*3.2. Mechanical Property*

The variation in relative density, hardness and compressive strength of the Ti3SiC2/Cu composites with the sintered temperature are shown in Figure 5. As seen in Figure 5, the relative density of the Ti3SiC2/Cu composites sintered at 950 ◦C was 95.35%, which was slightly lower than that sintered at 1000 ◦C (96.43%). However, the relative density of the composite sintered at 1050 ◦C was 93.85%, which was the lowest among the three samples. The higher relative density of the Ti3SiC2/Cu composites sintered at 1000 ◦C was attributed to the synergetic effect of high temperature, high pressure and pulse current during the sintering process. The most important thing was that the quasi-liquid Cu possessed proper mobility, which was beneficial for the densification of the composites as well. At 1050 ◦C, its lowest relative density was also related to the flowability of Cu. In such circumstances, Cu flowed easily and released rapidly, causing the aggregation of Cu (see Figure 4c) and the loss of Cu (see Figure 3c).

**Figure 5.** The variation in relative density, hardness and compressive strength of the Ti3SiC2/Cu composites versus the sintered temperature.

As shown in Figure 5, both the hardness and the compressive strength of the Ti3SiC2/Cu composites increased with the increase in the sintered temperature. It was clearly seen that the deviation of the hardness of the composites sintered at 950 ◦C was the highest among the three samples, which originated from the agglomeration of Cu in the composites (see Figures 3a and 4a). Compared with the polycrystalline Ti3SiC<sup>2</sup> (5.5 GPa) [21], the higher hardness of the composites at different temperatures came from the formation of hard TiC product, which was detected by XRD analysis (see Figure 1).

Additionally, in comparison with the polycrystalline Ti3SiC2, the compressive strength of the composites sintered at different temperatures exhibited an equivalent or higher value [22], which was due to the appropriate reaction between Ti3SiC<sup>2</sup> and Cu. As mentioned above, during the sintering process, the de-intercalation of Si from Ti3SiC<sup>2</sup> and thereafter the dissolution of it in the liquid Cu phase led to the formation of TiC and Cu3Si. Moreover, the Cu3Si is uniformly distributed along the grain boundary of Ti3SiC2. The as-obtained fine TiC and Cu3Si grains were uniformly distributed along the boundary of Ti3SiC<sup>2</sup> grains, which was a benefit for the higher compressive strength of the composite. As seen in Figure 6a, the Ti3SiC2/Cu composites showed a brittle fracture character, which was identical with polycrystalline Ti3SiC2. From the compression fracture morphology of the composites (see Figure 6b–d), it was apparently seen that both the intergranular fracture and transgranular fracture were present on the compression fracture surface of the

composites sintered at different temperatures. It indicated that the fracture mode of the composites was independent of the sintered temperature.

**Figure 6.** (**a**) Compressive stress–strain curve of Ti3SiC2/Cu composites, and the fracture morphology of Ti3SiC2/Cu composites sintered at (**b**) 950 ◦C (inset showed the structure at a higher magnification), (**c**) 1000 ◦C and (**d**) 1050 ◦C.
