Effect of in situ graphene-doped nano-CeO2 on microstructure and electrical contact properties of Cu30Cr10W contacts

The graphene oxide (GO)-doped nano-CeO2 was introduced into Cu30Cr10W electrical contact composites by ball milling dispersion, freeze-drying, and spark plasma sintering, resulting in excellent mechanical strength and high arc erosion resistance. The effects of GO and CeO2 on the microstructure and properties of the composites were investigated. The arc erosion behavior was investigated by the JF04C electrical contact testing apparatus. Consequently, the uniform distribution of CeO2 nanoparticles hinders the movement of dislocations, GO transformed into reduced graphene oxide (rGO) during high-temperature sintering, and the in situ formation of Cr3C2 between trace carbon atoms and chromium particles at the C–Cu interface, which enhanced the interface combination. Compared with Cu30Cr10W composites, the tensile strength of the two composites was increased by 47 and 36% by importing GO and nano-CeO2, respectively. Finally, electrode material migrated from the cathode to the anode, and the rGO delayed the formation of pits and sharp bumps on the contact surface of the electrode and inhibited diffusion of molten metal; when compared with Cu30Cr10W, the GO/ CeO2–Cu30Cr10W composites have better welding force.


Introduction
Owing to its high electrical and thermal conductivity, along with remarkable processing performance, copper is widely used in the aerospace, electronics industries, and power transmission, including high-speed rail contact wires and electrical contact materials [1]. Electric contact is the bridge between electrical and electronic components and power transmission, playing a vital role in electrical circuits [2,3]. At present, all kinds of equipment are developing toward miniaturization and lightweight, which requires the comprehensive properties of copper matrix composites to improve vastly [4][5][6].
Recently, copper matrix composites with two or more reinforcing phases have been deeply studied, which have better properties than copper matrix composites with single reinforcing phase [7]. Through the combination of matrix and reinforcing phase, the excellent properties of different materials are superimposed to realize the complementary properties among the materials [8,9]. For the time being, the commonly utilized reinforcing phases are Al 2 O 3 [10], SiC [11], AlN [12], and TiB 2 [13]. Lee et al. [14] prepared nanometer Al 2 O 3 dispersed copper matrix composites by the internal oxidization-hot extrusion process and explained the strengthening mechanism with the Orowan reinforcement theory. However, the increase in strength can destroy the conductivity [15]. Therefore, the strategy of using graphene as reinforcement arises at the historic moment, the introduction of graphene into copper matrix composites is rare, and there must be matching advanced preparation technology. The schematic representation of the process is shown in graphical abstract.
Graphene, known as "the wonder material of today's scientific world," is not only the strongest material to be discovered to date but also is the thinnest material [16]. In recent years, graphene and its derivatives have drawn attention due to their excellent mechanical and electrical properties [17,18]. It is two-dimensional honeycomb hexagonal lattice structure composed of a single layer of carbon atoms tightly packed through sp 2 hybridized orbitals, which can be coated to form fullerenes and carbon nanotubes [19,20]. It has ultra-high breaking strength (130 GPa) [21], excellent Young's modulus (1 TPa) [22], high carrier mobility (15,000 cm 2 V −1 s −1 ) [23], high thermal conductivity (5,000 W m −1 K −1 ) [24], and large specific surface area (2,630 m 2 g −1 ) [25]; its application prospect in various fields, such as energy storage, radiator, sensors, aviation and space, and anti-bacterial [26]. However, most of these features cannot be achieved in practice. Theoretically modeled physicochemical parameters of graphene plane are modified by impurities, defects, admixtures, presence of substrate, and connection of several layers or chemical functionalization [27].
At present, the majority of the research on electric contact materials focuses on copper composite materials, which can be prepared by chemical vapor deposition, mechanical alloying, in situ synthesis, and powder metallurgy. Compared with traditional copper alloys and silverbased electric contact materials, the comprehensive performance was significantly improved [28,29]. Li et al. [4] adopted chemical vapor deposition (CVD), and severe plastic deformation graphene was prepared in situ reinforced copper matrix composites. The uniformly distributed graphene pinned dislocations and provided a high-speed channel for the movement of electrons; the material tensile strength and electric conductivity reached 595 MPa and 5.46 × 10 7 S/m, respectively.
Under laboratory conditions, the research of graphene and its derivatives reinforced copper matrix composites has achieved remarkable results, but they are still not ready for industrial production. To date, the problems of homogenous dispersion, agglomeration, and interfacial bonding have still not been explored comprehensively [30,31]. The van der Waals forces result in the agglomeration of graphene in the process of dispersion, which results in the limited strengthening effect on the metal matrix or even the negative toughening effect [32,33].
In order to solve this problem, graphene derivatives have been extensively investigated. GO contains a large number of hydrophilic functional groups, which improves GO wettability, dispersibility, and surface activity [34] as well as significantly improves the binding of graphene to the matrix. However, the existence of a large number of functional groups greatly reduces the excellent properties of graphene, such as electrical conductivity and mechanical properties [35]. Wang et al. [36] found that GO lost some functional groups and converted to rGO during discharge plasma sintering, which improved the conductivity. Using the combination of preloading and CVD, Chen et al. [37] prepared excellent three-dimensional (3D) interlinked CNTs/Cu composites, which provided a new and effective way for the preparation of high performance copper matrix composites.
Another key issue is how to enhance interface cohesion. Nonetheless, graphene itself has a weak affinity with copper matrix, which is the bottleneck restricting the development of graphene copper matrix composites. The results show that under certain conditions, the in situ formation of strong carbides can effectively improve the interface bonding. Zhang et al. [38] prepared GO/Al 2 O 3 -Cu35W5Cr copper matrix composites by vacuum hotpressing sintering method. The larger thermal expansion coefficient difference between graphene and copper leads to high-density dislocations at the grain boundaries, and the Cr 3 C 2 generated in situ can significantly improve the interfacial bond strength. However, in the research process of graphene-doped rare earth oxide synergistic strengthening of copper matrix composites, there were few reports of electrical contacts. In the research of contact materials, Shao et al. [39] prepared multi-layer GNPs/Cu composites by electrostatic self-assembly and electroless copper plating. The GNPS coating effectively inhibits the splash of liquid metal, and the arc erosion resistance of the materials was significantly improved.
At present, rare earth oxide with high ionic conductivity has become a potential reinforcing phase of copper matrix composites, which has little harm to the conductivity while improving the material strength [40][41][42]. At present work, GO and CeO 2 particles were introduced into copper matrix, the GO/CeO 2 -Cu30Cr10W electrical contact material was successfully prepared by spark plasma sintering (SPS). X-ray diffraction (XRD) and Raman spectroscopy were used to analyze the powder materials. The structural changes of graphene oxide were investigated by X-ray photoelectron spectroscopy (XPS). The microstructure of the materials was analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution electron microscopy (HRTEM). Finally, the electrical contact properties of the composites were studied by using the JF04C contact test system.

Preparation of composite materials
An appropriate amount of anhydrous ethanol was added into the mixed metal powder and ball-milled in the QM-3SP2 planetary ball mill at 350 rpm for 4 h and then removed. Pure copper balls were used as a grinding medium (ball to material ratio was 5:1) to prevent particles from uniting. GO was prepared by the modified Hummers' method [43], and then GO and nano-CeO 2 were dispersed in 500 mL deionized water for 1.5 h to prepare a suspension. The ball-milled powder was transferred to suspension and mechanically stirred for 6 h. The obtained turbid liquid was placed into the LGJ-12 freeze dryer with a vacuum degree of 6 × 10 −2 Pa and a cold trap temperature of −60°C. After 72 h, the sample was transferred to the YH-10 mixer (the ratio of the ball to the material was 10:1) and mixed in an atmospheric environment at the speed of 50 rpm for 4 h to obtain a composite powder.
The composite powders were transferred into the high strength graphite mold, and the sintering of the composite was carried out in the SPS-20T-10 SPS furnace (vacuum degree 6 × 10 −2 Pa, heating rate 100°C/min). After sintering, the samples were cooled to below 100°C in the furnace and removed to obtain the ∅ 50 mm × 20 mm samples.

Microstructure and properties characterization
The morphology of the powders and the microstructure of the sintered samples were observed by SEM, TEM, and HRTEM. Powders were characterized by XRD and Raman spectra. X-ray photoelectron spectroscopy (XPS) was used to make a thorough inquiry into the state changes of carbon atoms in GO before and after sintering. Sigma 2008B1 conductivity meter measures the conductivity of the sintered samples. Nine areas were selected on the surface of the sintered sample, and the hardness was measured using the HV-100 Vickers hardness tester under 200 g load. The relative density of sintered samples was measured and calculated by the Archimedes method with deionized water at 21°C. Tensile tests were performed using Shimadzu AG-I 250 kN universal testing machine at room temperature.

Electrical contact tests
After sintering, the composite material was processed into electrical contact with a smooth end face and ∅ 3.8 × 8 mm size. Under DC load, 5,000 test cycles were completed on the JF04C contact test device. The action contact was used as the anode and the static contact as the cathode. The test voltage was set at 25 V, the current was 10, 20, 25, and 30 A, and the contact force was between 40 and 60 cN. Before and after the electrical contact test, the samples were weighed five times, and the average value was calculated to calculate the mass change of the two electrodes. As shown in Formula (1): Δ is the quality change of electrical contact, M 2 represents the quality of electrical contact after the electrical contact test, and M 1 represents the quality of electrical contact before the electrical contact test. After the electrical contact test, the arc erosion morphology of the electrical contact was analyzed by the JSM-7800f field emission scanning electron microscope and nano focus three-dimensional topography instrument.

Morphology of GO and composite powder
TEM can show the edge wrinkle morphology of graphene oxide, and the number of layers of GO can be measured at the edge of the multilayer region [44,45]. Figure 1(a and b) is a TEM image of graphene oxide after ultrasonic dispersion by vacuum freeze-drying transferred to microgrid copper mesh. The edge of GO presents convolution like folds and several Fresnel stripes can be seen on the edge, which can indicate the number of layers of graphene oxide. Figure 1(c-f) shows that part of the powders have been alloyed, and the presence of GO flakes is observed in both composite powders. Meanwhile, the original structure of GO was not destroyed after freezedrying and ball milling. According to the energy dispersion spectra (EDS) in Figure 1(g) and (h), the surface of GO sheets after ball milling and freeze-drying contains cerium oxide and copper, chromium, and tungsten metal particles. It can be seen from the figure that the 2θ peak of graphite is 26.5°and the 2θ peak shifts to 10.5°due to the strong oxidation, which corresponds to the characteristic diffraction peaks of natural graphite (PDF#75-2078) and GO [36]. Layer spacing can be calculated from the Bragg's law equation:

XRD, Raman spectroscopy, and XPS analysis
Here, d is the crystal plane spacing, θ is the diffraction angle, n is the order 1, and λ is the wavelength of X-ray produced by the copper target, which is 0.154 nm. According to the Bragg's equation, the interlayer distance of GO was 0.833 nm. Similarly, the interlaminar distance of graphite is 0.335 nm. The graphite was completely oxidized to GO by opening the interlayer space under strong oxidation [46]. Compared with graphite diffraction peak, GO shows wider and weaker diffraction peak due to a large number of functional groups in GO destroying the original crystal structure [47,48]. Figure 2(b) shows the XRD test pattern of the composite powder. The characteristic diffraction peaks of Cu-W-Cr correspond to the standard peaks, and the crystal planes corresponding to each peak are shown in the figure.
Different substances have Raman spectra corresponding to their molecular structure. Figure 2(c) shows the Raman spectra of pure GO and composite powders. The results show that the D-peak and the G-peak of GO appear around 1,348 and 1,580 cm −1 , respectively. The D peak at 1,348 cm −1 is caused by the destruction of the translational symmetry of the hexagonal lattice, which reflects the disorder degree and structural defects of GO [36]. The G peak at 1,580 cm −1 is caused by the in-plane vibration of the sp 2 carbon atom [49]. The I D /I G ratio is used to characterize the structural integrity [50]. After freeze-drying, the D and G peaks of GO shift slightly, and the I D /I G ratio is 0.98, 0.96, and 0.94, respectively. The original structure of GO remained after dispersion, freezedrying, and ball milling.
XPS can be often used to characterize and analyze the chemical state of material elements, the percentage of functional groups, and the surface chemical composition [51]. In order to study the structural changes of graphene oxide before and after sintering, XPS was performed on pure graphene oxide and composite materials. Figure 3(a) is the XPS full spectrum analysis of pure GO and the two composites. According to the results of the spectrum, W 4f, C 1s, O 1s, Cr 2p, and Cu 2p peaks appeared at 35.8, 284.8, 532.8, 576.8, and 932.8 eV, respectively. Figure 3(b-d) shows the degree of oxidation of carbon atoms in GO and the transition of the state of carbon atoms after sintering, respectively. Figure 3 Figure  3(b-d) that the content of sp 2 and sp 3 hybrid oxygenfree carbon (C-C/C]C) in the two composites increased significantly, while the content of all oxygen-containing functional groups decreased sharply, which indicated that most of GO was converted to rGO in the process of SPS, which had a significant impact on the electrical conductivity. The O 1s spectra of graphene oxide and composites are shown in Figure 3

Comprehensive properties of composite materials
The density, Vickers hardness, and conductivity of the composites are listed in Table 1. By calculation, the relative density of 0.5GO/0.5CeO 2 -Cu30Cr10W and 1.0 GO/0.5CeO 2 -Cu30Cr10W composites is above 98%. In addition, compared with the Cu30Cr10W composite prepared under the same conditions, the electrical conductivity of the two composites containing graphene is increased by 16.8 and 10%, respectively. Although the existence of functional groups in GO will weaken the electrical conductivity, some functional groups will be reduced during the sintering process, which makes graphene oxide transform into reduced graphene oxide with excellent electrical conductivity. According to XPS results, GO will not be completely transformed into rGO. However, rGO can significantly improve the conductivity of the material. Therefore, the conductivity of GO-doped composites will be significantly improved. However, when 1 wt% GO is doped, the conductivity decreases due to the agglomeration of GO. In addition, the Vickers hardness of GO-doped composites is increased by 16 and 19.6%, respectively.
Welding characteristics of electrical contact materials are closely related to the strength of materials, and the welding force decreases when tensile strength decreases [7]. Nevertheless, when the strength is lower than the threshold value, the wear resistance of electrical contact materials will be reduced, thus accelerating the failure of electrical contact materials [37]. Figure 4 shows the stress-strain curve of the studied material. The tensile strength of the three composites is 262, 387, and 358 MPa, respectively. The tensile strength of the composites with the strengthening phase is increased by 47 and 36%, respectively. The main reason behind this is that the strong bond between the functional groups of graphene oxide and copper provides their strong adhesion, graphene oxide transforms into reduced graphene oxide under the action of high discharge temperature, and rGO causes the redistribution effect of high load [19]. In addition, part of the carbon atoms and chromium form Cr 3 C 2 nanoparticles, which enhance the interface bonding. Moreover, CeO 2 nanoparticles hinder the movement of dislocations, resulting in the stacking of dislocations, which improves the mechanical strength.

SEM and TEM analysis
SEM images of the 0.5GO/0.5CeO 2 -Cu30Cr10W and 1.0GO/ 0.5CeO 2 -Cu30Cr10W composites are shown in Figure 5. Cr particles are evenly distributed in the copper matrix, while tungsten particles are slightly agglomerated, the carbon element dispersed on the metal matrix, and the components of the composite did not have obvious oxidation behavior. The surface of the matrix is compact without obvious defects. Figure 6 shows the TEM and HRTEM images of the 0.5GO/0.5CeO 2 -Cu30Cr10W composite. This is a typical deformation microstructure, including dislocation entanglement and twinning. The entanglement of dislocations and the formation of dislocation packets can be seen in Figure 6(a), which significantly improves the strength of the copper matrix. As shown in Figure 6(c), cerium oxide   nanoparticles are dispersed on the rGO. Twins are formed in the copper matrix after plasma sintering, which is of great help to improve the elongation of the composite material. However, this is not the contribution of the twin itself, but because the twin changes the phase of the crystal, leading to the start of the new slip system, which indirectly contributes to the deformation of the statue. Fourier transforms the selected A 1 and A 2 regions in Figure 6(d). Copper and rGO were identified, which perfectly matched the XPS test results of the existing state of carbon atoms. The particles formed in situ between copper and rGO can be identified as Cr 3 C 2 in Figure 6(d and e) by fast Fourier transform analysis. Figure 6(f) shows the orientation of two crystal planes of Cr 3 C 2 with orthorhombic structure: 230 Cr C 3 2

( )
and 140 Cr C 3 2 ( ) . According to the periodic contrast difference of the high resolution atomic images in Figure 6(g), the crystal plane spacing of 0.425 nm for 230 Cr C 3 2 ( ) . It can be seen from Figure 6(f) that there were obvious edge dislocations in Cr 3 C 2 particles. In the sintering process, the formation and deformation of the compound occur simultaneously. The movement of one atomic plane ends in the Cr 3 C 2 grain, and the other atomic planes continue to move, forming a stress field centered on the edge dislocation core, as shown in Figure  6(h). Therefore, Cr 3 C 2 particles have a positive effect on the interface bonding of coupled rGO-Cu. This results in a significant improvement in mechanical properties.

Arc ablation morphology
Arc ablation refers to the discharge between two contacts in the process of circuit making and breaking contact and produces a lot of arc heat, which leads to the increase of contact temperature. The contact surface material melts and flows under the action of heat flow, and the flowing liquid metal will separate the components of the composite material [52,53]. Serious arc ablation will lead to material evaporation, sputtering and contact deformation, and then failure. Figure 7 shows the low power images and corresponding three-position profiles of the two kinds of doped graphene and ceria contact materials after 25 V, 30 A, and 5,000 switching cycles. Canyon-like pits and mountain-like protrusions appear on the surface of the anode and cathode, respectively, which indicate that the material on the contact surface migrates from cathode to anode. Figure 8 shows the high-power arc ablation morphology and EDS analysis results of 0.5GO/0.5CeO 2 -Cu30Cr10W contact material. Figure 8(a)-(d) shows typical arc ablation patterns such as dispersed droplets, peak-like protrusions, and coral-like structures on the contact surface. As shown in Figure 8(a), due to the high temperature generated by the arc, the copper with a low melting point will melt preferentially, and the liquid copper will splash to the low-temperature area of the contact and solidify. Cr particles strongly absorb oxygen at high temperature, forming the small particles of chromium oxide in Figure  8(c). Due to the flow and splash of liquid metal and the fracture of the liquid metal bridge when the contact is disconnected, the composition distribution on the contact surface is uneven. Moreover, the accumulation of refractory metals leads to the appearance of a coral-like structure, as seen in Figure 8(d).

Morphology evolution and mass transfer of arc erosion
A large number of migrations and loss of anode and cathode materials are fatal to electrical contacts, which will directly affect the service life of components. Figure 9 shows the results of the quality change of the electrical contact material after the electrical contact test at 25 V DC and 10-30 A. The electrode materials migrate from cathode to anode. The total mass of the electrode shows a decreasing trend compared with that before the electrical contact test, and this trend becomes more and more obvious with the increase of the current, which indicates that the electrode material diffuses to the surrounding environment. In the process of contact opening and closing, there are two stages: the formation of a melting bridge and the formation of an arc. According to previous reports, the arc mainly experienced the metal phase and gas phase [54].
Under the action of an external electric field, a large amount of heat melts the material. When the electrical contact is disconnected, the molten liquid bridge will be formed. When the electrical contact is further disconnected, the liquid bridge will be randomly disconnected, resulting in the uneven migration of the electrode material to the electrode contact surface. The vaporized metal particles in the contact gap ionize and generate an arc. Due to the further influence of the electric field, metal ions are produced in the process of the collision between the electrons emitted from the cathode and the metal vapor, leading to the migration of a small amount of electrode materials from the anode to the cathode. During the arc discharge progress process, the properties of the arc change substantially. This is because the gas in the environment enters the arc and becomes the main ionized plasma column, which leads to the transformation of arc properties from metal arc to gas arc [55,56]. The falling gas particles bombard the cathode surface, resulting in a large amount of erosion of cathode contact surface materials. At the same time, the contraction of the arc in the cathode region increases the heat flux into the cathode, and the cathode surface material will be more eroded than the anode. Therefore, the net anode gain can be observed instead of the migration of anode material to the cathode. Reduced graphene oxide will increase the viscosity of molten metal, and the density of reduced graphene oxide is smaller than the molten metal, so it will float on the surface of flowing molten metal, which is conducive to stabilizing the molten pool, inhibiting the flow and splashing of molten metal, to reduce arc erosion. In addition, composite resulted in favorable mechanical and thermal property, which suppressed the mass loss under arc erosion [57]. At the same time, the mass transfer and mass loss of the electrical contact doped with 0.5 wt% GO are optimal under each test current condition. This is because the conductivity of the other two kinds of electrical contact materials is low, and the heat generated by the arc will directly lead to the aggravation of arc erosion.

The relationship between welding force, strength, and arcing energy
When the contacts split and close the circuit, the arc discharge will occur when the arcing condition is satisfied, which leads to arc erosion on the contact surface of the electric contacts, accompanied by electric contact physical phenomena such as arc energy and fusion welding force [54]. In the interaction between the arc and the contact, the arc inputs energy to the contact, so that the contact is heated and melted, forming a molten pool. When the molten metal solidifies, it is easy to weld the contact, so that it cannot be disconnected. Welding is one of the main forms of contact failure. The force that separates the welding contact is called the fusion welding force. Figures 10 and 11 show the variation of arc burning energy and welding force of different composites under 25 V DC and 10-30 A test conditions. The results indicate that the arc burning energy and welding force of the composite is significantly reduced with the strengthening phase. It should be noted that the 1.0GO/0.5CeO 2 -Cu30Cr10W composite has the minimum arc energy and welding force under each test condition. This is because GO and nano-CeO 2 disperse the arc and improve the arc extinguishing ability of the material. There is a close relationship between the welding force, arc energy, and tensile strength. It is necessary to clarify the relationship between the arc energy, tensile strength, and welding force in order to solve the problem of contact failure. According to the mathematical evaluation model of the welding force [58], it can be expressed as follows: where F m represents the maximum welding force, K is the coefficient of contact material, and W is the sum of arc energy and Joule heat. It is generally believed that the energy produced by the arc is much greater than Joule heat, so the total energy W can be equal to the arc energy. The total energy input into the contact area of the contact and the coefficient K are as follows: where V A and V C are the current and voltage loaded at both ends of the electrical contact material, respectively; W C and W A are Joule heat and arc energy, respectively; Г and δ represent the ultimate tensile strength of the electrical contact material and the density of the molten material, respectively; T m and T b are the melting point and initial temperature, respectively; and C L and C V are the melting latent heat and specific heat. For the three composites, the relative density and specific heat C V are almost the same. Therefore, the ratio of contact material coefficient K is close to the ratio of ultimate tensile strength. Therefore, it can be concluded from the analysis of the expression that the maximum welding force F m is related to the arc energy W A and the tensile strength Γ.
From this, one can get the mathematical expression for the maximum welding force: Here, N is a constant. The welding force is determined by the tensile strength and the arc energy. The Γ of Cu30Cr10W and 1.0GO/0.5CeO 2 -Cu30Cr10W are 262 and 358 MPa, respectively. The average arc energy is 198 and 67 mJ at 25 V DC and 20 A, respectively. From formula (6) calculation, it can be concluded that the welding force of Cu30Cr10W composite is 1.5 times that of 1.0GO/0.5CeO 2 -Cu30Cr10W composite, which can verify that the theoretical calculation results are consistent with the experimental results (37.29 and 25.24 cN). Based on the analysis of formula (6), the arc energy plays a dominant role in determining the welding force. Therefore, in situ graphene doped with nano-CeO 2 can play an important role in reducing the welding force.

Conclusions
(1) Freeze-drying retained the structure of graphene oxide to the greatest extent and reduced the degree of agglomeration. In the process of the high-temperature sintering, GO transforms to rGO, and rGO causes the redistribution effect of load. Cr 3 C 2 particles formed in situ at the interface of rGO-Cu enhance the interface bonding. CeO 2 nanoparticles not only improve the mechanical strength but also do not degrade electrical conductivity. The tensile strength of the composites doped with the strengthening phase is increased by 47 and 36%, respectively. In addition, twins also play an active role in plastic deformation. (2) Arc energy and tensile strength affect the welding force, and the arc energy is the dominant factor. The arc energy of the material decreases with the addition of the reinforcing phase, and 1.0GO/0.5CeO 2 -Cu30Cr10W has the best welding resistance. At the same time, the cathode materials migrate to the anode, and reduced graphene oxide can inhibit the flow of molten metal and weaken the splash. (3) At present, researchers have understood the effect of graphene on the mechanical and electrical properties of copper-based materials and have a preliminary understanding of the reinforcement mechanism. In the next stage, we focus on the research of go surface modification, focusing on improving the dispersion of go, and deeply studying the strengthening mechanism Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.