## 3 Results
### The Linear Tribological Contact  
According to Table 5, the average CoF values of 0.44 for neat PI and 0.38 for neat PEI were 1.4 times higher than those for the point tribological contact (Table 3). Their reinforcement with CCFs reduced these levels by 1.6 times for the PI/10 CCF composite and by 1.7 times for the PEI/10 CCF one. Subsequent loading with CNTs and HNTs almost did not change the integral CoF values in all the cases studied. Figure 8 presents the time dependence in the CoF values for the three-component PI- and PEI-based nanocomposites (for the two-component ones and the neat polymers, similar data are reported in the Supplementary Materials). For neat both PI and PEI, the CoF levels were consistently high (0.44 and 0.37) with a pronounced \"high-frequency\" oscillation after a short running-in stage (Figure S3a,b). Loading with CCFs (Figure S3c,d), as well as subsequent filling with CNTs and HNTs, reduced both the oscillation levels and CoF values down to 0.24 (Figure 8a-d). This fact was previously associated with the formation of tribological layers on the wear track surfaces [43]. However, the average CoF value was lower for the PEI-based nanocomposites than for the PI-based ones (\(\sim\)0.23 versus \(\sim\)0.28, respectively) in the current study. Note that reinforcing PEI with CCFs reduced the WR level by three times compared to the neat polymer (Table 5). However, subsequent filling with CNTs and HNTs further lowered the WR values by up to approximately five times. In contrast to the point tribological contact, the WR levels were close for the nanocomposites based on both polymer matrices. Nevertheless, the minimum values were typical after loading with HNTs (0.77 and 0.95 for the PI- and PEI-based nanocomposites, respectively). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**No.** & **Composition (wt.\%)** & **Coefficient of Friction, CoF** &
\begin{tabular}{c} **Wear Rate, WR,** \\ **mm3/N-m, 10\({}^{-6}\)** \\ \end{tabular} & **Temperature, T, \({}^{\circ}\)C** \\ \hline
1 & PI/PEI & 0.442 \(\pm\) 0.063/0.379 \(\pm\) 0.044 & 20.37 \(\pm\) 1.03/14.60 \(\pm\) 1.30 & 32.9 \(\pm\) 0.4/32.7 \(\pm\) 0.4 \\
2 & PI/10 CCF/PEI/10 CCF & 0.279 \(\pm\) 0.030/0.217 \(\pm\) 0.027 & 1.06 \(\pm\) 0.26/5.15 \(\pm\) 0.32 & 24.3 \(\pm\) 0.2/23.3 \(\pm\) 0.2 \\
3 & PI/10 CCF/1 CNT/PEI/10 CCF/1 CNT & 0.275 \(\pm\) 0.031/0.222 \(\pm\) 0.028 & 2.56 \(\pm\) 0.10/1.13 \(\pm\) 0.12 & 25.4 \(\pm\) 0.1/25.1 \(\pm\) 0.2 \\
4 & PI/10 CCF/1 HNT/PEI/10 CCF/1 HNT & 0.276 \(\pm\) 0.028/0.230 \(\pm\) 0.027 & 0.77 \(\pm\) 0.75/0.95 \(\pm\) 0.10 & 24.0 \(\pm\) 0.1/25.1 \(\pm\) 0.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The tribological characteristics of neat both PI and PEI, as well as the PI- and PEI-based nanocomposites. Figure 7: SEM micrographs of the steel counterpart surface (**a**) and the tribological layer on the PEI/10 CCF/1 HNT nanocomposite (**b**). Point tribological contact. Digits in blue squares denote Spectra numbers in Table 4.  
Figures 9 and 10 show optical photographs of the counterpart surfaces, the wear tracks and their profiles for the studied nanocomposites. For PI/10 CCF/1 HNT (Figure 9e) and PEI/10 CCF/1 HNT (Figure 10e), the tribological layers were characterized by the smoothest surfaces. The same nanocomposites exhibited the minimum WR values of 0.77 \(\times\) 10\({}^{-6}\) and 0.95 \(\times\) 10\({}^{-6}\) mm\({}^{3}\)/N\(\cdot\)m, respectively. However, the difference in the WR levels did not exceed a couple of dozen of percent (Table 5) in contrast to the two-component nanocomposites. Since it was impossible to reveal any peaks or maxima for neat PI in the Raman spectra, the authors analyzed the spectra for the PEI/10 CCF/1 HNT nanocomposite (Figure 11). Curve 1 did not possess any noticeable peaks or maxima outside the wear track, similar to the above data for the \"ball-on-disk\" scheme (Figure 6b). On the wear track (curve 2), there was a pronounced range of increased intensity in wavenumbers 1000-1700 cm\({}^{-1}\).