Synergistically assembled RGO/Si3N4 whiskers hybrid aerogels to endow epoxy composites with excellent thermal and tribological performance

Epoxy resin (EP) composites with satisfactory thermal and tribological performance are highly required for engineering moving components. However, the simple addition of fillers leaded to the serious filler agglomeration and limited promotion in tribological properties. In this work, we constructed a new kind of three-dimensional (3D) reduced graphene oxide (RGO)/Si3N4 hybrid aerogel for EP composites, which was prepared by a facile hydrothermal self-assembly method followed by freeze-drying technique. As a result, the dispersibility of Si3N4 whiskers was greatly improved through wrapping of polydopamine—polyethyleneimine copolymer (PDA—PEI) copolymer and physical spacing of 3D skeleton. Furthermore, benefiting from the synergistic effect of RGO and Si3N4@PDA—PEI in the thermal network, the thermal conductivity of RGO/Si3N4 hybrid aerogel (GSiA)—EP increased by 45.4% compared to that of the neat EP. In addition, the friction coefficient and wear rate of GSiA—EP decreased by 83.7% and 35.8%, respectively. This work is significant for opening a tribological performance enhancement strategy though constructing 3D hybrid architecture.


Introduction
Epoxy resin (EP) is widely employed as mechanical system in the aerospace field due to its low cost, light weight, and excellent processability [1][2][3]. Owing to the special environment of high temperature and high altitude, the EP urgently need to further satisfy the demand for excellent thermal properties, outstanding mechanical properties, and good tribological performance simultaneously [4][5][6]. Thus, numerous attentions have been focused on adding the high-performance micro/nanofillers into the EP matrix, such as carbon nanostructures, diamond-like carbon, and ceramic phases [7][8][9][10]. Among them, the carbon nanostructures, including zero-dimensional carbon nanoparticles (CNPs), one-dimensional carbon tubes (CNTs), and two-dimensional graphene oxide sheets (GO) have been evaluated as a good candidate because of its excellent thermal conductivity and outstanding lubricating properties [11][12][13][14]. However, due to the surface effect, the fillers agglomeration, resulting in uneven dispersion in polymer matrix, prevents these carbon nanostructures from reaching their full potential. In addition, these fillers are hard to interlaced together to construct continuous thermal networks into composites. As a result, they cannot dissipate the friction heat in time, leading to an extremely high instantaneous interface temperature and the decline of epoxy matrix [15][16][17][18].
Recently, graphene aerogels (GAs), having a three-dimensional (3D) honeycomb-like structure, can solve these adverse factors perfectly with interconnected porous architectures, low bulk density, and high porosity [19][20][21]. However, due to the weak connection and stacking between GO flakes, the concentration of GO dispersion had certain limitations during the fabrication process of GAs. Moreover, single GAs has the inevitable structural defects, leading to incomplete thermal conduction pathways and mechanical properties [23][24][25]. As a result, the researchers are committed to embedding different-dimensional reinforcing materials into the GAs to improve the mechanical stability and thermal properties of GAs as anchoring materials [26,27]. Apparently, taking 3D hybrid aerogel as the reinforcing skeleton of EP matrix, the thermal and mechanical properties as well as tribological properties of EP composites could be improved effectively.
The ceramic-phase materials, including SiC nanowires, and Si 3 N 4 ceramic whiskers, are often employed to improve the thermal and mechanical properties of polymer composites because of their high thermal properties, outstanding mechanical strength, and hardness [28][29][30][31][32]. Among them, Si 3 N 4 ceramic whiskers, with special aspect ratio between powder and fiber, attracted great attentions to enhance the thermal, mechanical properties of polymer composites. However, the dispersion of Si 3 N 4 whiskers is not fascinating due to the lack of active groups on the surface, resulting in the agglomeration and precipitation. Numerous studies are devoted to improving the dispersibility of Si 3 N 4 whiskers in composites via surface modification [28,29,33]. Recently, as a surface modifier without any host selectivity, the polydopamine-polyethyleneimine copolymer (PDA-PEI) has demonstrated its immense capability and potential for modifying the surface of Si 3 N 4 whiskers [34,35]. Besides, interspersing or embedding the modified Si 3 N 4 whiskers between the voids of GAs via hydrogen bonding, covalent grafting, and mechanical interlocking could further alleviate the agglomeration and precipitation [36,37]. In this way, it can be assumed that graphene sheets could act as physical spacers to separate the Si 3 N 4 whiskers. Meanwhile, the incorporating of one-dimensional Si 3 N 4 whiskers into GAs can strengthen the skeleton of GAs by serving as anchoring bridges and build continuous heat-dissipation highways into the 3D conduction network. Most importantly, as a reducing agent, the PDA-PEI coated onto the Si 3 N 4 whiskers surface can promote the fabrication of GAs during hydrothermal reduction to some extent. Thus, the 3D graphene aerogels and one-dimensional Si 3 N 4 whiskers may have a potential synergistic effect on the tribological properties of EP composites. Moreover, the above assumption has not been investigated yet, and it is necessary to attempt further in the present research.
Based on above motivation, in this work, for the first time the Si 3 N 4 whiskers were modified with the PDA-PEI copolymer, and a 3D porous graphene aerogel-supported Si 3 N 4 @PDA-PEI (reduced graphene oxide (RGO)/Si 3 N 4 hybrid aerogel) was prepared via a facile and scalable hydrogel route. Finally, the epoxy resin was poured into the hybrid aerogel by vacuum-assisted impregnation method to prepare the EP composites. The composition and structure of obtained RGO/Si 3 N 4 hybrid aerogel were thoroughly investigated via scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR), and X-ray photoelectron spectroscopy (XPS). The skeleton structure of RGO/Si 3 N 4 hybrid aerogel in epoxy resin were explored by optical microscope. The obtained RGO/Si 3 N 4 hybrid aerogel-EP composites presented excellent thermal, mechanical, and tribological properties. Moreover, the tribological behavior and wear mechanism of EP composites were further investigated in detail via analyzing the wear surface and transfer film. This work opens a new avenue to promote the tribological performance of polymer composites by constructing 3D hybrid aerogel.

Preparation of RGO/Si 3 N 4 hybrid aerogel and
GSiA-EP composites Figure 1(a) displayed the whole fabrication process of the GSiA-EP composites. Firstly, Si 3 N 4 whiskers, DA, and PEI were dispersed into 500 mL tris buffer solution by ultrasonicating for 8 h (all the sample concentrations were 2 mg/mL). The obtained suspension was centrifuged and the collected sediment was alternately washed by deionized water and ethanol for several times, and followed by drying in vacuum for 24 h to obtain the Si 3 N 4 @PDA-PEI whiskers. Afterward, a certain amount of Si 3 N 4 @PDA-PEI whiskers were dispersed into 25 mL GO dispersion (3mg/mL) by magnetic stirring for 12 h. And then 0.300 g LAA was dispersed into the above solution by re-sonicating for 1 h. Then, the prepared mixture was transferred into a sealed bottle and hydrothermally reduced for 2 h in an oven at 80 °C to obtain the RGO/Si 3 N 4 hybrid hydrogel. The self-assembled hybrid hydrogel was repeatedly washed by deionized water to remove the impurities. Subsequently, the hybrid hydrogel was first frozen with liquid nitrogen for 5 min, and followed by lyophilizing for 24 h in a freeze-drier to obtain the RGO/Si 3 N 4 hybrid aerogel (GSiA). The RGO/Si 3 N 4 hybrid aerogels with different mass ratio (the mass ratio of RGO flakes and Si 3 N 4 @PDA-PEI whiskers were designed as 2:1, 1:1, and 1:2) were coded as GSiA 1 , GSiA 2 , and GSiA 3 , respectively. In addition, the neat graphene aerogels (GA) without Si 3 N 4 @PDA-PEI whiskers were prepared by the same procedure via using different concentration of GO dispersion (1, 2, 3, and 4 mg/mL) and designated as GA 1 , GA 2 , GA 3 , and GA 4 , respectively.
Finally, the EP and C 6 H 18 N 4 (the mass ratio was 3:1) were magnetic stirred for 10 min. Sequentially, the GSiA was immersed into the above mixture under vacuum for 0.5 h. The products were left at room temperature for 4 h and kept for 6 h in an oven at 100 °C in order to prepare the RGO/Si 3 N 4 hybrid aerogel-EP composites (GSiA-EP) composites. The GSiA-EP composites with different GSiA (GSiA 1 , GSiA 2 , and GSiA 3 ) were designated as GSiA 1 -EP, GSiA 2 -EP, and GSiA 3 -EP, respectively. Meanwhile, the RGO aerogel-EP composites (GA-EP) were fabricated in the same way. The GA-EP composites with different GA (GA 2 , GA 3 , and GA 4 ) were designated as GA 2 -EP, GA 3 -EP, and GA 4 -EP, respectively. As a contrast, the GO/Si 3 N 4 -EP composites were prepared by simply mixing GO and Si 3 N 4 whiskers into the EP matrix.
The optical microscope (OM, OLYMPUS UC90) was taken to explore the distribution of RGO/Si 3 N 4 hybrid aerogel in the epoxy resin. The thermal conductivities of the composites were obtained via laser thermal conductivity apparatus (LFA-457) and PerkinElmer Pyris diamond thermal analyzer performed the thermal gravimetric analysis (TGA) under N 2 atmosphere. The thermal imaging camera (Fotric323pro) was conducted for monitoring the real-time temperature of the composites. The hardness experiments were measured by Vickers microhardness tester (HVT-1000A) and the hardness of all samples were performed at least five tests to calculate the average values. The friction and wear tests were performed by the ball-on-disk apparatus (MS-T3001, load 10 N, speed 200 r/min). The wear volume was investigated by 3D profilometer (MicroXAM-800, America) and the specific wear rate W S was calculated by the formula (1): where ∆V is the wear volume; F is the applied load; and L is the sliding distance. All key experiment results were measured three times and the average values were adopt to minimize the error.

Characterization of GSiA and GSiA-EP composites
The synthetic route of RGO/Si 3 N 4 hybrid aerogel via 2094 Friction 11(11): 2091-2106 (2023) | https://mc03.manuscriptcentral.com/friction combing the Si 3 N 4 whiskers and RGO sheets was exhibited in Fig. 1. According to the previous research work [34], owing to the hydrophilicity of oxygencontaining group, the helical carbon nanotubes could be attached to the GO nanosheet surface in the water solution. On this account, the Si 3 N 4 @PDA-PEI whiskers could be evenly parasitized in the GO sheets because of the reactive groups on Si 3 N 4 @PDA-PEI whiskers surface in aqueous dispersion. After the hydrothermal procedure, the RGO hydrogels with or without Si 3 N 4 @PDA-PEI whiskers kept a cylindrical shape and displayed a slight volume shrinkage except for the GO concentration of 1 mg/mL (Figs. S1 and S2 in the Electronic Supplementary Material (ESM)). Subsequently, the freeze-drying method was taken to obtain the RGO/Si 3 N 4 hybrid aerogels and then the RGO/Si 3 N 4 -EP composites were obtained by vacuumassisted impregnation. As showed in Fig. 1(b), the PDA-PEI copolymer were obtained by the selfpolymerization of DA and the copolymerization with PEI. Through π-π accumulation and hydrogen bonding, the Si 3 N 4 whiskers were coated by the PDA-PEI copolymer ( Fig. S3 in the ESM) and then the dispersion of Si 3 N 4 @PDA-PEI whiskers in deionized water was greatly improved (Fig. S4 in the ESM). In the process of hydrothermal reduction ( Fig. 1(c)), the -OH functional groups provided by PDA-PEI and LAA were changed into ketones. Meanwhile, the oxygen-containing functional groups on GO flakes (i.e., -OH and -COOH) were partly removed and the sp 2 lattice structure was restored to some extent. The light RGO/Si 3 N 4 hybrid aerogels with the weight of 0.021 g could be allowed to stand on a dandelion flower and support a load of 500 g without collapse ( Fig. 1(d)). Figure 1(e) displayed the possible binding force in RGO/Si 3 N 4 -EP composites during the curing reaction. In addition to the π-π accumulation and hydrogen bonding, the active -NH 2 , from current agent and PDA-PEI copolymer, undergone an epoxy ring-opening reaction with the epoxy group on the EP matrix and esterification reaction with the residual groups (-COOH) on RGO sheets surface to form a covalent bond. As a result, the Si 3 N 4 @PDA-PEI whiskers could be stably embedded into the 3D skeleton constructed by RGO sheets and the binding strength between EP matrix and RGO/Si 3 N 4 hybrid aerogel could be improved markedly.
The internal porous structures of RGO aerogel and RGO/Si 3 N 4 hybrid aerogel were comparatively investigated by the SEM and TEM images. Without www.Springer.com/journal/40544 | Friction Si 3 N 4 @PDA-PEI whiskers, the GA 3 displayed the traditional cellular graphene walls, which were thin and soft (Figs. 2(a)-2(c)). Meanwhile, it could be observed in Figs. 2(d) and 2(e) that Si 3 N 4 @PDA-PEI whiskers were uniformly parasitized in the cross-linked network, suggesting that the RGO sheets served as a good dispersant for Si 3 N 4 whiskers. However, the pore size of GSiA 1 was bigger than that of neat RGO aerogel and the cellular graphene walls turned more solid and thick (Figs. 2(d) and 2(e)), implying that the Si 3 N 4 @PDA-PEI whiskers could also protect RGO sheets from stacking to some extent. Furthermore, the high-magnification SEM (HM SEM) images of GSiA 1 showed that the Si 3 N 4 @PDA-PEI whiskers welded the holes (Fig. 2(f)), as well as was wrapped by RGO sheets entirely ( Fig. 2(g)) or embedded incompletely ( Fig. 2(h)) throughout the RGO cross-linked skeleton. From Fig. 2(i), both the continuous RGO wrinkles and PDA-PEI co-deposition coating could be observed in the TEM images, corresponding to the 3D hybrid structure of RGO/Si 3 N 4 . However, as the content of Si 3 N 4 @PDA-PEI whiskers increased, the more Si 3 N 4 @PDA-PEI whiskers in GSiA 2 were exposed outside the skeleton compared to the GSiA 1 (Figs. 2(j) and 2(k)). The aggregation of Si 3 N 4 @PDA-PEI whiskers got much worse and then damaged the structure uniformity due to the excessive contents (Figs. 2(m) and 2(m 1 )).
The chemical composition of RGO/Si 3 N 4 hybrid aerogel was displayed in Fig. 3. It can be seen that the XRD pattern of Si 3 N 4 whiskers have a good agreement with β-phase Si 3 N 4 [34]. When GO sheets were reduced to GA, the characteristic peak appearing at 2θ of 9.6 o corresponding to (002) disappeared and a new broad peak occurring at 2θ of 25 o corresponding to (100) could be observed, indicating the effective removal of oxygen-containing functional groups and poor ordering of RGO sheets [22,38]. Meanwhile, the XRD   Figures 3(c) and S5 in the ESM further presented the FTIR spectra of aerogel and Si 3 N 4 samples. The broad band between 3,200 and 3,600 cm -1 could be attributed to the -OH asymmetric stretching peak of GA. Two typical absorption peaks of Si 3 N 4 were clearly observed at 400-600 cm -1 and 800-1,100 cm -1 , corresponding to the bending and stretching vibration of Si-N bond, respectively. Meanwhile, compared to the unmodified Si 3 N 4 whiskers, the FTIR spectra of Si 3 N 4 @PDA-PEI whiskers exhibited the stronger -OH asymmetric stretching peak at 3,436 cm -1 , and the N-H absorption peak at 3,270 cm -1 . In addition, the new C=O, C-N, C-O/-NH 2 characteristic absorption peaks at 1,727, 1,388, and 1,274 cm -1 , respectively, were attributed to the PDA-PEI co-deposition coating [36]. In addition, both the Si 3 N 4 whiskers and GA characteristic absorption peaks could be well discerned in the spectrum of RGO/Si 3 N 4 hybrid aerogel, suggesting the successful assembly of RGO sheets and Si 3 N 4 @PDA-PEI whiskers.
The chemical bond type of RGO/Si 3 N 4 hybrid aerogel sample was further analyzed by XPS measurements. As could be seen from Fig. 3(c), the C 1s and O 1s signals presented in the spectrum indicated the Si 3 N 4 whiskers with weak oxidation. While there was a significant reduction of Si 2p and N 1s signals and enhancement of C/O ratio for Si 3 N 4 @PDA-PEI owing to the co-deposition of the PDA-PEI coating [36]. As for GO sheets, the C1s spectra was resolved into four component peaks C=C/C-C (284.3 eV), C-OH (285.2 eV), C-O-C (286.5 eV), and C=O (288.6 eV) (Fig. 3(d)). As a contrast, C-OH component in GA became weak, revealing the incomplete reduction of GO sheets in GA (Fig. 3(e)). Meanwhile, a new C-N (285.7 eV) bond peak appeared in GSiA accounting for the PDA-PEI co-deposition coating (Fig. 3(f)). According to the N 2p spectrum for GSiA, except for the N-Si (397.5 eV), -NH 2 (401.5 eV), -N-C- www.Springer.com/journal/40544 | Friction (395.7 eV), and -N-H (399.8 eV) peaks could also be found due to the PDA-PEI coating. Moreover, the high-resolution spectrum of Si 2p could be divided into three peaks Si-N (101.9 eV), Si-O-Si (102.3 eV), and Si-C (101.2 eV) (Fig. 3(h)). All these results indicated the effective coating of PDA-PEI for Si 3 N 4 whiskers and successful assembly of Si 3 N 4 and GO sheets in GSiA.
In order to evaluate the dispersion of RGO/Si 3 N 4 hybrid aerogel with 3D structure in the epoxy matrix, the optical microscopy images and SEM cross-sectional morphologies of the EP composites were exhibited in Fig. 4. As shown in Figs. 4(a)-4(d), it could be clearly seen that there existed a continuous and interconnected RGO/Si 3 N 4 hybrids with 3D structure in EP composites. By a contrast, the obvious agglomeration of GO sheets and Si 3 N 4 whiskers could be found in Figs. 4(e) and 4(f). Moreover, the uneven granular particulates, consisting of agglomerated GO sheets and Si 3 N 4 whiskers, were exposed to the cross-sectional surface of the GO/Si 3 N 4 -EP (Fig. 4(i)), suggesting that the GO sheets and Si 3 N 4 whiskers presented a poor dispersion in the epoxy resin by simple mechanical blending. In the meanwhile, compared to the pure epoxy resin with river-like patterns, the cross-sectional surface of GSiA-EP became visibly rough, which indicated the RGO/Si 3 N 4 hybrids constructed a 3D cross-linking network inside the resin and then limited the brittle fracture of epoxy resin effectively (Figs. 4(g) and 4(h)). Figure 5 exhibited the TGA curves and the thermal conductivity to investigate the thermal stability and heat conduction performance of EP composites. It could be found from Fig. 5(a) that the TGA curves between neat EP and EP composites had the similar change trend, suggesting the analogous thermal decomposition behaviors. While it could be found that the temperatures of GA 3 -EP and GSiA 1 -EP for a 5 wt% mass loss increased by 2.1 and 5.1 o C compared to that of neat EP, respectively. It was due to the lamellar barrier effect of RGO sheets and the 3D developed physical blocking structure of GSiA that the escape of the volatile pyrolysis products could be retarded [39]. Moreover, the Si 3 N 4 @PDA-PEI whiskers in GSiA could combine with RGO sheets to produce an even strong interfacial interaction with EP matrix and further limit the chain segment movement and removal of decomposition products [40]. However, the thermal stability of GO/Si 3 N 4 -EP  presented little effective enhancement compared to neat EP due to the serious agglomeration of GO sheets and Si 3 N 4 whiskers. Figure 5(c) depicted the thermal conductivity of EP, GA 3 -EP, GO/Si 3 N 4 -EP, and GSiA-EP composites. Thereinto, due to the formation of effective 3D thermal network constructed by RGO sheets, the thermal conductivity of GA 3 -EP was 0.282 W/mK, showing an increase by 45.4% compared to that of the neat EP. Furthermore, the thermal conductivity of GSiA 1 -EP and GSiA 2 -EP reached 0.31 and 0.351 W/mK, which presented a 60% and 81% increase compared to that of the neat EP, respectively. It could be described to that the Si 3 N 4 @PDA-PEI whiskers inside GSiA acted as the heat-conducting highways and bridges between the RGO sheets, which accelerated the heat diffusion and endowed the GSiA-EP composites with optimal thermal conductivity. But the excessive contents of Si 3 N 4 @PDA-PEI whiskers would result in poor thermal conduction performance due to the severe agglomeration and poor interfacial adhesion. In addition, compared with GSiA 1 -EP, the thermal conductivity of GO/Si 3 N 4 -EP was decreased by 22.6%, indicating that the simple blending was difficult to form an effective heat conduction network inside EP.

Thermal properties
Because it is generally believed that the large amount of fillers should be required to exceed the percolation threshold to form a thermal network by simple blending in the previous research work [41]. In order to observe the thermal conduction performance of different EP composites intuitively, all samples were placed onto a heating plate and the surface temperature was recorded by a thermal imaging camera. As displayed in Fig. 5(d), the surface temperature of GSiA 1 -EP dramatically increased and firstly reached 68 o C after 90 s, which was faster than that of GA 3 -EP and GO/Si 3 N 4 -EP. In contrast, the surface temperature of neat EP increased slowly to 41 o C at the same heating time due to its poor thermal conductivity. As a result, the GSiA 1 -EP had the optimal thermal stability and thermal conductivity performance.

Microhardness
The Vickers hardness of neat EP and EP composites were shown in Fig. 5(e). It could be seen that the hardness of GA 3 -EP, GSiA 1 -EP, GSiA 2 -EP, and GSiA 3 -EP was 151.0, 177.4, 165.7, and 156.0 MPa, respectively, which was 29.4%, 48%, 42%, and 35.3% higher than that of the pristine EP. It was contributed to that the load could be effectively transferred through the 3D RGO/Si 3 N 4 hybrid aerogel framework when subjected to the normal loading. In particular, www.Springer.com/journal/40544 | Friction the matrix-filler adhesion could improve the hardness of GSiA 1 -EP due to the great surface area and tremendous contact points caused by roughening the contact surface between the hybrid aerogel and polymer matrix. Besides, the reactive functional groups on the RGO sheets and Si 3 N 4 @PDA-PEI whiskers could produce the chemical reactions and promote the strong chemical bonding with the epoxy resin chains, which could make the chains of epoxy resin have a better resistance to external force [42]. By a contrast, the hardness of GO/Si 3 N 4 -EP was even lower than that of neat EP. It could be ascribed to that the RGO sheets and Si 3 N 4 whiskers had an aggregation effect and then weaken the interface bonding between the fillers and epoxy resin, which could act as many mechanical weakness during the elastic deformation [43]. It could be found that the average friction coefficient of GA 2 -EP, GA 3 -EP, and GA 4 -EP was 0.33, 0.2934, and 0.3201, which was decreased by 5.8%, 16.2%, and 8.6% compared to that of the neat EP, respectively. In particular, the GA 3 -EP had the lowest average friction coefficient, which could be ascribed to the continuous lubrication supply by 3D RGO sheets. The wear rate and wear width of GA-EP and neat EP were characterized in Fig. 6(b). Compared with the neat EP, the wear rate of GA 2 -EP, GA 3 -EP, and GA 4 -EP was 7.083 × 10 −5 , 4.67 × 10 −5 , and 7.5 × 10 −5 mm 3 /(N·m), which was reduced by 49%, 66.4%, and 46%, respectively, which indicated that GA 3 -EP had the optimal tribological properties. As for GSiA-EP composites, owing to the superior self-lubricating properties of RGO sheets and the outstanding load-bearing capacity of Si 3 N 4 @PDA-PEI whiskers, it could be seen that the friction coefficient of GSiA 1 -EP was as low as 0.2249, presenting a decrease by 35.8% as compared to neat EP. In contrast, the friction coefficient of GO/Si 3 N 4 -EP was higher than that of GSiA 1 -EP, indicating that 3D cross-linked structure of GSiA 1 played an important role in the synergistic lubricating effect between RGO sheets and Si 3 N 4 @PDA-PEI whiskers. Perceptibly, compared to the simple blending of GO/Si 3 N 4 fillers with EP, the 3D structure of GSiA 1 could effectively avoid the agglomeration of GO sheets and Si 3 N 4 whiskers inside EP and promote the interfacial bonding between fillers and matrix. As a result, the | https://mc03.manuscriptcentral.com/friction lubricating performance of GO sheets and carrying capacity of Si 3 N 4 whiskers could be fully released to construct a high-quality transfer film. Furthermore, the excellent thermal properties of GSiA 1 -EP could relieve the unexpected local softening of epoxy matrix and serious abrasion due to the friction heat. In the meanwhile, the wear rate of GSiA 1 -EP reached the minimum value of 2.27 × 10 −5 mm 3 /(N·m), which was reduced by 83.7% compared to the pristine EP. It was worth noting that the wear rate of GSiA 2 -EP and GSiA 3 -EP tended to increase gradually with the further increase in the content of Si 3 N 4 @PDA-PEI whiskers. Even more remarkably, the wear rate of GO/Si 3 N 4 -EP was raised to 10.8 × 10 −5 mm 3 /(N·m), which was ~4.8 times higher than that of GSiA 1 -EP. The above results implied that the GSiA 3 -EP presented the optimal lubricating and abrasion resistance properties.

Tribological properties
The coefficient of friction curves versus sliding time in the sliding process were exhibited in Fig. 6(e). The friction process could be categorized into two stages: running-in stage and steady state. The friction coefficient of all EP samples tended to increase sharply in the running-in stage. During steady state, the friction coefficient of neat EP was continuous to rise steadily. While the friction coefficient of GA-EP kept steady and was lower than the neat EP. Furthermore, the GSiA 1 -EP had the most stable and the lowest friction coefficient. In contrast, with the increase of Si 3 N 4 @PDA-PEI whiskers content, the friction coefficient curve of GSiA 2 -EP became unstable and the sever fluctuation was more obvious in GSiA 3 -EP caused by the agglomeration of Si 3 N 4 @PDA-PEI whiskers.

Anti-wear mechanism
The morphologies of worn surfaces were used to investigate the anti-wear mechanism of various EP composites. Perceptibly, the wear area of pure EP was the highest among all EP composites ( Fig. 7(a 1 )). In particular, the worn surface was very uneven and rough ( Fig. 7(a)), and a mass of delamination, cracks, and adhesive debris could be found on the worn surface ( Fig. 8(a)), confirming the serious plowing damage and plastic deformation occurring in the sliding process. This phenomenon corresponded to the typical adhesive and fatigue wear, which could be attributed to the high brittleness and poor thermal properties of the pure epoxy resin [44]. As for GA-EP, it could be found that the wear area of GA 3 -EP was narrower and the worn surface became relatively smooth (Figs. 7(b) and 7(b 1 )). But some spalling pits, tiny wrinkles, and adhesive pits still appeared on GA 3 -EP worn surface, corresponding to the slight adhesive wear and fatigue wear ( Fig. 8(b)). As a contrast, the worn surfaces of GA 2 -EP and GA 4 -EP in Specially, the GSiA 1 -EP presented the lowest wear depth and the narrowest wear width (Figs. 7(d) and 7(d 1 )), and no obvious adhesive trace except for some slight furrows and tiny holes could be found on its worn surface (Fig. 8(d)), corresponding to slight wear. By contrast, the wear area of GO/Si 3 N 4 -EP was obviously larger than that of GSiA 1 -EP, the scratch surface was filled with serious holes and cracks corresponding to the serious adhesive wear and peeling wear (Fig. 8(c)). Compared to GO/Si 3 N 4 -EP, the RGO/Si 3 N 4 hybrid aerogel with outstanding porous structure could elevate the dispersion of RGO sheets and Si 3 N 4 @PDA-PEI whiskers inside EP effectively, which could dissipate the friction heat timely through 3D heat conduction network. Moreover, the optimized interfacial interaction between hybrid aerogel and resin matrix could further improved the hardness of EP composites. Meanwhile, the improved dispersion and interfacial interaction contributed to a high-quality tribofilm by the continuous and steady replenishment of RGO sheets and Si 3 N 4 @PDA-PEI whiskers. However, the serious delamination and furrows could also be found on the GSiA 2 -EP and GSiA 3 -EP scratch surface (Figs. 8(e) and 8(f)), implying that the further increase in Si 3 N 4 @PDA-PEI whiskers content had a negative effect on anti-wear performance of EP matrix due to the unexpected agglomeration.
The wear morphologies and chemical composition of ball counterface after the friction process were characterized by SEM, Raman, and XPS analysis. As shown in Fig. 9(a), there was a thick and uneven transfer film formed on counterface corresponding to pure EP, indicating the rapid transfer and shedding of EP, which was consistent with severe abrasion of pure EP. As for GA-EP, the thin and flattened transfer film except for some cracks was found on counterpart surface, suggesting that the RGO sheets could participate in the friction transfer of EP and then  promote the bonding performance of transfer film ( Fig. 9(b)). It was noteworthy that the counterpart surface was covered by a thin, uniform, and dense transfer film by GSiA 1 -EP ( Fig. 9(d)), which still remained flat and sturdy under the alternating stress. And the EDS results showed that the N and Si elements corresponding to Si 3 N 4 whiskers were dispersed in the transfer film uniformly (Figs. 9(e) and 9(f)). However, the transfer film, uneven and loose, was formed by GO/Si 3 N 4 -EP, suggesting that the exposed Si 3 N 4 whiskers were not easily attached to the steel ball surface (Fig. 9(c)), while the RGO sheets could contribute to the friction transfer of Si 3 N 4 @PDA-PEI whiskers through RGO/Si 3 N 4 wrapping structure (Figs. 2(f)-2(h)). In summary, the combination of RGO sheets and Si 3 N 4 @PDA-PEI whiskers could improve the carrying capacity and bonding strength of transfer film effectively. The Raman spectroscopy ( Fig. 9(e)) demonstrated that the I D /I G ratio of RGO in GSiA 1 -EP was 1.14, while the transferred RGO sheets on countersurface was 0.84. It implied that the defect of RGO sheets could be slightly reduced and the structure of RGO sheets became more ordered due to the friction heat, which could display a significant tribological performance. That obtained results were consistent with the previous study [45]. Moreover, the XPS spectra were detected to further investigate the chemical composition of transfer film for GSiA 1 -EP. High-resolution C 1s spectrum ( Fig. 9(f)) indicated the presence of C=C/C-C (284.8 eV), C-OH (285.1 eV), C-O-C (286.5 eV) and C=O (288.4 eV), which were attributed to the epoxy resin, PDA-PEI copolymer, and the residual groups on RGO sheets. Furthermore, the N 1s could be divided into four peaks appearing at 395.7 eV (-N-C-), 397.5 eV (N-Si), 399.8 eV (-N-H), and -NH 2 (401.5 eV), respectively. Meanwhile, the Si 2p spectrum showed two peaks centered at 101.8 eV (Si-N) and 102.4 eV (Si-O-Si), which were attributed to the Si 3 N4@PDA-PEI whiskers (Figs. 9(g) and 9(h)). Moreover, compared to the initial Si 3 N 4 @PDA-PEI whiskers ( Fig. 3(h)), the peak intensities of Si-O-Si in Si 2p spectra of transfer film improved significantly, suggesting that some exposed Si 3 N 4 @PDA-PEI whiskers were oxidized to SiO 2 during the process of friction in air ambient. Different from the previous research [46], the oxidation rate of Si 3 N 4 @PDA-PEI www.Springer.com/journal/40544 | Friction whiskers was faster, suggesting that the friction heat could offer energy for that chemical process to complete faster. This process could be presented in the reaction (2): Hence, it implied that both of the releasing of RGO/Si 3 N 4 wrapping hybrids and the production of SiO 2 could form the high-quality transfer film, greatly elevating the tribological performance of GSiA 1 -EP.
Overall, the primary friction-reducing and anti-wear model of GSiA 1 -EP could be seen in Fig. 10. As shown in Fig. 10(b), the 3D RGO/Si 3 N 4 thermal network could dissipate timely the friction heat generated in the procedure of friction, which effectively avoided the large plastic deformation and local softening of epoxy matrix. Moreover, the RGO/Si 3 N 4 wrapping hybrid could be transferred to the counterpart surface continuously and formed the high-quality transfer film, which avoided the direct contact between the hard steel ball and GSiA 1 -EP composites (Fig. 10(c)). And the RGO sheets could further reduce the friction coefficient of GSiA 1 -EP composites by interlayer gliding (Fig. 10(d)). As a result, the RGO/Si 3 N 4 hybrid aerogel with 3D structure could endow the EP composites with optimal tribological properties.

Conclusions
In this work, we fabricated the reduced graphene oxide (RGO)/Si 3 N 4 hybrid aerogel with fascinating three-dimensional (3D) porous structure via a facial hydrothermal and freeze-drying strategy. And the dispersibility of Si 3 N 4 @polydopamine-polyethyleneimine copolymer (PDA-PEI) whiskers was dramatically optimized through combination of wrapping of PDA-PEI and physical spacing of 3D skeleton. The 3D continuous conductive frameworks of RGO/Si 3 N 4 hybrid aerogel endowed the epoxy resin (EP) composites with the better thermal conductivity. Furthermore, the thermal conductivity of GSiA 3 -EP composites (the mass ratio of RGO flakes and Si 3 N 4 @PDA-PEI whiskers was 1:2) increased about 60% in comparison to that of the pure EP material. The specific wear rate and friction coefficient of GSiA 1 -EP composites reached 2.27 × 10 −5 (mm 3 /(Nm)) and 0.2249, about 83.7% and 35.8% lower than that of pure EP material, respectively. Meanwhile, the GSiA-EP composites also possessed the excellent thermal stability and microhardness due to the physical blocking and interfacial interaction between RGO/Si 3 N 4 hybrid aerogel and epoxy matrix. The results indicated that the prominent thermal properties, enhanced microhardness, and high-quality transfer films were responsible for the remarkable tribological performance of EP composites. Moreover, the successful obtained RGO/Si 3 N 4 hybrid aerogel-EP composites would further widen the applications of epoxy resin composites in friction field.