## 4 Concept Verification and Discussion  
The key result of the study should be considered the fact of identifying the tribological layer in the point tribological contact. Both PI and PEI, as representatives of high-performance polymers, are not anti-friction materials. Therefore, to improve wear resistance (as well as to minimize wearing of a counterface), they are conventionally loaded with solid lubricant fillers (in order to form a TF), as well as nanoparticles [48; 49; 50]. The latter, due to their small sizes, do not damage the steel counterpart, but somehow strengthen polymer composites. Under dry sliding friction, the chemical activity of the polymer matrix is extremely important, which determines the possibility of its interaction with the counterface material. This process, among other things, can be estimated using CoF values, as well as the development of tribo-chemical reactions between polymers and nanofiller particles. In doing so, of particular importance are the specific pressure in the tribological contact and the counterpart roughness. In this research, a phenomenon of interest was the threefold higher WR level found for the neat PEI in tribological contact with the GCr15 steel counterpart. While the physical and mechanical properties of both polymer matrices were equal, the authors attributed this fact to the greater activity of PEI that contained, among other things, oxygen atoms. However, the difference in the molecular structure and activity of both matrices was further leveled out through loading with CCFs, which abrasively wore the steel counterface during sliding friction in the point tribological contact (even if the surface of the two-component nanocomposite was polished before testing). Thus, it was required to form a protective layer through additional loading with nanofillers. This was only possible for the PEI matrix by loading it with 1 wt.% HNTs, resulting in a CoF value of \(\sim\)0.24. However, it was not a pronounced manifestation of the solid lubricant effect, when CoF levels of polymer nanocomposites could be \(\sim\)0.1, for example, after loading with PTFE particles [48]. Note that only the loading of PEI with HNTs contributed to the reduction of wear resistance. Table 7 shows that the WR value for the PEI/1 HNT nanocomposite was more  
Figure 13: SEM micrographs of the wear track surfaces on the PI/10 CCF/1 CNT (**a**), PI/10 CCF/1 HNT (**b**), PEI/10 CCF/1 CNT (**c**) and PEI/10 CCF/1 HNT (**d**) nanocomposites. Digits in blue squares denote Spectra numbers in Table 6.  
than 500 times higher in contrast to the PEI/10 CCF/1 HNT one (in the point tribological contact). At the same time, the CoF level of the PEI/10 CCF sample was at the same level as neat PEI. According to Figure 13(a), a rather high CoF oscillation was registered during the tribological test of the PEI/1 HNT nanocomposite, which corresponded to the sample's abrasive wearing (Figure 13(c),d). In this case, a large amount of formed debris adhered to the steel counterpart surface as a thick transfer layer (Figure 13(b)), intensifying the wear process. In the point tribological contact, the aspect of fixing a tribolayer (TL) on the wear track surface was substantially dependent on the loading conditions. Figure 6 shows the variation kinetics of the CoF value in the tribological tests of the PEI/10 CCF/1 HNT nanocomposite at the increased loads of 7.5 and 10 N, while Figure 6b-g present micrographs of the counterpart surfaces, as well as the wear tracks and their profiles on the nanocomposites. The integral results of the tribological tests are shown in Table 8. When the load was increased above 5 N, the CoF value rose to \(\sim\)0.3 (Figure 14(a)) and its pronounced oscillation was observed at about half the test distance. Then, it was not manifested; however, the CoF value did not decrease as well. At the same time, the WR level increased by approximately three times compared to the result with a load of 5 N (Figure 14(d),g; Table 8).