Development and optimization of high-performance PEEK/CF/ Nanosilica hybrid composites

In the present study, tribological properties of PEEK/CF/nanosilica composites with distinct amounts of silica nanoparticles against steel were studied by using a block-on-ring tribometer followed by the characterizations of associated transfer films and polymer worn surfaces. The results demonstrate that the content of silica nanoparticles exerts an obvious influence on the friction and wear properties of PEEK/CF/nanosilica composites. Under low-load conditions, the friction coefficient and specific wear rate exhibit opposite dependence on the nanosilica content. The friction coefficient decreases with increasing nanofiller content, while the specific wear rate increases with enhancing nanosilica loading. When the load conditions were changed toward high values, the divergence of the tribological properties becomes insignificant, which show less dependence on the nanosilica loading. Taking into account the practical applications of such composites, the composite containing 2 wt.% silica nanoparticles can serve as an excellent candidate for manufacturing tribological components in the practical applications.


| INTRODUCTION
Polyetheretherketone (PEEK) is one of the mostly used highperformance polymers for industrial applications, which exhibits excellent mechanical properties and thermal resistance. [1][2][3] Its glass transition temperature and melting temperature are 150 C and 343 C, respectively. 4 As a representative of high-performance polymers used as tribological materials for producing different components in mechanical and automotive engineering, for example, sliding bearing bushings, cages of high-precision ball bearings, polymer gears, etc., PEEK has been paid much attentions. [5][6][7][8] It is generally accepted that a high-performance polymer-based tribomaterial consists of a highperformance polymer, such as PEEK, polyimide, 9,10 polyphenylene sulfide, 11 internal solid lubricants (polytetrafluoroethylene, graphite, molybdenum disulfide), and reinforcing fibers, for example, carbon fibers, glass fibers, and aramid fibers. The solid lubricants reduce the friction, while the reinforcing fibers enhance the wear resistance of the polymer composite.
Recently, the friction and wear properties of such tribocomposites have been successfully controlled by adding functional fillers at different length scales for further increase of their tribological properties, for example, submicro-and nano-sized particles. [12][13][14][15][16][17] In the study of Zhang et al., 18 they studied the roles of low-loading nano-sized silica particles (1 vol.%) on the friction and wear behavior of short carbon fiber (SCF)/ PTFE/graphite (micro-sized)-filled PEEK. It was found that the nanoparticles remarkably reduced the friction coefficient in the studied range of pressure (p) and velocity (v) conditions up to 7 MPa and 2 m/s. With respect to the wear performance, the wear resistance of the PEEK composite was greatly improved under high pv-conditions by addition of nanosilica. In a more recent study, 18 Chang and co-workers investigated the impact of nanofiller content on the tribological properties of PEEK. It was reported that there was an optimum filler loading for the specific wear rate. More importantly, like other properties of polymer nanocomposites, homogeneous distribution of nanoparticles within the PEEK matrix and strong filler/matrix interphase is responsible for the improvement of the tribological performance. 19,20 In summary, it can be concluded that addition of nano-sized inorganic particles can reduce the friction and wear of the polymer materials.
To guide the designing of PEEK components with high reliability, tailored friction, and wear performance for tribological applications, studies on the friction and wear mechanisms of PEEK composites have been the pursuit of researchers. Transfer films have been found to be a crucial factor in governing the tribological functionality of PEEK composites. 21,22 As for the role of rigid particle fillers, it was revealed that the particles may slide or roll between the fiber and counterpart interface depending on the loading parameters. 23 The movement pattern of the rigid particles may be closely related to the near surface properties of the mating polymer materials, which also showed great contribution to the tribological characteristics of polymer materials. 24 In our recent study, 25 it was revealed that the internal solid lubricants were no more than the indispensable component for ensuring superior tribological performance of polymer composites owing to the synergetic effect between the carbon fibers and the rigid particles. Meanwhile, such tribocomposites with high-carbon-fiber content exhibited excellent mechanical properties, which can broaden their applications. In this work, tribological properties of PEEK/CF/ nanosilica composites with different nanosilica contents were investigated in a wide range of pressure and sliding velocity conditions and compared with those of a conventional PEEK-based tribocomposite, in which only carbon fibers and graphite particles were incorporated.
In addition, the friction and wear mechanisms were characterized based on the comprehensive analysis of the polymer worn surfaces and transfer films formed on the steel counterpart. The aims of the present study are, on the one hand, to explore the optimum nanofiller loadings for the tribological performance of PEEK composites and, on the other hand, to reveal the dominant mechanisms in governing the tribological performance of the composites studied. Graphite, Sweden, and nano-sized silica (Aerosil R9200) from Evonik Industries, Germany, were utilized as reinforcing fillers. Their specifications according to the manufacturers are given in Table 1. In addition, morphology images of the as-received fillers are shown in Figure 1.
The nanosilica content in the composites was varied from 1 to 10 wt.%. The designations and detailed formulations of the composites are introduced in Table 2 chopped carbon fibers and graphite particles was prepared by feeding pure PEEK and chopped carbon fibers in the main feeder followed by feeding the graphite particles through a side feeder under same processing conditions. This multistep compounding process can lead to a high dispersion and distribution quality of the particulate fillers according to early studies. 29,30 After compounding of the designed composites, they were injectionmolded to sheets with a dimension of 50 mm × 50 mm × 4 mm by using an injection molding machine (Engel victory 200/80 spex, ENGEL Austria GmbH, Austria), from which the mechanical and tribological specimens were milled. During injection molding, the cylinder temperature was kept at 385 C, 395 C, 395 C, and 395 C in different zones. The mold temperature was chosen as 195 C. The processing chain from the row materials to the testing specimens is shown in Figure 2. After injection molding of all the composites, the density of each composite was determined according to Archimedes' principle by using a precision balance (Kern ABT 220-50M, Germany). The results are shown in Table 3. The density of the composites was used to calculate the specific wear rate of the studied composites.
T A B L E 1 Specifications of different fillers according to the manufacturers 26

| Tribological tests
Tribological investigations were carried out on a block-on-ring (BoR) testing device at room temperature under dry sliding conditions, as it is described in an early study. 31 in which, Δm is the mass loss of the composite material, ρ presents its density, and F N denotes the normal load. v and t are the sliding speed and time, respectively. For each pv-combination, at least three tests were conducted for determining the average friction coefficient and specific wear rate.

| Analysis of the worn surfaces transfer films
Worn surfaces of polymer samples and transfer films formed on the steel ring were characterized by using a Keyence laser scanning microscope VK-X1050, Japan, and a JEOL scanning electron microscope (SEM, JSM-6460 LV SEM, Japan) with energy-dispersive X-ray (EDX) spectroscopy. In addition, the thickness of the transfer films was determined by applying a focused ion beam (FIB) system (FEI Altura 875 Dualbeam, USA). In order to avoid the damage of the transfer films during the FIB cutting process, the counterbody surface was sputtered with a thin layer of platinum prior to the cutting process.

| Mechanical properties
Mechanical performance of polymer materials is of vital importance for their applications, which is also very important for tribological applications of such materials. Generally, high mechanical properties can significantly enhance their load-bearing capacity and broaden the pv-limit of polymer-based tribomaterials. Figure 1 shows the tensile properties of PEEK composites. As is seen in Figure 3A,B, addition of high amounts of carbon fibers and nanosilica into PEEK matrix leads to enhanced stiffness and strength compared to that of PEEK-Tr.
However, high filler loading impairs the ductility of the PEEK/CF/ nanosilica composites ( Figure 3C). The elongation at break of PEEK/ CF/nanosilica composite with 10 wt.% nanosilica content is about 3%.

| Friction and wear properties
The steady-state friction coefficient and specific wear rate of PEEK-Tr are shown in Figure 4. As can be seen, the friction coefficient exhibits less dependence on the pv-conditions except at 8 MPa and 4 m/s, which is between 0.3 and 0.45 in the studied range of pv-conditions ( Figure 4A). More surprisingly, PEEK-Tr presents the lowest friction coefficient of 0.26, once the pv-product was raised to 32 MPaÁm/s.
Considering the specific wear rate of PEEK-Tr, it is observed that, unlike the friction coefficient, the wear resistance of PEEK-Tr is much susceptible to the pv-level. Under pv-conditions higher than 6 MPaÁm/ s, an increase in the pressure at same velocity leads to an decrease of the specific wear rate up to 51%, as is shown in Figure 2B.
With respect to the friction performance of PEEK/CF/nanosilica composites, it can be clearly seen from Figure 5A    can also contribute to the friction reduction, as reported in our early studies. 8,36 The accumulation of the wear debris and inorganic particles adjacent to the carbon fibers serves as third bodies between the sliding pair, which might prevent the severe wear of the polymer matrix.

| Tribological mechanisms
It has been generally accepted that the formation of transfer films on the steel counterpart, that is, structure and properties, plays a vital role on determining the tribological performance of polymer/steel tribosystems. 33,37,38 Analysis of the transfer films formed on the steel counterface reveals that the wear particles of PEEK/CF/nanosilica composites are entrapped into the deep grooves (dark areas in SEM images in Figure 10) on the steel surface in the earlier sliding process that reduces the surface roughness and thus decreases the abrasion of the counterpart asperities. More interestingly, it is also manifested that PEEK/CF/nanosilica composites present similar morphology of transfer films independent on the load conditions and nanofiller content ( Figure 10).
Inspection of the transfer film structure in its thickness direction by using FIB cut of the steel counterpart clearly reveals that the transferred materials are entrapped into the deep grooves of the steel counterbody ( Figure 11) and smooth the counterface. In addition, transfer films can be also found on the smooth areas of the counterbody surface, as is shown in Figure 11. The thickness of the transfer films can attain hundreds of nanometers. As is seen, there is almost no difference of the transfer films between PEEK/CF/nanosilica composites with different particle contents under the same pvcondition. It can be therefore concluded that the distinction of the friction and wear properties of the PEEK/CF/nanosilica composites

| CONCLUSIONS
In the present study, tribological properties of PEEK/CF/nanosilica composites with distinct nanosilica loadings were systematically investigated under dry sliding conditions on a BoR apparatus. The following conclusions can be drawn: 1. It is revealed that PEEK composites filled with carbon fibers and silica nanoparticles exhibit much better friction and wear performance compared to those of PEEK-Tr. More importantly, their tribological performance is strongly dependent on the nanofiller content.
2. PEEK/CF/nanosilica composite filled with 2 wt.% nanosilica exhibits excellent tribological properties in the whole pv-range studied, which can meet the requirements of the industrial applications.
3. It is of great interest to reveal that the transfer film shows similar morphology and composition independent on the nanosilica contents and pv-conditions. The dominant tribological mechanisms, which determine the friction and wear behavior of the PEEK/CF/ nanosilica composites, are the abrasive or adhesive wear,