Fig. 3 displays the XRD patterns of Cu/SiC nanocomposite powders with different SiC contents (i.e., 0wt%, 1wt%, 2wt%, 4wt%, and 8wt%) after 20 h of milling. The XRD patterns were analyzed according to JCPDS 85-1362 and 89-2225 standard cards. The patterns were mainly composed of Cu and SiC phases (primary phases of the starting mixture) considering that Cu and SiC powders exhibit cubic and rhombohedral crystal structures, respectively. Importantly, the absence of the characteristic SiC peaks in the nanocomposites with 1wt% and 2wt% SiC is attributed to the low weight percentages of SiC, which are below the XRD detection limit. After the milling process, a significant broadening with a decrease in the intensity of peaks of the Cu matrix occurred while the intensity of the SiC peaks increased with increasing weight percentage of the nano-SiC particles .
In order to characterize the microstructure of the prepared nanopowders, the Cu matrix crystallite size, dislocation density, and microstrain of the Cu/SiC nanocomposite powders after 20 h of milling were calculated using the broadening of their diffraction peaks. The effects of the SiC content on the crystallite size, dislocation density, and microstrain of the Cu/SiC nanocomposite powders are illustrated in Fig. 4. Notably, with the increase in SiC weight percentage, the crystal size decreased while the dislocation density and microstrain increased because of severe plastic deformation and grain size refinement during mechanical alloying process due to the added nano-SiC particles [6,27]. The crystal sizes of Cu–0wt%, 1wt%, 2wt%, 4wt%, and 8wt% SiC milled powders were 21.03, 20.69, 19.95, 18.1, and 14.33 nm, while the microstrains were 0.3333%, 0.3383%, 0.3522%, 0.3857%, and 0.4915%, respectively.
Generally, the existence of metal (ductile) particles in ceramics (brittle) forms a ductile–brittle system, which leads to several changes during the milling process. First, the ductile particles suffer from deformation, while brittle particles suffer from fragmentation. Based on this behavior, brittle particles tend to stay between ductile particles at the instant of ball collision [5,19,28]. As a result, a real composite is formed. The morphology of the Cu matrix and Cu/SiC nanocomposite powders were investigated using SEM as shown in Fig. 5. With the increase in nano-SiC reinforced particles, a finer nanocomposite powder was obtained with homogenous distribution of SiC particles in the Cu matrix, which was substantial for improving the mechanical, electrical, and thermal properties. Surprisingly, SEM images are an insufficient method to ensure uniform distribution of SiC particles into Cu matrix because of the incorporation of SiC particles into grain boundaries of Cu. Therefore, chemical element distribution maps of samples containing 4wt% and 8wt% SiC were obtained as shown in Figs. 6(a) and 6(b).
Figure 5. SEM images of (a) Cu–0wt%SiC, (b) Cu–1wt%SiC, (c) Cu–2wt%SiC, (d) Cu–4wt%SiC, and (e) Cu–8wt%SiC milled powder specimens.
To obtain Cu/SiC nanocomposites with high densification, their powders were compacted and sintered. During the sintering process, the necks at the boundaries between the particles grew, which produced the sintered nanocomposites at the desired densification. Theoretical, bulk, and relative densities and the apparent porosities of the nanocomposite samples sintered for 1 h at 775 and 875°C with various SiC contents are shown in Table 1. The relative densities of the specimens were remarkably reduced with an increase in the weight percentage of the SiC reinforced particles. Alternatively, the apparent porosity increased by the same factor. With increasing sintering temperature from 775 to 875°C, the relative densities of the specimens increased, while the apparent porosity decreased. The major reason behind this behavior is the presence of SiC, which has lower compressibility than that of the Cu matrix and a large difference in the melting temperatures (Cu: ~1080°C; SiC: ~2730°C), which leads to weak bonding and subsequently limits diffusion. Furthermore, the SiC reinforcement particles act as a barrier during the diffusion step of sintering process [29–30]. Similarly, increased amounts of ceramic particles (i.e., ZrO2, SiC, and TiC particles) in the Cu matrix reduce the relative density of the sintered nanocomposites [4–6,31]. Generally, the ceramics are responsible for low diffusion at the interface between the matrix and reinforcement; therefore, the densification of the sintered nanocomposites decreases [29,32]. The measurable improvement in the density, i.e., reduced porosity with increasing sintering temperature to 875°C, may be because of the increasing diffusion rate that contributes to increase the contacts between particles, grain growth, and decreasing the pore volume [33–34]. Hence, higher relative densities (lower porosity) were achieved at higher sintering temperature. This trend can be more clarified using Eq. (11), which shows that the sintering temperature plays an essential role in the diffusion process :
SiC content /
Sintering temperature /
Theoretical density /
Bulk density /
Relative density /
Apparent porosity /
0 775 8.96 8.49 94.7 3.78 1 8.80 8.28 94.1 4.52 2 8.65 8.06 93.2 5.71 4 8.36 7.65 91.5 7.66 8 7.84 7.01 89.4 9.11 0 875 8.96 8.73 97.4 2.11 1 8.80 8.55 97.1 2.51 2 8.65 8.32 96.2 3.37 4 8.36 7.95 95.1 4.99 8 7.84 7.34 93.6 6.94
Table 1. Theoretical, bulk, and relative densities and apparent porosities of nanocomposite samples sintered for 1 h in argon
where D, D0, Q, R, and T are the diffusion coefficient, constant, activation energy, Boltzmann’s constant, and temperature, respectively.
SEM micrographs of the nanocomposite specimens with 2wt% and 8wt% SiC sintered at 775 and 875°C are shown in Figs. 7 and 8. An increase in nano-SiC particles contributed to an observable change in the microstructure of the nanocomposites, which led to an increase in porosity and considerable weakness in the contact between Cu particles. As shown in Fig. 7, the bonding between SiC and Cu was weak because of the insufficient sintering temperature. However, by increasing the sintering temperature to 875°C, a suitable bonding strength between SiC and Cu was reached (Fig. 8). The sintering temperature is responsible for better densification behavior, as shown by the grain growth and decrease in the number of pores .
Fig. 9 illustrates the relative expansion versus temperature of Cu/SiC nanocomposites with various weight percentages of nano-SiC particles. Generally, the relative expansion (dl/l0) of the nanocomposites with various SiC contents increased as the temperature increased significantly. Furthermore, it reduced with increasing SiC weight percentage. The CTE values of samples with different sintering temperatures are shown in Fig. 10. The CTE value decreased with increased SiC content and increased with increasing sintering temperature. The theoretical CTE values for Cu–0wt%, 1wt%, 2wt%, 4wt%, and 8wt% SiC nanocomposites calculated using the simple rule of mixtures were 17 × 10−6, 16.64 × 10−6, 16.28 × 10−6, 15.61 × 10−6, and 14.40 × 10−6 °C−1, respectively. The CTE of unreinforced Cu sintered at 875°C was 15.67 × 10−6 °C−1 and the addition of 1wt%, 2wt%, 4wt%, and 8wt% SiC reduced the CTE values to 14.81 × 10−6, 14.07 × 10−6, 12.9 × 10−6, and 11.1 × 10−6 °C−1, respectively. Interestingly, the theoretical CTE values of the sintered samples were greater than the experimental values because of the high bonding between Cu matrix and SiC particles, which indicated the excellent interfacial bonding also imposes effective constraint to the expansion of the Cu matrix [10,37]. Generally, the lower CTE of the Cu matrix with the addition of SiC particles should be attributed to the lower CTE of SiC (3.7×10−6 °C−1) than that of Cu matrix (17×10−6 °C−1), which comes from the interfacial bonding between the SiC nanoparticles and Cu matrix. Furthermore, for nanocomposite samples, residual stresses (compression and tensile stresses) resulting from thermal mismatch between the Cu matrix and SiC particles play a strong role in determining the behavior of thermal expansion . Alternatively, increasing the thermal expansion of nanocomposites with increased sintering temperature from 775 to 875°C is compatible with the relative density and apparent porosity because of the microstructure development and densification . The thermal mismatch generates a residual stress field in the Cu matrix and SiC reinforcement where the pores undergo a compression stress that causes shrinkage of the pore volume. Subsequently, the thermal expansion and CTE value of the composite decrease with an increase in the porosity and vice versa .
Figure 9. Variations of the thermal expansion of nanocomposites sintered for 1 h at (a) 775°C and (b) 875°C.
Figure 10. Effects of SiC weight percentage and sintering temperature on CTE values of nanocomposites samples.
An efficiency factor (i.e., F) is used to assess the influence of SiC content and sintering temperature on the CTE of the Cu matrix nanocomposite according to Eq. (12) :
where αc and αm are the CTE values of the nanocomposite and Cu, respectively; V is the SiC volume percentage. The relationship between F and SiC content are represented in Fig. 11, where the F value decreased with increasing SiC content but increased with temperature.
Fig. 12 shows the microhardness values of the Cu/SiC nanocomposite samples. The microhardness values of the nanocomposites were greater than that of the unreinforced Cu sample processed in the same sintering temperature. Furthermore, the microhardness increased with an increase in SiC weight percentage and sintering temperature. The standard hardness values of Cu and SiC range from 343 to 369 MPa and 23.5 to 25.5 GPa, respectively. The microhardness increased and reached 808.9 MPa for the composite sample containing 8wt% SiC sintered at 775°C. With increasing sintering temperature up to 875°C, the microhardness further increased to 958.7 MPa for the composite sample. The longitudinal and shear ultrasonic velocities (i.e., VL and VS) are represented in Fig. 13, and the elastic moduli of the sintered samples are listed in Table 2. Notably, VL and VS increased with SiC weight percentage and sintering temperature. The results in Fig. 13 indicated that with an increase in SiC content from 0wt% to 8wt%, the VL values of the samples sintered at 775 and 875°C increased from 4301 to 6202 m·s−1 and 4419 to 6514 m·s−1, respectively, and the VS values ranged from 2167 to 3082 m·s−1 and 2210 to 3214 m·s−1, respectively. As a result, the elastic moduli had the same trend as the ultrasonic velocities (Table 2). The elastic modulus of the nanocomposite samples with 8wt% SiC sintered at 775 and 875°C (261.9 and 309.9 GPa, respectively) were greater than those of unreinforced Cu matrix sintered at the same temperatures (154.5 and 171.1 GPa, respectively).
SiC content / wt% Sintering temperature / °C E / GPa L / GPa K / GPa G / GPa ʋ 0 775 104.3 154.5 39.2 39.2 0.3299 1 109.8 162.7 41.3 41.3 0.3301 2 113.1 167.8 42.5 42.5 0.3304 4 137.5 204.9 51.6 51.6 0.3316 8 172.9 261.9 64.7 64.7 0.3360 0 875 114.1 171.2 128.3 42.8 0.3332 1 121.4 182.4 136.9 45.5 0.3337 2 128.5 193.2 145.0 48.2 0.3340 4 157.3 237.9 179.0 58.9 0.3355 8 202.1 309.9 234.5 75.5 0.3391
Table 2. Young’s modulus (E), elastic modulus (L), bulk modulus (K), shear modulus (G), and Poisson’s ratio (ʋ) of nanocomposite samples sintered for 1 h in argon
Figure 13. (a) Longitudinal and (b) shear ultrasonic velocities of Cu/SiC nanocomposites at different sintering temperatures.
Fig. 14 displays the room temperature (i.e., 30°C) compressive stress–strain curves for the Cu/SiC nanocomposites with different SiC contents and sintered at 775 and 875°C. The ultimate strength, yield strength, and elongation are listed in Fig. 15. The strength revealed significant improvement with the addition of nano-SiC particles and the increased sintering temperature. Moreover, the addition of SiC particles was responsible for a considerable decrease in the ductility of the Cu matrix while it increased with increased sintering temperature. The elongation was reduced from ~29.7% for Cu matrix to ~19.2% for the Cu with 8wt% SiC nanocomposite sintered at 775°C; however, when the sintered temperature was 875°C, the elongation was reduced from 33.7% to 21%. The apparent strengthening efficiency (i.e., Ra) of nanocomposites samples as a function of the SiC content is presented in Fig. 16. The improvement in Ra was a result of increasing both the SiC content and sintering temperature. The Ra value can be calculated as follows :
Figure 15. (a) Ultimate strengths, (b) yield strengths, and (c) elongations of Cu/SiC nanocomposites samples.
where σc and σm are the yield strength of nanocomposite and Cu matrix, respectively; V is the volume fraction of the reinforcement particles. Based on these results, adding various weight percentages of nano-SiC to the Cu matrix remarkably improved the mechanical properties, including the Vickers microhardness, elastic moduli, ultimate strength, yield strength, and strengthening efficiency. These improvements may be a result of the addition of very hard SiC particles to the ductile Cu matrix, uniform distribution of nano-SiC particles, and grain refinement .
The addition of SiC particles to the Cu matrix also contributes to transferring the applied load from the Cu matrix to SiC, which consequently increases the resistance to the plastic deformation of the sintered nanocomposite samples because of the variation in CTE value of the Cu matrix and SiC reinforcement particles. Accordingly, the thermal mismatch increases the dislocation density in the vicinity of SiC (Section 3.1.1), increasing the strength of the nanocomposite samples. The thermal stresses enhance the microhardness of the nanocomposites and flow stresses in the samples, making the plastic deformation more difficult . Furthermore, the grain boundaries increase because of grain refinement that occurs after milling . The interaction between the reinforcement and dislocations is a major reason for the strength of the composite materials. The "Orowan mechanism" describes that the passage of dislocations via particles introduces residual dislocation loops around each particle. Indeed, these particles act to enhance the strength of the material by prohibiting the migration of dislocations [5,44].
Alternatively, the decrease rate in ductility of the nanocomposite sample exhibits a considerable increase because of the successive addition of SiC particles. The main reason behind this reduction in ductility with the addition of SiC is the weak bonds between SiC and the Cu matrix, which act as stress concentrators and contribute to the formation of micro cracks, propagation, and fracture of the nanocomposite samples in compression . In general, the enhancement in the mechanical properties with increasing sintering temperature from 775 to 875°C is closely correlated to the densification of the studied sample. The increasing sintering temperature causes a decrease in the inter-atomic spacing, which consequently increases the propagation of ultrasonic waves in the nanocomposites samples and therefore increases the value of the elastic moduli and acceleration of the diffusion process .
The dependence of electrical conductivity of the sintered nanocomposites on SiC content, at two different sintering temperatures, is illustrated in Fig. 17. Notably, the electrical conductivity decreased with an increase in SiC particle content, while it increased with increasing sintering temperature. The conductivity of metal is highly dependent on the movement of electrons into the structure. However, the addition of SiC particles contributed to distorting this structure and hindering the movement of the Cu electrons, and accordingly, the conductivity was reduced . After the milling process, the grain boundaries increased as a result of grain refinement, which prohibited the electron path. In addition, the dislocations and porosity increased as a result of the ceramic particles, leading to an electron scattering phenomenon [3,48]. When increasing the sintering temperature, the conductivity of the nanocomposite samples increased as a result of the reduced porosity. Similar results were reported by Islak , Ayyappadas et al. , and Taha and Zawrah . Moreover, Moustafa and Taha  reported that the electrical conductivity is directly proportional to the sintering temperature of the Cu matrix composites.
Evaluation of the microstructure, thermal and mechanical properties of Cu/SiC nanocomposites fabricated by mechanical alloying
11 June 2020
Revised: 24 August 2020
Accepted: 25 August 2020
Available online: 27 August 2020
Abstract: Nano-sized silicon carbide (SiC: 0wt%, 1wt%, 2wt%, 4wt%, and 8wt%) reinforced copper (Cu) matrix nanocomposites were manufactured, pressed, and sintered at 775 and 875°C in an argon atmosphere. X-ray diffraction (XRD) and scanning electron microscopy were performed to characterize the microstructural evolution. The density, thermal expansion, mechanical, and electrical properties were studied. XRD analyses showed that with increasing SiC content, the microstrain and dislocation density increased, while the crystal size decreased. The coefficient of thermal expansion (CTE) of the nanocomposites was less than that of the Cu matrix. The improvement in the CTE with increasing sintering temperature may be because of densification of the microstructure. Moreover, the mechanical properties of these nanocomposites showed noticeable enhancements with the addition of SiC and sintering temperatures, where the microhardness and apparent strengthening efficiency of nanocomposites containing 8wt% SiC and sintered at 875°C were 958.7 MPa and 1.07 vol%−1, respectively. The electrical conductivity of the sample slightly decreased with additional SiC and increased with sintering temperature. The prepared Cu/SiC nanocomposites possessed good electrical conductivity, high thermal stability, and excellent mechanical properties.