*3.4. Tribological Mechanisms*

Figure 6 showed the SEM assessment of tribosurfaces of TSC-PA/Inconel 718 tribocouple after tribological testing at elevated temperature. TSC-PA was full of scars resulting from the abrasive wear, while the Inconel 718 surface was covered with an even tribolayer delivered from the TSC-PA surface. The surface was surrounded by the tribolayer made up of incompletely oxidized TSC-PA (Table 3 and Figure 6). With the rise in temperature, the

tribofilm formed on the surface of TSC-PA was smoother and denser. This film was formed undoubtably by a coalescence of diverse mechanical blending, fracturing, resintering, and mechanically vitalized oxidation and other chemical processes, perhaps, continuously refined during the heat cycle. The EDS data showed that the elements of Ni, Cr, and Fe from Inconel 718 alloy appeared on the surface of TSC-PA (Figure 6a,c,e,g,i in Table 3) and the elements of Ti, Si, Pb, and Ag from TSC-PA appeared on the surface of Inconel718 alloy (Figure 6b,d,f,h,j in Table 3). Moreover, tribo-oxidation was a predominant factor that influenced, and even controlled, the wear mechanism and finally brought about the change of friction and wear features [31]. The chemical states of Ti, Si, Pb, and Ag elements on the worn surface of TSC-PA at RT–800 ◦C were investigated in detail by XPS (see Figure 7).

**Table 3.** The chemical components of samples corresponding to Figure 6 as determined by EDS.


At RT, some wear debris dispersed on the worn surface of the TSC-PA with several dark continent regions (see Figure 6a). It was worth mentioning that some loose cracks and pores scattered on the worn surface, which was due to the peeling-off and decentralization of the granules. Such an episode was in accordance with Ti3SiC<sup>2</sup> [6]. Therefore, it can be deduced that the addition of Pb and Ag did not affect the wear behavior of Ti3SiC<sup>2</sup> at RT. Though the weak bonding between Ti3SiC<sup>2</sup> grains in the TSC-PA led to the fracture and pulling out of Ti3SiC<sup>2</sup> granules, soft metals (including Pb and Ag) distributed around the Ti3SiC<sup>2</sup> grains, inhibiting the peeling-off and decentralization of Ti3SiC<sup>2</sup> matrix; therefore, the wear rate of TSC-PA was only one tenth of Ti3SiC<sup>2</sup> (see Table 2). A third body was formed and entrapped between the pin and disk, leading to the severe wear of TSC-PA and Inconel 718 at RT. The obvious evidence was that wear grooves were found on the worn surface of Inconel 718 after tribology testing at RT (see Figure 6b) and mutual material transfer between the pin and disk was detected by EDS analysis (see Table 3). The abrasive wear was the primary wear mechanism for TSC-PA. A small amount of SiO<sup>2</sup> and PbO was detected on the worn surface of TSC-PA (see Figure 7). However, the temperature was too low to form an enough thick tribo-oxidation film between the contact surface of TSC-PA and Inconel 718, so high friction coefficients and high wear rates were present at RT (see Figures 4 and 5a).

At 200 ◦C, some compacted wear debris was found on the worn surface of TSC-PA (see Figure 6c). Compared with RT, besides SiO<sup>2</sup> and PbO, other oxides, TiO<sup>2</sup> and Ag2O, were formed on the surface of TSC-PA (evidently owing to the XPS at 200 ◦C in Figure 7), generating a thicker tribo-oxidation film. The formation of the tribo-oxidation film did not inhibit the fracture and pulling-out of Ti3SiC<sup>2</sup> grains. The formation and entrapment of the

third body (the formed wear debris) between the pin and disk contributed to the wear of TSC-PA and Inconel 718. The mutual material transfer between the pin and disk at 200 ◦C was detected by EDS analysis (see Table 3). A tribo-oxidation film with some wear grooves was transferred to the Inconel 718, so the counterpart showed relatively flat region and slight negative wear (see Figures 5b and 6d). At 200 ◦C, the abrasive wear mechanism was the main wear mechanism with very slight adhesive wear. ductility of TSC-PA. On other hand, when the asperities of the tribo-pair were in contact with each other, the instant flash temperature may reach 3000 °C [32]. Therefore, a high temperature was beneficial for the formation of oxides, such as TiO2, SiO2, PbO, and Ag2O. Therefore, the adhesive wear and plastic flow was the main wear mechanism with slight abrasive wear (as distinct on account of the slight wear trenches on the Inconel 718 disk).

TSC-PA; therefore, the wear rate of TSC-PA at 400 °C exhibited a distinct decline in contrast to those at RT and 200 °C. Moreover, the addition of Ag and Pb could strengthen the

*Materials* **2022**, *15*, x FOR PEER REVIEW 10 of 13

**Figure 6.** SEM micrographs of (**a**) TSC-PA at 25 °C, (**b**) Inconel 718 alloy at 25 °C, (**c**) TSC-PA at 200 °C, (**d**) Inconel 718 alloy at 200 °C, (**e**) TSC-PA at 400 °C, (**f**) Inconel 718 alloy at 400 °C, (**g**) TSC-PA at 600 °C, (**h**) Inconel 718 alloy at 600 °C, (**i**) TSC-PA at 800 °C, and (**j**) Inconel 718 alloy at 800 °C after tribological testing. **Figure 6.** SEM micrographs of (**a**) TSC-PA at 25 ◦C, (**b**) Inconel 718 alloy at 25 ◦C, (**c**) TSC-PA at 200 ◦C, (**d**) Inconel 718 alloy at 200 ◦C, (**e**) TSC-PA at 400 ◦C, (**f**) Inconel 718 alloy at 400 ◦C, (**g**) TSC-PA at 600 ◦C, (**h**) Inconel 718 alloy at 600 ◦C, (**i**) TSC-PA at 800 ◦C, and (**j**) Inconel 718 alloy at 800 ◦C after tribological testing.

of Pb, Ag, Ti, and Si to form TiO2, SiO2, PbO, and AgO (see Figure 7).

this temperature.

**Figure 7.** X-ray photoelectron spectroscopy for (**a**) Ti2p, (**b**) Si2p**c**, (**c**) Pb4f, (**d**) Ag3d, and (**e**) C1s on the worn surface of TSC-PA after sliding against Inconel 718 alloy with different temperature. **Figure 7.** X-ray photoelectron spectroscopy for (**a**) Ti2p, (**b**) Si2pc, (**c**) Pb4f, (**d**) Ag3d, and (**e**) C1s on the worn surface of TSC-PA after sliding against Inconel 718 alloy with different temperature.

At 600 °C, the worn surface of TSC-PA was surrounded by a relatively smooth and continuous tribolayer after tribology testing (see Figure 6g). This surface was covered with a sufficiently thick tribo-oxidation film containing TiO2, SiO2, PbO, and AgO (see Figure 7). In comparison with 400 °C, AgO was found on the worn surface of TSC-PA at 600 °C instead of Ag2O. Interestingly, the existence of AgO played a significant role in the decrease of friction and wear of the tribopair. The worn surface of Inconel 718 was embraced by a tribo-oxidation film, which was transferred from TSC-PA (see Figure 6h). The serious plastic flowing resulted in the adhesive wear. Mutual transfer of matter between the pin and the disk was verified by the EDS results in Table 3. Moreover, with the increase in temperature, especially at above 400 °C, oxygen contents of the pin and the disk were higher, which indicated the thicker thickness of the tribo-oxidation film (see Table 3). Therefore, the friction coefficient of TSC-PA was lower and more stable and its wear rate was low due to the moderately thick oxides. The wear mechanism was adhesive wear at

At 800 °C, the worn surface of TSC-PA was surrounded by a tribo-oxidation film, which was smoothest and densest among all temperatures (see Figure 6). With the thickness of the tribofilm increasing, the covering of the Inconel 718 surface increased. Matter accumulation on the Inconel 718 led to seizure of the contact. The tribofilm reached a critical thickness so that it was easily peeled off from TSC-PA (see Figure 6i). The Inconel 718 showed a smooth tribofilm with large negative wear (see Figures 5b and 6j). Of course, mutual transfer of matter between the pin and the disk was also confirmed by the EDS data in Table 3. The wear mechanism was adhesive wear and tribochemistry of elements

Conclusively at RT and 200 °C, the abrasive wear ascribed to the peeling-off and pulled-out of Ti3SiC<sup>2</sup> grains dominated the wear mechanism of TSC-PA, leading to the indispensable friction and wear. As the temperature increased, the formation of tribo-At 400 ◦C, obviously, the TSC-PA was covered with some large wear debris after tribology testing (see Figure 6e). It was expected that, initially, a tribo-oxidation film containing TiO2, SiO2, PbO and Ag2O was formed on its surface (as evident owing to the XPS at 400 ◦C in Figure 7), which was similar to the oxidation composition at 200 ◦C. Then, the peel-off and removal of the tribo-oxidation film occurred for TSC-PA. As the sliding finished, the mutual material transfer between the pin and disk was detected by EDS analysis (see Table 3). It was seen from Figure 6f that a relatively smooth film was built up on the surface of the Inconel 718. The plastic flow, in some way, retarded the abrasive wear of TSC-PA; therefore, the wear rate of TSC-PA at 400 ◦C exhibited a distinct decline in contrast to those at RT and 200 ◦C. Moreover, the addition of Ag and Pb could strengthen the ductility of TSC-PA. On other hand, when the asperities of the tribo-pair were in contact with each other, the instant flash temperature may reach 3000 ◦C [32]. Therefore, a high temperature was beneficial for the formation of oxides, such as TiO2, SiO2, PbO, and Ag2O. Therefore, the adhesive wear and plastic flow was the main wear mechanism with slight abrasive wear (as distinct on account of the slight wear trenches on the Inconel 718 disk).

At 600 ◦C, the worn surface of TSC-PA was surrounded by a relatively smooth and continuous tribolayer after tribology testing (see Figure 6g). This surface was covered with a sufficiently thick tribo-oxidation film containing TiO2, SiO2, PbO, and AgO (see Figure 7). In comparison with 400 ◦C, AgO was found on the worn surface of TSC-PA at 600 ◦C instead of Ag2O. Interestingly, the existence of AgO played a significant role in the decrease of friction and wear of the tribopair. The worn surface of Inconel 718 was embraced by a tribo-oxidation film, which was transferred from TSC-PA (see Figure 6h). The serious plastic flowing resulted in the adhesive wear. Mutual transfer of matter between the pin and the disk was verified by the EDS results in Table 3. Moreover, with the increase in temperature, especially at above 400 ◦C, oxygen contents of the pin and the disk were higher, which indicated the thicker thickness of the tribo-oxidation film (see Table 3). Therefore, the friction coefficient of TSC-PA was lower and more stable and its wear rate was low due to the moderately thick oxides. The wear mechanism was adhesive wear at this temperature.

At 800 ◦C, the worn surface of TSC-PA was surrounded by a tribo-oxidation film, which was smoothest and densest among all temperatures (see Figure 6). With the thickness of the tribofilm increasing, the covering of the Inconel 718 surface increased. Matter accumulation on the Inconel 718 led to seizure of the contact. The tribofilm reached a critical thickness so

that it was easily peeled off from TSC-PA (see Figure 6i). The Inconel 718 showed a smooth tribofilm with large negative wear (see Figures 5b and 6j). Of course, mutual transfer of matter between the pin and the disk was also confirmed by the EDS data in Table 3. The wear mechanism was adhesive wear and tribochemistry of elements of Pb, Ag, Ti, and Si to form TiO2, SiO2, PbO, and AgO (see Figure 7).

Conclusively at RT and 200 ◦C, the abrasive wear ascribed to the peeling-off and pulled-out of Ti3SiC<sup>2</sup> grains dominated the wear mechanism of TSC-PA, leading to the indispensable friction and wear. As the temperature increased, the formation of tribooxidation films was favorable for the reduction in friction and wear of the tribopair. As for the reason why the Inconel 718 disk showed negative wear at elevated temperatures. We proposed that with the temperature increasing, especially at 800 ◦C, the formed oxidation film was so thick that it was easily peeled-off from the TSC-PA pin and transferred to the Inconel 718 disk, leading to the negative wear of the disk. Additionally, the transition of wear mechanism from abrasive wear to adhesive wear occurred with the increasing temperature. The co-addition of PbO and Ag worked together in the improvement of the tribological behavior of TSC-PA at elevated temperatures, which shed light on the effectiveness of co-addition of solid lubricants in MAX phases at elevated temperatures.
