Evaluation of tribo-mechanical measurements and thermal expansion of Cu-based nanocomposites reinforced by high strength hybrid ceramics

It is known that Copper’s (Cu) electrical conductivity makes it a desirable material for use in industry. Due to poor properties such as hardness, thermal expansion, and corrosion resistance, its applications are limited. This manuscript solves these problems while maintaining no breakdown in electrical conductivity. In this study, high-strength ceramics (SiC nanoparticles and graphene nanosheets) were used as reinforcements in the manufacture of Cu-based hybrid nanocomposites using powder metallurgy technique. X-ray diffraction analysis (XRD) was used to investigate phase composition and crystal size of the milled powders. Transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM), respectively examined the microstructure of the prepared powder powders and sintered nanocomposites. Then, various properties of the sintered samples are measured, including physical, electrical and thermal properties and wear resistance. The obtained XRD technique and TEM images showed decreases in the crystal and particle size of milled samples reaching up to 14.08 and 28.30 nm, respectively for the sample contained 8 vol. % SiC + 0.8 vol. % graphene (SG8). A surprising improvement in the mechanical properties of up to 809.15, 341.84 MPa and 336.56 GPa for microhardness, strength and longitudinal modulus for the sample containing the highest reinforcements, achieving an improvement of up to 122, 61.37 and 41 percent compared to the Cu matrix. Moreover, there was a noticeable improvement in the coefficient of thermal expansion (CTE) and wear rate values of the samples by increasing the percentages of hybrid reinforcements in the examined sintered nanocomposite samples. The Sample SG8 recorded the lowest value, decreasing by about 50.2 and 76.5% compared to the SG1 sample. Finally, adding reinforcements to the Cu matrix had a negative effect on the relative density and electrical conductivity, and the lowest values was 92.94% and8.59 × 106 S/m, respectively for the SG sample.


Characterization of the sintered nanocomposites
The milled powders were pressed by a hydraulic press using a load of 30 MPa, After that, the sintering process was carried out at a temperature of 700, 800 and 850 °C in argon gas for 1 h and a heating rate of 5 °C/min.

Physical properties
The archimedes method (ASTM: B962-13) was used to study the relative density and apparent porosity of nanocomposites at different sintering temperature, as detailed in our recent work 26,27 .

FHSEM investigation
FHSEM coupled with energy dispersive X-ray analysis (EDX) (type Quanta FEG250 with an accelerating voltage of 30 kV and a magnification of 10 × up to × 300,000) was used to examine their microstructure.

Thermal properties
Thermal expansion of nanocomposites samples was measured from 30 up to 500 °C using an automatic Netzsch DIL402 PC (Germany) with a heating rate of 5 °C min −1 using rectangular bars.

Mechanical properties
As reported in our recent work, the Vickers tester was used to determine the microhardness (Hv) of the sintered samples in accordance with ASTM: B933-09, with an applied stress of 1.9 N for 10 s 28,29 .
where P is applied load (1.9 N) and d is the diagonal of indentation.
A hydraulic machine examined the ultimate strength according to ASTM E9.Ultrasonic longitudinal (V L ) and shear wave velocities (V S ) were measured in the nanocomposites, using the pulse-echo technique system.The values of Lame's constants are obtained from V L and V S as follows 30,31 : The longitudinal modulus (L), Young's modulus (Y), shear modulus (G), bulk modulus (B) and Poisson's ratio (ν) of the nanocomposite were calculated according to the formula 32,33 : Table 1.Scheme of the prepared composites referring to the sample name and its composition (vol.%).www.nature.com/scientificreports/

Wear test
The wear rate (W) measure was calculated by the following Eqs.( 12) and ( 13) using a pin-on-disc wear-testing apparatus 34 .The wear test process conditions included a speed of 0.8 m/s, a time 10 min, and applied loads of 10, 20, and 40 N.

Electrical conductivity
The electrical conductivity (σ) of the nanocomposites was measured e using Keithley device according to the formula 35 : where R, h and A are the electrical resistance, the sample diameter and the sample surface area.

X-ray analysis
The XRD patterns of powdered of milled Cu hybrid nanocomposites contain SiC nanoparticles and graphene nanosheets concentrations are shown in Fig. 1.According to the representation, only two phases (i.e.Cu and SiC) appear to have developed.These phases correspond to JCPDS-ICDD card numbers 85-1326 and 89-2225, respectively.Because the volume percentages of grapheme are small (≥ 0.8 vol.%) and also SiC in the SG1 and SG2 samples (1 and 2 vol.%, respectively) are below the XRD detection limit, there are no peaks representing the reinforcement 36 .Figure 2 shows how D, Ɛ, and δ change depending on the makeup of the milled powders.It is shown that the inclusion of both hard ceramic reinforcements, which function as milling balls and transmit more energy to the Cu matrix, causes the crystallite size to drop as SiC and graphene contents rise while lattice strain and dislocation density increase.Moreover, it has been demonstrated that because of extreme plastic deformation and grain size refinement during the milling process, the intensity of the peaks decreases and broadens as the volume percentage of ceramics reinforcements increases 37 .

TEM observations
Figure 3a,b shows TEM photos of SiC nanoparticles and graphene nanosheets reinforcements.The graphene powder appears in the form of sheets, while the SiC powder appears in the form of particles with a size of 27.4 nm.TEM images of powdered milled copper and its hybrid nanocomposites are shown in (Fig. 4a-c).The  www.nature.com/scientificreports/SG0 sample, as shown in (Fig. 4a), clusters together due to its flexibility and its 76.9 nm particle size.In contrast, the agglomeration has somewhat diminished, and the particle sizes of the SG4 and SG8 samples are 51.03 and 33.1 nm, respectively, in (Fig. 4b,c).Figure 5 shows the effect of added hybrid ceramic particles on the particle size of the Cu matrix.Generally, the ductile particles (Cu) suffer from deformation, and the brittle ones (SiC and graphene) suffer from fragmentation.Therefore, at the beginning of the milling process, the Cu particles start to weld, while the SiC and graphene ones come between two or more Cu particles.This means that the www.nature.com/scientificreports/fragmented ceramic particles stay at the interfacial boundaries of the welded Cu particles, and consequently, the nanocomposite powders are actually formed with reduced particle size.

Relative density and apparent porosity
The effect of sintering temperature and adding different volume contents of hybrid ceramics on the relative density and apparent porosity of the samples was measured and shown in Fig. 6 (a,b).It is evident that a rise in the volume percentage of ceramics in the nanocomposites causes the specimens' apparent porosity to increase and their densities to decrease.This outcome might be the result of the greater hardness of the reinforcement particles in the Cu matrix decreasing the pressing capacity of the sintered samples as the reinforcement contents increase.This result can be explained by knowing that the density of SiC and graphene (3.21 and 2.27 g/ cm 3 , respectively) is more than half that of Cu matrix (8.96 g/cm 3 ).Therefore, replacing a heavier element with a lighter heavier one leads to decreased densities and an increase the apparent porosity of sintered samples.Conversely, by forming necks between the particles and strengthening the connections between them, raising the sintering temperature effectively improves the relative density.Additionally, it has been observed that higher sintering temperatures accelerate solid-state diffusion, which improves densification behavior 38,39 .For example, at a sintering temperature of 700 °C, the relative density of the samples SG0, SG1, SG2, SG4, and SG8 are 92.38,91.81, 89.60, 87.77, and 84.89%, respectively, while the relative density of the same samples are 96.75, 96.54, 95.33, 94.06, and 92.41%, respectively.This trend can be more clarified using Eq. ( 11), in which it appears that the sintering temperature plays an essential role in the diffusion process 40 .
where D, D 0 , Q, R, and T represent for the diffusion coefficient, constant, activation energy, Boltzmann's constant, and temperature, respectively.www.nature.com/scientificreports/

Morphology of the sintered nanocomposites
Figure 7 illustrates the SEM images of SG1, SG2, SG4, and SG8 nanocomposites sintered at 850 °C for 1 h in an argon atmosphere.Generally, SiC nanoparticles and graphene nanosheets are found at the Cu matrix's grain borders.As for samples SG1 and SG2, the amount of hybrid reinforcement is small, therefore not clearly visible, and more homogeneous.On the other hand, with an increase in the amount of hybrid reinforcements, as is evident in samples SG4 and SG8, it appears clearly with a decrease in homogeneity.Notably, it was also shown that in the specimens under study, the porosity rose as the number of reinforcing particles rose.Nevertheless, better densification behavior is produced by raising the sintering temperature to 850 °C, which promotes the diffusion   www.nature.com/scientificreports/process throughout the heating phase.Strong reinforcement matrix interfacial bonding was achieved during the sintering of the nanocomposites samples, as seen by the expanding contact border between the particles.Figure 8 shows the EDX spectrum and mapping of the distribution of each component in SC8 sample.Based on the data obtained, one can conclude that there are no other components, and thus the presence of contamination during milling or sintering processes can be ruled out.Another conclusion obtained from this figure is that: A wellhomogeneous distribution of SiC nanoparticles and graphene nanosheets was obtained in Cu matrix.

Thermal expansion
The relative thermal expansion (ΔL/L) behavior of Cu and its nanocomposites was sintered at 850 °C at temperatures between 50 and 500 °C, as displayed in (Fig. 9).In general, as the temperature rose noticeably, the ΔL/L of the nanocomposites with different reinforcement concentrations increased.Additionally, it decreased as the amount of reinforcements increased.Figure 10 displays the CTE values of nanocomposites as calculated from the previous figure.The results showed that as the proportion of hybrid ceramics in the nanocomposites increased, the CTE value decreased.The CTE value for sample SG0 is 16.3 × 10 −6 /° C, which decreases to 15.9, 15, 13.6, and 11.4 × 10 −6 /° C for samples SG1, SG2, SG4, and SG8, respectively, which decreases by about 2.5, 8, 16.6, and 30.1% compared to the Cu matrix.The lower CTE of SiC and graphene (3.7 and 3.2 × 10 −6 /° C, respectively) than that of the Cu matrix (17 × 10 −6 /° C), which results from the bonding between the reinforcement and Cu matrix, is generally responsible for the lower CTE of the Cu matrix after the addition of hybrid reinforcements.Moreover, residual stresses arising from the thermal mismatch between the ceramics and the Cu matrix significantly influence the thermal expansion behavior of nanocomposite samples.The results agree with other literature [41][42][43] .

Mechanical properties
Figures 11-13 show the effect of hybrid ceramics and sintering temperature on microhardness, ultimate strength, and ultrasonic velocities, while the values of elastic moduli are listed in Table 2 of the sintered samples.It is evident that there are significant improvements in microhardness, ultrasonic velocities, and elastic moduli after adding hybrid ceramics compared to Cu matrix.On the other hand, increasing the sintering temperature has a positive effect on improving the previous properties.The microhardness values of the SG0, SG1, SG2, SG4, and Vol:.( 1234567890      www.nature.com/scientificreports/This noticeable improvement in the mechanical properties of the nanocomposites after adding hybrid ceramic is due to the good distribution of SiC nanoparticles and graphene nanosheets reinforcements in the Cu matrix, leading to the refinement of grain sizes and an increase in the number of grain boundaries, which act as barriers against dislocation movement 44,45 .This improvement can be better by observing this Eq.( 14) Ref. 46 .Moreover, a significant discrepancy in the CTE of the reinforcements (graphene nanosheet and SiC nanoparticle) and the Cu matrix contributes to the generation of thermally induced residual stresses.The thermal stresses produced in the Cu matrix, even at low temperatures, greatly increase the dislocation density close to the interface, strengthening the nanocomposite samples [47][48][49] .On the other hand, raising the sintering temperature causes an increase in atom diffusion, which expands the particle's surface area and, as a result, raises the density and decreases the porosity of sintered nanocomposites.

Wear behavior
Table 3 illustrates how the applied load and hybrid reinforcement contents affect the weight loss of nanocomposite specimens sintered at 700, 800, and 850 °C for one hour.Interestingly, the table shows that significant increases in the weight loss of all samples under examination occur when the applied load is increased during sliding wear testing.On the other hand, raising the amount of hybrid ceramics and the sintering temperature decreases weight loss.Based on the finding that samples SG0, SG1, SG2, SG4, and SG8 sintered at 850 °C had determined weight loss values of 13.87, 8.98, 5.93, 4.22, and 2.76 mg, respectively, when the applied force equals 10 N.After increasing the applied load to 40 N, the weight loss for the same sample were 18.71, 13.69, 12.24, 9.01, and 6.65 mg, respectively.Figure 14 shows that the wear rate of nanocomposite samples was calculated using previous weight loss results.The results of the study led to the conclusion that adding hybrid reinforcements to nanocomposites and increasing the sintering temperature are more beneficial for wear resistance, while increasing the load is the opposite.The wear rate of SG1, SG2, SG4, and SG8 samples sintered at 700 °C was 0.0231, 0.0190, 0.0162, 0.0115, and 0.0075 mg/s, respectively, when the applied force equals 10 N which decreased by about 17.7, 30.1, 50.2, and 67.50%, respectively, compared to the SG0 sample (0.0231 mg/s).When the sintering temperature rises to 850 °C, The wear rate of previous samples was 0.0248, 0.0205, 0.0159, 0.0115, and 0.0110 mg/s, respectively, when the applied force equals 10 N which decreased by about 13, 28.20, 44.43, 61.48, and 6.65 mg, respectively, compared to the SG0 sample (0.0286 mg/s).It is essential to highlight that the addition of SiC nanoparticles and graphene nanosheets to the Cu matrix enhances the microhardness and strength of nanocomposites samples, as previously discussed, and as a result, the weight loss and wear rate decreases in accordance with Archad Eq. ( 16) Ref. 50 .
W is wear rate, K is a wear coefficient (constant value).This can be demonstrated by showing the effect of microhardness on the wear rate of the samples under load 10 N as shown in (Fig. 15).Moreover, a decrease in the actual area of contact is associated with an increase in microhardness.Reduced actual area of contact results in significant reductions in wear rate since it is widely acknowledged that real area of contact may be described as the ratio of the normal load to the hardness of the pin material 51,52 .On the other hand, the noticeable improvement in weight loss and wear rate with increasing sintering temperature is a result of the improvement in relative density, which is an important reason for the improvement in microhardness.Moreover, it is widely acknowledged ( 16) that a rise in the load can result in a heating effect that causes convulsions and thermal softening.Moreover, it increases the surface area that slides when in contact, increasing the rate of wear 53 .
Using FESEM measurements, the worn surfaces of the SG0 and SG8 samples were examined under an applied stress of 40 N to evaluate the wear processes of the Cu matrix and its nanocomposites, as shown in (Fig. 16a,b).For the Cu matrix, only loose layers and grooves appear on the wear track as shown in (Fig. 16a).Surface delamination reveals adhesive wear, which includes crack initiation and propagation as well as final fracture of the material in the vicinity of the surface.Figure 16b shows that the SG8 sample has a smoother surface than the SG0 sample, and there is only sporadic debris and slight grooves on the worn surface.Some Cu matrix debris has flattened in the wear process because of its low microhardness.Very low cracks appear in this wear track, thus, the dominant wear mechanism is abrasive wear.

Electrical conductivity
The conductivity of each component, the amount, distribution, and size of reinforcement, the bulk density and Cu-reinforcements bonding, and the manufacturing process all unquestionably influence the electrical conductivity of nanocomposites.The electrical conductivity of sintered samples is shown in Fig. 17.Cu's electrical conductivity somewhat reduces as the hybrid reinforcement's volume percentage rises while increasing the sintering temperature has a positive effect on the electrical conductivity.Based on the finding that samples SG0, SG1, SG2,   S/m, respectively.The apparent decrease in conductivity is due to the moving electrons into the structure is a major factor in Cu conductivity.Nevertheless, the addition of graphene nanosheets and SiC nanoparticles helped to distort this structure and impair the flow of electrons, which decreased the conductivity.Furthermore, refining the grains during the milling process increased the grain boundaries, hindering the movement of electrons 54 .In contrast, the noticeable increases in conductivity of samples with increasing sintering temperature are a result of the decrease the porosity.

Conclusion
In this current study, the manufacturing of Cu-based hybrid nanocomposites with better mechanical properties, wear resistance, and CTE and using SiC nanoparticles and graphene nanosheets as reinforcements using the powder metallurgy (PM) technique was shown.After milling, the addition of hybrid reinforcement has an obvious effect on grain refinement, as the crystal and particle size decrease to 14.1 and 33.1 nm, respectively, for the sample containing high hybrid refinement contents (SG8).The finding showed that the addition of both hybrid reinforcements to the Cu base had a positive effect on improving mechanical properties, CTE value, and wear resistance and a slight negative effect on electrical conductivity.The apparent porosity of the nanocomposites increases with the addition of hybrid ceramics, while the relative density decreases.The maximum improved of microhardness, ultimate strength and bulk modulus about 122.1, 61.4, and 103.7%, respectably for the sample SG8 compared to Cu base (SG0).For the same sample, the maximum decrease in CTE value and wear rate about 50.2 and 80.13%, respectively Finally, increasing the sintering temperature has a wonderful effect on improving all

Figure 2 .
Figure 2. Effect of adding hybrid reinforcements on the, (a) crystal size, (b) lattice strain, and (c) dislocation density of milled powders.

Figure 6 .
Figure 6.(a) Relative density, and (b) apparent porosity of samples at different sintering temperatures.

Figure 8 .
Figure 8.(a) EDX mapping of all constituents of GS8 sample.

Figure 9 .
Figure 9. Relative thermal expansion of the sample sintered at 850 °C.

Figure 10 .
Figure 10.CTE value of the sample sintered at 850 °C.

Figure 11 .
Figure 11.Microhardness of the samples at different sintering temperatures.

Figure 12 .
Figure 12.Ultimate strength of the samples at different sintering temperatures.

Figure 13 .
Figure 13.(a) Longitudinal and (b) shear ultrasonic velocities of all samples at different sintering temperatures.

Figure 15 .
Figure 15.The effect of microhardness on the wear rate of the samples under load 10 N.

Figure 16 .
Figure 16.SEM images of wear tracks for (a) SG0 and (b) SG8 samples sintered at 850 and applied load 40 N.

Figure 17 .
Figure 17.Electrical conductivity of the samples at different sintering temperatures.

Table 2 .
The values of elastic moduli of nanocomposites sintered at 700, 800 and 850 °C., H C , H Cu, H C, H Graphene and H SiC represent the microhardness of the nanocomposite, Cu, graphene, and SiC, respectively, while F Cu , F Graphene and F SiC represent the volume fraction of Cu matrix, graphene nanosheets and SiC nanoparticles, respectively.

Table 3 .
The values of weight loss of nanocomposites sintered at different applied loads and sintering temperatures.