The failure of metal parts due to friction and wear on the surface of metal parts. Laser cladding is an effective surface modification technology that can improve the friction and wear performance of metal parts, thereby extending the service life of metal parts. Nickel-based alloy powder has become a research focus in laser cladding materials due to its good wettability with the metal matrix, excellent performance improvement, and suitable price. The regulation of the composition of the cladding layer is the fundamental reason affecting the performance of the nickel-based alloy cladding layer. The wear resistance of the coating is improved by refining the grains and regulating the coating structure by utilizing the special properties of different substances. Therefore, this paper introduces the research status of the influence of different element additions, self-lubricating phase additions, ceramic phase additions, rare earth additions, solid solution strengthening, fine grain strengthening, and second phase strengthening on the microstructure of the cladding layer and reducing internal defects in the laser cladding nickel-based alloy cladding layer, summarizes and prospects the future development direction of nickel-based alloy cladding layers, and serves as a reference for people to understand the current status of the development of friction and wear of nickel-based alloy cladding layers.

1 Introduction
Under complex and harsh working conditions, friction and wear on the surface of metal parts lead to surface failure and reduced service life of metal parts, resulting in irreparable economic losses and, in serious cases, a series of safety accidents[1]. With the development of science and technology, surface modification technologies such as plasma spraying[2], physical vapor deposition (PVD)[3], chemical vapor deposition (CVD)[4], cladding[5], carburizing[6], nitriding[7], and laser cladding[8] have been widely used to enhance the surface hardness and friction and wear performance of metal parts. Laser cladding technology is a surface modification technology that uses a high-energy laser beam to melt powder or wire and then deposit it on the surface of a metal substrate to repair and strengthen it. Compared with traditional surface modification technologies, laser cladding has a wide range of cladding materials. Metals, alloys, ceramics and composite materials can all be used as cladding materials. By controlling the parameters, the morphology and properties of the cladding layer can be precisely controlled to prepare a high-quality cladding layer. By preparing a high-performance cladding layer, the service life of metal parts can be extended, damaged metal parts can be repaired, and the replacement cost can be reduced. It has become one of the most important surface modification technologies in modern industry and has broad application prospects [9-12]. Nickel-based alloys have good wettability, oxidation resistance, friction and wear performance, corrosion resistance, self-solubility and other comprehensive properties, as well as a suitable price, and are the preferred materials for laser cladding [13-14]. With the development of social industry and the increasing requirements of application environment for the friction and wear performance of metal parts, the performance of a single nickel-based alloy cladding layer is limited when dealing with different harsh working conditions. In order to improve the adaptability of the nickel-based alloy cladding layer to the friction and wear of different harsh environments, different substances can be added to the nickel-based alloy cladding layer to further improve the friction and wear performance of the cladding layer. Therefore, by introducing different elements, ceramic phases, self-lubricating phases and rare earths into the nickel-based alloy cladding layer, the microstructure is improved and the reduction of internal defects is optimized through the special properties of different substances such as second phase strengthening, solid solution strengthening, and fine grain strengthening, so as to introduce the current status of optimizing and improving the friction and wear performance of the nickel-based alloy cladding layer, and to look forward to and summarize the future development direction of the friction and wear performance of the nickel-based alloy cladding layer.

2 Effect of element addition on friction and wear properties of cladding layer
Different elements can use their different properties to act on the cladding layer to improve its friction and wear properties. In order to improve the performance of Inconel 625 cladding layer, WANG et al. [15] took 0.5% SiC addition as a constant value and changed the addition amount of Nb element to further improve the performance of the cladding layer. Compared with Inconel 625 cladding layer, with the increase of Nb element, the precipitation of NbC and Cr23C6 hard phases was promoted, the grains were refined, and the wear mechanism changed from severe abrasive wear, adhesive wear and oxidation wear of the matrix to slight abrasive wear and oxidation wear of the coating. As shown in Figure 1, the coating generates an oxide film during friction and wear, and the hard phase is dispersed between grains and grain boundaries. Under the action of fatigue stress during friction and wear, the hard phase falls off, resulting in abrasive wear and oxidation wear. However, excessive addition of Nb element is not conducive to the uniform distribution of the precipitated phase, which reduces the friction and wear performance. HUANG et al. [16] studied the effect of Ta addition on the performance of Ni60A/WC cladding layer. With the addition of Ta, the TaC content increased, which promoted the decomposition of WC during the cladding process. Ta is a strong carbide element and is easy to combine with C, which reduces the WC content and crack sensitivity, refines the grains, and promotes the uniform distribution of carbides. When the Ta addition is 10%, it has the best friction and wear performance. SUN et al. [17] added different elements to Ni60AA powder to prepare three types of cladding layers: Ni60AACu, Ni60AACuMo, and Ni60AACuMoW, to study the influence of element addition on the performance of Ni60AA cladding layers. The addition of elements resulted in a decrease in the hardness of the cladding layer, but Ni60AACu had the best friction and wear performance at room temperature and 600°C. During the friction and wear process, Cu replaced part of Ni to form a CuMo solid solution with good friction reduction performance, resulting in better friction and wear performance at room temperature and high temperature than Ni60AA cladding layers when the hardness decreased.

LI et al. [18] studied the improvement of the performance of the cladding layer by adding Mo element in Ni60AA/WC. During the friction process, a dense oxide film was formed on the surface of the cladding layer. The oxide film used its own hardness as the contact medium between the ball and the cladding layer, thereby optimizing the wear resistance of the cladding layer. With the increase of Mo element addition, the cracks showed a trend of first increasing and then decreasing. The high melting point Mo element and other elements formed carbides and precipitated as nucleation points, which refined the grains and inhibited the generation of cracks. Excessive Mo element increased the affinity of the cladding layer with O element, introduced impurities and oxides, caused the number of cracks to increase, and improved the strength of the M23(C,B)6 phase in the cladding layer, and promoted the generation of the new Mo2C phase. When the Mo element addition was 1%, it had the minimum friction coefficient and wear amount, and improved the friction and wear performance of the cladding layer. FENG et al. [19] improved the performance of the cladding layer by adding Al element to Inconel625. The addition of Al element promoted the increase of BCC content and synergistically improved the hardness and friction and wear performance of the cladding layer with the solid solution strengthening effect. HONG et al. [20] used 20% WC ceramic particles as the addition amount and changed the addition amount of Nb element to further optimize the performance of Ni60 cladding layer on the surface of Q550 steel. The addition of Nb element promoted the decomposition of WC residual particles deposited at the bottom of the cladding layer, inhibited the generation of Cr7C3 phase, and promoted the precipitation of NbC phase. However, the hardness of the cladding layer gradually decreased with the addition of Nb element. The addition of appropriate amount of Nb element was beneficial to the improvement of the performance of the cladding layer. NbC was transformed from coarse strips to fine needles, and the microstructure distribution was more uniform, which strengthened the bonding between the cladding layer and the substrate. When 3% Nb was added, it effectively prevented the shedding of the hard phase during the friction process, further improving the friction and wear performance of the cladding layer. The content and proportion of elements in the nickel-based cladding layer are controlled, and the wear resistance of the cladding layer is achieved through the introduction of hard phase, solid solution strengthening and grain refinement.

3 Effect of ceramic phase addition on friction and wear properties of cladding layer
Nickel-based alloy powder has good toughness and wettability. Single nickel-based alloy powder has limitations in improving the performance of the cladding layer. The ceramic phase has excellent properties such as high hardness, wear resistance, and high temperature resistance, but it is far from the thermal physical properties of the matrix, and it is prone to cracks and peeling. Adding the ceramic phase to the nickel-based alloy powder can combine the advantages of both to prepare a high-performance metal-ceramic composite cladding layer. The generation method of the ceramic phase can be divided into direct addition and in-situ generation. LI et al. [21] used Ti powder and nickel-coated graphite powder as the element sources for in-situ synthesis of TiC to enhance the performance of Ni45 cladding layer on U71Mn steel. With the increase of Ti powder and nickel-coated graphite powder content, the TiC diffraction peak in the cladding layer was enhanced, and the microhardness of the cladding layer was significantly improved compared with the Ni45 cladding layer. The friction and wear performance showed a trend of first increasing and then decreasing. Excessive addition led to increased brittleness of the cladding layer and decreased toughness. Large-area peeling occurred during the friction process, which was not conducive to the improvement of friction and wear performance. When the Ti+C content was 15%, it had the best friction and wear performance. HU et al. [22] clad nickel-based alloy cladding layers with different WC contents on 304. Figure 2 shows the wear mechanism of different WC contents. The addition of WC inhibits the growth of dendrites, and the dissolved W and C elements are dissolved and promote the formation of reinforcement phases. Under the combined effect of second phase strengthening, solid solution strengthening and fine grain strengthening, the friction and wear performance is improved linearly with the increase of WC content. LIU et al. [23] studied the friction and wear performance of nickel-based alloy cladding layers reinforced with WC ceramic particles in Ni50 coatings. The addition of WC helps to refine the microstructure of the cladding layer and improve the microhardness. The remaining WC is embedded in the cladding layer, which can effectively improve the friction and wear performance of the cladding layer. When the WC addition is 20%, it has the best friction and wear performance. ZHAO et al. [24] prepared cladding layers by adding TiC-TiN-B4C in different proportions as reinforcing phases in Ni204. The nickel-based alloy cladding layer with 10% TiC-10% TiN-10% B4C had the highest microhardness and the lowest friction coefficient. Under the combined action of the ceramic phase and the reinforcing phase that promoted its formation, the friction and wear performance of the cladding layer was enhanced. NING et al. [25] found through finite element simulation analysis that stress concentration occurred at the junction of the Inconel 625/SiC cladding layer and the substrate, which affected the forming quality of the cladding layer. The performance of the cladding layer was further analyzed through experiments. The experiment showed that with the increase of SiC content, the residual stress of the cladding layer showed a trend of first decreasing and then increasing, the microstructure was refined, and the carbide precipitation increased, which had a positive effect on improving the hardness and friction and wear performance of the cladding layer. Compared with direct addition, in-situ synthesis of carbides is beneficial to reduce the thermal performance gap between the ceramic phase and the matrix powder and improve the bonding strength of the ceramic phase in the cladding layer. However, different ways of introducing the ceramic phase will inevitably bring about internal defects such as cracks, thus affecting the further improvement of the wear resistance of the cladding layer.

4 Effect of rare earth addition on the friction and wear properties of the cladding layer

The addition of rare earth elements can improve the forming quality and friction and wear properties of the nickel-based alloy cladding layer by refining the grains, purifying the structure, promoting the precipitation of hard phases, and producing solid solution strengthening. SHI et al. [26] further improved the performance of the cladding layer by adding rare earth oxide La2O3 to the Ni60A/SiC cladding layer on the surface of 65Mn steel. The addition of La2O3 promoted the flow of the molten pool and the formation of hard phases, making the distribution of elements and hard phases more uniform, refining the grains of the cladding layer structure, and further improving the hardness and friction and wear properties of the Ni60A/SiC cladding layer. ZHANG et al. [27] improved the performance of Ni60/WC cladding layer by adding CeO2. The addition of CeO2 reduced the internal defects of the cladding layer, refined the grain size, and promoted the uniform distribution of the organization and elements. When the CeO2 content was 2%, the hardness and friction and wear performance reached the best. LIANG et al. [28] added 4%CeO2, 5%Y2O3 and 5%La2O3 to Ni60 to prepare the cladding layer and compared it with Ni60. As shown in Figure 3, the rare earth modified cladding layer has reduced pores and cracks, improved toughness, refined grains, and uniform element distribution, which is conducive to the improvement of friction and wear performance. SU et al. [29] added different amounts of Y2O3 to the Ni60A/Cr2C3 cladding layer on the surface of 60Si2Mn to explore the effect of rare earth oxides on the performance of the cladding layer. With the addition of Y2O3, the internal defects such as cracks and pores in the Ni60A/Cr2C3 cladding layer were effectively reduced, the cladding layer and the substrate had a better metallurgical bond, the toughness of the cladding layer was improved, the composite coating was not easy to fall off, and the precipitation and uniform distribution of the hard phase were promoted. When the Y2O3 addition amount was 1.5% (see sample M5 in Figure 4), it had the lowest friction coefficient, the friction coefficient curve was smoother, the wear amount was low, and it showed the best comprehensive friction and wear performance. Gao et al. [30] added different amounts of La2O3 to improve the performance of Ni60 cladding. With the addition of La2O3, the surface cracks of the cladding layer decreased, the XRD diffraction peak shifted, the lattice distortion occurred, the precipitation of hard phases in the cladding layer decreased, the microhardness decreased, but the element structure distribution was more uniform. When the La2O3 content was 1.6%, the hardness curve was the most gentle, with the lowest friction coefficient and the lowest wear depth. Compared with the Ni60 cladding layer, it showed excellent friction and wear performance. The addition of rare earth elements did not significantly change the composition of the phases, but enhanced the convection of the molten pool, purified the structure by adsorbing inclusions, and refined the grains to improve the wear resistance of the cladding layer.

5 Effect of self-lubricating phase addition on the friction and wear performance of the cladding layer

Adding solid lubricants such as h-BN, graphite, and WS2 to the nickel-based alloy cladding layer can improve the friction and wear performance of the cladding layer by reducing the friction coefficient during the friction process. LUO et al. [31] prepared Ni60/CNTs by laser cladding and compared them with Ni60. As shown in Figure 5, CNTs decomposed under the action of a high-energy laser beam. The generated C element and other elements in Ni60 formed carbides, which improved the hardness of the Ni60/CNTs cladding layer. CNTs have good self-lubricating properties and refined the grain size. They synergize with carbides to improve the friction and wear properties of the Ni60/CNTs cladding layer. ZHAO et al. [32] prepared two self-lubricating cladding layers of h-BN/Ni60 and Nano-Cu/h-BN/Ni60 and compared them with Ni60. The addition of solid lubricant caused the self-lubricating cladding layer to soften at high temperature between 25 and 600 °C. The microhardness was lower than that of the Ni60 cladding layer. h-BN had excellent lubrication properties. Cu coated h-BN could reduce the burning of h-BN by high-energy laser beam during the cladding process. Cu had certain lubrication performance. High temperature could generate CuO solid lubricant, which worked together with h-BN to significantly reduce the friction coefficient of the cladding layer at 25 to 600 °C, thereby improving the friction and wear performance of the cladding layer. Zhang et al. [33] investigated the effect of different NbC contents on the performance of Ni60/NbC cladding layers. On the basis of the cladding layer with the optimal NbC content, the quality of the cladding layer was further improved by adding solid lubricant graphene. When the addition amount of graphene was 10%, the cladding layer and the substrate showed good metallurgical bonding, and no obvious cracks and holes were found. Graphene increased the C content in the cladding layer and promoted the precipitation of hard phases. The high thermal conductivity of graphene increased the fluidity of the molten pool. The fine NbC particles melted and recondensed to form petals and rod-shaped large particles, which worked together with other hard phases in the cladding layer to inhibit the growth of grains and increase the microhardness of the cladding layer, which was beneficial to the improvement of the friction and wear performance of the cladding layer. FENG et al. [34] prepared three types of cladding layers: NiCrBSi, single-layer NiCrBSi-30% WS2, and double-layer NiCrBSi-30wt% WS2. The addition of WS2 reduced the hardness of the cladding layer, but formed a sulfide lubricating film during the friction process, which effectively reduced the friction coefficient and improved the friction and wear performance. The double-layer NiCrBSi-30% WS2 had the best friction and wear performance. The addition of solid lubricants can be evenly distributed in the cladding layer, acting as a lubricating medium in friction and wear, and also forming a lubricating protective film to reduce wear during the friction process. During the laser cladding process, the solid lubricant is decomposed by heat. On the one hand, the series of reactions between the decomposed elements and the cladding layer help to improve the performance. On the other hand, it leads to a reduction in the content of the lubricating phase. The loss of the lubricating phase can be reduced by plating Ni, Cu, etc. on the surface of the solid lubricating phase to reduce direct contact with the laser. Therefore, the self-lubricating performance under high temperature conditions needs further study.

6 Conclusion
For the composition control of nickel-based alloy cladding layer, the friction and wear performance of the cladding layer is improved by adding rare earth, self-lubricating phase, elements, and ceramic phase. However, the current nickel-based alloy system is not rich enough and focuses on common additives. The research scope of different additives can be further expanded in the future. The laser cladding of nickel-based alloy cladding layer on the surface of metal parts is limited to the shape of parts. The shape of the metal substrate is mostly a plane or a circular shaft substrate. There is a lack of research on the friction and wear performance of nickel-based alloy cladding layers on irregular surfaces such as curved surfaces and arcs. In addition, the research on the application environment of the friction and wear performance of nickel-based coatings is mostly focused on dry friction. The actual nickel-based alloy wear-resistant cladding layer is used in more severe and complex environments. There is a lack of research on the friction and wear performance in extreme environments such as high temperature and high pressure environments and strong acid corrosion, and the conversion rate of research results is low. The effects of different material additions on the friction and wear properties of nickel-based coatings are different. Currently, there are a lot of research on improving the friction and wear properties of various common nickel-based alloy cladding systems, but there is a lack of corresponding databases and related software to summarize them, which can provide convenient query support for further research on laser cladding nickel-based alloys and actual industrial repair use, thereby further promoting the standardization process of nickel-based alloy cladding layers.