shown in Figure 1. The particle sizes are between 60~160, 80~200, 10~30, and 10~25 µm, respectively. The EDS test results of TC4 and Ni-MoS2 alloy powders are shown in Tables 3 and 4.
The purity of WC and Y2O3 powders is higher than 99%.

1.3 Experimental plan
The laser cladding equipment selected was TRUMPF TRULASER Cell 7040 machining center, in which the laser model was Laser TruDisk4002. During the coating preparation process, helium was used to feed powder, argon was used to protect the molten pool, and the coaxial powder feeding cladding process was shown in Figure 2. After preliminary exploratory experiments and literature analysis and research [22-23], the optimized cladding process parameters are shown in Table 5. After the coating was prepared, a 10 mm × 10 mm square area was cut out in the middle, as shown in Figure 3a. Figure 3b shows the surface penetration inspection, XRD, SEM, EPMA, EBSD and friction and wear characterization surfaces of the cut test block. Before the test, grinding and polishing or etching were required according to the respective inspection standards and requirements. TEM characterization samples require the use of special equipment, and the main steps include cutting, punching (φ=3 mm), grinding and polishing (to 50 μm) and ion thinning (to 5 μm). Table 6 lists the names, models and purposes of the equipment (materials) characterized by the above tests.

2 Results and analysis

2.1 Surface morphology
Figure 4 shows the results of color penetration testing of coatings with different Y2O3 additions. It can be seen from the figure that the number of surface cracks of the coating without Y2O3 addition is the largest, and the distribution is in a disordered network structure; the crack defects of the 1% and 2%Y2O3 coatings gradually decrease, and the forming quality of the 3%Y2O3 coating is the best; when the mass fraction of Y2O3 reaches 4%, sporadic cracks appear again at the laser arc starting and arc ending ends of the coating. The above results show that as the Y2O3 content gradually increases, the number of cracks on the coating surface shows a trend of first decreasing and then increasing, and the appropriate amount of Y2O3 can effectively improve the forming quality of the coating.

2.2 Macroscopic morphology
Figure 5 shows the cross-sectional SEM morphology of coatings with different Y2O3 contents. As can be seen from the figure, 1% and 2% Y2O3 coatings have obvious cracks, 3% Y2O3 has no obvious defects, and 4% Y2O3 coating has no cracks but has holes, which is consistent with the change law of the above coating color penetrant flaw detection test results. After measurement, the average thickness of 0%~4% Y2O3 coatings is 0.975, 1.161, 1.189, 1.202, and 1.447 mm, respectively. By calculation [24], the dilution rates of 0%~4% Y2O3 coatings are 41.36%, 45.74%, 46.81%, 48.33%, and 55.98%, respectively, indicating that the thickness and dilution rate of the cladding layer gradually increase with the increase of Y2O3 addition [25-26].

2.3 Phase and synthesis mechanism analysis
Figure 6 shows the XRD analysis results of coatings with different Y2O3 additions. As can be seen from the figure, all coatings contain TiC, Ti2Ni, Ti2S, matrix phase β-Ti and WC. The addition of Y2O3 does not change the type of coating phase. WC is detected in the coating XRD spectrum, mainly because the high melting point and refractory WC powder is easy to produce residues in the molten pool [27-30]. No MoS2 diffraction peak was found in the analysis results, indicating that the material has been completely decomposed in the molten pool. It can also be found from the figure that with the increase of Y2O3 content, the diffraction peak intensity of TiC, Ti2Ni, Ti2S and WC gradually decreases, and the β-Ti diffraction peak gradually increases. The main reason is that the addition of Y2O3 increases the absorption rate of the molten pool to the laser energy, resulting in an increase in the dilution rate of the coating and an increase in the mass proportion of the matrix phase, which in turn causes a decrease in the mass fraction of TiC, Ti2Ni, Ti2S and WC.

During the laser cladding process, the molten pool formed after the cladding powder and the TC4 substrate are melted mainly contains elements such as Ti, C, Ni and S. Since the cooling process of the molten pool is in a non-equilibrium solidification state, the microscopic distribution of elements in the molten pool is uneven. Therefore, the possible synthesis reactions of the in-situ TiC, Ti2Ni and Ti2S phases are:
Ti+C=TiC (1)
Ti +S=TiS →Ti+TiS=Ti2 S (2)
2Ti+S=Ti2 S (3)
Ti+Ni=TiNi → Τi+ΤiΝi=Τi2Νi (4)
2Ti+Ni=Ti2 Ni (5)
However, the Gibbs free energy analysis results of the above reactions [33] (Figure 7) show that the reaction Gibbs free energy of Ti+TiS above 518 °C is ΔG>0 kJ/mol, so the Ti2S synthesis method in reaction (2) is not easy to occur in the molten pool. In addition, the Gibbs free energy of the Ti+TiNi=Ti2Ni reaction is close to 0 and higher than reaction equation (5). Therefore, the synthesis of the Ti2Ni and Ti2S in-situ phases in the coating is mainly based on reaction equations (3) and (5).

2.4 Microstructure of laser cladding layer
Figure 5 shows that white spherical particles appear in different Y2O3 coatings. EDS detection of these residual particles shows that they are residual WC powder. Figure 8a shows the microstructure morphology of the WC-containing area of ​​the 0%Y2O3 coating. Figures 8b and 8c are the enlarged images of Figure 8a. It can be seen from the figure that the coating is distributed with a large number of strip phases A, irregular block phases B and matrix phases D. In addition, a large number of spherical phases C are enriched around WC. EDS detection of the above phases is performed, and the results are shown in Table 7. Comprehensive XRD analysis shows that the strip phase A is Ti2S, the irregular block phase B is the intermetallic compound Ti2Ni, the spherical phase C is the reinforcement phase TiC, and the matrix phase D is β-Ti. In addition, Table 7 shows that the Mo element is mainly dissolved in the matrix phase β-Ti, which further shows that MoS2 has been completely dissolved.

Figures 9a, 9b, 9c, 9d, and 9e are respectively the microstructures of the middle area of ​​the 0%~4%Y2O3 coating without WC particles. It can be seen from the figure that a bright white micron spherical phase E appears at the grain boundary of the 3%~4%Y2O3 coating. EDS detection (Table 7) shows that it is Y2O3. In addition, only a very small amount of spherical TiC appears in the local area of ​​the coating, indicating that the coating area away from the WC particles is in the C-poor area. Combined with Figure 8, it can be seen that the C element only forms a C-rich area near the residual WC area. Combined with Figure 9, it can be seen that when the mass fraction of Y2O3 is 1%, the maximum diameter of Ti2S (about 0.39 μm) and the maximum area of ​​Ti2Ni (about 102.26 μm2) are reduced by about 40% and 30% respectively compared with the coating without Y2O3 (the maximum diameter of Ti2S is about 0.65 μm, and the maximum area of ​​Ti2Ni is about 146.02 μm2), and the exposed area of ​​the matrix increases; when the mass fraction of Y2O3 is 2%, except for the matrix phase, all the in-situ phase morphological characteristics of the coating have basically disappeared, and a local agglomeration trend has appeared in the coating; when the mass fraction of Y2O3 is 3%~4%, the exposed area of ​​the matrix phase is further increased, and the morphological characteristics of the remaining precipitated phases have completely disappeared, leaving only the remaining grain boundary structure wrapped around the matrix phase. The occurrence of the above experimental phenomena is mainly related to the special role of Y2O3 in the cooling process of the molten pool. Studies have shown that Y2O3 can first increase the supercooling of the molten pool, thereby increasing the nucleation rate of the molten pool and refining the precipitation phase of the coating; secondly, Y2O3 is easy to pin at the grain boundary of the coating [34-35]. During the solidification of the molten pool, Y2O3 can act as a heterogeneous nucleation center to attract the in-situ phase to nucleate at the grain boundary, forming a coating structure in which the precipitation phase grows continuously along the grain boundary (the structural characteristics disappear). It is worth noting that although the microstructure characteristics of the coating with a Y2O3 mass fraction of 3% and a Y2O3 mass fraction of 4% are similar, careful observation shows that (Figure 9e) the precipitation phase segregation phenomenon occurs in the local area of ​​the 4%Y2O3 coating, which may be the main reason for the re-initiation of cracks in the coating, and it also shows that the content of Y2O3 added in the coating material system needs to be reasonably controlled. 2.5 Calculation of two-dimensional mismatch theory
In order to verify whether Y2O3 can serve as a heterogeneous nucleation center in the molten pool to refine the coating structure, the two-dimensional lattice mismatch theory is used to calculate the coherent growth relationship of Y2O3 as the nucleation substrate and TiC, Ti2S and Ti2Ni as the nucleation phases. The two-dimensional lattice mismatch calculation method [36] is shown in formula (6). (Substrate Phase), n represents the nucleation phase; (hkl)s is a low-index crystal plane of the substrate phase; s [ ] uvwi is a low-index crystal direction of (hkl)s; s [ ] uvwi d is the atomic spacing along the s [ ] uvwi crystal direction; (hkl)n, n [ ]i uvw and n [ ] uvwi d are the low-index crystal plane, low-index crystal direction and atomic spacing along the crystal direction corresponding to the nucleation, respectively; θ is the angle between [uvw]s and [uvw]n. The unit cell model of Ti2S, Ti2Ni, TiC and Y2O3 is shown in Figure 10, the lattice parameters are shown in Table 8, and the two-dimensional lattice mismatch calculation results and low-index crystal plane mismatch relationship are shown in Table 9 and Figure 11, respectively.

It can be seen from Table 9 that the two-site lattice mismatch between the (001) crystal plane of Y2O3 and the (100) crystal plane of Ti2Ni, the (100) crystal plane of Y2O3 and the (001) crystal plane of Ti2S, and the (100) crystal plane of Y2O3 and the (100) crystal plane of TiC are 5.77%, 3.671% and 1.974% respectively.
According to the Bramfitt two-site lattice mismatch theory[37], Y2O3 forms a coherent dependent growth relationship with Ti2Ni, Ti2S and TiC, that is, Y2O3 can be used as a nucleation substrate to refine Ti2Ni, Ti2S and TiC.

There are two issues worth noting here. First, after TiC, Ti2Ni and Ti2S were refined by Y2O3, their preferred growth directions were significantly inhibited, the structural features gradually disappeared and large mask agglomeration was produced, as shown in Figure 9c; second, when the mass fraction of Y2O3 in the coating reached 3% and 4%, except for the matrix, the various precipitated phases after refinement grew continuously along the grain boundary structure around the β-Ti matrix. From the distribution and action characteristics of Y2O3 in the coating, pinning at the grain boundary is the main reason for attracting the growth of various phases along the grain boundary, but the large-area agglomeration and continuous growth of the in-situ phase may be related to the dependent growth relationship between Ti2Ni, Ti2S and TiC.

Therefore, the two-dimensional lattice mismatch theory is used again to determine whether a heterogeneous nucleation relationship can be formed between Ti2Ni, Ti2S and TiC. Before calculation, we must first clarify the nucleation base phase. According to the melting point relationship, TiC is the first to precipitate in the molten pool, so it is the nucleation base phase of Ti2Ni and Ti2S; Ti2S has a higher melting point than Ti2Ni and is the nucleation base phase of Ti2Ni. Based on the above analysis, Table 10 lists the low-index crystal planes, crystal phases and two-dimensional lattice mismatch calculation results selected in the calculation process. It can be seen from Table 10 that a coherent mismatch growth relationship is formed between TiC, Ti2S and Ti2Ni, which also reasonably explains their agglomeration and continuous growth behavior.

As can be seen from Table 10, the mismatch between TiC, Ti2S and Ti2Ni is less than 6, and the TiC-Ti2S, TiC-Ti2Ni and Ti2S-Ti2Ni coherent composite structure phases are effectively synthesized in the coating. During the molten pool circulation stirring process, the above three in-situ phases drag, hold hands and grow together, which also reasonably explains the experimental phenomenon of large-area agglomeration of refined precipitate phases.

2.6 Confirmation of laser cladding layer phase
Figure 12 shows the EBSD test results of the middle area of ​​0%Y2O3 coating (away from WC particles). As can be seen from the figure, the coating mainly includes yellow matrix phase β-Ti, green strip phase Ti2S, blue irregular block phase Ti2Ni and a very small amount of red granular phase TiC. The precipitated phase accounts for 39.7%, 15.7%, 44.3% and 0.3% in the coating respectively. The distribution pattern is consistent with the SEM results, which again shows that the C element provided by the partially molten WC has not formed an enriched state in the non-WC residual area.

In order to confirm the coating phase again, Figure 13 lists the transmission electron microscopy mapping of the C-rich area (near residual WC) of the 3%Y2O3 coating. It can be seen from Figure 13i that the micro-area mainly includes strip phase A, dark irregular block phase B, petal phase C, spherical phase E and matrix phase D filled around the above phases. According to the results of XRD, EDS and EBSD analysis, combined with Figures 13a and e, it can be concluded that the strip phase A is Ti2S, combined with Figures 13a and f, it can be known that the irregular block phase B is Ti2Ni, combined with Figures 13a and d, it can be known that the petal-shaped phase is TiC, combined with Figures 13g and h, it can be confirmed that
E is Y2O3, combined with Figures 13a~c, it can be known that the matrix phase D is β-Ti with Al and V elements dissolved. It should be emphasized that it can be seen from Figure 13i that TiC and Y2O3 form a composite structure phase with Y2O3 as the flower core and TiC as the petals, which matches the calculation results of the two-dimensional lattice mismatch.

In order to re-determine the coating phase from the perspective of compound crystallography, Figure 14 shows the selected area electron diffraction pattern calculation results of the above five precipitated phases. From the results in the figure, it can be determined that the strip phase A is Ti2S (pdf # 72-0013, space group Pnnm), the irregular block phase B is Ti2Ni (pdf # 72-0442, space group Fd-3ms), the petal phase C is TiC (pdf # 71-0298, space group Fm-3m), the matrix phase D is β-Ti (pdf # 89-4913, space group Im-3m), and the spherical phase E is Y2O3 (pdf # 72-0927, space group La-3). From the above
results, it can be seen that although the precipitated phase in the 3% Y2O3 coating is largely segregated at the grain boundary, it does not affect the phase composition and dependent growth relationship.

2.7 Microhardness of laser cladding layer
Figure 15 shows the microhardness test results of TC4 and coatings with different Y2O3 additions, where Figure 15a is the comparison result of microhardness change curves, and Figure 15b is the comparison result of average microhardness. It can be seen from Figure 15a that, except for the 3% Y2O3 coating, the microhardness values ​​of the other coatings fluctuate slightly, which may be related to the uneven distribution of precipitated phases in the coating or the occurrence of local agglomeration. As can be seen from Figure 15b, the average microhardness of the 0%~4%Y2O3 coating is 60.42%, 52.84%, 47.91%, 43.85% and 39.89% higher than that of the TC4 substrate, respectively, indicating that the microhardness value of the coating shows a negative correlation with the Y2O3 content. This is mainly due to the fact that after the Y2O3 content in the coating increases, the precipitate phase particle size decreases and gradually refines, but the coating dilution rate increases, and the exposure area of ​​the toughening phase β-Ti gradually increases.

2.8 Tribological properties of laser cladding layer
Figures 16 and 17 are the time-varying curves of the friction coefficient of the substrate and the coating with different addition amounts of Y2O3 and the comparative analysis results of the average friction coefficient. It can be seen from the figure that the friction coefficient of all coatings is higher than that of the substrate, indicating that the coating does not have a friction reduction function, that is, Ti2S with unknown tribological properties does not belong to the self-lubricating phase. In addition, by comparison, it is found that the friction coefficient jitter amplitude of the 3%Y2O3 coating is the most stable, which has the same law as the amplitude change of the microhardness value of the 3%Y2O3 coating, indicating that after the coating structure is fully refined, the friction resistance and stress concentration of the grinding ball during the sliding and rolling process on the coating surface are small.

The results of non-contact white light interferometry analysis of the wear section of the TC4 substrate and coating are shown in Figure 18, and the average wear rate is shown in Figure 19. It can be seen from Figure 18 that the wear depth and roughness of all coatings are lower than those of the TC4 substrate, and with the increase of Y2O3 content, they show a change law of first decreasing and then increasing. Among them, the wear depth (67.11 μm) and roughness (Ra=21.24 μm) of the 3%Y2O3 coating are the lowest; when Y2O3 reaches 4%, the coating roughness and wear depth are higher than those of the 3%Y2O3 coating. Figure 19 shows that the wear rate of the coating is lower than that of the TC4 substrate, and with the addition of Y2O3, the wear rate of the coating also shows a trend of first decreasing and then increasing. Among them, the wear rate of the 3%Y2O3 coating is the lowest. When the mass fraction of Y2O3 increases to 4%, the wear rate of the coating increases instead of decreasing. The overall change trend is the same as the wear depth and roughness, indicating that the Y2O3 content must be reasonably controlled. After excessive Y2O3 is added, the coating forming quality and distribution uniformity decrease. The possible reason is that there is too much Y2O3 in the molten pool and it fails to form a uniform distribution in the molten pool, resulting in Y2O3 agglomeration behavior and segregation of Y2O3 refined phase in local micro-areas, which leads to increased stress concentration and aggravated performance fluctuations during the friction and wear process of the coating. The tribological performance is lower than that of the 3% Y2O3 coating.

Figure 20 shows the SEM morphology of the worn surfaces of the substrate and the coatings with different Y2O3 additions. Comparative analysis shows that the worn surface of the TC4 substrate exhibits the typical characteristics of poor wear resistance, with obvious flaking, plastic deformation and local cracking, and the wear mechanism is mainly adhesive wear. It can be seen from Figure 20b~f that when the mass fraction of Y2O3 is 0%~4%, the plastic tearing and deformation phenomena on the wear surface disappear completely, the wear mechanism is abrasive wear, and the number and particle size of wear debris on the wear surface show a change pattern of first decreasing and then increasing; when the mass fraction of Y2O3 is 1% and 2%, parallel plow-shaped friction scratches gradually appear, and the number and particle size of wear debris are reduced compared with the coating without Y2O3; when the mass fraction of Y2O3 is 3%, the wear surface is the smoothest and flattest compared with other Y2O3 coatings, with the least number of residual wear debris and the smallest average particle size (the maximum particle size has dropped to about 5 μm), which is consistent with the change pattern of the non-contact white light interferometer detection results; when the mass fraction of Y2O3 is 4%, although the wear surface of the coating shows abrasive wear characteristics as a whole, a small amount of agglomerated wear debris appears locally.

In summary, if the problems of large particle size, agglomeration and segregation of the coating phase are not effectively solved during the friction and wear process, a large number of brittle reinforcing phases are easily broken and fragmented under the extrusion and collision of the grinding balls. After these unevenly distributed and hard particles of different sizes enter the wear track, they are crushed by the grinding balls and unevenly embedded in the wear surface, which not only increases the wear resistance, but also increases the stress concentration and shaking and bumping during the wear process. Although adding Y2O3 to the cladding material system can improve the above situation, the amount of addition needs to be scientifically and reasonably controlled.

3 Conclusions

1) TC4+Ni-MoS2+WC+xY2O (3 x=0%, 1%, 2%, 3%, 4%) titanium-based wear-resistant composite coatings were prepared on the surface of TC4 alloy by coaxial powder feeding laser cladding technology. All coatings contain Ti2S, Ti2Ni, TiC, β-Ti matrix and a small amount of residual unmelted WC powder.

2) With the increase of Y2O3 content, the coating forming quality shows a trend of first increasing and then decreasing, and the 3% Y2O3 coating has the best forming quality; the two-dimensional lattice mismatch theory calculation shows that TiC, Ti2S and Ti2Ni can be refined by Y2O3 heterogeneous nucleation, and the precipitation process can form a TiC-Ti2S-Ti2Ni composite structure phase; with the increase of Y2O3 content, except for the matrix phase, the remaining phases gradually refine, agglomerate and continuously segregate at the coating grain boundary with Y2O3.

3) With the increase of Y2O3 content, the coating dilution rate gradually increases and the microhardness gradually decreases; the friction coefficient of all coatings is higher than that of TC4 substrate, and the coating does not have self-lubricating properties; the wear resistance of all coatings is higher than that of TC4 substrate, and the wear mechanism is abrasive wear, among which the 3%Y2O3 coating has the lowest wear rate and the best wear surface quality.