Introduction

With the emergence of new industries, the working conditions for mechanical moving parts under lubrication have become increasingly challenging due to factors such as frequent start-stop cycles, speed variations, and high loads [1, 2]. To ensure the dependability of motion systems in such environments, there are two primary approaches. One is to improve the characteristics of the lubricating medium, while the other is to develop efficient inorganic sliding pairs as alternatives to the conventional metal-to-metal friction pairs [3, 4]. Over the past two decades, extensive research has been conducted on the tribological properties and processes of polymer composites, particularly fiber-reinforced composites [5,6,7,8]. In recent years, it has been found that the tribological performance of fiber-reinforced polymer composites can be further enhanced by incorporating sub- and nanoparticles, such as TiO2, ZnS, SiO2, B4C, etc. Polymer-based composites, due to their self-lubricating properties, have been frequently employed to replace metallic components in dry friction environments [9]. In a study by Feng [10], it was reported that the addition of MoS2-modified carbon fiber and nano SiO2 in polyether ether ketone (PEEK) composites resulted in a 31.43% reduction in friction coefficient and a 79.18% reduction in wear rate for PEEK/SiO2/SCF-MoS2 composites.

The presence of fibers has been found to facilitate the development of a friction layer on the counterpart surface, as demonstrated in the investigation of Xue et al. [11]. The tribological properties of polymer composites in an oily environment have been a central focus of research [11]. Furthermore, studies have examined the influence of fiber surface treatment on the tribological properties of polymer composites in oil lubrication, highlighting the significance of the interface bonding between fibers and the polymer matrix in understanding their tribological behavior [12,13,14]. Previous studies have already recognized the critical role of friction film development on the sliding surfaces in determining the tribological properties of peek-based materials operating under mixed and boundary lubrication conditions [1].

Although epoxy resin (EP) composites with excellent properties have been extensively studied, most of these studies have focused on dry friction conditions [15,16,17,18,19,20]. Consequently, there has been limited research on the friction and wear characteristics of EP composites in lubricated conditions [21, 22]. Therefore, it is crucial to thoroughly investigate the tribological behavior of EP composites with lubricants. Among the various types of reinforcing fillers used in polymer substrates, carbon fiber and glass fiber (GF) are the most commonly employed. It is generally believed that carbon fiber exhibits better tribological properties in dry friction environments compared to short carbon fiber (SCF) [9]. Studies have reported that carbon particles are transferred to the opposing surface, along with other wear products, to enhance the tribological properties during dry friction [23]. On the other hand, due to its high hardness, short glass fiber (SGF) has the potential to scratch metal surfaces [24]. Thus, in practical applications involving SGF as a fiber reinforcement in thermoplastic-based composites, the introduction of solid lubricants such as graphite or PTFE is often necessary to prevent SGF from scratching the metal surfaces. These solid lubricants form a lubricating transfer film that acts as a protective barrier. However, the friction and wear properties of SGF-reinforced thermoplastic composites are still not well understood.

The tribological characteristics of SCF-reinforced EP composites under both water and oil conditions have been studied [24]. SCF was found to significantly improve the anti-wear properties of EP. The improvement in anti-wear properties and the formation of a friction film are crucial factors contributing to the enhanced wear resistance. However, the friction coefficient of EP under boundary and mixed lubrication does not immediately decrease upon the addition of SCF to the EP matrix [25, 26]. Therefore, it is of great practical significance to investigate EP composites that exhibit both low friction and high anti-wear properties.

In this study, the tribological behavior of SGF-reinforced EP composites under oil lubrication was investigated. The effects of different types of SGF, graphite, PTFE, and B4C nanoparticles on the friction and wear properties of EP were examined using a ball-on-block machine. The worn surfaces were characterized using optical microscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The anti-wear mechanisms were proposed based on the experimental observations and analysis.

Materials and Methods

Raw Materials

The raw materials used in this study include bisphenol epoxy resin (DER331, Dow) as the base material for EP composites and a cycloaliphatic amine hardener (HY 2954, Huntsman). Short glass fibers (SGF) were employed as reinforcement, with a diameter ranging from 9 to 15 μm and an average length of 200 μm. Supplementary fillers such as polytetrafluoroethylene (PTFE) (TF 9207, Dyneon), B4C nanoparticles (Nanopox F400, Evonik), and graphite (RGC39TS, Superior graphite) were added to the composites. The average particle size of PTFE, B4C nanoparticles, and graphite are 9 μm, 20 nm, and 5 μm, respectively. Three series of composite materials were investigated: pure EP, SGF-reinforced EP, B4C-reinforced EP, and graphite-reinforced EP composites. The composition of the EP composites is shown in Table 1. The process involved combining the resin with SGF, graphite, and PTFE in a sequential manner, followed by dispersion in a vacuum dissolver. The mixture was then stirred with the hardener in the dissolver under vacuum. The EP composite material was prepared by pouring the liquid mixture into an aluminum mold, holding it at a gel temperature of 70℃ for 8 h, and curing it at 120℃ for 8 h [21].

Table 1 Compositions of EP composites (vol%)
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Tribology Tests

The MFT-3000 ball friction and wear equipment (BOB) of Rtec was used for tribological test. Figure 1 displays a schematic illustration of the BOB contact setup. The size of the polymer samples under investigation was measured at 20 × 5 × 5 mm3. The corresponding Cr15 steel ball is 12.7 mm in diameter and 0.1 ~ 0.2 μm in roughness. Under the force of gravity, lubricant (PAO4, supplied by China National Petroleum Corporation) drips through elastomer tubes onto the worn track, covering the surface of the track throughout the sliding test. The test parameters were normal load of 200 N, sliding speed of 0.44 m/s [11]. Each operation takes 6 h.

Fig. 1
figure 1

Schematic diagram of the ball-on-block contact configuration

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After each test, tissue paper was used to gently wipe away extra oil from the surfaces of the counterpart and composite samples. The remaining oil was then removed from the sample and its counterpart by ultrasonic cleaning for one minute in a petroleum ether bath. Using a force transducer, the friction coefficient was recorded online. Using a digital-reading optical microscope, the wear scar width of the block was assessed at six random locations along its length. The Eq. (1) is used to estimate the specific wear rate of the average sample.

$${{\text{W}}_{\text{s}}}=\frac{l}{{L \cdot N}}\left[ {{r^2} \cdot \arcsin \left( {\frac{W}{{2r}}} \right) - \frac{W}{4}\sqrt {4{r^2} - {W^2}} } \right]$$
(Eq. 1)

where Ws is the wear rate (mm3/Nm), r is the steel ball’s radius, l and W are the wear scar of the composite’s breadth and length (mm), L is the cumulative sliding distance (m), and N is the normal load (N) delivered towards the composites.

Nano-Scratch Tests

Nano-scratch tests were conducted on pure EP and SGF-enhanced EP composites using an in situ nanomechanics testing system (TI-950, Hysitron Inc.). These tests aimed to gain a comprehensive understanding of the tribological mechanism of SGF-enhanced EP. The smooth surfaces of the pure EP and SGF-reinforced EP composites were subjected to scratch tests. During the test, a normal load of 1000 µN was applied, and the sliding speed was set to 0.33 μm/s with a sliding distance of up to 10 μm. The normal displacement and lateral force were measured during the scratch test. The friction coefficients were determined from the “on-load” scratch test, and the scratch profile was obtained by performing “off-load” topography scans. The friction coefficient and anti-wear properties of the short glass fibers in the EP matrix were evaluated.

Worn Surface Analysis

The morphology of the worn surfaces of the samples and corresponding balls was analyzed using an optical microscope and a field emission scanning electron microscope (FE-SEM). Furthermore, energy-dispersive X-ray spectroscopy integrated within the FE-SEM (Energy 350, Oxford) was employed to identify the elemental distributions on the worn surfaces. XPS is a valuable technique for determining the chemical states of elements on the surface of a sample. In this study, XPS was used to analyze the chemical composition of the tribofilm formed on the sample surface. Additionally, the nanostructure of a selected tribofilm was examined using a standard high-resolution TEM.

Results and Discussions

Tribological Properties

The trend of friction coefficients (COF) for the various composites as a function of sliding time is shown in Fig. 2a. The introduction of SGF significantly reduces the friction coefficient of EP. In the running-in stage, the COF of pure EP increases from 0.07 to 0.1, indicating a mixed lubrication regime where solid-solid contact bears a significant load. The high friction coefficient of pure EP in this stage can be attributed to the bonding strength between the EP matrix and the metal counterface. However, the addition of SGF, as well as the volume fraction of SGF, has a substantial impact on the COF of EP. Compared to pure EP, the COF of SGF-reinforced EP composites consistently decreases over an extended running-in period, eventually stabilizing at approximately 0.03. It is noteworthy that the friction coefficient of EP increases significantly when the fiber percentage is increased from 5 to 10%. However, when the SGF content is further increased from 10 to 15%, there is a noticeable improvement in the COF. The presence of 10% SGF reduces the friction coefficient of EP by 66.30%. The friction coefficients tend to be similar for composites with 10% SGF reinforcement and other compositional variations. It is important to note that the addition of B4C, graphite, and PTFE did not have any noticeable impact on the COF of the 10% SGF-reinforced EP composite under the given conditions. Overall, these findings suggest that the incorporation of SGF in EP composites significantly reduces the friction coefficient, with the most substantial decrease observed at an optimal SGF content of 10%. Further additions of supplementary fillers did not have a significant effect on the COF in the presence of 10% SGF-reinforced EP composite.

Fig. 2
figure 2

COF of the composites with various sliding times (a) and mean friction coefficient and wear rates of EP composites (b)

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The average friction coefficients and wear rates of SGF-reinforced EP composites are compared in Fig. 2b. It is evident that the inclusion of SGF significantly reduces the friction coefficient and wear rate of the EP composites under oil lubrication conditions. A slight increase in friction coefficient is observed when the fiber percentage is increased from 5 to 10%. The highest friction coefficient among the SGF-reinforced EP composites, while still lower than that of pure EP, is observed in the EP composite filled with 15% SGF. The higher friction coefficient of the 15% SGF composite is attributed to the increased presence of flaws at the SGF-EP interface as the fiber fraction increases, leading to an elevated friction coefficient in the composites. However, due to the excellent abrasion resistance of SGF, the variation in the SGF fraction within the studied range does not result in a significant difference in wear rate. The addition of 10% SGF reduces the wear rate of EP by 90.93%. Interestingly, the inclusion of graphite, B4C nanoparticles, and PTFE in the SGF-reinforced EP does not significantly alter the friction coefficient and anti-wear properties of the EP composite containing 10% SGF in the presence of lubricants, contrary to the trends observed under dry sliding conditions [27, 28]. This indicates that the performance of SGF is primarily governed by the tribological characteristics of the SGF-reinforced EP. The variations in the friction coefficient observed during the running-in period for EP and other composites can be attributed to the formation of the tribofilm.

The worn surface near the wear track of the steel counterface is compared between pure EP and SGF-reinforced EP composites in Fig. 3. The wear track on the SGF-reinforced EP composite surface is significantly different from the original surface (Fig. 3b, c, and d), whereas the difference is not as noticeable for the worn surface resulting from rubbing against pure EP (Fig. 3a). This suggests that the introduction of SGF promotes the development of a tribofilm, which may be the primary cause of the reduction in friction coefficient observed during the running-in period in the SGF-reinforced EP composites. Therefore, the tribological characteristics of SGF-reinforced EP composites are significantly influenced by the presence and development of the tribofilm.

Fig. 3
figure 3

Q1

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Micro-friction studies were conducted on the EP matrix and 10% SGF to gain a deeper understanding of how SGF reduces friction and wear on a microscopic scale. The results of nano-scratch tests performed on the surface of the EP matrix and 10% SGF are shown in Fig. 4. It is evident that the scratched EP matrix (Fig. 4a1) exhibits a deep groove, indicating significant material removal. In the case of 10% SGF, the scratch initiates in the EP matrix and extends within the matrix by elongating the tip along the surface of the glass fiber while applying a constant load at a constant rate. Deep grooves can be observed on both sides of the SGF within the EP matrix. However, the scratch trails on the SGF surface appear much lighter (Fig. 4a2), with only a few visible scratches. This suggests that SGF exhibits significantly higher scratch resistance and hardness compared to the EP matrix. The size of the grooves indicates that the normal displacement of SGF (100–300 nm, Fig. 4c2) is much smaller than that of the EP matrix (700–1000 nm, Fig. 4c1), and the SGF surface appears smoother. The friction coefficients were measured during the scratch tests using the ratio of the lateral force to the normal force. The friction coefficient on the SGF surface (Fig. 4b2) is around 0.16. However, the friction coefficient increases on both sides of the SGF to 0.8-1.0, which is comparable to the friction value of the EP matrix (Fig. 4b1). This increase in friction coefficient is attributed to the greater penetration depth of the tip into the EP matrix compared to the SGF matrix. The adherence of the matrix to the tip and the resistance of the matrix to plowing are responsible for the increased COF observed in the EP matrix.

Fig. 4
figure 4

Topography images (al, a2), the friction coefficients (b1, b2), normal displacements (c1, c2), and normal forces (d1, d2) of the EP matrix (al, b1, c1, d1) and SGF surface (a2, b2, c2, d2) scratches vary with the various scratch displacement. The scratching direction is noted by red arrows

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Microfriction processes might differ somewhat from macrofriction processes, which are controlled by adhesion, roughness, plasticity, furrow impacts, and other factors [29]. While the microfriction characteristics of different phases within composites can provide clues for comprehending the macro-scale tribological causes, we believe that the superior low-friction and antiwear properties of SGF-reinforced composites are mostly due to the high load transfer capacity and wear resistance of SGF. Glass fibers support a significant percentage of the load applied directly by solid-solid contact when the composite rubs during the mixed and boundary lubrication systems.

Microfriction processes can differ from macrofriction processes, as they are influenced by factors such as adhesion, roughness, plasticity, and the impacts of furrows [29]. While the microfriction characteristics of different phases within composites can provide insights into understanding the macro-scale tribological behavior, we believe that the superior low-friction and antiwear properties of SGF-reinforced composites are primarily attributed to the high load transfer capacity and wear resistance of SGF. When composite materials undergo rubbing in mixed and boundary lubrication systems, glass fibers bear a significant portion of the applied load due to direct solid-solid contact. Thus, the tribological characteristics of the glass fibers play a crucial role in determining the properties of the composites.

Analysis of Worn Surfaces

Figure 5 presents SEM images and EDX spectra of the worn surfaces of SGF-reinforced EP composites. The tribological behavior of fiber-reinforced polymer composites under dry conditions is influenced by phenomena such as fiber thinning, fracture, delamination, and scratching [30]. The worn surface of pure EP appears rough, exhibiting fractures and pressure wave deflections caused by repeated sliding and the residual stress on the sliding surface in the presence of oil lubrication [26]. In this study, the majority of glass fibers on the worn surface are fragmented into small pieces due to the residual stresses acting on the fibers (Fig. 5a2). However, once the broken glass fibers are removed, large aggregates of wear debris become entangled and compacted on the surface of the composites, preventing further stripping of the glass fibers (indicated by red arrows in Fig. 5a, b, and c). This behavior contributes to the self-healing abilities, improved strength, and ductility of SGF-reinforced EP composites. Similar findings have been reported in various other studies on EP composites under water and oil lubrication conditions [21, 31].

The surface tribofilm, observed as white arrows in Fig. 4a, b, and c, is composed of elements such as Si (from the glass fibers), and C and O (from the glass fibers and EP matrix), as indicated by the corresponding EDX spectrum in Fig. 5a1.

Fig. 5
figure 5

Q2

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From Fig. 5 (b) and (c), it can be observed that when a steel ball slides against an SGF/G/PTFE/B4C composite, several surface wear modes occur, including abrasive wear, adhesive wear, and fatigue wear. This is consistent with previous studies on composite materials, where the presence of hard particles can lead to abrasive wear on the material surface. The PTFE component of the composite has also been found to contribute to adhesive wear, where the steel ball and the composite surface adhere to each other, resulting in material transfer. Fatigue wear, caused by repeated sliding motion, is also a common wear mode observed in composite materials.

However, it is important to note that the wear modes observed in this study may be unique to the SGF/G/PTFE/B4C composite, as the combination of these materials may result in specific interactions and behaviors that are not observed in other composite materials. Therefore, it is necessary to compare and contrast the results of this study with other relevant literature to gain a more comprehensive understanding of the wear modes and mechanisms of SGF/G/PTFE/B4C composite materials.

Comparing the results of this study with current literatures can provide valuable insights into the tribological behavior of SGF/G/PTFE/B4C composite materials [11,12,13,14]. It can help identify commonalities and differences in wear modes and mechanisms between different composite materials, allowing for a more comprehensive understanding of the tribological properties of these materials. This knowledge can then be used to guide future research and development efforts in the field of tribology, leading to improved materials with enhanced tribological properties.

The micrograph in Fig. 6 presents the worn counterface that experienced sliding against SGF/G/PTFE. Under dry sliding conditions, a lubricating tribofilm is formed when the matrix and reinforcement from the composites transfer to the counterface surface [32]. However, in this study with oil lubrication, material transfer was hindered on the wear track. It can be observed that the counterface’s roughness grooves contain only a small portion of wear particles. On the counterpart, a patch-like tribofilm has formed, appearing thin and non-uniform. The EDX data indicates that the main components trapped in the roughness grooves are Si (from the glass fibers), Fe (from the steel counterpart), F (from PTFE), and C and O (from the glass fibers and EP matrix). This suggests that the EP matrix and damaged glass fibers migrate to the counterface during the sliding process. Furthermore, a super-thin tribofilm is observed between the rough grooves, which exhibits a high Fe intensity originating from the ball. Based on this observation, it can be hypothesized that the patch-like tribofilm, which carries the majority of the load, not only prevents direct contact between the sliding surfaces but also facilitates the entry of lubricant into the spaces within these areas during sliding. Consequently, the lubricated system with composite materials performs well in tribological tests. By employing SEM/EDX, XPS, and FIB-TEM analyses, a detailed understanding of the microstructure and composition of the tribofilm formed on the counterface was obtained. This information contributes to elucidating the lubrication mechanism and tribological performance of the SGF/G/PTFE composite system.

Fig. 6
figure 6

SEM image (a) and EDX spectra b) of worn surfaces of the SGFG/PTFE composite. The scratching direction is noted by the arrow

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Comparing the results of this study with current literatures can provide valuable insights into the tribological behavior of SGF/G/PTFE composite materials. Previous studies have shown that the tribological behavior of composite materials is influenced by various factors such as the type of composite materials, sliding conditions, lubrication conditions, and surface roughness of sliding surfaces [33]. The results of this study suggest that under oil lubrication conditions, the tribofilm formed on the counterface plays a crucial role in reducing excessive wear of the SGF/G/PTFE composite materials. This is consistent with previous studies that have shown that tribofilms can effectively protect sliding surfaces from wear. However, it is important to note that different composite materials may have different tribofilm formation mechanisms and behaviors. Therefore, it is necessary to compare and contrast the results of this study with other relevant literature to gain a more comprehensive understanding of the tribofilm formation mechanisms and behaviors in SGF/G/PTFE composite materials. In addition, this study also contributes to a better understanding of the lubrication mechanism and tribological performance of SGF/G/PTFE composite materials. Previous studies have shown that different lubricants can have different tribological behaviors in composite materials. The results of this study suggest that under oil lubrication conditions, the tribofilm formed on the counterface plays a crucial role in reducing excessive wear of the SGF/G/PTFE composite materials. This is consistent with previous studies that have shown that tribofilms can effectively protect sliding surfaces from wear. However, it is important to note that different composite materials may have different tribofilm formation mechanisms and behaviors. Therefore, it is necessary to compare and contrast the results of this study with other relevant literature to gain a more comprehensive understanding of the lubrication mechanism and tribological performance of SGF/G/PTFE composite materials. This study provides valuable insights into potential applications of SGF/G/PTFE composite materials in tribological systems. The results suggest that these materials can effectively reduce wear and improve tribological performance under oil lubrication conditions. This information can be used to guide future research and development efforts in the field of tribology, leading to improved materials with enhanced tribological properties.

A TEM image depicting a cross-section of the tribofilm formed on the ball is shown in Fig. 7a. Prior to preparing the cross-section using a focused ion beam (FIB), a layer of Pt was deposited on the worn surface. The tribofilm appears structureless and has a thickness of approximately 50 nm. High-resolution TEM (HR-TEM) images of the tribofilm are presented in Fig. 7b and c. The HR-TEM analysis reveals the presence of hematite (Fe2O3) and graphite nanocrystals within the tribofilm, which are integrated into an amorphous matrix. The formation of Fe2O3 suggests that the friction process leads to oxidation of the steel counterface. However, the presence of graphite indicates that, despite the limited material transfer compared to dry sliding conditions, some graphite is still being transferred to the counterface. However, as mentioned earlier, the addition of solid lubricant has no significant effect on the friction and wear properties of the SGF-reinforced EP composite. The macroscopic lubricating ability of the tribofilm is not greatly influenced by the presence of graphite in the tribofilm. Nevertheless, the FIB-TEM results indicate that the development of the tribofilm on the surface is crucial to the overall tribological behavior of the EP composites.

Fig. 7
figure 7

Cross-section TEM images (a) and high-resolution TEM images (b, c) of tribofilm on steel ball sliding against SGF/G/PTFE composite

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Figure 8 presents XPS spectra of the worn surface of the SGF/G/PTFE/B4C composite. The C spectrum is adjusted to a binding energy of 283.4 eV. The spectra reveal the presence of C, Fe, O, B, F, Si, and other elements on the worn surface. The oxidized steel substrate is identified by the presence of iron oxide (710.9 eV and 724.0 eV peaks of Fe 2p) on the surface. The Si 2p spectrum with peaks at 101.1 eV and 102.2 eV corresponds to SiO2 and silicate, confirming the presence of these species [33]. The O 1s spectra also indicate the presence of iron oxide, SiO2, and silicate compounds on the worn surface. The F1s spectrum reveals the presence of PTFE on the counterface, as evidenced by the peak at 688.9 eV. The tribofilm formed on the worn surface contributes to improved wear resistance and friction reduction by preventing direct contact between the sliding surfaces. It is evident that the tribofilm contains wear debris from the composite and iron oxide particles, which are absorbed by the base oil or deposited on the worn surface.

Fig. 8
figure 8

XPS spectra of the worn surface of SGF/G/PTFE/B4C composite

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Based on the experimental findings, Fig. 9 presents a schematic diagram illustrating the tribological process of SGF/G/PTFE/B4C composites sliding with oil lubrication. In the SGF/G/PTFE/B4C composite material, G exhibits good lubricity and a low friction coefficient, which can reduce the friction of the composite material. Graphite also has good thermal conductivity, which promotes heat dissipation and reduces the temperature rise of the material during friction. The addition of graphite particles can enhance the strength and stiffness of the composite material, resulting in reduced surface wear during the friction process. On the other hand, B4C nanoparticles have high hardness and stiffness, contributing to increased compressive strength and stiffness of the composite material. They also improve the wear resistance, reducing wear and friction. Additionally, B4C nanoparticles possess high thermal stability, enhancing the high-temperature resistance of the composite material. In the SGF/G/PTFE/B4C composite material, PTFE serves as a solid lubricant, reducing the friction coefficient and providing solid lubrication effects. PTFE enhances the wear resistance and reduces friction, while also improving the high-temperature performance of the composite.

Fig. 9
figure 9

Schematic diagram of tribological process in SGF/G/PTFE/B4C composites

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Due to the pressure and abrasion caused by surface irregularities, the glass fibers (GFs) initially fracture into smaller fragments during ball-on-block sliding with oil lubrication. Iron oxide particles transfer from the SGF of the counterpart and wear debris from the composite, eventually being absorbed by the base oil or deposited on the worn surface. A tribofilm develops on the surfaces of both counterparts, consisting of wear debris composed of the composite and iron oxide particles. This tribofilm acts as a protective layer, isolating the direct contact between the sliding surfaces and filling wear troughs with compacted particles, thereby reducing severe wear of the GFs and protecting the counterparts from excessive wear.

Conclusions

The addition of SGF to oil lubricants significantly reduces COF and improves wear resistance of EP composites due to SGF’s high load-bearing capacity and wear resistance. Detailed analysis shows the tribofilm on worn composites surfaces comprises silica from glass fibers, transferred EP, and graphite, contributing to a nano-structured tribofilm in mixed and boundary lubrication. The nano-structured tribofilm plays a key role in reducing excessive wear by preventing direct sliding surface contact. It also deposits compacted wear debris in troughs, further enhancing wear resistance.