## 4 Concept Verification and Discussion
The absence of CoF oscillations in the second half of the test distance was accompanied by the formation of a TL on the wear track surface (Figure 14(c),f). Before starting this process, the steel counterpart surface had been damaged partly (Figure 14(b),e). Therefore, the protective TL was formed from the very beginning of the tribological tests only at loads of \(\leq\)5 N. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Composition (wt.\%)** & **Coefficient of Friction, CoF** & **Wear Rate, WR, mm\({}^{3}\)/N: m, 10\({}^{-6}\)** \\ \hline PEI/1 HNT & 0.295 \(\pm\) 0.043 & 2722.5 \(\pm\) 256.05 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The tribological properties of the PEI/1 HNT nanocomposite. Point tribological contact. Figure 14: The time dependence of the CoF values for the PEI/1 HNT nanocomposite (**a**), optical photographs of the steel counterpart surfaces (**b**), the wear track (**c**) and the wear track profile (**d**) after the tribological tests of the PEI/1 HNT (**d**) nanocomposite. Point tribological contact. Similar to the point tribological contact, as well as the recent experimental data reported by the authors of [43], another test was carried out in the linear tribological contact at more \"severe\" load-speed conditions (\(P=180\) N, \(V=0.5\) m/s). Its results are presented in Table 9. Compared to the data at \(P=60\) N and \(V=0.3\) m/s, the CoF level decreased from 0.23 to 0.19, while the WR value increased from 0.95 to 1.87. Note that such a WR increase was expected in this case. A time dependence of the CoF values for the PEI/10 CCF/1 CNT nanocomposite is presented in Figure 16. It could be stated that the pattern of its variation clearly indicated the formation of a TL. The same was evidenced using a micrograph of the wear track surface (Figure 17b), which was quite similar to that of the \"mild\" tribological test conditions (Figure 10e). The wear track profile was quite smooth (Figure 17c), and the \"polished\" steel counterpart surface had a uniform shade (Figure 17a), which could indicate the  
Figure 15: The time dependence of CoF values for the PEI/10 CCF/1 HNT nanocomposite in the tribological tests at the loads of 5, 7.5 and 10 N and a sliding speed of 0.3 m/s (**a**). Optical micrographs of the steel counterpart surface (**b**,**e**), wear tracks (**c**,**f**) and their profiles (**d**,**g**). Loads were 7.5 (**b–d**) and 10 N (**e–g**). Point tribological contact. \begin{table}
\begin{tabular}{c c c c} \hline \hline \(P\), N & **Composition (wt.\%)** & **Coefficient of Friction, CoF** & **Wear Rate, WR, mm3/N-m, 10\({}^{-6}\)** \\ \hline
7.5 & PEI/10 CCF/1 HNT & 0.283 \(\pm\) 0.005 & 16.02 \(\pm\) 1.07 \\
10.0 & PEI/10 CCF/1 HNT & 0.283 \(\pm\) 0.007 & 17.45 \(\pm\) 0.97 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: The tribological properties of the PEI-based nanocomposites. Point tribological contact. formation of a thin transfer film, identical in composition to the tribological layer on the wear track surface. Note that the steel counterpart protection was achieved precisely by loading PEI with 1 wt.% HNTs. According to the literature, the degree of nanofilling could be varied over a wide range of 5-10 wt.% [12], but the nanofiller efficiency was determined by the possibility of forming and fixing a tribological layer or a transfer film. On the wear track surface of the PEI/10 CCF/1 HNT nanocomposite, a thin transfer film was not formed, but a mixing layer was, which could be classified as a tribological one. This fact was confirmed by its non-smooth surface according to the SEM micrograph (Figure 7b), as well as the relatively high CoF value of ~0.24. This phenomenon was not characteristic of the three-component PI-based nanocomposites, nor the PEI/10 CCF/1 CNT one. According to the authors, the reasons for this were the specifics of both the chemical structure and activity of this polymer (which was consistent with the high WR value of 390.2 \(\times\) 10\({}^{-6}\) mm\({}^{3}\)/N\(\cdot\)m for neat PEI reported in Table 3).