3. Results and Discussion
3.1. Microstructure characterization
Fig. 3 shows the bright-field TEM micrographs of the unreinforced pure nickel together with the Ni–SiCNP composites. Evident dislocation lines are observed in the unreinforced pure nickel, as shown in Fig. 3(a) and no other defects are observed in the sample. Nevertheless, a large number of well-dispersed SiCNP and defects are seen in the Ni–SiCNP composites as shown in Fig. 3(b–e). The SiCNP is bright in the bright-field TEM micrographs due to the low atomic numbers of Si and C atoms in comparison to Ni. The SiCNP size in the Ni–SiCNP composite with a milling time of 8 h is 30–100 nm, whereas a milling time of 24 h increases the size to around 30–150 nm due to an obvious coarsening effect of the SiCNP in the Ni–SiCNP composites. The SiCNP are uniformly dispersed in this latter case due to the increase in milling time. In addition, a visible coarsening of the SiCNP is seen, likely associated to the large size of the SiCNP which transpires with extended milling time from 24 to 48 h, as shown in Fig. 3(c–e). Taking into account the high melting point and high stability of SiCNP in the Ni matrix, it can be concluded that the coarsening of the SiCNP is likely due to their agglomeration during the ball milling process due to strong adhesion forces and the sintering of these agglomerates into single particles during SPS and heat treatment[37]. In addition, the phenomenon becomes more significant with an increase in milling time, which can be explained by the increased probability of contact between interspersed SiCNP that can agglomerate with the extension of milling time.