A number of such materials are described in excessive detail in the literature, in particular, neat polyetheretherketone (PEEK) and PEEK-based composites, while a similar polyaryletherketone (PAEK) polymer has rarely been the subject of research. At the same time, polytetrafluoroethylene (PTFE) is not included in this list as a matrix material, despite the fact that it is extremely widely used as solid lubricant filler. The authors are aware that such an effect is associated primarily with the difference in conditions of tribological tests, including a loading scheme (a contact type), load-speed (P·V) parameters, temperatures and environments, and the counterpart material and its roughness, etc. For this reason, the conditions of the tribological tests are characterized in the description and analysis of the obtained results.
As a key criterion (the top level of the classification), a contact scheme of the polymer composites and counterparts (namely, point, line, and flat ones) is adopted since it determines the macroscale level of the tribological interaction of the rubbing bodies. The influence of other parameters is considered when interpreting reported values of the coefficient of friction (CoF) and wear rate (WR), as well as the characteristic mechanisms that determine an increase/decrease in wear resistance.
The prospect of using HPPs to design materials for friction units is determined by their two key properties: high melting points and great strength properties. In addition to an acceptable level of melt flow index and the ability to provide interfacial adhesion to filler particles/fibers (including at a sufficiently high degree of their content), these materials are also attractive because of their ability to process and improve the stiffness/load-bearing capacity. However, advanced strength properties of this class of materials simultaneously determine great CoF levels, which in most cases can exceed 0.3 on steel under dry sliding friction (DSF) conditions. According to Brian J. Briscoe, a typical method for solving this problem is loading polymers with various types of fillers using the ‘hard and strong fillers in a softer matrix’ or ‘soft and lubricating fillers in a hard and strong matrix’ principles 
Recently, special attention has been paid to loading metals and nonmetal oxides (in particular, SiO2
, SiC, ZnO, TiO2
, CuO) with nanoparticles as a way to improve the load-bearing capacity and wear resistance of the polymer composites against counterparts 
. In addition, a large number of researchers studying the composites have loaded carbon nanotubes (CNTs) with the goal of enforcing polymers since CNTs show very high strength and stiffness.
Regardless of the types of loaded fillers, the invariable condition for enhancing wear resistance is the formation of a thin and uniform transfer film (TF) or transfer film layer (TFL) and its reliable adherence on a counterpart. Thinner TFs tend to adhere stronger to counterpart surfaces than thicker ones. In this regard, the emphasis in interpreting the results of tribological tests of polymer composites is shifting toward the specifics of the TF formation and fixation on the counterparts. Additionally, there are two conditions that determine the ability of a polymer composite to form such a film: (1) loss of the polymer material by attrition due to interaction with metal asperities; and (2) its ability to be adhered and retained on the counterpart surface for a long time. The latter process is hindered by the loss of the TFs via debonding from the counterparts 
. A third important effect on the wear process should also be added, namely, the formation of oxidized debris in the tribological contact zones, including from the fractured TFs. They can both exert an abrasive effect and act as a third body that facilitates sliding.
As an effective approach for designing antifriction polymer composites, Li Chang 
analyzed the formation of hybrid materials that simultaneously include both solid lubricant particles and reinforcing fibers. The synergetic principle of their action characterizes a scheme in Figure 1
. The following are noted as key tribological aspects 
: (1) The pin-on-disc (P-o-D) and block-on-ring (B-o-R) schemes are two generally used methods for sliding wear tests. (2) The presence of reinforcing fibers strengthens the polymer composites and protects them against irregularities (asperities) on the counterpart surfaces on the one hand. On the other hand, hard fibers can damage the counterpart surfaces and prevent TFs from attaching to them. Finally, the author concluded that ‘understanding of the growth of TFLs and their tribological behavior during steady state wearing stage is still limited owing to the lack of quantitative techniques’ 
. Note that researchers of HPP-based composites put very different concepts into the TF term despite the obviousness of the wear process development. For this reason, the TF role in the formation of the tribological properties can be both negative and positive, even providing friction in the ‘wearless’ mode.
A summary of the structural components of the wear resistant polymer composites. Figure is reproduced with permission from reference 
Meghashree Padhan suggested some examples of hybrid ‘PEEK + PAEK’-based composites loaded with 30% short carbon fibers (SCFs) and 10% graphite (Gr) nanoparticles as primary solid lubricants (SLs), as well as 10% noncarbon secondary SLs such as WS2
, h-BN, etc. When interpreting the results, the effect of synergism between both types of SLs and the best efficiency of nanoparticles over microsized inclusions were considered.
The widespread use of nanofillers to improve wear resistance of polymer composites has prompted a significant number of reviews. For example, Qihua Wang emphasized 
that nanoparticles alter friction and wear behavior in different ways. As a result, low CoF levels do not necessarily correspond to decreased WR values. At the same time, the factors affecting wear resistance are types of the combined polymer and nanoparticles, the filler content, and the friction contact size and shape in addition to operating conditions. Due to so many influencing factors, the effect of nanoparticles on the tribological properties of the antifriction polymer composites is still an open field for further research. This is especially the case when nanoparticles in combination with conventional fillers are considered. In doing so, a detailed analysis of the tribological behavior of various types of the composites with explicit consideration of the specific conditions of tribological tests is required.
A modification of M. Ashby’s concept on ranking structural materials over multidirectional functional properties 
has been suggested for designing polymer composites for tribological purposes 
. It considers WR, CoF and tensile strength levels of the polymer matrix, etc. (Figure 2
). This approach is important due to its versatility since it enables the solution of the practically important issues of choosing polymer composites (including multicomponent ones) for given operating conditions. However, as noted above, the tribological properties depend on a number of parameters, so it is almost impossible to offer a completely universal approach within the framework of such data. As a result, a promising research area is the implementation of machine learning algorithms 
The tribological properties of different polymer matrices and fillers. Figure is reproduced with permission from reference 
The demand for HPPs in various industries has prompted a variety of reviews on their functional properties. In particular, Kurdi, A. et al. discussed the wear mechanisms under sliding friction for designing high-performance composites 
. In this case, special attention was also paid to environmental factors, namely lubrication conditions, temperature, and even the possibility of developing vibration-induced phenomena. It was noted that loading with fillers may cause discontinuities in the polymer matrix, which promote the formation of debris. As a result, the critical filler contents should be limited to 5–15 vol.%. The role of nanoparticles in changing wear resistance narrows to both probable rolling effect and topographic smoothening of the friction surfaces. Similar to other authors in this field of science, Kurdi, A. and Chang, L. noted that the desired effect is achieved mainly by using a combination of reinforcing particles and fibers. However, an increase in both strength and toughness of HPPs does not always reduce the intensity of the wear mechanism development and the formation of debris.
Neat HPPs are not antifriction materials due to their high strength, which inhibits easy sliding on the counterpart (primarily steel) surfaces., This challenge is overcome in the design of composites by loading solid lubricant particles, followed by TF formation on the counterparts. However, the number of asperities on the sliding surfaces, determining CoF levels, also depends on the contact area. Three contact types are typically distinguished in tribology: point, line, and area (Figure 3
. It is not too difficult to form a TF that is uniform in thickness and relatively firmly adhered on the surface of a steel ball at a point contact; however, this task becomes highly complicated for area contact types. The reason is lower specific pressures for this tribological loading scheme (as a rule). Temperature is another important aspect, since it is easier to realize heating followed by tribological oxidation and adhesion in a point contact than in both line and area ones.
Various tribo-configurations and tribo-contacts: point contact (a-1
), line contacts (a-6
) and area contacts (b-2
). Rotary motion (a-1
), Reciprocating linear motion (a-5
), linear forward motion (a-3
). Figure is reproduced with permission from reference 
HPPs include polyethersulfone (PES), polyetherimide (PEI), polyphenylene sulfide (PPS), PEEK, fluoropolymers, etc. Their attractiveness lies in the possibility of replacing metal parts with nonmetallic ones in high-tech industries 
. However, most papers on HPPs for tribological applications focus on a limited number of thermoplastics such as PPS, PEEK, and polyimide (PI). In this regard, numerous loaded fillers, their combinations, as well as schemes and conditions of tribological tests should be investigated on this topic.
A significant number of papers on the use of PEEK-based composites are devoted to orthopedic applications. The authors are looking for ways to replace metal implants (components) with polymer ones, for which neat PEEK is often loaded with CFs 
. In the design of PEEK-based composites for biomedical engineering applications, aspects of the polymer-filler interfacial interaction and their fabrication methods also play an important role. Oladapo et al. 
noted perspectives of the PEEK-based composites from the point of view of the implementation of additive manufacturing (AM) procedures. Also, cellular calcium hydroxyapatite (CHAp) was analyzed as an advanced excipient. Another review 
is devoted to the use of PEEK for designing biomedical composites and highlights its relevance in dental applications as well. In addition, PEEK is widely used for manufacturing knee, hip, spine and other implants in orthopedics. A review on dental applications of PEEK is given in 
. Further analysis of the issue in its application for scaffolds was carried out by the authors of 
. In a similar paper 
Ma, H. et al. summarized some aspects of the performance requirements, the composite design process, and the surface modification when using PEEK as a material for the manufacture of orthopedic implants.
Because the tribological properties of the polymer composites are determined by their friction surfaces, some data on the PEEK surface modification have also been reviewed. In 
, Singh, S. analyzed plasma treatment of PEEK in terms of its influence on the biological, surface (adhesion and wettability), mechanical, and tribological properties. As noted above, some similar materials are gaining popularity in the development of HPP-based composites. In their review 
Veazey, D., et al. proposed a roadmap for high-performance PAEK-based composites reinforced with long fibers (Figure 4
). They emphasized that one of the key challenges in designing these composites is improving the fiber-matrix interfacial bond strength.
Typical tensile properties of common metallic alloys and CF-reinforced PAEK-based composites: UD, unidirectional fibers; HCM, hot compression molded fibers; INJ, injection molded fibers. Figure is reproduced with permission from reference 
The unique properties of PI have determined the prospects for some practical applications of PI-based composites. The most detailed review 
covers the issues of their synthesis. It has been shown that the presence of functional groups and the formation of dendritic structures enable unique properties in PI-based composites. Their key advantage is the possibility of high-temperature applications. Nevertheless, some design challenges remain for the PI-based nanocomposites. For example, Ogbonna, V.E. emphasized 
some issues of interfacial adhesion and surface degradation, which affect their mechanical properties and abrasive wear resistance in tribological applications. The following are considered fillers for such PI-based nanocomposites: CNTs, GN, graphene oxide (GO), boron nitride (BN), MoS2
, silica (SiO2
), titania (TiO2
), alumina (Al2
), CFs, aramid fibers (AFs), GF, zinc dioxide (ZnO2
), zirconium dioxide (ZrO2
), silicon nitride (Si2
), and carbon nitride (C3
). In 
nanofillers of the mentioned types were reviewed from the standpoint of their influence on the structure and mechanical properties of the PI-based nanocomposites. Also, similar to the PEEK-based composites, the PI-based ones are used in medical applications. Some chemical, physical, and manufacturing aspects of designing the PI-based biocompatible composites are summarized by Catalin P. Constantin in