*3.1. Phase Composition and Microstructure*

Figure 1 shows XRD patterns of the as-synthesized Ti3SiC2/Cu composites at different sintered temperatures. It was clearly observed that the as-synthesized Ti3SiC2/Cu composites were all composed of Ti3SiC2, Cu3Si and TiC, which was independent of the sintered temperature.

**Figure 1.** XRD patterns of Ti3SiC2/Cu composites at different sintered temperatures.

Based on the unique sandwich structure of Ti3SiC2, it exhibited high reactivity when contacting with metal phases [5,11,15–19]. On the one hand, the Si atoms were easily deintercalated from Ti3SiC2. On the other hand, Si-containing solid solution or intermetallic

compounds was prone to form when the metal phases contacted with Ti3SiC2. The presence of contacted metal phases accelerated the decomposition of Ti3SiC2. As for Ti3SiC2/Cu composites, Cu can form Cu (Si) solid solution or react with Si to form CuxSi<sup>y</sup> intermetallic compounds [18]. Therefore, the composition of Ti3SiC2/Cu composites consisted of Cu(Si) solid solution, CuxSi<sup>y</sup> intermetallic compounds, and TiCz, which were commonly examined by researchers [5,11,13].

In this study, with the change in sintered temperatures, Cu3Si was the only Cu-Si intermetallic compound, and TiC was the only decomposed product of Ti3SiC2. According to Guo et al. [20], when the content of Cu was less than that of Si, Cu3Si was the preferential product of the Cu-Si system. Because based on the binary phase diagram of Cu-Si, the content of Si in Cu3Si (22.2–25.2%) was the maximum among six copper silicides (Cu (Si), Cu7Si, Cu5Si, Cu4Si, Cu15Si<sup>4</sup> and Cu3Si) [20]. This explained the reason why Cu3Si was the single Cu-Si intermetallic compound in our present study.

The effect of sintered temperature on the microstructure is shown in Figure 2. As seen in Figure 2a, at 950 ◦C, the typical plate-like morphology of Ti3SiC<sup>2</sup> grains was evidently inhibited by the addition of Cu. A considerable amount of Ti3SiC<sup>2</sup> equiaxed grains appeared due to the reaction between Ti3SiC<sup>2</sup> and Cu. Additionally, with the increase in the sintered temperature from 950 ◦C to 1050 ◦C, the Ti3SiC<sup>2</sup> granules decreased, which was accompanied by fewer pores or holes, indicating higher reactivity of Ti3SiC<sup>2</sup> and Cu.

**Figure 2.** Optical micrographs of polished and etched Ti3SiC2/Cu composites sintered at 950 ◦C (**a**), 1000 ◦C (**b**) and 1050 ◦C (**c**).

The back scattering electron images of polished and etched Ti3SiC2/Cu composites sintered at 950 ◦C, 1000 ◦C and 1050 ◦C are shown in Figure 3. As mentioned above, the main composition of Ti3SiC2/Cu composites were Ti3SiC2, Cu3Si and TiC. Ti3SiC<sup>2</sup> was located at the dark grey area in Figure 3; both Cu3Si and TiC were located at the light grey area in Figure 3. It was clearly seen from Figure 3 that the as-formed Cu3Si distributed along the grain boundary of Ti3SiC2, and it was accompanied by the formation of hard TiC particles. Moreover, with the increase in the sintered temperature, the reaction between Ti3SiC<sup>2</sup> and Cu became more severe, resulting in the formation of Cu3Si with a non-negligible amount, especially at 1000 ◦C. The reaction mechanism between Ti3SiC<sup>2</sup> and Cu was proposed as follows. After milling, the Cu powder is relatively evenly distributed in Ti3SiC<sup>2</sup> powder. At elevated temperatures (for example, 900 ◦C), the Si atoms deintercalated from Ti3SiC<sup>2</sup> grains, diffused rapidly around Cu and reacted with Cu to form Cu3Si. Meanwhile, the original Ti3SiC<sup>2</sup> skeleton structure transformed to TiC structure, which was concomitant with the formation of pores or holes due to the mismatch of skeletal density between Ti3SiC<sup>2</sup> and TiC. When the sintered temperature raised (for example, 1000 ◦C), the de-intercalation of Si atoms from the Ti3SiC<sup>2</sup> skeleton was accelerated by the defects (pores or holes) and high temperature. On the other hand, Cu had a tendency to melt and flow around the grain boundaries of Ti3SiC<sup>2</sup> grains and the as-formed defects mentioned above. All these led to more violent reactions between Ti3SiC<sup>2</sup> and Cu, causing the Ti3SiC<sup>2</sup> grains to become smaller and smaller. This reaction process well coincided with

Zhou et al. [13]. Theoretically, with the increase in temperature, the reactivity of Ti3SiC<sup>2</sup> and Cu increased, producing more Cu3Si. However, when the sintered temperature reached 1050 ◦C, a large amount of Cu melted and released from the graphite die, leading to the loss of Cu. As seen in Figure 3, visually, the content of Cu3Si was largest at 1000 ◦C, not at 1050 ◦C.

**Figure 3.** BSE micrographs of the Ti3SiC2/Cu composites sintered at (**a**) 950 ◦C, (**b**) 1000 ◦C (inset showed the structure at a higher magnification) and (**c**) 1050 ◦C (inset showed the structure at a higher magnification).

Figure 4 illustrates the mapping of elemental distribution for Ti3SiC2/Cu composites sintered at different temperatures. It was irrefutable evidence for the explanation of the sintering effect of the composites. At 950 ◦C, the Cu considerably aggregated in the Ti3SiC2/Cu composites (see Figure 4a). At this temperature, a solid–solid sintering process occurred. The agglomeration of Cu relied on the uniformity of its distribution in Ti3SiC<sup>2</sup> during ball milling. As we know, ball milling cannot avoid material agglomeration. At 1000 ◦C, it was obviously observed that Cu was relatively evenly distributed in the Ti3SiC2/Cu composites (see Figure 4b). It was speculated that solid–liquid sintering process took place at 1000 ◦C. The appropriate flowability of the quasi-liquid Cu contributed to not only its reaction with Si atoms but also its uniform distribution in the composites. At 1050 ◦C, the Cu melted and rapidly flowed around the Ti3SiC<sup>2</sup> grains, leading to the relatively obvious aggregation of Cu (see Figure 4c). Moreover, the release of liquid Cu during sintering caused the loss of Cu to a certain extent. Therefore, it was concluded that the optimal sintering temperature was 1000 ◦C for the Ti3SiC2/Cu composites in our study.

**Figure 4.** Mapping of elemental distribution for Ti3SiC2/Cu composites sintered at 950 ◦C (**a**), 1000 ◦C (**b**) and 1050 ◦C (**c**).
