*3.3. Sintering Behaviors of the Ti3SiC2/Cu Composites*

It is instructive to explain the sintering process of the Ti3SiC2/Cu composites at different temperatures. The relationships of temperature, current and axial dimension with time is illustrated in Figure 7. In comparison, although the temperature difference for the three sintering temperatures was the same (950–1000 ◦C and 1000–1050 ◦C), the change in the axial dimension was different. The change in the axial dimension for 950–1000 ◦C was obviously lower than that for 1000–1050 ◦C.

The profile of temperature, current and axial dimension with time was insensitive with the sintering temperature (see Figure 7a–c). We took 1000 ◦C as an example to elaborate the related connections among the temperature, the current, the axial dimension and time. As seen in Figure 7b, from room temperature to 600 ◦C (Note: without monitoring temperature by the infrared thermometer), the heating rate was fast (approximate 100 ◦C/min) in order to remove moisture to dry powder completely, thus the current increased rapidly to 2300 A. During this period, the decrease in the axial dimension was due to the action of pressure and the softening due to the heating, and the first maximum of the axial dimension at about 5 min resulted from the thermal expansion and contraction. Hereafter, from 600 to the sintering temperature (for example, 1000 ◦C), the heating rate was set as 50 ◦C/min; therefore, the current rapidly adjusted to a low value (about 750 A) and then increased at a proper rate to make sure the sintering temperature intelligently controlled. At the same time, the axial dimension was controlled by a comprehensive impact of pressure, current and the thermal expansion and contraction, and it kept relatively stable before 10 min, then it remarkably decreased from 800 ◦C to the sintering temperature (for example, 1000 ◦C). The rapid decrease in the axial dimension was mainly attributed to the densification effect due to the continuous pressure at the sintering temperature and possibly due to the reaction between Ti3SiC<sup>2</sup> and Cu. Finally, in the holding period, both the temperature and the current were constant, and the axial dimension slowly decreased as a result of

the further densification. Additionally, an extensive reaction between Ti3SiC<sup>2</sup> and Cu was partly beneficial for the decrease in the axial dimension.

**Figure 7.** The relationship of the temperature, the current and the dimensional change with sintering time for the Ti3SiC2/Cu composites sintered at (**a**) 950 ◦C, (**b**) 1000 ◦C and (**c**) 1050 ◦C.

The proposed sintering process of the Ti3SiC2/Cu composites is shown in Figure 8. In the initial state (see Figure 8a), Ti3SiC<sup>2</sup> and Cu were relatively uniformly distributed in the graphite die, which corresponded to the state of the first 10 min in Figure 7. During the initial stage, there was no reaction between Ti3SiC<sup>2</sup> and Cu, and the softening of the powder and the thermal expansion and extraction were due to the action of the current and the pressure. In the transition state (see Figure 8b), under the synergetic effect of the current, temperature and pressure, Cu locally melted and was extruded into the grain boundary of Ti3SiC2.

On the other hand, Ti3SiC<sup>2</sup> grains underwent plastic deformation under the same condition. All these factors increased the contact area of Ti3SiC<sup>2</sup> and Cu. Additionally, the de-intercalation of Si atoms from Ti3SiC<sup>2</sup> and its diffusion contributed to the reaction between Ti3SiC<sup>2</sup> and Cu, leading to the formation of Cu3Si and TiC, which was distributed along the grain boundary of Ti3SiC<sup>2</sup> grains. Moreover, the quasi-liquid Cu diffused into the inner part of Ti3SiC<sup>2</sup> grains through the holes or pores produced by the mismatch of skeleton density of the original Ti3SiC<sup>2</sup> and the as-formed TiC and reacted further with the Si atoms de-intercalated from the inner Ti3SiC<sup>2</sup> grains. For the Ti3SiC<sup>2</sup> grain surrounded by Cu, its surface was continuously transformed into TiC, which was accompanied by the formation of Cu3Si. Accordingly, the grain consisting of Cu3Si and TiC, which was embedded by Ti3SiC2, was expected (see the inset of Figures 3b and 8b). Therefore, the grain size of Ti3SiC<sup>2</sup> became smaller and smaller, which was inconsistence with the result in Figure 2. This state (the transition state in Figure 8b) corresponded to the rapid decrease in the axial dimension in Figure 7. In the final state (see Figure 8c), the reaction of Ti3SiC<sup>2</sup> and Cu continued, and the pores were filled by the product, densifying the composites. This

final state corresponds to the holding period in Figure 7. Consequently, the Ti3SiC2/Cu composites with superior mechanical properties were obtained at 1000 ◦C for 20 min by SPS. The proposed sintering process undoubtedly inspired us to further explore the sintering of Ti3SiC2/Cu composites in detail.

**Figure 8.** Proposed schematic illustration showing the sintering process of the Ti3SiC2/Cu composites: (**a**) initial state, (**b**) transition state and (**c**) final state.
