# Production and Characterization of Cu/CNT Nanocomposites
## 3 Results and Discussion
### Strengthening Mechanisms  
The nanocomposites were produced with different volume fractions of CNTs. Figure 5 shows the volume fraction's influence on the hardness of the nanocomposites. There is an increase in hardness for 1.0% vol. of CNTs, and a decrease in hardness is observed for higher fractions of CNTs. The rise of CNT content above 1.0% vol. leads to larger clusters, increasing the heterogeneous distribution of CNTs and thus softening the nanocomposites. These results are similar to those already observed for Ni/CNT and Al/CNT nanocomposites produced by the same route [9; 20; 21; 22; 23; 24]. The effectiveness of the dispersion process in these nanocomposites can be seen in the results presented in Figure 6. Figure 6a shows the evolution of the percentage of CNT pores and agglomerates with the volume fraction of the reinforcement. Approximately 0.5% pores can be observed for the Cu sample, but with the addition of 1.0% vol. CNTs or more, the  
Figure 4: (**a**) SEM and (**b**) TEM images of ultrasonication CNTs, (**c**) distribution of outer diameter of the as-received and dispersed CNTs by ultrasonication, (**d**) HRTEM image of the CNT dispersed by ultrasonication showing in the region marked the magnification image the structure and the number of the walls, and (**e**) Raman results for the as-received CNTs and CNTs dispersed by ultrasonication and ball milling for 1 h.  
pore fraction increases, which is associated with agglomerates of CNTs. CNTs are present in the Cu matrix as agglomerates, mainly in the grain boundaries. Still, some can also be observed individually in the matrix, as previously reported [20,21]. In the SEM image of Figure 6b, a nanocomposite's microstructure and a CNT cluster's detail can be observed. Measurements of CNT pores and clusters for the nanocomposite samples showed that their percentage increases with an increase in the reinforcement volume fraction. Increasing the reinforcement fraction makes it challenging to obtain a homogeneous dispersion of CNTs, as a more significant amount of pores and clusters of CNTs appear in the microstructure. This behavior can be explained by the processing conditions, especially dispersion, which limits the fraction of CNTs successfully dispersed in metal matrices. Since the increase in hardness was not significant with the addition of CNTs (a 17% increase over the hardness of the copper matrix), tensile tests were performed to further investigate the effect of CNT strengthening on the nanocomposites. Figure 7 and Table 1 show the tensile test results and the relative density of the sintered samples. The relative density  
Figure 5: Evolution of the microhardness of Cu/CNT nanocomposites with the fraction of CNTs. Figure 6: (**a**) Evolution of the fraction of pores and agglomerates with the volume fraction of the CNTs and (**b**) SEM image of the cross-section of a nanocomposite with 1.0% vol. the marked region in the figure represents the magnified area showing the CNTs. is somewhat lower for the nanocomposites than for the Cu matrix, which agrees with what was already mentioned. However, the cold rolling process mainly aims to increase these values for the nanocomposites. As for the tensile test results, a more significant increase in tensile strength is observed when CNTs are added to the matrix (67% increase over the Cu matrix value). Based on these results, it can be indicated that the process proved to be effective in the production of Cu/CNT nanocomposites since, for instance, [PERSON] et al. [18] report a 44% increase in the mechanical strength of Cu/CNT nanocomposites produced by ball milling as a dispersion process followed by the sintering and cold rolling processes. The improvement in the mechanical properties of the nanocomposites with 1.0% vol. can be attributed to different strengthening mechanisms that can act simultaneously. Different characterizations were performed to determine the strengthening mechanisms acting simultaneously on these nanocomposites.