Fabrication, Microstructure, and Properties of In Situ V 2 C-Reinforced Copper Composites

: In this paper, in situ V 2 C-reinforced Cu composites were successfully fabricated by hot pressing at 750 ◦ C under 25 MPa using Cu and V 2 SnC powders. Due to decomposition of V 2 SnC to V 2 C and Sn during sintering, Sn atoms entered the crystal structure of Cu. Therefore, ﬁnal compositions of composites consisted of Cu(Sn) and V 2 C phases. Here, copper composites with 0, 5, 10, 20, and 30 vol.% V 2 C were designed. Their microstructures and physical and mechanical properties were systematically investigated. It was observed that with increasing V 2 C content, electrical conductivity decreased from 0.589 × 10 8 S · m − 1 to 0.034 × 10 8 S · m − 1 and thermal conductivity decreased from 384.36 W · m − 1 · K − 1 to 24.65 W · m − 1 · K − 1 , while Vickers hardness increased from 52.6 HV to 334 HV. Furthermore, it was found that composites with 20 vol.% V 2 C had the highest tensile strength (440 MPa).


Introduction
Copper has high electrical conductivity, high ductility, high chemical stability, and excellent thermal properties [1]. To date, Cu has been widely used in electrical devices, electrodes, and pantography [2]. However, low strength and weak wear resistance have greatly limited its application. In order to solve this problem, dispersion strengthening (DS) has been widely employed [3]. It was found that by adding a second phase to the copper matrix, dispersed particles can reinforce the substrate by crack bridging, crack deflection, second-phase pulling out, and grain refinement [4][5][6][7]. Because traditional ceramics have high hardness and Young's modulus, ceramic-particle-reinforced Cu composites have been extensively investigated. For instance, Tao et al. prepared Ni@Al 2 O 3 /Cu composites by utilizing spark plasma sintering (SPS) to densify Ni coated Al 2 O 3 with copper. By modifying the poor wettability between Cu and Al 2 O 3 , the fracture toughness of composites was significantly improved. Oanh, N.T.H et al. significantly enhanced the hardness of Cu by the addition of TiC [8][9][10][11][12][13][14]. However, traditional ceramics (Al 2 O 3 , SiC et al.) are usually detrimental to the electrical and thermal properties of copper, as they are all insulators. Therefore, research on the development of alternative materials with good electrical properties, high hardness, and good strength has become important.
Recently, layered compounds such as M n+1 AX n phases, which can reinforce copper while maintaining excellent electrical and thermal properties, have attracted huge research interest. M n+1 AX n phases where M is an early transition metal, A is a IIIA or IVA group element, X is carbon, nitrogen, or boron, and n is an integer commonly equal to 1, 2, or 3 are good additives to reinforce a Cu matrix. They contain both metal bonds and covalent bonds in crystal structures, exhibiting combined properties of metals and ceramics, such as high bending strength, high Young's modulus, and high thermal and electrical conductivities [15][16][17][18][19][20][21][22]. In addition, they have a good wettability with copper [23][24][25]. In previous works, it was reported that the introduction of MAX phase and MAX-derived MX into copper effectively enhanced the hardness and tensile strength of composites [26]. MAX-derived MX is formed due to precipitation of A atoms under high temperatures and pressures. As a result, a similar layer structure to that of MAX phase can be obtained. Dudina et al. [27] prepared Ti 3 SiC 2 (5 vol.%)-Cu composites by spark plasma sintering (SPS). The electrical conductivity and hardness of composites were 21.2% IACS and 152 HV100, respectively. Wu, J., et al. [28] also prepared Ti 2 SnC (1 vol.%)-Cu composite by hot pressing and increased the tensile strength of Cu to 336 MPa, with only 24.4% of the electrical conductivity lost. Zhang, J., et al. [26] reported on TiC x -Cu composites with enhanced tensile strength obtained by sintering of Cu with Ti 3 AlC 2 .
In this work, in order to reinforce copper composites, a new MAX phase of V 2 SnC was introduced. Sn-based MAX phase (V 2 SnC) exhibits high electrical conductivity, selflubrication and low hardness, which are promising to be used as addtions to reinforce Cu [29]. It is believed that this research will enrich the design and fabrication of Cubased composites. Interestingly, it was found that V 2 SnC completely decomposed to V 2 C and Sn, forming V 2 SnC-derived V 2 C, and Sn dissolved into copper to form a solid solution [30][31][32][33]. The microstructure and physical and mechanical properties of composites were systematically investigated. Moreover, the positive reinforcement effect of in situ V 2 SnC-derived V 2 C particles on the Cu matrix was examined.
Commercial copper powder (300 mesh, 99.9%, Qinhuangdao ENO High-Tech Material Development Co., LTD., Qinhuangdao, China) and as-prepared V 2 SnC powder (200 mesh) were mixed by ball milling in an agate jar for 12 h with a rotating speed of 150 rpm. The powder-to-ball ratio was 1:10. The content of V 2 C in the copper composites was 0 vol.%, 5 vol.%, 10 vol.%, 20 vol.%, and 30 vol.%, respectively. After drying in an oven (101-WSB, Supo Corp., Shaoxing, China) for 24 h, the powder mixture was put into a graphite die and consolidated at 750 • C for 60 min under a pressure of 25 MPa in a hot pressing furnace (ZT-50-24Y, Chenhua Corp., Shanghai, China). The whole sintering process was carried out in vacuum with a heating rate of 20 • C/min. After sintering, samples were cooled naturally in the hot pressing chamber. Finally, contaminations on the surface of samples were removed by a diamond grinding wheel. All samples were machined by electrical discharged machining (EDM) and polished down to 1.0 µm diamond grids.
Phase composition of samples was examined by an X-ray diffractometer (D8 AD-VANCE, Bruker, Germany) with Cu Kα radiation (λ = 1.54178 Å). The crystal parameters of copper were calculated using two basic relationships. Firstly, for face-centered cubic: , where h, k, and l are indices of crystallographic plane; α is a constant that equals 1 when h, k, and l are all odd numbers; and a is the lattice constant. The second relationship is the Bragg equation: 2dsinθ = n λ, where d is the crystalline interplanar spacing, θ is the diffraction angle, and λ is the wave length of X-ray [34]. By utilizing XRD data, lattice constants of copper were roughly calculated. Elemental distribution of the composites was analyzed by a field emission scanning electron microscope (Inspect Density of as-prepared Cu composites was measured by Archimedes' method in distilled water. Electrical conductivity of specimens (dimension: 1 mm × 1 mm × 10 mm) was measured by a resistivity tester machine (FT-300A1, Ningbo Rooko Instrument Co., Ltd., Ningbo, China). Thermal conductivity of samples (dimension: Ø12.7 mm × 3 mm) was measured by a laser thermal conductivity meter (NETZSCH LFA467, Selb, Germany, reference material: Cu) at room temperature.
Hardness of specimens was measured by a Vickers hardness tester (HVS-50, Lianer Corp., Shanghai, China) by a load of 10 N and dwelling 15 s. Samples were machined by wire-electrode cutting to a dimension of 24 mm × 10 mm × 1 mm (3 samples) for tensile strength measurements (YC-100KN, Yice Corp., Ningbo, China, gauge length: 17.80 mm, moving speed: 1 mm/min). To investigate the damage mechanisms, the fracture surface of specimens was examined by SEM. Figure 1 compares XRD patterns of the initial powder mixture and as-prepared copper composites. It can be seen that in the initial mixture, diffraction peaks of Cu were strong, and weak peaks of V 2 SnC and Sn were detected [35] (Figure 1a). Figure 1b-e show XRD spectra of S1, S2, S3, and S4, respectively, corresponding to 5%, 10%, 20%, and 30% volume content of V 2 C. Diffraction peaks of V 2 SnC and Sn disappeared, and those of V 2 C occurred. It seems that during the hot pressing, V 2 SnC was completely decomposed into V 2 C and Sn, and Sn entered the crystal structure of Cu to form a solid solution.

Phase Composition and Microstructure
Here, the space group of V 2 C is P6 3 /mmc (PDF#73-1302), which is the same as that of V 2 SnC. With increasing the content of V 2 C, diffraction peaks of Cu shifted to lower angles. The shift of diffraction peaks of Cu to lower angles indicates that the lattice constant of Cu (Face centered cubic, a = b = c) increased by forming a Cu-Sn solid solution, considering that the atomic radius of copper is 1.57 Å and that of Sn is 1.72 Å [36]. By calculating, it was confirmed that with the rise in of V 2 C content, the crystal parameter of copper increased from 3.6001 Å to 3.7192 Å (Table 1), corresponding to the higher solid-solution content of Sn [37].    Figure 2 shows the element distribution of V, Sn, and Cu. It can be seen that V element existed in the region of black particles, corresponding to the V 2 C phase (Figure 2a,b). Sn and Cu elements had the same distribution region, presenting a homogeneous, solid solution (Figure 2c,d). These results support the conclusion that Sn dissolved into copper to form a solid solution.  The microstructure of the etched surface of pure Cu and Cu-5 vol.% V2C composite is shown in Figure 3a,b. As shown, there was no obvious pore on the surface of pure Cu or the Cu composite. High sintering temperature and long holding time were conductive to the diffusion of Cu. The grain size of pure Cu prepared by hot pressing was as large as 50 μm (Figure 3a). The introduction of V2C into the Cu matrix can refine the grain size. Consequently, the grain size of the Cu composite was smaller than that of pure Cu, as shown in Figure 3b. The microstructure of the etched surface of pure Cu and Cu-5 vol.% V 2 C composite is shown in Figure 3a,b. As shown, there was no obvious pore on the surface of pure Cu or the Cu composite. High sintering temperature and long holding time were conductive to the diffusion of Cu. The grain size of pure Cu prepared by hot pressing was as large as 50 µm (Figure 3a). The introduction of V 2 C into the Cu matrix can refine the grain size. Consequently, the grain size of the Cu composite was smaller than that of pure Cu, as shown in Figure 3b. The tensile fracture surface of pure copper and Cu composites was examined ( Figure  4). As shown in Figure 4a, the fracture surface of pure copper was characterized by dimples, presenting a typical fracture feature of metals. The diameter of dimples was about 8 μm. For the Cu-5 vol.% V2C composite, tear ridges were clearly observed, indicating a  The tensile fracture surface of pure copper and Cu composites was examined ( Figure 4). As shown in Figure 4a, the fracture surface of pure copper was characterized by dimples, presenting a typical fracture feature of metals. The diameter of dimples was about 8 µm. For the Cu-5 vol.% V 2 C composite, tear ridges were clearly observed, indicating a great plastic deformation of Cu grains before fracture. In addition to the fractured V 2 C particles, some holes formed during the tensile process (Figure 4b). Figure 4c-e show the tensile sections of Cu composites with 10, 20, and 30 vol.% V 2 C respectively. With increasing V 2 C content, the number of tear ridges decreased gradually. Especially in the Cu composite with 30 vol.% V 2 C, tear ridges disappeared, and only fractured V 2 C particles were observed. The tensile fracture surface of pure copper and Cu composites was examined ( Figure  4). As shown in Figure 4a, the fracture surface of pure copper was characterized by dimples, presenting a typical fracture feature of metals. The diameter of dimples was about 8 μm. For the Cu-5 vol.% V2C composite, tear ridges were clearly observed, indicating a great plastic deformation of Cu grains before fracture. In addition to the fractured V2C particles, some holes formed during the tensile process (Figure 4b). Figure 4c-e show the tensile sections of Cu composites with 10, 20, and 30 vol.% V2C respectively. With increasing V2C content, the number of tear ridges decreased gradually. Especially in the Cu composite with 30 vol.% V2C, tear ridges disappeared, and only fractured V2C particles were observed.  Figure 5 shows the measured density of composites as a function of V2C content. With increasing content of V2C, the density of pure Cu, S1, S2, S3, and S4 was 8.870 g·cm −3 , 8.706 g·cm −3 , 8.521 g·cm −3 , 8.095 g·cm −3 , and 7.709 g·cm −3 , respectively. The calculated relative density of the composites was 99%, 98.9%, 98.7%, 97.6%, and 96.8%, respectively. Compared with pure Cu, the density of Cu composites with 5, 10, 20, and 30 vol.% V2C was decreased by 1.8%, 3.9%, 8.7%, and 13.1%, respectively. The continuous decrease in density was due to the lower density of V2C (5.63 g·cm −3 ) compared to that of Cu (8.96 g·cm −3 ). The introduction of more V2C undoubtedly reduces the density of composites.  Figure 5 shows the measured density of composites as a function of V 2 C content. With increasing content of V 2 C, the density of pure Cu, S1, S2, S3, and S4 was 8.870 g·cm −3 , 8.706 g·cm −3 , 8.521 g·cm −3 , 8.095 g·cm −3 , and 7.709 g·cm −3 , respectively. The calculated relative density of the composites was 99%, 98.9%, 98.7%, 97.6%, and 96.8%, respectively. Compared with pure Cu, the density of Cu composites with 5, 10, 20, and 30 vol.% V 2 C was decreased by 1.8%, 3.9%, 8.7%, and 13.1%, respectively. The continuous decrease in density was due to the lower density of V 2 C (5.63 g·cm −3 ) compared to that of Cu (8.96 g·cm −3 ). The introduction of more V 2 C undoubtedly reduces the density of composites.

Physical Properties
The measured electrical resistivity and electrical conductivity of pure Cu, S1, S2, S3, and S4 samples is shown in Figure 6. As can be seen, electrical resistivity showed a linear change with increasing V 2 C content. The electrical conductivity of pure Cu was 0.589 × 10 8 S·m −1 , and that of Cu composites with 5 vol.% V 2 C, 10 vol.% V 2 C, 20 vol.% V 2 C, and 30 vol.% V 2 C was 0.180 × 10 8 S·m −1 , 0.099 × 10 8 S·m −1 , 0.052 × 10 8 S·m −1 , and 0.034 × 10 8 S·m −1 , respectively. Interestingly, it was observed that electrical conductivity of the composite had a considerable decrease when V 2 C content was 5 vol.%. Then, the electrical conductivity of composites decreased continuously with increasing V 2 C content. V 2 C in the matrix increased the scattering of electrons. It is known that electrical conductivity of metallic materials mainly depends on the scattering of electrons [38]. Consequently, the continuous decrease in electrical conductivity of Cu composites with more V 2 C content was probably related to three reasons: (1) electrical conductivity of V 2 C is lower than that of copper; (2) boundaries of refined copper grains enhanced the electron scattering; (3) a solid solution of Sn in the crystal structure of Cu induced the lattice distortion. What is worth mentioning is that the electrical conductivity (0.034 × 10 8 S·m −1 ) of the Cu composite with 30 vol.% V 2 C still met the requirement of a Cu-based slider (≥0.0286 × 10 8 S·m −1 , TB/T1842. . As a result, the composites retained good electrical properties. The measured electrical resistivity and electrical conductivity of pure Cu, S1, S2, S3, and S4 samples is shown in Figure 6. As can be seen, electrical resistivity showed a linear change with increasing V2C content. The electrical conductivity of pure Cu was 0.589 × 10 8 S·m −1 , and that of Cu composites with 5 vol.% V2C, 10 vol.% V2C, 20 vol.% V2C, and 30 vol.% V2C was 0.180 × 10 8 S·m −1 , 0.099 × 10 8 S·m −1 , 0.052 × 10 8 S·m −1 , and 0.034 × 10 8 S·m −1 , respectively. Interestingly, it was observed that electrical conductivity of the composite had a considerable decrease when V2C content was 5 vol.%. Then, the electrical conductivity of composites decreased continuously with increasing V2C content. V2C in the matrix increased the scattering of electrons. It is known that electrical conductivity of metallic materials mainly depends on the scattering of electrons [38]. Consequently, the continuous decrease in electrical conductivity of Cu composites with more V2C content was probably related to three reasons: (1) electrical conductivity of V2C is lower than that of copper; (2) boundaries of refined copper grains enhanced the electron scattering; (3) a solid solution of Sn in the crystal structure of Cu induced the lattice distortion. What is worth mentioning is that the electrical conductivity (0.034 × 10 8 S·m −1 ) of the Cu composite with 30 vol.% V2C still met the requirement of a Cu-based slider (≥0.0286 × 10 8 S·m −1 , TB/T1842. . As a result, the composites retained good electrical properties.   The measured electrical resistivity and electrical conductivity of pure Cu, S1, S2, S3, and S4 samples is shown in Figure 6. As can be seen, electrical resistivity showed a linear change with increasing V2C content. The electrical conductivity of pure Cu was 0.589 × 10 8 S·m −1 , and that of Cu composites with 5 vol.% V2C, 10 vol.% V2C, 20 vol.% V2C, and 30 vol.% V2C was 0.180 × 10 8 S·m −1 , 0.099 × 10 8 S·m −1 , 0.052 × 10 8 S·m −1 , and 0.034 × 10 8 S·m −1 , respectively. Interestingly, it was observed that electrical conductivity of the composite had a considerable decrease when V2C content was 5 vol.%. Then, the electrical conductivity of composites decreased continuously with increasing V2C content. V2C in the matrix increased the scattering of electrons. It is known that electrical conductivity of metallic materials mainly depends on the scattering of electrons [38]. Consequently, the continuous decrease in electrical conductivity of Cu composites with more V2C content was probably related to three reasons: (1) electrical conductivity of V2C is lower than that of copper; (2) boundaries of refined copper grains enhanced the electron scattering; (3) a solid solution of Sn in the crystal structure of Cu induced the lattice distortion. What is worth mentioning is that the electrical conductivity (0.034 × 10 8 S·m −1 ) of the Cu composite with 30 vol.% V2C still met the requirement of a Cu-based slider (≥0.0286 × 10 8 S·m −1 , TB/T1842. . As a result, the composites retained good electrical properties.  Furthermore, in order to analyze the effect of V 2 C content on thermal properties of Cu composites, the thermal conductivity and thermal diffusivity of composites were tested (Figure 7). Pure copper prepared by hot pressing had a high thermal conductivity of 384.36 W·m −1 ·K −1 , and those of the S1, S2, S3, and S4 specimens were 135.75 W·m −1 ·K −1 , 73.10 W·m −1 ·K −1 , 37.46 W·m −1 ·K −1 , and 24.65 W·m −1 ·K −1 , respectively. The noticeable decrease in thermal conductivity might be attributed to following reasons [39][40][41]: (1) the thermal conductivity of V 2 C is lower compared to that of copper; (2) more grain boundaries of Cu and V 2 C inhibited the movement of electrons; (3) the solid solution of Sn raised the crystal mismatch of Cu. In addition, thermal diffusivity of Cu composites shows the same decreasing tendency, reducing from 104.99 mm 2 ·s −1 for pure Cu to 7.61 mm 2 ·s −1 for Cu composite with 30 vol.% V 2 C. 73.10 W·m −1 ·K −1 , 37.46 W·m −1 ·K −1 , and 24.65 W·m −1 ·K −1 , respectively. The noticeable decrease in thermal conductivity might be attributed to following reasons [39][40][41]: (1) the thermal conductivity of V2C is lower compared to that of copper; (2) more grain boundaries of Cu and V2C inhibited the movement of electrons; (3) the solid solution of Sn raised the crystal mismatch of Cu. In addition, thermal diffusivity of Cu composites shows the same decreasing tendency, reducing from 104.99 mm 2 ·s −1 for pure Cu to 7.61 mm 2 ·s −1 for Cu composite with 30 vol.% V2C. Figure 7. Thermal conductivity and thermal diffusivity of Cu composites as a function of V2C content measured at room temperature. Figure 8 shows the Vickers hardness of Cu composites as a function of V2C content. The hardness increased from 56.2 HV for pure Cu to 334 HV for the Cu composite with 30 vol.% V2C. The increase in hardness can be attributed to the fact that hardness of V2C is higher than that of copper. Figure 9 shows variations in tensile strength of Cu composites with different V2C content. For the pure copper sample, the tensile strength was only 160 MPa. With the addition of more V2C, the tensile strength of composites effectively rose to 322 MPa for Cu-5 vol.% V2C composite, 362 MPa for Cu-10 vol.% V2C composite, and 440 MPa for Cu-20 vol.% V2C composite, whereas when 30 vol.% V2C was added, the tensile strength of the composite degraded to 349 MPa. We speculate that addition of V2C particles refined the Cu grains in the composites, as discussed in Figure 3. Consequently, according to the Hall-Petch rule, when the grain size of copper was decreased, the strength of the composite increased correspondingly [42]. Additionally, solid solution strengthening may also play another important role in reinforcing Cu composites due to the formation of a Cu(Sn) solid solution [43,44]. However, the tensile strength of the composite decreased when V2C content exceeded a certain value (30 vol.%). More V2C can embrittle the matrix, and defects distributed along the V2C made cracks spread easily. Therefore, the tensile strength decreased.  Figure 8 shows the Vickers hardness of Cu composites as a function of V 2 C content. The hardness increased from 56.2 HV for pure Cu to 334 HV for the Cu composite with 30 vol.% V 2 C. The increase in hardness can be attributed to the fact that hardness of V 2 C is higher than that of copper. Figure 9 shows variations in tensile strength of Cu composites with different V 2 C content. For the pure copper sample, the tensile strength was only 160 MPa. With the addition of more V 2 C, the tensile strength of composites effectively rose to 322 MPa for Cu-5 vol.% V 2 C composite, 362 MPa for Cu-10 vol.% V 2 C composite, and 440 MPa for Cu-20 vol.% V 2 C composite, whereas when 30 vol.% V 2 C was added, the tensile strength of the composite degraded to 349 MPa. We speculate that addition of V 2 C particles refined the Cu grains in the composites, as discussed in Figure 3. Consequently, according to the Hall-Petch rule, when the grain size of copper was decreased, the strength of the composite increased correspondingly [42]. Additionally, solid solution strengthening may also play another important role in reinforcing Cu composites due to the formation of a Cu(Sn) solid solution [43,44]. However, the tensile strength of the composite decreased when V 2 C content exceeded a certain value (30 vol.%). More V 2 C can embrittle the matrix, and defects distributed along the V 2 C made cracks spread easily. Therefore, the tensile strength decreased.   Properties of different in situ MX-Cu composites are listed in Table 2. The in situ TiCxreinforced copper composites were prepared by hot pressing, and TiCx was derived from Ti3AlC2. As a term of comparison, although both V2C-Cu and TiCx-Cu composites have similar electrical properties, the tensile strength of the V2C-Cu composite is slightly higher. Accordingly, the V2C-Cu composite in the present study achieved a high tensile strength and obtained excellent electrical properties.  Properties of different in situ MX-Cu composites are listed in Table 2. The in situ TiC x -reinforced copper composites were prepared by hot pressing, and TiC x was derived from Ti 3 AlC 2 . As a term of comparison, although both V 2 C-Cu and TiC x -Cu composites have similar electrical properties, the tensile strength of the V 2 C-Cu composite is slightly higher. Accordingly, the V 2 C-Cu composite in the present study achieved a high tensile strength and obtained excellent electrical properties.

Conclusions
Dense Cu composites with different V 2 C contents (0 vol.%, 5 vol.%, 10 vol.%, 20 vol.%, and 30 vol.%) were successfully fabricated by hot pressing Cu and V 2 SnC as initial materials. The phase composition, microstructure, physical, and mechanical properties of composites were systematically investigated, and the obtained conclusions are summarized as follows: i. V 2 SnC completely decomposed into V 2 C and Sn because the Sn-based MAX phase easily precipitated Sn atoms under high temperatures and pressures. Sn entered the crystal structure of copper to form a solid solution. As a result of increasing V 2 SnC content, the lattice constant of copper increased from 3.6001 Å to 3.7192 Å.
Due to the decomposition of V 2 SnC, phase compositions of the composite consisted of V 2 C and copper. ii. With increasing V 2 C content, the scattering of electrons was improved. Consequently, the electrical and thermal conductivities of the composites decreased from 0.589 × 10 8 S·m −1 to 0.034 × 10 8 S·m −1 and from 384.36 W·m −1 ·K −1 to 24.65 W·m −1 ·K −1 , respectively. iii. The introduction of V 2 C resulted in reinforcement of the Cu matrix by dispersion strengthening, grain refinement, and solid-solution strengthening. As a result, the hardness and strength of composite were improved. The Vickers hardness of composites increased from 52.6 HV to 334 HV with increasing V 2 C content. The Cu composite with 20 vol.% V 2 C had the highest tensile strength of 440 MPa, which was nearly three times that of pure Cu (160 MPa). When the content of V 2 C exceeded 30 vol.%, the tensile strength decreased due to embrittlement.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to all the datasets created during this research belonging to the funder, according to the contract.