**1. Introduction**

Ti3SiC<sup>2</sup> belongs to one of the MAX phases (polycrystalline nanolaminates of ternary carbides and nitrides, over 60 + phases) [1]. It possesses the combined characteristics of metal and ceramics, such as being easily machinable, being electrically and thermally conductive, being oxidation resistant, and having a high melting point. Ti3SiC<sup>2</sup> can be a potential candidate for structural functional components. Similar to graphite and MoS2, it has a hexagonal structure, endowing it with an exceptional lubricating property. Research on the tribological properties of Ti3SiC<sup>2</sup> demonstrated that it is not a special solid lubricant. Contrarily, Ti3SiC<sup>2</sup> generally has a high friction coefficient (µ > 0.6) and a high wear rate (WR: in the order of 10−2–10−<sup>3</sup> mm3/N m), especially at medium and low temperatures (RT) [2–8]. It has been reported that, in some circumstances (such as with temperature > 600 ◦C, at a speed of >5 m/s, or sliding against some particular tribopair material), polycrystalline Ti3SiC<sup>2</sup> displays the relatively low µ and wear rate, which mainly results from the tribo-oxidation transfer films generated on the contact surface [2,6,8–10]. In other situations, the fracture and pulling-out of Ti3SiC<sup>2</sup> grains causes three-body abrasive wear, which is the main wear mechanisms for Ti3SiC2.

Subsequently, most of scientific enthusiasts concentrate on the intensifying research of polycrystalline Ti3SiC<sup>2</sup> by uniting metal and/or ceramic to enhance its friction and wear behaviors. Ren et al. [11] prepared Ti3SiC2/Cu/Al/SiC composites, and they found that the tribological behaviors of the composite were better than those of polycrystalline Ti3SiC<sup>2</sup> at RT and 200 ◦C, while the wear properties of the composite were worse than

**Citation:** Zhang, R.; Zhang, H.; Liu, F. Microstructure and Tribological Properties of Spark-Plasma-Sintered Ti3SiC2-Pb-Ag Composites at Elevated Temperatures. *Materials* **2022**, *15*, 1437. https://doi.org/ 10.3390/ma15041437

Academic Editor: Tomasz Trzepieci ´nski

Received: 29 December 2021 Accepted: 10 February 2022 Published: 15 February 2022

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that of polycrystalline Ti3SiC<sup>2</sup> at elevated temperatures (≥400 ◦C). The incorporation of Al, Cu, SiC and Al2O<sup>3</sup> played an important role in reinforcing the bonding of Ti3SiC<sup>2</sup> grains and in fixing the soft Ti3SiC<sup>2</sup> matrix around them during reciprocate sliding at RT–200 ◦C, where the wear mechanism was abrasive wear. At elevated temperatures (≥400 ◦C), the plastic flow of tribo-oxides layers led to the higher wear rate of the composite than that of Ti3SiC<sup>2</sup> where the wear mechanism was adhesive wear. Yang et al. [12] prepared Cu-Ti3SiC<sup>2</sup> co-continuous composites. They showed excellent electrical conductivity and wear resistance due to the addition of Cu. Yang et al. [13] explored the tribological behaviors of (TiB<sup>2</sup> + TiC)/Ti3SiC<sup>2</sup> composites. It was discovered that µ of the composites was bigger compared with that of Ti3SiC2. The addition of TiB<sup>2</sup> and TiC could fix the soft matrix around them and scatter the shear stresses. In our earlier work [14], we discussed the tribological behavior of Ti3SiC2/CaF<sup>2</sup> composites at elevated temperatures. The prepared Ti3SiC2/CaF<sup>2</sup> exhibited better friction and wear property than Ti3SiC<sup>2</sup> in a wide temperature range due to the tribo-oxide competition. Islak et al. [15] investigated the effect of reinforcing TiB<sup>2</sup> particles (5, 10, and 15 wt.%) on the properties of as-synthesized Ti3SiC2. The addition of TiB<sup>2</sup> improved the mechanical properties and thermal diffusivity of the composites. Moreover, the addition of TiB2 significantly increased the wear properties of Ti3SiC<sup>2</sup> matrix. Magnus et al. [16] prepared the Ti3SiC2-TiSi2-TiC composites by spark plasma sintering (SPS). They discovered that the tribological property of the composites was attributed to the intrinsic lubricity and the addition of second phase TiC particles. During the sliding, the tribo-oxidative wear was the main wear mechanism; then, it changed to the deformationinduced wear. The transition resulted from the breakdown of the formed tribofilm.

Silver shows some outstanding characteristics (such as good thermal conductivity, favorable ductility, and special mechanical behaviors) and can be widely applied in many fields, such as air-foil bearings, and biomedical and thermal interface materials. It is commonly utilized as a solid lubricant at temperatures below 500 ◦C. Additionally, it has a high diffusion coefficient and easily generates a lower shear stress intersection at the sliding interface [17,18]. Thus, it was significant and practical to incorporate good properties of Ag with Ti3SiC2. The goal of this work was to synthesize Ti3SiC2/Ag composites and to uncover the mechanical and tribological behaviors of the composite at raising temperatures. F. AlAnazi et al. [19] prepared metals-based (Ag and Bi) composites incorporation with 5 vol%, 10 vol%, 20 vol%, and 30 vol% Ti3SiC<sup>2</sup> and investigated their tribological properties. They found that the incorporation of Ti3SiC<sup>2</sup> improved the hardness and compressive yield strength of the two composites. Additionally, the addition of Ti3SiC<sup>2</sup> enhanced the tribological performance of the two composites. Zeng et al. [20] inspected the tribological behaviors of Ti3SiC2/Ag composites with Inconel 718 as the tribopair at RT. At different sliding speeds, the composites showed lower µ and wear rate than that of Ti3SiC2.

Pb is lubricous due to its low shear strength. In our previous work [21,22], we found that the incorporation of PbO (melting point: 888 ◦C) instead of Pb (melting point: 327 ◦C) in Ti3SiC<sup>2</sup> can successfully prepare Ti3SiC2/Pb composites and avoid the loss of Pb during the sintering process. The as-prepared composite showed better friction and wear behaviors than Ti3SiC<sup>2</sup> at elevated temperatures.

As a result of the above-mentioned facts, it was necessary to suggest some novel composites that can move forward a single step in strengthening the tribological performance of Ti3SiC<sup>2</sup> in a wide temperature range. Therefore, in this work, the metal Pb was added to the Ti3SiC2/Ag composite to prepare Ti3SiC2-Pb-Ag composites (TSC-PA). To prevent the loss of Pb during the process of preparing the TSC-PA, PbO was chosen as the source of Pb. The target of this job was to synthesize TSC-PA and to discuss the mechanical and tribological behaviors of the as-synthesized compound at elevated temperatures.
