## 3 Results
### The Point Tribological Contact

Table 3: The tribological properties of neat PI and PEI as well as the PI- and PEI-based nanocomposites. To identify the reasons for the observed differences, an analysis of micrographs of the wear track surfaces was carried out for both the counterparts and the nanocomposites. In the neat PI case (Figure S1a), a film of oxidized polymeric debris was attached to the counterpart and some grooves, oriented along the sliding direction, were observed. The formation and adherence of the thick transfer layer on the counterpart was associated with the high WR level of the polymer. Regardless of the presence of CNTs and HNTs, loading with CCFs exerted a damaging effect on the steel counterpart surface, causing its abrasive wear (Figures S1d and 4a,d). Thus, the presence of the nanofillers was not accompanied by the formation of a stable TF on the steel counterpart's surface. Nevertheless, debris was observed on the wear track surfaces of the PI-based nanocomposites, which, however, did not protect the steel counterpart against the protrusion of CCFs above the surface (Figures S1e and 4b,e). This fact was consistent with the rough (ragged) wear track profiles (Figures S1f and 4c,f). Figure 5 presents optical photographs of the wear tracks on the PEI-based nanocomposites and the counterparts. In the neat PEI case, a thick transfer film also adhered on the steel counterpart surface, with scratches oriented along the sliding direction (Figure S2a). Although the polymer surface topography was similar to that of neat PI (Figure S1b), the WR value was three times higher for neat PEI (Table 3; Figure S2c). Loading with CCFs deteriorated the tribological characteristics of the two-component PI- and PEI-based composites (Table 3), whose wear tracks were similar (Figure S2e) due to the abrasive damage of the steel counterpart (Figure S2d). Loading with CNTs did not change the wear pattern substantially (Figure 5a-c). The abrasive wear of the steel counterpart was only excluded when loaded with HNTs (representing kaolinite double-layer packages) (Figure 5d). In this case, a stable (\"compacted\") tribological layer was formed on the wear track surface of the PEI/10 CCF/1 HNT nanocomposite. Note that it is more correct to name it \"the tribological layer\" rather than \"the tribological film\" [(13; 43)]. At the same time, there was no significant decrease in the CoF level. Figure 4: Optical photographs of the steel counterpart surfaces (**a**,**d**), wear tracks (**b**,**e**) and wear track profiles (**c**,**f**) after the tribological tests of the PI/10 CCF/1 CNT (**a**–**c**) and PI/10 CCF/1 HNT (**d**–**f**) nanocomposites. Point tribological contact. White arrows denote sliding direction. According to the authors, it was precisely the facilitated possibility of straining the thin surface layer (representing mixture of PEI, CCFs and HNTs) smoothed by the rotated steel counterpart, which was gradually transformed into the tribological one, that protected the steel counterpart from the damaging effect of CCFs. On the other hand, loading with CNTs with the stable and chemically inert structure prevented the formation of such a TL. Note that a thin (optically translucent) fixed TF was observed on the counterpart surface (Figure 5d) after the tribological test of the PEI/10 CCF/1 HNT nanocomposite. Its composition is analyzed below. However, judging by the rather high CoF value of \(\sim\)0.24, it included both oxidized polymer and halloysite, which did not facilitate mutual sliding, as it took place after loading with PTFE microparticles, for example, see [44; 45; 46]. Thereby, HNTs were the only nanofiller that made it possible to exclude the metal counterpart wear and ensure low WR values at relatively high CoF levels. Figure 6 shows Raman spectra taken on the wear track of the PEI/10 CCF/1 HNT nanocomposite as well as on the steel counterpart. The intensities are plotted in relative units.