Fabrication, Microstructure, and Properties of In Situ V₂C-Reinforced Copper Composites

Abstract

In this paper, in situ V₂C-reinforced Cu composites were successfully fabricated by hot pressing at 750 °C under 25 MPa using Cu and V₂SnC powders. Due to decomposition of V₂SnC to V₂C and Sn during sintering, Sn atoms entered the crystal structure of Cu. Therefore, final compositions of composites consisted of Cu(Sn) and V₂C phases. Here, copper composites with 0, 5, 10, 20, and 30 vol.% V₂C were designed. Their microstructures and physical and mechanical properties were systematically investigated. It was observed that with increasing V₂C content, electrical conductivity decreased from 0.589 × 10⁸ S·m⁻¹ to 0.034 × 10⁸ 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.% V₂C had the highest tensile strength (440 MPa).

1. 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@Al₂O₃/Cu composites by utilizing spark plasma sintering (SPS) to densify Ni coated Al₂O₃ with copper. By modifying the poor wettability between Cu and Al₂O₃, 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 (Al₂O₃, 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+1AX_n phases, which can reinforce copper while maintaining excellent electrical and thermal properties, have attracted huge research interest. M_n+1AX_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 Ti₃SiC₂ (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 Ti₂SnC (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 Ti₃AlC₂.

In this work, in order to reinforce copper composites, a new MAX phase of V₂SnC was introduced. Sn-based MAX phase (V₂SnC) exhibits high electrical conductivity, self-lubrication 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 Cu-based composites. Interestingly, it was found that V₂SnC completely decomposed to V₂C and Sn, forming V₂SnC-derived V₂C, 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 V₂SnC-derived V₂C particles on the Cu matrix was examined.

2. Experimental Procedure

Commercial element powders of V (300 mesh, 99.9%, Qinhuangdao ENO High-Tech Material Development Co., LTD., Qinhuangdao, China), Sn (300 mesh, 99.9%, Qinhuangdao ENO High-Tech Material Development Co., LTD., Qinhuangdao, China), and C (1500 mesh, 99.9%, Qinhuangdao ENO High-Tech Material Development Co., LTD., Qinhuangdao, China) were utilized as raw materials to synthesize V₂SnC. The process of synthesis of V₂SnC is the same as in previous work researched by the author’s lab [29].

Commercial copper powder (300 mesh, 99.9%, Qinhuangdao ENO High-Tech Material Development Co., LTD., Qinhuangdao, China) and as-prepared V₂SnC 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 V₂C 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 ADVANCE, 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: $\mathrm{d}\left(\mathrm{hkl}\right)=\frac{1}{\mathsf{\alpha }}\frac{a}{\sqrt{h2+k2+l2}}$, 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: $2\mathrm{dsin}\mathsf{\theta }=\mathrm{n} \mathsf{\lambda }$, 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 F50, FEI, Hillsboro, OR, US) equipped with an energy-dispersive spectrometer (EDAX, Super octane, Hillsboro, OR, US). The polished and etched surfaces (5g FeCl₃, 50 mL HCl, 100 mL H₂O) of composites were examined by an optical microscope (XZJ-L2030, Phenix, Shangrao, China).

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.

3. Results and Discussion

3.1. Phase Composition and Microstructure

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 V₂SnC 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 V₂C. Diffraction peaks of V₂SnC and Sn disappeared, and those of V₂C occurred. It seems that during the hot pressing, V₂SnC was completely decomposed into V₂C and Sn, and Sn entered the crystal structure of Cu to form a solid solution. Here, the space group of V₂C is P6₃/mmc (PDF#73-1302), which is the same as that of V₂SnC. With increasing the content of V₂C, 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 V₂C content, the crystal parameter of copper increased from 3.6001 Å to 3.7192 Å (

Table 1. Calculated lattice constant of Cu based on the XRD patterns.

Composites	Pure Cu	S1	S2	S3	S4
Lattice constant (Å) (a = b = c)	3.6001	3.6181	3.6553	3.6806	3.7192

), 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 V₂C 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 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.

3.2. Physical Properties

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.

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 × 10⁸ 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 × 10⁸ S·m⁻¹, 0.099 × 10⁸ S·m⁻¹, 0.052 × 10⁸ S·m⁻¹, and 0.034 × 10⁸ 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 × 10⁸ S·m⁻¹) of the Cu composite with 30 vol.% V₂C still met the requirement of a Cu-based slider (≥0.0286 × 10⁸ S·m⁻¹, TB/T1842.1-2002). As a result, the composites retained good electrical properties.

Furthermore, in order to analyze the effect of V₂C 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 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 mm²·s⁻¹ for pure Cu to 7.61 mm²·s⁻¹ for Cu composite with 30 vol.% V₂C.

3.3. Mechanical Properties

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.

Properties of different in situ MX-Cu composites are listed in

Table 2. Tensile strength and electrical conductivity of the different in situ MX-Cu composites.

Composites	Tensile Strength (MPa)	Electrical Conductivity (%IACS)	Preparing Method	Reference
Pure Cu	160	98	Hot pressing	This work
V2C (5 vol.%)-Cu	332	31.4
V2C (10 vol.%)-Cu	362	17
V2C (20 vol.%)-Cu	440	9
V2C (30 vol.%)-Cu	349	5.8
Pure Cu	130	84.7	Hot pressing	Ref. [25]
TiCx (5 vol.%)-Cu	315	30
TiCx (10 vol.%)-Cu	356	16
TiCx (20 vol.%)-Cu	392	9
TiCx (30 vol.%)-Cu	299	6

. The in situ TiC_x-reinforced copper composites were prepared by hot pressing, and TiC_x was derived from Ti₃AlC₂. As a term of comparison, although both V₂C-Cu and TiC_x-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.

4. Conclusions

Dense Cu composites with different V₂C contents (0 vol.%, 5 vol.%, 10 vol.%, 20 vol.%, and 30 vol.%) were successfully fabricated by hot pressing Cu and V₂SnC 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.

V₂SnC completely decomposed into V₂C 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 V₂SnC content, the lattice constant of copper increased from 3.6001 Å to 3.7192 Å. Due to the decomposition of V₂SnC, phase compositions of the composite consisted of V₂C and copper.

ii.

With increasing V₂C content, the scattering of electrons was improved. Consequently, the electrical and thermal conductivities of the composites decreased from 0.589 × 10⁸ S·m⁻¹ to 0.034 × 10⁸ 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 V₂C 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 V₂C content. The Cu composite with 20 vol.% V₂C had the highest tensile strength of 440 MPa, which was nearly three times that of pure Cu (160 MPa). When the content of V₂C exceeded 30 vol.%, the tensile strength decreased due to embrittlement.