The low hardness and poor wear resistance of titanium alloys seriously limit their service life as working parts under friction conditions. The use of laser cladding technology to prepare wear-resistant and self-lubricating coatings on the surface of titanium alloys can greatly improve the wear resistance of titanium alloys. First, the influence of laser cladding process (laser power, scanning speed, spot diameter, specific energy, post-heat treatment, high-frequency vibration, etc.) on the wear resistance of coatings is reviewed; secondly, the characteristics of different types of hard phases, matrix phases, and self-lubricating phases and their influence on the wear resistance of coatings are discussed; finally, the wear-resistant self-lubricating coatings prepared on the surface of titanium alloys by laser cladding technology are summarized and prospected.

Titanium alloys are widely used in aerospace, marine engineering, biomedicine and other fields due to their light weight, high specific strength, good corrosion resistance and biocompatibility. In these application fields, titanium alloys inevitably have friction and wear problems, and the poor wear resistance of titanium alloys will seriously Affects its reliability and service life as a working part.

Surface modification technology is the main method to improve the wear resistance of titanium alloy. Existing surface modification technologies mainly include physical vapor deposition, chemical vapor deposition, spraying, nitriding, carburizing, micro-arc oxidation, etc. However, these technologies usually have problems such as poor bonding between the coating and the substrate, thin coating thickness, and easy peeling of the coating under harsh friction and wear conditions. Compared with the above technologies, laser cladding technology has the advantages of dense coating structure and unlimited thickness, high bonding strength between the coating and the substrate, and not easy to peel off. It is widely used to improve the wear resistance of the titanium alloy surface.

The main method of using laser cladding technology to improve the wear resistance of titanium alloy surface is to prepare wear-resistant and self-lubricating coatings on the titanium alloy surface. In the process of preparing wear-resistant and self-lubricating coatings on the titanium alloy surface, by adjusting the cladding process parameters, the cladding powder is quickly melted under the action of laser high temperature. Therefore, the laser cladding process is an important factor in determining the wear resistance of the coating. In addition, the composition of the coating is also an important factor affecting the wear resistance of the coating. The wear-resistant coating is composed of a hard phase and a matrix phase, and the self-lubricating coating is composed of a hard phase, a matrix phase and a self-lubricating phase. The hard phase can improve the hardness of the coating and thus improve the wear resistance of the coating; the matrix phase can improve the toughness and wettability of the coating and thus improve the comprehensive performance of the coating; the self-lubricating phase can reduce the friction coefficient of the coating and thus improve the wear resistance of the coating. Therefore, this paper reviews the influence of the laser cladding process and the characteristics of the coating components (hard phase, matrix phase and self-lubricating phase) on the wear resistance of the coating.

1 Effect of laser cladding process on the wear resistance of the coating

The wear-resistant and self-lubricating coatings prepared by laser cladding technology have relatively different physical properties (elastic modulus, thermal expansion coefficient, melting point, etc.) from those of the substrate. There is a big difference between the laser cladding process and the laser cladding process, so the coating is prone to defects such as cracks and pores. The appropriate laser cladding process can reduce various defects in the coating and improve the wear resistance of the coating. The laser cladding process includes cladding process parameters and auxiliary processes. The cladding process parameters mainly include laser power, scanning speed, spot diameter, specific energy and other parameters.

1.1 Laser power

The laser power has a significant effect on the macroscopic morphology, defects, structure and hardness of the coating. Cui Aiyong et al. studied the effect of laser power on the macroscopic morphology of the coating (see Table 1). It can be seen from Table 1 that the dilution rate of the coating and the depth of the molten pool increase with the increase of laser power, while the macroscopic morphology of the coating is basically not affected by the laser power. Weng Fei studied the effect of laser power on coating defects and found that lower laser power prevents the gas in the molten pool from escaping in time to form pore defects; higher laser power High laser power makes the cladding material fully melted and reduces pore defects. Ma Yong studied the effect of laser power on the coating structure and hardness and found that high laser power makes the coating structure dense, evenly distributed and hardness improved. Under normal circumstances, the principle of selecting the laser power is to increase the laser power as much as possible while ensuring that the coating morphology is relatively flat and the coating dilution rate is less than 5%.

1.2 Scanning speed

The scanning speed will affect the melting state of the cladding powder, and then affect the wear resistance of the coating. When the scanning speed is low, the cladding powder can be fully melted; if the scanning speed is too low, the cladding powder will be overburned and the alloy elements in the powder will evaporate; if the scanning speed is too high, the cladding powder will not be completely melted. Li et al. studied the influence of scanning speed on the dilution rate and wear resistance of the coating prepared by Ti + TiBCN cladding powder. The results are shown in Figure 1. As the scanning speed increases, The dilution rate of the coating decreases, the microhardness increases first and then decreases, the friction coefficient, wear mass loss and wear volume decrease first and then increase, and the comprehensive performance of the coating is optimal when the scanning speed is 7 mm/s. Tan Jinhua et al. [23] studied the influence of scanning speed on the coating prepared by TC4+Ni60+h ￣ BN cladding powder. The results showed that the coating with a scanning speed of 10 mm/s has the best comprehensive performance. Therefore, in different cladding powder systems, the optimal scanning speed is different.

1.3 Spot diameter and specific energy

The spot diameter determines the coating molten pool width and the energy input per unit area of ​​the spot. A large spot diameter can increase the molten pool width, but reduce the energy input, while a small spot diameter reduces coating defects and makes the structure dense, but it will increase the laser cladding time, which is not conducive to laser cladding technology. Industrial application.

In order to study the combined effect of spot diameter D, scanning speed v and laser power P on the coating, researchers proposed the concept of specific energy E, which represents the amount of energy per unit area of ​​the coating irradiated by the laser (E = P / DV). Sui et al. studied the influence of specific energy on Ti3Al composite TiN + Ti3AlN coating. The results showed that an increase in specific energy will improve the comprehensive performance of the coating, but the coating dilution rate will also increase; a decrease in specific energy will lead to uneven distribution of coating tissue and increased defects; when the specific energy is 58.3 J/mm2, the coating has the least pores and cracks and the best wear resistance. However, Liu et al. studied the influence of specific energy on TiC + TiB2 coating. The results showed that when the specific energy is 45 J/mm2, the coating has the least pores and cracks and the best wear resistance. The wear resistance of the coating with a thickness of 2.0 mm2 is the best. In different cladding material systems, the type and powder size of the cladding materials are different, which makes the energy required for the coating to achieve the best performance different. Therefore, the specific energy can only be used as a reference in similar cladding material systems.

1.4 Auxiliary processes

The auxiliary processes of laser cladding include the introduction of rotating magnetic field, ultrasonic vibration and post-heat treatment. The introduction of rotating magnetic field can reduce the depth and width of the molten pool, while having little effect on the macroscopic morphology and wear resistance of the coating [28]. Appropriate ultrasonic vibration power can significantly reduce the grain size of the coating. Wang Wei et al. [29] found that 2.2 W ultrasonic vibration made the coating macroscopic morphology smoother, and the grain size was reduced by about 42% compared with the coating without ultrasonic vibration.

Post-heat treatment can reduce the residual stress of the coating and improve the fracture toughness of the coating [30-33]. However, different post-heat treatment processes have different effects on the wear resistance of the coating. Li et al. [31] heat-treated the coating prepared by laser cladding (mainly composed of WC, W2C, α-Ti, Ti2Ni and TiNi) at 500 °C for 1 h and 2 h respectively, and then cooled in air. The microhardness and wear resistance of the coating decreased slightly. Chen et al. [32] heat-treated the prepared titanium-based composite TiC+TiB coating at different temperatures (400 °C, 600 °C and 800 °C) for 3 h, and then cooled in air. As the heat treatment temperature increased, the hardness and wear resistance of the coating increased.

2 Effect of hard phase characteristics on the wear resistance of coatings
The wear-resistant coating prepared by laser cladding on the surface of titanium alloy is usually composed of hard phase and matrix phase. The wear resistance of the coating is mainly determined by the content, characteristics and formation method of the hard phase. The higher the proportion of the hard phase, the better the wear resistance of the coating, but too high a proportion of the hard phase will cause large-area cracks in the coating and even peeling. When the content of the hard phase is limited, the characteristics and formation method of the hard phase become the key factors determining the wear resistance of the coating [34-36]. There are two ways to form the hard phase: directly adding hard phase particles to the cladding powder and using laser high temperature to generate the hard phase in situ. This paper introduces the influence of different types of hard phases on the coating according to the different ways of hard phase formation.

2.1 Direct addition of hard phase

The method of directly adding hard phase is to directly add high melting point ceramic phase As cladding powder, a smaller laser power and a higher scanning speed are used during the laser cladding process to avoid a large amount of ceramic phase decomposition. The undecomposed ceramic phase after laser cladding is used as the hard phase of the coating to improve the wear resistance of the coating. Common high melting point ceramic phases include C-BN (cubic boron nitride) [21, 37], WC [35], etc. Samar et al. [35] selected WC + NiCrBSi powder for laser cladding. As shown in Figure 2, the microhardness of WC particles in the coating is as high as 3338 HV, which significantly improves the wear resistance of the coating. However, the edge of the WC particles is affected by the high temperature of the laser and decomposes to produce many small particles, which increases the cracking tendency of the coating.

Fu et al. [38] used the coating method to improve the direct addition of hard phase. The problem of easy decomposition and cracking under the action of laser high temperature. As shown in Figure 3, the uncoated c-BN particles decompose and crack under the action of laser high temperature. During the dry friction test, the cracks cause some c-BN particles to break and form abrasive wear, and narrow and deep wear marks appear on the coating. However, after the Ni-coated c-BN particles are clad with laser, the c-BN particles have almost no cracks, and the wear resistance of the coating is significantly improved[38].

2.2 In-situ generation of boride ceramic phase

The method of directly adding hard phase particles is prone to cracks. Although adding a coating layer to the hard phase particles can reduce the occurrence of cracks, there are problems such as a small number of coating materials and increased costs. The in-situ generation method does not have the above problems. The in-situ generation of hard phases uses the high temperature effect of laser to make the cladding powder react in situ in the molten state to generate hard phases. The hard phases generated in situ mainly include boride ceramic phases, carbide ceramic phases, oxide ceramic phases, etc.

Boride ceramics have high thermal conductivity and good high temperature stability. They are also highly hard and wear-resistant [36]. The boride ceramic phases in the wear-resistant coating prepared by laser cladding technology are mainly TiB2 and TiB ceramic phases [39, 40]. The reaction Gibbs free energy and reaction enthalpy of the ceramic phase are both negative and both are exothermic reactions. Therefore, TiB2 and TiB ceramic phases generally appear simultaneously in the coating. In addition, the Gibbs free energy of the reaction to generate TiB is lower. When the reaction is sufficient, the reaction to generate TiB is more likely to occur [41-44]. As shown in Figure 4, the morphology of the TiB phase tends to be hexagonal needle-shaped, and the morphology of the TiB2 phase tends to be hexagonal plate-shaped [41]. Liu Di et al. [45] prepared a wear-resistant coating with TiB and TiN as the main hard phases. Dry friction tests showed that TiB and TiN have a pinning strengthening effect and significantly inhibit the peeling of hard phase particles, thereby improving the wear resistance of the coating.

2.3 In-situ generation of carbide ceramic phase

The in-situ generated carbide ceramic phases are mainly (Ti, W) C1-x [46], TiCx [47], etc. In the formation process of cladding coating, when the molten pool contains titanium, carbon and tungsten elements, carbon element reacts with titanium element first to form TiCx. When carbon element is supersaturated, it will react with tungsten element to form WC. Then WC and TiCx react to form a single solid solution (Ti, W) C1-x. Therefore, the content of (Ti, W) C1-x in the coating is extremely low, and the effect on the wear resistance of the coating is small [46, 48].

TiCx ceramics have high hardness, high elastic modulus, and thermodynamic parameters and physical parameters are similar to those of titanium alloys. Therefore, it is a hard phase that is widely used in laser cladding to prepare wear-resistant coatings [46]. TiCx is a non-stoichiometric compound. Affected by the rapid melting and solidification characteristics of the laser cladding process, the morphology of TiCx varies. As shown in Figure 5, TiCx has dendrites, petals, spheres or irregular shapes. However, the influence of TiCx with different morphologies on the wear resistance of coatings has not been studied in depth [49]. The wear-resistant coating with TiCx as the hard phase prepared by Zhao et al. [50] has a maximum microhardness of 540 HV. The wear-resistant coating with TiB+TiC as the hard phase prepared by Ma Yong [20] has a maximum microhardness of 1 404.6 HV, and the wear loss is reduced by 66.67% compared with the substrate. TiCx ceramics When used as a hard phase in the coating, it is necessary to add other types of hard phases to significantly improve the wear resistance of the coating.

2.4 In-situ generation of oxide ceramic phase

Due to the large interface energy between oxide and liquid metal, most oxide ceramic phases have poor wettability in the coating. Therefore, there are few studies on the in-situ generation of oxide ceramics by laser cladding. Only some scholars have studied ZrO2 ceramics and Al2O3 ceramics [51-53]. In addition to high strength and high hardness, ZrO2 ceramics also have the function of eliminating residual stress [51, 54]. Luo Ya et al. [51] prepared TiNi+Ti2Ni composite ZrO2 coating on the surface of TA15 alloy. The microhardness of the coating reached up to 1070 HV, the wear rate is much lower than that of the substrate.

In addition, the auxiliary process of ultrasonic vibration can reduce the adverse effects caused by poor wettability of oxides. Wang et al. [52] added the auxiliary process of ultrasonic vibration during laser cladding and prepared a coating containing Al2O3 and W2(C, O) oxide ceramic phases. Ultrasonic vibration refines the grains of the coating, improves the wettability of the oxide hard phase Al2O3 and W2(C, O) in the coating, and the average microhardness of the coating reaches 1 029.4 HV, with excellent wear resistance.

3 Effect of matrix phase characteristics on coating wear resistance

In the wear-resistant coating prepared by laser cladding technology, the phase with the highest content is the matrix phase. The matrix phase can improve the toughness and wettability of the coating and avoid excessive cracks, pores and other defects in the coating. The matrix phase of the wear-resistant coating is mainly formed by titanium-based, nickel-based, cobalt-based, aluminum-based and their composite material systems. Therefore, according to the type of the coating matrix phase, the wear-resistant coating is divided into metal-based composite ceramic coating and intermetallic compound composite ceramic coating.

3.1 Metal matrix phase

The matrix phase of the metal-based composite ceramic coating is formed by a metal element with a very high content. Common metal matrix phases include titanium-based, nickel-based, cobalt-based, etc. Therefore, the metal-based composite ceramic coating can be divided into titanium-based, nickel-based, and cobalt-based composite ceramic coatings. The matrix phase of the titanium-based composite ceramic coating is similar to the physical properties of the substrate, so it can significantly reduce various defects of the coating and has good wettability [55-57]. The common titanium matrix phase is formed by titanium powder during laser cladding. Lin Peiling et al. [58] selected Ti+B powder to prepare titanium-based composite TiB The microhardness of the ceramic coating is relatively low (650~770 HV). The matrix phase of the titanium-based composite TiOx coating prepared by Zhao et al. [13, 59] and Lu et al. [60] is formed by TiO2 powder. As shown in Figure 6, the coating has a dense structure and uniform distribution. There is no crack at the interface between the substrate and the coating. The aluminum and vanadium elements in the substrate diffuse into the coating, indicating that the coating and the substrate have achieved good metallurgical bonding. The hard phase TiOx makes the average microhardness of the coating reach 1583 HV1N, and the wear rate of the coating is only 0.1 times that of the substrate.

The matrix phase of the nickel-based composite ceramic coating is formed by nickel-based self-fluxing alloy powder. The nickel-based self-fluxing alloy powder used for laser cladding mainly includes F101 Nickel-based alloy, Ni60, Ni45A, NiCrBSi and other powders [36, 61-64], their chemical element compositions are shown in Table 2. Nickel-based self-fluxing alloy powder contains elements such as boron and silicon, which have a deoxidizing effect during laser cladding and improve the wettability of the coating [36]. The matrix phase of the nickel-based composite ceramic coating is composed of γ-Ni, which can form a grid-like interdendritic eutectic structure with silicon, chromium and boride, thereby significantly improving the wear resistance of the coating [65, 66]. Samar et al. [35] selected WC + NiCrBSi powder to prepare a nickel-based composite WC + W2C coating, and the average microhardness reached 1 384 HV1N. However, there is also a small amount of intermetallic compound phase TiNi in the nickel-based composite ceramic coating. Adding an appropriate amount of rare earth elements can reduce the content of TiNi phase in the coating, increase the content of α-Ti phase, and reduce the cracking tendency of the coating interface [61, 62].

The matrix phase of the cobalt-based composite ceramic coating is formed by cobalt-based self-fluxing alloy powder. The price of cobalt-based self-fluxing alloy powder used for laser cladding is relatively high. It mainly includes Co42, Co-01 and other alloy powders. Its chemical composition is shown in Table 3 [40, 67]. The matrix phase of the cobalt-based composite ceramic coating is mainly γ-Ni/Co solid solution and a small amount of intermetallic compounds CoTi, CoTi2 and NiTi[68, 69]. γ-Ni/Co solid solution, CoTi, CoTi2 and NiTi are highly brittle, which can easily lead to cracks in the coating. At the same time, it increases the probability of cracking in the coating during dry friction and reduces the wear resistance of the coating[70-74]. Weng et al.[41, 68, 69] used the method of adding rare earth elements to solve the brittleness problem of the cobalt matrix phase. They selected Co42+B4C+SiC+Y2O3 powder, Co42+B4C+CeO2 powder and Co42+TiN powder to prepare wear-resistant coatings. The results showed that the three coatings were metallurgically bonded to the substrate. A small amount of intermetallic compounds in the coating would not cause the interface between the coating and the substrate to crack. The coating is free of cracks, and the coating grains are refined and the number of microcracks in the coating is significantly reduced by adding appropriate amounts of rare earth elements Y2O3 and CeO2. Therefore, the wear resistance of the coating containing rare earth elements is improved.

3.2 Intermetallic compound matrix phase

The matrix phase of the intermetallic compound composite ceramic coating is the intermetallic compound phase, which mainly includes Ti-Al-based, Ti-Ni-based, Co-Ni-based, and Ni-Al-based. Therefore, the intermetallic compound composite ceramic coating is divided into Ti-Al-based, Ti-Ni-based, and Co-Ni-based intermetallic compound composite ceramic coatings.

The matrix phase of the Ti-Al intermetallic compound composite ceramic coating is the Ti3Al intermetallic compound. The Ti3Al intermetallic compound has low density, The advantages of Ti-Al intermetallic compounds are high elastic modulus, high yield strength, good thermal conductivity and the formation of a dense oxide film at high temperature to improve oxidation resistance, but there are also disadvantages such as poor toughness, poor ductility at room temperature and sensitivity to microcracks [75-77]. The advantages of Ti-Al intermetallic compounds make the coating have high hardness and wear resistance, but the poor toughness of Ti-Al intermetallic compounds makes the coating inevitably have cracks, which are difficult to completely eliminate even if an appropriate amount of rare earth elements are added to the cladding powder. For example, Li et al. [78] added Y2O3 to the cladding powder and successfully prepared a Ti3Al intermetallic compound composite ceramic coating with a microhardness between 1250 and 1400 HV2N, but the coating still has many microcracks.

Ti-Al intermetallic compounds have high hardness and wear resistance, but the poor toughness of Ti-Al intermetallic compounds makes the coating inevitably have cracks, which are difficult to completely eliminate even if an appropriate amount of rare earth elements are added to the cladding powder. For example, Li et al. [78] added Y2O3 to the cladding powder and successfully prepared a Ti3Al intermetallic compound composite ceramic coating with a microhardness between 1250 and 1400 HV2N, but the coating still has many microcracks. The matrix phase of the TiNi and Ti2Ni intermetallic compound composite ceramic coating is TiNi and Ti2Ni phases. TiNi and Ti2Ni intermetallic compounds have good hardness and wear resistance[79]. When the Ti content in the cladding powder is high, the matrix phase of the coating is dendritic Ti2Ni; when the Ni content is high, the matrix phase of the coating is TiNi[80]. Compared with other intermetallic compounds, TiNi and Ti2Ni do not show obvious brittleness. The coating mainly composed of TiNi and Ti2Ni phases has no obvious cracks, the structure is relatively dense, and the coating is well bonded to the substrate. However, compared with the Ti-Al intermetallic compound composite ceramic coating, the hardness of the coating is lower (580~900 HV)[34,80]. 。

In addition, there are less studied Co ￣Ni and Ni ￣Al intermetallic compound matrix phases. During the formation process, the Co ￣Ni intermetallic compound matrix phase will simultaneously generate Co ￣Ti phases with greatly different physical and thermodynamic properties from the substrate, resulting in cracks at the interface between the coating and the substrate [81]. The Ni ￣Al intermetallic compound matrix phase has the advantages of high temperature oxidation resistance and wear resistance, but has the disadvantage of high greenhouse brittleness [82].

3.3 Comparison of wear resistance of coatings formed by different matrix phases Since different scholars used different friction test conditions (friction mode, friction pair material, load, friction time, etc.) when testing the wear resistance of coatings, the wear-resistant coatings they prepared cannot be directly compared using the test results such as wear rate and friction coefficient. The microhardness is the same as the wear resistance of the coating. The microhardness of different types of wear-resistant coatings can reflect the wear resistance of the coating to a certain extent, so the microhardness of different types of wear-resistant coatings is summarized, as shown in Table 4.

4 Effect of self-lubricating phase characteristics on wear resistance of coatings

The self-lubricating coating prepared by laser cladding technology is based on the components of wear-resistant coatings and adds self-lubricating phases. Therefore, compared with wear-resistant coatings, the friction coefficient of self-lubricating coatings is lower.

4.1 Materials forming self-lubricating phases

In the self-lubricating coatings prepared by laser cladding technology, some common solid lubricating materials are used to form self-lubricating phases during the laser cladding process, mainly including graphene[84], hexagonal boron nitride (h-BN)[66] and various sulfides[85, 86]. Graphene, as a new type of two-dimensional material, has the characteristics of high strength, toughness and good self-lubricating properties[87, 88]. h-BN is a hexagonal crystal system with a layered structure. The layers are connected by van der Waals bonds, so it is a good Good solid lubricating materials [66, 89]. Various sulfides such as MoS2, WS2, TiS, Ti2SC have layered structures, and shear slip easily occurs between layers. Under medium and low temperature dry friction conditions, they form transfer films and have self-lubricating effects [85, 86]. However, the above solid lubricating materials have poor wettability and are easy to decompose under the high temperature of laser as cladding powders. Therefore, the content of self-lubricating phase in the coating is relatively low [85, 87-89]. In order to solve the problems of poor wettability and easy decomposition of solid lubricating materials, there are two main solutions: directly adding solid lubricating materials to cladding powders to form self-lubricating phases and using laser high temperature to generate self-lubricating phases in situ.

4.2 Direct addition of self-lubricating phases

Directly adding solid lubricating materials to cladding powders requires the use of low The cladding process parameters of laser power and high scanning speed are adjusted to avoid the complete decomposition of solid lubricating materials during the laser cladding process. Shi Gaolian et al. [66] studied the self-lubricating coating formed by laser cladding of Ni60 + hn-BN powder. The undecomposed hn-BN was used as the self-lubricating phase. Under high temperature dry friction test conditions, the hn-BN particles softened and spread to form a lubricating transfer film. The wear amount was significantly reduced compared with the coating formed by Ni60 powder. Zhao et al. [50] and Zhang et al. [84] selected titanium + graphene powder to prepare self-lubricating coatings on the surface of TC4 alloy. During the laser cladding process, most of the graphene reacted with titanium to form TiC hard phase, and a small amount of graphene was converted into graphite at high temperature. A small amount of graphite and undecomposed graphene form a self-lubricating phase. In the dry friction test, the mechanical mixed layer composed of the self-lubricating phase and the hard phase on the surface of the coating reduces the contact stress between the friction pair and the coating and improves the wear resistance of the coating [84].

4.3 In-situ generation of self-lubricating phase

The self-lubricating phase content of the in-situ reaction using laser high temperature is higher and has a better wear reduction effect. Liu Xiubo et al. [85] and Liu et al. [86] prepared a coating with NiCr + Cr3C2 + WS2 powder to generate Ti2SC+CrS self-lubricating phase in situ, which can form a lubricating transfer film under friction conditions from room temperature to 600 ℃, reducing the friction coefficient and wear rate; while Ti2SC+TiS+TiC+WS2 powder was used to generate Ti2SC+TiS+TiC … The self-lubricating phase has a good self-lubricating effect at low and medium temperatures, but the self-lubricating phase will oxidize and fail to form an oxide film above 500 °C.

Usually, graphene will react with Ti element to generate TiCx in the laser cladding process, so it is difficult to use graphene as an in-situ self-lubricating phase. Weng et al. [90] adjusted the powder ratio and cladding process parameters, and used Ni60+B4C powder to perform laser cladding on the surface of TC4, and generated spherical graphite with a structure similar to graphene in situ. The mechanism of in-situ generation of spherical graphite self-lubricating phase during laser cladding is shown in Figure 7. After titanium and carbon elements generate TiCx, the excess carbon atoms rapidly solidify along the interface between the bubble and the melt to form spherical graphite. The spherical graphite makes the coating The friction coefficient of the coating is reduced and the wear resistance is significantly improved (the wear resistance of the coating is 43.67 times that of the substrate)[90].

5 Summary and Prospects

In summary, laser cladding on the surface of titanium alloy to prepare wear-resistant and self-lubricating coatings can effectively solve the problem of poor wear resistance of titanium alloys. The laser cladding process and coating components (hard phase, matrix phase, self-lubricating phase) are the main factors that determine the wear resistance of the coating. The laser cladding process parameters are mainly set by trial and error method to determine the cladding process parameters; ultrasonic vibration can significantly reduce the grain size of the coating, and heat treatment of the coating can effectively improve the fracture toughness of the coating. The hard phase is the key factor in improving the wear resistance of the coating. The hard phase formed by the in-situ generation method has the advantages of no cracks and no broken small particles at the edge of the hard phase, and is widely used in the formation of hard phase in wear-resistant coatings. Nickel matrix phase and titanium matrix The bulk phase has good wettability, which can significantly reduce the cracks and pore defects of the coating and improve the comprehensive performance of the coating. The formation of the self-lubricating phase requires the addition of materials that can generate the self-lubricating phase in situ in the cladding powder to avoid the large-scale decomposition of the self-lubricating phase under the high temperature of the laser.

In order to further improve the wear resistance of the wear-resistant and self-lubricating coatings prepared by laser cladding technology, future research should focus on the following aspects. First, establish a mathematical model that can comprehensively consider various factors (laser type, cladding powder type and size, etc.) to set the cladding process parameters to achieve the best wear resistance of the coating. Secondly, develop more cladding powder materials to solve the problem of low proportion of hard phase and self-lubricating phase in the coating. Finally, in-depth study of the various complex chemical reactions of different cladding powders during the laser cladding process to further improve the wear resistance of the coating.