*3.1. Phase Composition and Microstructure of TSC-PA*

The XRD pattern of TSC-PA is displayed in Figure 1. As noticed in Figure 1, the main phases of TSC-PA were Ti3SiC2, Pb, and Ag. The Ti3SiC<sup>2</sup> was temporarily stable and did not break down during the sintering process of TSC-PA. To the best of the authors' knowledge, the Ti3SiC<sup>2</sup> revealed high reaction activity when in contact with the metal phases, such as Cu [23–25], Al [26,27], and Fe [28]. In this study, the Ti3SiC<sup>2</sup> did not decompose with the

co-addition of Ag and PbO at the sintering temperature of 1130 ◦C. In a previous paper [21], we reported that PbO in the Ti3SiC2-PbO system was deoxidized to Pb by C or Si during the preparation process and Ti3SiC<sup>2</sup> did not decompose at 1200 ◦C. The relevant chemical reactions were described in Reference [21]. In comparison, the addition of Ag did not change the composition of the Ti3SiC2-PbO system. compose with the co-addition of Ag and PbO at the sintering temperature of 1130 °C. In a previous paper [21], we reported that PbO in the Ti3SiC2-PbO system was deoxidized to Pb by C or Si during the preparation process and Ti3SiC<sup>2</sup> did not decompose at 1200 °C. The relevant chemical reactions were described in Reference [21]. In comparison, the addition of Ag did not change the composition of the Ti3SiC2-PbO system. compose with the co-addition of Ag and PbO at the sintering temperature of 1130 °C. In a previous paper [21], we reported that PbO in the Ti3SiC2-PbO system was deoxidized to Pb by C or Si during the preparation process and Ti3SiC<sup>2</sup> did not decompose at 1200 °C. The relevant chemical reactions were described in Reference [21]. In comparison, the addition of Ag did not change the composition of the Ti3SiC2-PbO system.

The XRD pattern of TSC-PA is displayed in Figure 1. As noticed in Figure 1, the main phases of TSC-PA were Ti3SiC2, Pb, and Ag. The Ti3SiC<sup>2</sup> was temporarily stable and did not break down during the sintering process of TSC-PA. To the best of the authors' knowledge, the Ti3SiC<sup>2</sup> revealed high reaction activity when in contact with the metal phases, such as Cu [23–25], Al [26,27], and Fe [28]. In this study, the Ti3SiC2 did not de-

The XRD pattern of TSC-PA is displayed in Figure 1. As noticed in Figure 1, the main phases of TSC-PA were Ti3SiC2, Pb, and Ag. The Ti3SiC<sup>2</sup> was temporarily stable and did not break down during the sintering process of TSC-PA. To the best of the authors' knowledge, the Ti3SiC<sup>2</sup> revealed high reaction activity when in contact with the metal phases, such as Cu [23–25], Al [26,27], and Fe [28]. In this study, the Ti3SiC2 did not de-

*3.1. Phase Composition and Microstructure of TSC-PA*

*Materials* **2022**, *15*, x FOR PEER REVIEW 4 of 13

*Materials* **2022**, *15*, x FOR PEER REVIEW 4 of 13

**3. Result and Discussion**

*3.1. Phase Composition and Microstructure of TSC-PA*

**3. Result and Discussion**

**Figure 1.** The XRD pattern of TSC-PA. **Figure 1.** The XRD pattern of TSC-PA. **Figure 1.** The XRD pattern of TSC-PA.

As observed in Figure 2, the morphology and the distributions of elements of the cross section of the TSC-PA was analyzed using SEM. It can be observed that the Pb particles were homogeneously scattered in the Ti3SiC<sup>2</sup> matrix, while the Ag particles were agglomerated to a certain extent. As the melting point of silver was merely 961 °C, some amount of molten silver nearby flowed together during sintering. Therefore, the agglom-As observed in Figure 2, the morphology and the distributions of elements of the cross section of the TSC-PA was analyzed using SEM. It can be observed that the Pb particles were homogeneously scattered in the Ti3SiC<sup>2</sup> matrix, while the Ag particles were agglomerated to a certain extent. As the melting point of silver was merely 961 ◦C, some amount of molten silver nearby flowed together during sintering. Therefore, the agglomeration of Ag occurred in the TSC-PA. As observed in Figure 2, the morphology and the distributions of elements of the cross section of the TSC-PA was analyzed using SEM. It can be observed that the Pb particles were homogeneously scattered in the Ti3SiC<sup>2</sup> matrix, while the Ag particles were agglomerated to a certain extent. As the melting point of silver was merely 961 °C, some amount of molten silver nearby flowed together during sintering. Therefore, the agglomeration of Ag occurred in the TSC-PA.

**Figure 2.** SEM micrograph (**a**) and distribution of Ti(**b**), Si(**c**), Pb(**d**), Ag(**e**) elements in **Figure 2.** SEM micrograph (**a**) and distribution of Ti(**b**), Si(**c**), Pb(**d**), Ag(**e**) elements in the cross section of TSC-PA **Figure 2.** SEM micrograph (**a**) and distribution of Ti (**b**), Si (**c**), Pb (**d**), Ag (**e**) elements in the cross section of TSC-PA.

### the cross section of TSC-PA *3.2. Mechanical Performance of TSC-PA*

The relative density, microhardness, flexural strength, and compressive strength of TSC-PA and TSC are listed in Table 1. It can be seen that the relative densities of TSC-PA and TSC were 95.35% and 98.24%, respectively. The microhardness of TSC-PA was less than half that of TSC, and their microhardness values were 2.24 and 5.5 GPa, respectively. Pb and Ag in the TSC-PA belong to soft metals and caused the low hardness of TSC-PA. The flexural strength of TSC-PA was less than that of TSC, and their values were 311 and

428 MPa, respectively. The compressive strength of TSC-PA was only half that of TSC, and their values were 654 MPa and 1230 MPa, respectively. **Table 1.** Properties of TSC and TSC-PA. **Compressive** 

**Flexural Strength**

**Strength**

**Microhardness**

The relative density, microhardness, flexural strength, and compressive strength of TSC-PA and TSC are listed in Table 1. It can be seen that the relative densities of TSC-PA and TSC were 95.35% and 98.24%, respectively. The microhardness of TSC-PA was less than half that of TSC, and their microhardness values were 2.24 and 5.5 GPa, respectively. Pb and Ag in the TSC-PA belong to soft metals and caused the low hardness of TSC-PA. The flexural strength of TSC-PA was less than that of TSC, and their values were 311 and 428 MPa, respectively. The compressive strength of TSC-PA was only half that of TSC,


and their values were 654 MPa and 1230 MPa, respectively.

*Materials* **2022**, *15*, x FOR PEER REVIEW 5 of 13

**Table 1.** Properties of TSC and TSC-PA. **Sample No. Relative Density /%**

*3.2. Mechanical Performance of TSC-PA*

Note: The data in the first line were from Reference [21]. As shown in Figure 3a, the TSC-PA only suffered from elastic deformation before

As shown in Figure 3a, the TSC-PA only suffered from elastic deformation before fracture, which was a typical brittle fracture. The fracture surface of TSC-PA (see Figure 3b) indicated that the fracture surface of TSC-PA was featured by intergranular and transgranular fracture. This fracture mode of TSC-PA was similar to TSC. As well known, the diverse energy-absorbing strategies including the diffuse of the microcracks, lamination, crack deflection, and grain pull-out were discovered to be in charge of the high flexural strength of TSC [29,30]. In addition to intergranular and transgranular fracture, some pores and cracks resulting from the unsatisfactory compatibility (e.g., wettability) of the TSC matrix and Pb and Ag were observed (see Figure 3b). The thermal expansion coefficients (for instance: TSC: 9.1 <sup>×</sup> <sup>10</sup>−<sup>6</sup> , Pb: 29.3 <sup>×</sup> <sup>10</sup>−<sup>6</sup> , and Ag: 19.5 <sup>×</sup> <sup>10</sup>−<sup>6</sup> ◦<sup>C</sup> −1 ) of different phases were not matched with each other, leading to the weak bonding between granules and the formation of some defects (such as micro-pores and micro-cracks) during the preparation procedure. The as-formed defects were detrimental to the flexural strength. An additional major justification for the poor fracture strength of the TSC-PA was that the distribution of Pb and Ag around the TSC grains to a certain extent hindered the diverse energy-assimilating strategies, which let fracture take place between the grains. fracture, which was a typical brittle fracture. The fracture surface of TSC-PA (see Figure 3b) indicated that the fracture surface of TSC-PA was featured by intergranular and transgranular fracture. This fracture mode of TSC-PA was similar to TSC. As well known, the diverse energy-absorbing strategies including the diffuse of the microcracks, lamination, crack deflection, and grain pull-out were discovered to be in charge of the high flexural strength of TSC [29,30]. In addition to intergranular and transgranular fracture, some pores and cracks resulting from the unsatisfactory compatibility (e.g., wettability) of the TSC matrix and Pb and Ag were observed (see Figure 3b). The thermal expansion coefficients (for instance: TSC: 9.1 × 10−<sup>6</sup> , Pb: 29.3 × 10−<sup>6</sup> , and Ag: 19.5 × 10−<sup>6</sup> °C−<sup>1</sup> ) of different phases were not matched with each other, leading to the weak bonding between granules and the formation of some defects (such as micro-pores and micro-cracks) during the preparation procedure. The as-formed defects were detrimental to the flexural strength. An additional major justification for the poor fracture strength of the TSC-PA was that the distribution of Pb and Ag around the TSC grains to a certain extent hindered the diverse energy-assimilating strategies, which let fracture take place between the grains.

**Figure 3.** (**a**) Compressive stress–strain curve of TSC-PA and (**b**) SEM image of the frac-**Figure 3.** (**a**) Compressive stress–strain curve of TSC-PA and (**b**) SEM image of the fractured surface of TSC-PA after the three-point bending test.

## tured surface of TSC-PA after the three-point bending test. *3.3. Tribological Behavior of TSC-PA*

Figure 4a showed the plot of µ versus distance curve of the TSC-PA/Inconel 718 tribopair at elevated temperatures. It can be easily construed that the temperature had a direct effect on the µ of the tribopair. At RT and 200 ◦C, the µ had a big fluctuation. At 400, 600, and 800 ◦C, the incipient µ had a big fluctuation and, then, it soon achieved a steady state with low µ values. Specifically, at RT, the µ showed a larger fluctuation than that at other temperatures. Particularly, at 600 ◦C, the µ exhibited a turbulent behavior in contrasted with that at other temperatures, which indicated an incipient short breakin period with relatively low µ values; then, it reached high µ values and fluctuated largely. Later, it approached a stable state with relatively low µ values. In comparison, the fluctuation of the µ decreased with the temperature increasing from RT to 800 ◦C. Figure 4b compared the µmean of TSC-PA/Inconel 718 tribopair at elevated temperatures. It can be seen that the µmean decreased with the temperature increasing from RT to 800 ◦C.

Figure 4a showed the plot of μ versus distance curve of the TSC-PA/Inconel 718 tribopair at elevated temperatures. It can be easily construed that the temperature had a direct effect on the μ of the tribopair. At RT and 200 °C, the μ had a big fluctuation. At 400, 600, and 800 °C, the incipient μ had a big fluctuation and, then, it soon achieved a steady state with low μ values. Specifically, at RT, the μ showed a larger fluctuation than that at other temperatures. Particularly, at 600 °C, the μ exhibited a turbulent behavior in contrasted with that at other temperatures, which indicated an incipient short break-in period with relatively low μ values; then, it reached high μ values and fluctuated largely. Later, it approached a stable state with relatively low μ values. In comparison, the fluctuation of the μ decreased with the temperature increasing from RT to 800 °C. Figure 4b compared the μmean of TSC-PA/Inconel 718 tribopair at elevated temperatures. It can be

seen that the μmean decreased with the temperature increasing from RT to 800 °C.

*3.3. Tribological Behavior of TSC-PA*

**Figure 4.** Plot of (**a**) friction coefficient (μ) versus distance and (**b**) the average friction coefficients of TSC-PA sliding against Inconel 718 alloys with different temperature. **Figure 4.** Plot of (**a**) friction coefficient (µ) versus distance and (**b**) the average friction coefficients of TSC-PA sliding against Inconel 718 alloys with different temperature.

Figure 5a plotted the wear rates of the TSC-PA pin at elevated temperatures. The wear rates of the pin decreased with the temperature increasing from RT to 400 °C. Moreover, the wear rates of the pin were the same when the temperature was at 400, 600, and 800 °C. No other results were found in the literature for a comparison with the results in this work because Ti3SiC<sup>2</sup> composites with the co-addition of PbO and Ag have not been reported before. Moreover, it was not easy to make a comparison between tribology results as they were carried out under distinct conditions on the basis of the detailed demands of an application. For interpretative aspirations, Table 2 compared tribological behavior of Ti3SiC<sup>2</sup> composites with the addition of the different amount of Ag or the addition of the different amount of PbO. Comparatively, the incorporation of relative low content (5–20 vol%) of Ti3SiC<sup>2</sup> can diminish the wear rate of the Ag-based composite during sliding against Al2O<sup>3</sup> at 5N and 0.5 m/s [19]. In this work, the situation was different because the sample was Ti3SiC<sup>2</sup> based-composite with 70 vol% of Ti3SiC2. At RT, comparatively, Ti3SiC2 and composites of Ti3SiC<sup>2</sup> with 15vol% Ag, with 15vol% PbO, and with 15vol% PbO-15vol% Ag showed wear rates of 2 × 10−3 mm3/(N·m), 3 × 10−5 mm3/(N·m), 2 × 10−4 mm3/(N·m), and 2 × 10−4 mm3/(N·m), respectively, during sliding against Inconel 718 at 5N and 0.1 m/s [20,22]. These results showed that the incorporation of 15vol% Ag or 15vol% PbO or 15vol% PbO-15vol% Ag can decrease the wear rate of Ti3SiC<sup>2</sup> by at least an order of magnitude at RT. At elevated temperatures (>200 °C), the composite of Ti3SiC<sup>2</sup> with 15vol% PbO showed better wear resistance than both Ti3SiC<sup>2</sup> and the composite of Ti3SiC<sup>2</sup> with 15vol% PbO-15vol% Ag (TSC-PA). The tribological behavior of the composite of Ti3SiC<sup>2</sup> with 15vol% Ag at elevated temperature was unavailable and incomparable. It can be seen from Table 2 that TSC-PA showed a lower friction than TSC at > 200 °C and a lower friction than Ti3SiC2-15vol% PbO at <600 °C. It was concluded that the addition of Figure 5a plotted the wear rates of the TSC-PA pin at elevated temperatures. The wear rates of the pin decreased with the temperature increasing from RT to 400 ◦C. Moreover, the wear rates of the pin were the same when the temperature was at 400, 600, and 800 ◦C. No other results were found in the literature for a comparison with the results in this work because Ti3SiC<sup>2</sup> composites with the co-addition of PbO and Ag have not been reported before. Moreover, it was not easy to make a comparison between tribology results as they were carried out under distinct conditions on the basis of the detailed demands of an application. For interpretative aspirations, Table 2 compared tribological behavior of Ti3SiC<sup>2</sup> composites with the addition of the different amount of Ag or the addition of the different amount of PbO. Comparatively, the incorporation of relative low content (5–20 vol%) of Ti3SiC<sup>2</sup> can diminish the wear rate of the Ag-based composite during sliding against Al2O<sup>3</sup> at 5N and 0.5 m/s [19]. In this work, the situation was different because the sample was Ti3SiC<sup>2</sup> based-composite with 70 vol% of Ti3SiC2. At RT, comparatively, Ti3SiC<sup>2</sup> and composites of Ti3SiC<sup>2</sup> with 15 vol% Ag, with 15 vol% PbO, and with 15 vol% PbO-15 vol% Ag showed wear rates of 2 <sup>×</sup> <sup>10</sup>−<sup>3</sup> mm3/(N·m), 3 <sup>×</sup> <sup>10</sup>−<sup>5</sup> mm3/(N·m), <sup>2</sup> <sup>×</sup> <sup>10</sup>−<sup>4</sup> mm3/(N·m), and 2 <sup>×</sup> <sup>10</sup>−<sup>4</sup> mm3/(N·m), respectively, during sliding against Inconel 718 at 5N and 0.1 m/s [20,22]. These results showed that the incorporation of 15 vol% Ag or 15 vol% PbO or 15 vol% PbO-15 vol% Ag can decrease the wear rate of Ti3SiC<sup>2</sup> by at least an order of magnitude at RT. At elevated temperatures (>200 ◦C), the composite of Ti3SiC<sup>2</sup> with 15 vol% PbO showed better wear resistance than both Ti3SiC<sup>2</sup> and the composite of Ti3SiC<sup>2</sup> with 15 vol% PbO-15 vol% Ag (TSC-PA). The tribological behavior of the composite of Ti3SiC<sup>2</sup> with 15 vol% Ag at elevated temperature was unavailable and incomparable. It can be seen from Table 2 that TSC-PA showed a lower friction than TSC at > 200 ◦C and a lower friction than Ti3SiC2-15 vol% PbO at <600 ◦C. It was concluded that the addition of PbO and Ag apparently improved the tribological behavior of TSC-PA, in comparison with TSC and Ti3SiC2-15 vol% PbO. *Materials* **2022**, *15*, x FOR PEER REVIEW 7 of 13PbO and Ag apparently improved the tribological behavior of TSC-PA, in comparison with TSC and Ti3SiC2-15vol% PbO.

**Figure 5.** The wear rates of (**a**) pin and (**b**) disk for TSC-PA/Inconel 718 alloy tribo-pair as a function of temperature. **Figure 5.** The wear rates of (**a**) pin and (**b**) disk for TSC—PA/Inconel 718 alloy tribo-pair as a function of temperature.

Figure 5b compared the wear rates of the Inconel 718 disk sliding against TSC-PA at elevated temperatures. As the temperature raised from RT to 800 °C, the wear rates of the disk decreased. At RT, a positive wear rate was found for the disk. Interestingly, negative

**(mm<sup>3</sup>**

**/Nm)**

**μ Reference**

[19]

[20]

[22]

to 800 °C. Moreover, the wear rate of the disk at 600 °C was identical to that at 800 °C. The detection of Ti, Si, Pb, and Ag on the worn surface of Inconel 718 was in line with the

**Table 2.** Comparison of tribological behavior of different Ti3SiC2-based composites.

Block (Tab)-on-Disc, 5N, 0.5 m/s, air, RT 3.2 × 10−<sup>5</sup> 0.38

Pin-on-disk, 5N, 0.01 m/s, air, RT 4.0 × 10−<sup>3</sup> 0.62

Pin-on-disk, 5N, 0.1 m/s, air, RT 2.0 × 10−<sup>3</sup> 0.61 Pin-on-disk, 5N, 1 m/s, air, RT 6.0 × 10−<sup>3</sup> 0.57

Pin-on-disk, 5N, 0.01 m/s, air, RT 7.0 × 10−<sup>5</sup> 0.42 Pin-on-disk, 5N, 0.1 m/s, air, RT 8.0 × 10−<sup>5</sup> 0.51 Pin-on-disk, 5N, 1 m/s, air, RT 5.0 × 10−<sup>3</sup> 0.56

Pin-on-disk, 5N, 0.01 m/s, air, RT 4.0 × 10−<sup>5</sup> 0.4 Pin-on-disk, 5N, 0.1 m/s, air, RT 7.0 × 10−<sup>6</sup> 0.51 Pin-on-disk, 5N, 1 m/s, air, RT 3.0 × 10−<sup>3</sup> 0.5

Pin-on-disk, 5N, 0.01 m/s, air, RT 5.0 × 10−<sup>5</sup> 0.4 Pin-on-disk, 5N, 0.1 m/s, air, RT 3.0 × 10−<sup>5</sup> 0.5 Pin-on-disk, 5N, 1 m/s, air, RT 2.0 × 10−<sup>5</sup> 0.45

Pin-on-disk, 5N, 0.01 m/s, air, RT 2.0 × 10−<sup>5</sup> 0.31 Pin-on-disk, 5N, 0.1 m/s, air, RT 2.0 × 10−<sup>4</sup> 0.45 Pin-on-disk, 5N, 1 m/s, air, RT 1.0 × 10−<sup>5</sup> 0.4

Pin-on-disk, 5N, 0.1 m/s, air, RT 2.0 × 10−<sup>3</sup> 0.65

Pin-on-disk, 5N, 0.1 m/s, air, 200 °C 3.0 × 10−<sup>4</sup> 0.65 Pin-on-disk, 5N, 0.1 m/s, air, 400 °C 5.0 × 10−<sup>5</sup> 0.62 Pin-on-disk, 5N, 0.1 m/s, air, 600 °C 2.5 × 10−<sup>5</sup> 0.65

**-Surface Conditions Wear rate** 

**Composition Counter**

Al2O<sup>3</sup>

Inconel 718

Inconel 718 disk

Ag-5vol % Ti3SiC<sup>2</sup>

Ti3SiC<sup>2</sup>

Ti3SiC2-5vol% Ag

Ti3SiC2-10vol% Ag

Ti3SiC2-15vol% Ag

Ti3SiC2-20vol% Ag

Ti3SiC<sup>2</sup>

negative wear of the disk (see Table 3).

Ag-10vol % Ti3SiC<sup>2</sup> Block (Tab)-on-Disc, 5N, 0.5 m/s, air, RT 2.9 × 10−<sup>5</sup> 0.35 Ag-20vol % Ti3SiC<sup>2</sup> Block (Tab)-on-Disc, 5N, 0.5 m/s, air, RT 4.1 × 10−<sup>6</sup> 0.29 Ag-30vol % Ti3SiC<sup>2</sup> Block (Tab)-on-Disc, 5N, 0.5 m/s, air, RT 2.5 × 10−<sup>5</sup> 0.3

Ag Block (Tab)-on-Disc, 5N, 0.5 m/s, air, RT 4.9 × 10−<sup>5</sup> 0.38


**Table 2.** Comparison of tribological behavior of different Ti3SiC<sup>2</sup> -based composites.

Figure 5b compared the wear rates of the Inconel 718 disk sliding against TSC-PA at elevated temperatures. As the temperature raised from RT to 800 ◦C, the wear rates of the disk decreased. At RT, a positive wear rate was found for the disk. Interestingly, negative wear rates were detected for the disk and increased with the temperature rising from 200 to 800 ◦C. Moreover, the wear rate of the disk at 600 ◦C was identical to that at 800 ◦C. The detection of Ti, Si, Pb, and Ag on the worn surface of Inconel 718 was in line with the negative wear of the disk (see Table 3).
