**1. Introduction**

As a typical ternary layered MAX phase, Ti3SiC<sup>2</sup> exhibited combined characteristics of both metals and ceramics. It possessed good machinability, good electrical and thermal conductivity, good ductility, thermal stability, oxidation resistance, etc. [1–4]. It has potential for applications in electro friction materials, novel structural/functional ceramic materials and high-temperature lubricating materials.

The crystalline structure of Ti3SiC<sup>2</sup> can be described as a sandwich structure: a layer of Si and the twin boundary of TiC. The chemical bonding between the Si atom and the other atoms, such as Ti and C, was to a certain extent relatively weak compared to the strong Ti-C bonding [5,6]. Based on its unique structure, Ti3SiC<sup>2</sup> was chemically reactive when it contacted with metals at high temperatures. Li et al. [7] fabricated Ti3SiC2/Ni

**Citation:** Zhang, R.; Chen, B.; Liu, F.; Sun, M.; Zhang, H.; Wu, C. Microstructure and Mechanical Properties of Composites Obtained by Spark Plasma Sintering of Ti3SiC2-15 vol.%Cu Mixtures. *Materials* **2022**, *15*, 2515. https:// doi.org/10.3390/ma15072515

Academic Editors: Mattia Biesuz and Dina Dudina

Received: 13 February 2022 Accepted: 28 March 2022 Published: 29 March 2022

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and Ti3SiC2/Co through vacuum sintering. It was found that the metals (Ni and Co) were prone to aggregating towards their surfaces due to the poor wettability between Ti3SiC<sup>2</sup> and Ni or Co. Gu et al. [8] investigated the possibility of fabricating Ti3SiC2-Ti composites. They discovered that the as-obtained composites were mainly composed of Ti3SiC2, TiCx, Ti5Si3, and TiSi2. It was inferred that the decomposition of Ti3SiC<sup>2</sup> was owing to the de-intercalation of Si and the separation of carbon from Ti3SiC2. Gu et al. [9] studied the reactions between Ti3SiC<sup>2</sup> and Al in the temperature range of 600–650 ◦C, and they found that besides Ti3SiC<sup>2</sup> and Al, new phases Al3Ti, Al4SiC<sup>4</sup> and Al4C<sup>3</sup> were generated, the formation of which relied on the time, temperature and the relative amount of Al and Ti3SiC2. Kothalkar et al. [10] synthesized NiTi-Ti3SiC<sup>2</sup> composite, and they discovered that the composites showed higher damping up to applied stress of 200 MPa. For example, the energy dissipation of the composite was thirteen times larger than pure Ti3SiC<sup>2</sup> and two times larger than pure NiTi.

Except for the Ti3SiC2-metals composites described above, researchers concentrated on the investigation of Ti3SiC2/Cu composites. Dang et al. [11] fabricated Ti3SiC2/Cu composites with different contents of Cu by mechanical alloying and spark plasma sintering. They inferred that the presence of Cu leads to the decomposition of Ti3SiC<sup>2</sup> to form TiCx, Ti5Si3Cy, Cu3Si, and TiSi2Cz. In another paper, Dang et al. [12] synthesized Ti3SiC2/Cu/Al/SiC composites by powder metallurgy/spark plasma sintering and found that the addition of Al could inhibit the decomposition of Ti3SiC2. Lu et al. [5] found chemical reaction between Cu and Ti3SiC<sup>2</sup> contributes to the wettability. Zhou and his colleagues [13] investigated the chemical reactions and stability of Ti3SiC<sup>2</sup> in Cu for the Cu/Ti3SiC<sup>2</sup> composites. They found that at low content of Ti3SiC<sup>2</sup> or below 1000 ◦C, Cu (Si) solid solution and TiCx were generated, whereas at high temperature or high content of Ti3SiC2, Cu-Si intermetallic compounds, such as Cu5Si, Cu15Si4, and TiCx, were generated. In our recent publication [14], Ti3SiC2/Cu composites were synthesized by spark plasma sintering technique at various temperatures. The microstructure, composition and mechanical properties of the as-obtained composites were investigated. The results indicated that the Ti3SiC2/Cu composite sintered at 1100 ◦C exhibited superior mechanical properties. While these results give partial information on the reactions occurring between Ti3SiC<sup>2</sup> and Cu, they did not permit elucidation of the entire sintering behaviors occurring during the sintering process.

In this paper, the Ti3SiC2/Cu composites were fabricated by Spark Plasma Sintering (SPS) at different sintering temperatures. The sintering behaviors of the composites were explored by the characterization of the phase composition, microstructure and mechanical properties of the composites based on the relationship of the temperature, the current and the pressure with the time during the sintering process.
