## 1 Introduction
As a result, the thermal stability, glass transition temperature and tribological characteristics were improved after loading with 4 wt.% TiO\({}_{2}\). [PERSON] et al. [27] presented a thorough analysis of the application of carbon nanoparticles in tribology, including carbon nanotubes (CNT), graphene, nanodiamonds and fullerenes. They were compared in terms of their capability to reduce friction and wear when being loaded as a lubricant additive. In addition, the superlubricity effect was discussed in regard of CNT and graphene. Halloysite nanotubes (HNTs) are of wide practical interest due to their great strength, thermal stability, availability and low cost [28]. [PERSON] et al. [29] discussed the mechanisms of PI/HNT nanocomposite formation. It was shown that the tensile strength and tensile modulus could be increased by 62.8% and 63.7%, respectively, when the HNTs content was equal to 3 wt.%. [PERSON] et al. [30] loaded PI with HNTs in order to fabricate the polymer composites with high dielectric permittivity and thermal stability. In addition, KH550 sizing agent was employed for HNT surface treatment. However, HNTs have rarely been used in tribological applications. [PERSON] et al. [31] loaded 1 wt. HNTs into the epoxy matrix. In doing so, the elastic modulus increased by 50%, while the hardness increased by 100%. In addition, the tribological test showed that certain patterns of HNT orientations ensured a decrease in the scratches' volumes by 50%. To the best of our knowledge, HNTs have not been employed for the development of PI/PEI-based wear-resistant composites. Important factors determining the formation and adherence of a TF are the tribological contact scheme, namely, (i) point (\"ball-on-plate\", as an example), (ii) linear (e.g., \"block-on-ring\"), and (iii) planar (\"pin-on-disk\", for instance) contact, as well as the load speed conditions in tribological tests [32; 33; 34; 35; 36]. Consequently, they are also decisive for the performance characteristics of wear-resistant (anti-friction) (nano)composites. A recently published book on the tribology of polymer nanocomposites [37] analyzed the role of various types of nanofillers (metal nanoparticles, mineral fillers, carbon nanofibers (CNFs) and carbon nanotubes (CNTs), natural fibers, combined nanofillers) in terms of their functional characteristics. It was stated for various types of polymer matrices that, firstly, there is no single mechanism for the formation and adherence of a TF on both the counterpart and nanocomposite surfaces of different chemical natures. Secondly, the tribological contact conditions (including temperature, the test scheme, the counterface surface roughness, the presence of a lubricating medium, etc.) greatly determine the wear mechanism and the formation/adherence of the tribological film on the contacting surfaces. Thirdly (and most importantly), it was shown that designing polymer nanocomposites for tribological applications is a promising technical solution for maximizing economic efficiency by reducing the consumption of materials, production waste and energy [38]. Despite the unique structures and properties of various types of nanofillers, as well as their changes after loading into polymer matrices, the aspect of their cost still remains an important issue limiting wide industrial applications. In this regard, this study aimed to establish an inexpensive nanofiller (in particular, natural nanotubes) for wear-resistant PI- and PEI-based nanocomposites, additionally reinforced with chopped carbon fibers (CCFs) for metal-polymer tribological units. The relevance of a comparison of the two types of amorphous thermoplastics as matrices for the nanocomposites is also associated with their cost and technological characteristics. PEI remains stable only at temperatures up to 200 \({}^{\circ}\)C, but it is cheaper and easier to process than PI. Therefore, PEI is preferable (in the mentioned temperature range) when both types of the polymers have similar mechanical characteristics.