Enhanced tribological properties in core–shell structured SiO₂@GO hybrid fillers for epoxy nanocomposites

Abstract

SiO₂ coated with graphene oxide (GO) hybrids (SiO₂@GO) were fabricated by electrostatic self-assembly and introduced into an epoxy polymer (EP) matrix to prepare epoxy composites by a solvent-free curing process. The thermal stability, mechanical properties and tribological behavior of SiO₂@GO/EP composites were investigated and compared. It was found that the epoxy composites with the aligned core–shell structure of SiO₂@GO improved the thermal stability, as well as excellent mechanical and tribological properties. The low content 0.5 wt% SiO₂@GO/EP nanocomposites exhibited nearly 88.4% reduction in the friction coefficient and several orders of magnitude reduction in wear rate, constituting a potential breakthrough for future tribological applications. Compared with unfilled epoxy polymers or epoxy polymers with unmodified fillers, the major increase in the tribological properties of the SiO₂@GO/EP composites shows the synergy effect between nano-SiO₂ and GO during the wear process. The behavior of nano-SiO₂ and GO were probably mostly due to ball-bearing and an ultrathin lamellar effect, respectively. The obtained results indicate that the core–shell structure of SiO₂@GO as a more effective synergistic system is a valid method for greatly improving the friction and wear properties and mechanical properties of resin-based composites.

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1. Introduction

Epoxy resin as a highly cross-linked thermosetting plastic is widely used in coating, adhesive materials, supporting structural materials and so on. However, the three-dimensional network structure also makes epoxies intrinsically brittle and vulnerable to cracks, which limits their use in sliding wear applications.¹ Consequently, it is of great interest to improve the tribological performance of epoxies. There is typically achieved by modified EP composites with reinforcing nanofillers. Various nanoparticles has been employed to obtain high-performance EP nanocomposites, such as multi-walled carbon nanotubes (MWCNTs),² BN³ nanoparticles, which apparently enhanced wear resistance by enhanced thermal conductivity to reduce thermal softening. Likewise, nano-Al₂O₃ and conventional CaSiO₃ had synergistic effects on increasing mechanical properties and wear resistance of epoxy composites.⁴ Moreover, the spherical silica particles could improve the wear resistance of epoxy matrix even through the content of the fillers was at a relatively low level.⁵ In particular, nano-SiO₂ nanoparticles are introduced to epoxy resin, which can effectively improve the toughness and strength, enhance the glass transition temperature (T_g), reduce the coefficient of thermal expansion (CTE), lower shrinkage on curing, thereby result in tribological and mechanical reinforcement.⁶

For the preparation of high performance nanocomposite, the most property depends on fillers nature, fillers dispersed evenly, the spacing between filler particles and the interfacial interaction strength.⁷ The fillers/matrix interfacial cohesion directly influence the interfacial stress transfer in the composites structure, thereby significantly affects the integrated tribological and mechanical properties.⁸ In recent years, there are many methods have been applied to improve the interfacial interaction between fillers and polymer matrix, especially chemical modification on the surface of nanoparticles by great attention.

The lamellar materials such as graphite and metal dichalcogenides (MoS₂, WS₂) are widely used as solid lubricants or as additives to liquid lubricants.⁹ However, the lubricating property of graphite is extrinsic and in order to obtain low friction and wear, numerous amount of graphite is needed, which lowers the mechanical properties (i.e., hardness and fracture strength) of the composites.^10,11 Graphene has large surface area, superior tensile strength, high flexibility and good thermal conductivity, these advantages are beneficial for increase in tribological properties.¹² Moreover, considering the compatibility between graphene and polymer matrix, graphene oxide (GO) with a great deal of epoxy, hydroxyl, ketone groups and carboxyl functional groups, which is more compatible with organic polymers instead of pristine graphene.^8,13 With layer structure and some similar properties to graphite and graphene, graphene oxide shows an obvious superiority in enhancing the wear resistance of epoxy resins at very low contents.¹² In recent decades, it was reported that the GO is remarkably able to improve the mechanical and tribological properties of polymer composites, such as applied in epoxy resin,¹⁴ polyimide (PI),¹⁵ polyurethane,¹⁶ and polylactic acid.¹⁷ Until recently, take advantage of GO negatively charged surface, the GO encapsulated some positively charged organic/inorganic objects to form core–shell hybrids by electrostatic self-assembly strategy, which give them special properties and applications.⁸ The tribological properties of silica nanoparticles can be improved by functionalizing the layered materials with weak shearing inter-layer (typical van der-Waals) bonds.³⁶ Therefore a more effective synergistic system that a core–shell structure silica/graphene oxide has great potential in the tribology field. There are few researches investigate about core–shell structure of filler on the tribological properties of polymer composites.

In this work, we took advantage of the intrinsic attributes of nano-SiO₂ and GO, fabricate SiO₂@GO hybrids by electrostatic self-assembled, and introduced into epoxy resin by a solvent-free curing method to reinforce polymer composites. The SiO₂@GO/epoxy exhibited that the core–shell SiO₂@GO hybrids could improve the thermal stability, mechanical properties, and tribological properties, particularly a great increase in wear resistance at low level. Simultaneously, we compared the mechanical properties and tribological properties of polymer composites with addition appropriate content of SiO₂@GO hybrids, nano-SiO₂ particles and SiO₂ particles modified with APTES coupling agent (SiO₂–NH₂), found that reinforcement efficacy of SiO₂@GO is better than of other fillers. It can be attributed that external soft GO as a novel fillers surface modifier wrapped inner rigid SiO₂ nanoparticles, which play a good synergistic effect, can great reduce abrasive wear produced by rigid filler on the grinding surface and promote the transfer film production, thus improve the tribological properties of the polymer matrix, in addition better uniform dispersion and interfacial adhesion to improve the mechanical properties of composites.

2. Experimental section

2.1. Materials

Graphite (Gr) powders were purchased from Shanghai Huayi Group Huayuan chemical industry Co., Ltd. Nanoscale sized silica (average diameter: 30 nm) and 3-aminopropyltriethoxysilane (APTES) were purchased from Aladdin Industrial Co., Ltd. The epoxy resin, diglycidylether of bisphenol A (DGEBA, E-51, epoxy value = 0.51 and viscosity value = 12 Pa s at 25 °C) used in this study, was supplied Danbao Co., Ltd. (Zhengjiang, Jiangsu). This resin was used in combination with the commercial 4,4′-diaminodiphenylsulphone (DDS, Aladdin Chemistry Co., Ltd.) as a curing agent. Hydrogen peroxide 30% (H₂O₂ 30%) was obtained from Chengdu Kelong Chemical Reagent Company (Chengdu, China). Hydrochloric acid, ethanol and sulfuric acid (H₂SO₄, 98% v/v) were acquired from Chongqing Chuandong Chemical Reagent Factory (Chongqing, China). Others were bought from Taixing Chemical Company (Chongqing, China). Deionized water was used in this study, and all the analytical grade reagents were used without further purification.

2.2. Preparation of core–shell structured SiO₂@GO hybrid particles

Preparation of the core–shell structured SiO₂@GO hybrid particles could be simply divided into three steps:¹⁸ (1) preparation GO by the Hummers method; (2) the modification of silica particles with APTES coupling agent to obtain the positively charged SiO₂–NH₂ particles; (3) electrostatic assembly of negative exfoliated GO sheets with the SiO₂–NH₂ particles.

GO was prepared from natural graphite by the Hummers method as reported elsewhere.¹⁹ In short, 3 g graphite powder, 2.19 g (NH₄)₂S₂O₈, 2.5 g P₂O₅ and 12 mL H₂SO₄ were mixed in the beaker. The mixture was stirred in an oil bath at 80 °C for 4.5 h to complete the preliminary oxidation. Add 500 mL deionized water when the mixture cooled to the room temperature, place it overnight. In order to remove the residual acid, the slurry was filtered and dried at room temperature all night. Next, put the graphite powder mixed with 120 mL H₂SO₄ in a breaker at 0 °C. Then, add 15 g KMnO₄ slowly into the breaker under stirring and the temperature was controlled below 20 °C. The reaction system was kept under stirring at 35 °C for 2 h. Afterwards, 950 mL H₂O and 20 mL 30% H₂O₂ were added slowly, respectively. The mixture was yellow-brown after stirred for another 2 h. The product was collected by washing and centrifugation with 2 M HCl and water until the pH value neared 7. GO powder was obtained by freeze-drying and further dry in a vacuum oven at 60 °C overnight.

The surface modification of SiO₂ with APTES coupling agent was carried out in liquid solution. In a typical process, SiO₂ powder (10 g) was first dispersed well in a mixed solution of ethanol (190 mL) and H₂O (10 mL) by ultrasonication for 1 h. In order to promote the hydrolysis of APTES, the pH of suspension solution was adjusted to a range of 4–5 with formic acid before added 1 g APTES coupling agent. The mixture was stirred and refluxed at 95 °C for 24 h to complete the grafting reaction. The obtain SiO₂–NH₂ particles were centrifuged, washed with ethanol and deionized water three times, and dried under vacuum. The modification process is illustrated in Scheme 1.

Scheme 1 Schematic illustration of APTES modification process on the SiO₂ nanoparticle surface.

The SiO₂@GO hybrid particles were fabricated by simply mixing the SiO₂–NH₂ aqueous suspension and the GO solution. First, exfoliating GO in water with ultrasonic to obtain the well-dispersed GO aqueous solution. Then, 1000 mL SiO₂–NH₂ aqueous suspension (10 mg mL⁻¹) was added into the well-dispersed 1000 mL GO solution (0.2 mg mL⁻¹) under mild magnetic stirring for 1 h. When stirring was stopped, SiO₂@GO hybrid particles were precipitated at the bottom of the beaker, leaving the upper transparent aqueous solution. The sediment solid (SiO₂@GO) was collected and washed with deionized water several times to remove the unbound GO, and then freeze-dried under vacuum.

2.3 Preparation of epoxy-based composites

In order to avoid the solvent residual, the epoxy-based composites were fabricated by a solvent-free method. In a typical operation, DGEB-A monomer was heated to 60 °C in a drying oven to reduce its viscosity. A stoichiometric amount of SiO₂@GO nanofillers were added to the epoxy at 80 °C under vigorous stirring for 2 h. Then, the mixture was ultrasound for 1 h at 60 °C to remove the bubbles and make the nanofillers well dispersed. After cooling to room temperature, the DDS (32 wt% of epoxy) curing agent was added to the mixture and mixed thoroughly by stirring. Next, the mixture was degassed in a vacuum oven for 2 h at 80 °C to remove bubbles before they were cast into a mold. The samples were cured for 2 h at 100 °C, 2 h at 150 °C, 2 h at 180 °C and 2 h at 200 °C. All epoxy composites were prepared by the same procedure.

2.4 Friction and wear tests

The tribological tests were conducted using a reciprocating friction and wear testing machine (LSR-2M, Lanzhou Zhongke Kaihua Technology Development Co., Ltd) under dry conditions. The dual samples bearing stainless ball diameter is 4 mm, as provided by the supplier. Before each test, the counterpart steel ball and the materials were sanded using no. 1200 water-abrasive paper. The test was performed at room temperature, reciprocating frequency of 100 times min⁻¹, applied a load of 300 g with a test duration of 60 min. The friction coefficient was obtained by the computer automatically. At the end of each test, the depth of the wear scar was measured using a stylus surface profiler (Dektak 150, Veeco instruments Inc., USA), and the wear volume (ΔV, mm³) of the specimen was calculated according to the following equation:^15,20where ΔV is the wear volume (mm³), d the depth of the specimen (mm), L the length of stroke in one cycle (mm), and r the radius of the counterpart stainless ball (mm). The wear rate (K, mm³ N⁻¹ m⁻¹) of the specimen was calculated from the following equation:
where F is the applied load (N), t is the experimental duration (s), v the reciprocating frequency (Hz), and L the stroke length in one cycle (m). In this work, three readings of the friction and wear tests of steady-state sliding under dry conditions were taken, and the average values were adopted in our results.

2.5 Characterization

Fourier-transform infrared spectra (FT-IR) were recorded on a Nicolet 170SX Fourier transform infrared spectrometer (Madison, WI, USA) at room temperature. The wavelength range of FT-IR was 4000–500 cm⁻¹, using KBr pellets. Raman characterization was conducted on a Raman spectroscopy (Renishaw invia). For transmission electron microscopy (TEM) experiments, the suspension of SiO₂@GO was dropped onto a copper grid and dried in a vacuum drying oven for 2 h, then the morphology of the samples were examined using a JEM-1200EX TEM (JEOL, Tokyo, Japan) at an accelerating voltage of 120 kV. X-ray diffraction was performed on a XRD-3D, Puxi, (Beijing, China) X-ray diffractometer under the following conditions: Nickel filtered Cu Kα radiation (λ = 0.15406 nm) at a current of 20 mA and a voltage of 36 kV. The scanning rate was 4° min⁻¹ in the angular range of 3–80° (2θ). To study the thermal stability of the epoxy nanocomposites, thermo-gravimetric analysis (TG) was carried out at a heating rate of 10 °C min⁻¹ from room temperature to 800 °C under nitrogen flow (20 mL min⁻¹) using TA-STDQ600 (New Castel, USA). Differential scanning calorimetry (DSC 200 F3, Netzsch, Selb/Bavaria, Germany) was measured at temperature from 30 to 250 °C at heating rate of 10 °C min⁻¹ under a nitrogen atmosphere. Nitrogen at a rate of 20 mL min⁻¹ was used as the purge gas. An empty pan was used as a reference. The mechanical properties were measured using a Microelectronics Universal Testing Instrument Model Sans 6500 (Shenzhen, China) with cross-head speed of 2 mm min⁻¹ at room temperature. The data reported here represented the average of five tests. The tensile fractured surfaces of the samples were observed by scanning electron microscopy (SEM) using a Quanta FEG450 SEM instrument (FEI, USA) at an accelerating voltage of 20 kV. For the sake of investigating related tribological mechanisms, the morphologies of worn surfaces were also observed by using SEM. Before observation, the surfaces of the samples were coated with a conductive layer of gold.

3. Results and discussion

3.1 Structural and morphological characterization of SiO₂@GO hybrids

GO nanosheets with carboxylic acid groups on their surface are negatively charged in aqueous solution. The SiO₂ was modified by APTES, which could be ionization of amino groups to switch their surface charge state from negative to positive. When positively charged particles mixed with GO, the electrostatic force led to the mutual assembly.^18,21,22 Take the oppositely charged GO and the positive charged SiO₂–NH₂ particles into the water, the electrostatic assembly could be triggered. It was observed that the brown assembled core–shell SiO₂@GO hybrid particles quickly settle down to the bottom, the upper solution becomes transparent (Scheme 2).

Scheme 2 The process of electrostatic self-assembly of SiO₂@GO hybrid particles.

FTIR, Raman spectra and morphological analysis. Fig. 1A displays the FTIR of SiO₂-GO and SiO₂–NH_2. The FTIR spectra were used to confirm the covalent attachment of the APTES coupling agent and the GO wrapping on the silica surface. The most significant bands of SiO₂ were 3458, 1647, 1104, 799 and 472 cm⁻¹, which were due to the –OH stretching and bending vibration, Si–O–Si stretching vibrations, Si–OH bending vibration, Si–O–H bending vibration, respectively. After the amination reaction, several minor bands of SiO₂–NH₂ at around 2975 cm⁻¹, attributing to the C–H stretching vibration of the hydrocarbon chains of the grafting APTES.⁸ For the GO, characteristic peaks appeared at 3439, 1714, 1637, 1170 and 1036 cm⁻¹, which were assigned to the –OH stretching vibrations, C=O stretching, H–O–H bending, =C–H vibration and C–O stretching, respectively.^23–25 On account of the low content of GO in the core–shell particles, the peaks of GO are a little weak in the SiO₂@GO spectra curve.

Fig. 1 (A) FTIR spectra of SiO₂, SiO₂–NH₂, SiO₂@GO and GO, (B) Raman spectra of GO and SiO₂@GO hybrid, and (C) TEM images of a core–shell structured SiO₂@GO hybrids.

In order to further confirm the SiO₂ surface coated with GO, Raman spectroscopy was used to characterize the GO and SiO₂@GO. For carbon products, which conjugated and double carbon–carbon bonds lead to high Raman intensities. It is well known, the Raman spectrum of highly ordered graphite has a couple of bands, the in-phase vibration of the graphite lattice (G band) usually observed at ∼1575 cm⁻¹ as well as the disorder band caused by the graphite edges (D band) at approximately 1355 cm⁻¹.²⁶ Fig. 1B shows Raman spectra of GO and SiO₂@GO. The spectra of GO shown typical D and G bands at 1358 cm⁻¹ and 1602 cm⁻¹. The Raman spectrum of SiO₂@GO hybrids exhibits two main peaks at 1356 cm⁻¹ and 1608 cm⁻¹, which are attributed to the D and G bands of GO. This result indicates that the presence of GO sheets in the hybrids. Fig. 1C presents the TEM images of SiO₂@GO fillers dispersed in water. The TEM image of SiO₂@GO shows the wrinkles of GO sheets on the SiO₂–NH₂ surface are further confirmed that the ultrathin and flexible GO sheets have indeed successfully coated the modified SiO₂ particles.

XRD analysis. Fig. 2 shows the XRD patterns of Gr, GO, SiO₂, SiO₂@GO, neat EP and SiO₂@GO/EP specimens, to confirm the near-absence of a stacking order for SiO₂@GO and SiO₂@GO aggregation in the polymer matrix. The GO exhibits a relatively weak diffraction peak at 13.6° by contrast the strong diffraction peak of Gr located at 26.65°, which demonstrates the interlayer spacing of GO was 0.65 nm. Compared with the diffraction peak of SiO₂, that of SiO₂@GO displayed a relatively stronger broad band centered at 2θ = 22.2° suggesting the formation of a few layers of silanized silica functionalized GO sheets.²⁷ Moreover, the XRD patterns of neat EP and SiO₂@GO/EP specimens exhibit the same weak and broad diffraction peak, and no visible peak, assigned to SiO₂@GO, is observed.

Fig. 2 XRD plots of Gr, GO, SiO₂, SiO₂@GO, neat EP, and 0.5 wt% SiO₂@GO/EP specimens.

3.2 Thermal analysis

DSC. Fig. 3 shows the change in glass transition temperature (T_g) for the neat EP and SiO₂@GO/EP composites. It is well known that the T_g of epoxy usually related to the cross-linking density of the resin matrix.²⁸ Due to the degree of cross-linking in the epoxy matrix is not uniform, makes the glass transition temperature occurred in a wide range. There are three characteristic temperatures in the DSC curves, the onset temperature, the intermediate temperature and the end temperature.²⁹ And here the glass transition temperature was attributed to intermediate temperature. As shown in Fig. 5, the T_g of neat epoxy is 132 °C. The T_gs of the SiO₂@GO/EP composites with 0.05, 0.1, 0.5, 0.8, 1 and 3 wt% hybrid fillers are 133.5, 136.1, 152.8, 143.2, 143 and 142.5 °C, respectively. Meanwhile, the composites with 0.5 wt% SiO₂ and 0.5 wt% SiO₂–NH₂ nanofillers, the T_g are 143.5 and 145.7 °C. As we can see, the composite with same content 0.5 wt% SiO₂@GO, 0.5 wt% SiO₂–NH₂ and 0.5 wt% SiO₂ nanofillers, the T_g of the 0.5 wt% SiO₂@GO/EP composite is higher than two others. It can be attributed to SiO₂@GO nanofillers act as physical interlock points in the cured organic matrix, which provided a sterically hindered surroundings for curing reactions of composites and restricted the mobility of chains. Meanwhile, a mass of amine, epoxy and carboxyl functional groups on SiO₂–NH₂ and GO basal planes, respectively, which can participate in the curing reaction to form a strong interface with matrix and lead to a higher cross-linking density.

Fig. 3 DSC curves of the pure epoxy and epoxy composites.

TG. On account of its applications in many strict fields, thermal stability is another key parameter for EP nanocomposites. Fig. 4A and B shows the TG and DTG curves of neat EP and SiO₂@GO/EP nanocomposites. As shown in Fig. 4, the TG (Fig. 4A) curves show one thermal decomposition platform, indicating a one step process. All of the samples displayed similar degradation profiles, suggesting that the existence of the SiO₂@GO might act as a radical scavenger to alter the degradation temperature and hence result in improving thermal stability of the EP matrix. The summaries of TG results are listed in Table 1. It was found that all SiO₂@GO/EP nanocomposites show a higher decomposition temperature than neat EP, especially the incorporation of 0.5 wt% SiO₂@GO noticeably enhances thermal stability of EP, the reason can attributed to the uniform and oriented distributions of SiO₂@GO and strong covalent adhesion between two phases can make full use of excellent thermal stability of SiO₂@GO. Moreover, the weight losses of all the EP nanocomposites are lower than neat EP. Compared with neat EP, the DTG curves indicated that the maximum degradation temperature (T_max) also increased upon addition of the SiO₂@GO, as shown in Fig. 6B. The T_max of almost all EP nanocomposites was higher than neat EP, it clearly indicate that the thermal stability of EP nanocomposites was enhanced, the reason can attribute to the ultrathin GO are preferable to traditional coupling agent for the enhancement of silica-epoxy interfacial adhesion. It is ascribed to the intelligent construction of excellent thermal stability and compatibility of interface, as manifested above.⁸

Fig. 4 TG (A), and DTG (B), curves of EP, SiO₂@GO/EP, SiO₂–NH₂/EP, SiO₂/EP nanocomposites.

Table 1 Selected results from TGA

Samples	IDT^a	FDT^b	T50%^c	Tmax^d
	°C	°C	°C	°C
a The initial decomposition temperature.b The final decomposition temperature.c The temperature at 50% weight loss.d The temperature at the maximum rate of mass loss.
EP	373	782	426	423
0.1 wt% SiO2@GO/EP	393	768	436	439
0.5 wt% SiO2@GO/EP	417	695	445	440
0.8 wt% SiO2@GO/EP	410	769	439	439
1 wt% SiO2@GO/EP	414	574	442	435
3 wt% SiO2@GO/EP	400	783	431	424

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3.3 Mechanical properties

In this work, in order to study the effect of ultrathin GO shells decorated SiO₂ on the mechanical properties, the Young's modulus (E), tensile strength (σ_b) and elongation at break (ε_b) are used to evaluate the strength and the toughness of neat EP and its nanocomposites. The results are shown in Fig. 5A and B. It was found that SiO₂@GO, SiO₂–NH₂, and SiO₂ had an obvious reinforcing effect on the EP matrix. The Young's modulus (E), tensile strength (σ_b) and elongation at break (ε_b) of neat EP were 889.64 MPa, 34.36 MPa, and 2.41%, respectively. Compared with neat EP, the addition of 0.5 wt% SiO₂@GO results in a sharp increase of epoxy composites tensile strength and modulus, were 60.37 MPa and 1254.19 MPa. After the appropriate content of SiO₂@GO, the tensile strength of nanocomposites decreased slowly, this was due to the agglomeration of nanoparticles which can weaken the interfacial interaction. The result can be attributed to the good dispersion of SiO₂@GO within the EP matrix and the strong interfacial interactions between fillers and the EP matrix. In order to understand the effects of nanoparticles before and after the modification of the properties of composites, we compared the composites loaded with same content of 0.5 wt% SiO₂, 0.5 wt% SiO₂–NH₂, 0.5 wt% SiO₂@GO. The incorporation of 0.5 wt% SiO₂ results in a great improved tensile strength, the elongation at break and Young's modulus of epoxy composites, with average values of 53.19 MPa, 4.75% and 1207.46 MPa, respectively. The composites of 0.5 wt% SiO₂–NH₂/EP and 0.5 wt% SiO₂@GO/EP lead to simultaneously improved stiffness and toughness, especially the 0.5 wt% SiO₂@GO/EP increased in a more significant, the tensile strength. Compared with the 0.5 wt% SiO₂–NH₂/EP, the 0.5 wt% SiO₂@GO/EP results in a slightly improved. The results show that the mechanical enhancement effect of SiO₂@GO hybrid, it's attributed to the GO shell blanket the SiO₂ surface, act as the bridge between filler and matrix, enhances the interfacial interaction.

Fig. 5 Tensile strength and Young's modulus (A), and elongation at break-point (B) of SiO₂@GO/EP nanocomposites.

3.4 Morphology of SiO₂@GO/EP nanocomposites

The photograph of specimens was checked as the simplest qualitative test to observe the dispersion state of SiO₂@GO in the matrix.³⁰ In order to further study the dispersion, compatibility and interfacial interaction of hybrids in the EP matrix, we investigated the fracture surfaces of the neat epoxy and its composites by SEM. Fig. 6 shows SEM images depicts the specimen cross sections of neat EP, 0.5 wt% SiO₂/EP, 0.5 wt% SiO₂–NH₂/EP and 0.5 wt% SiO₂@GO/EP composite, respectively. It can be obviously observed from Fig. 6a and b that the smooth fracture surface regions and the river patterns appear on the fracture surface. This is a typical brittle fracture for thermosetting polymer. Other than the neat epoxy, the composites show a rough fracture surface, as shown in Fig. 6c–h. At 0.5 wt% SiO₂/EP composites, the fracture morphology is rather smooth, a mass of SiO₂ particles agglomerate are observed, revealing the weak interface interaction between the matrix and SiO₂ nanoparticles. While 0.5 wt% SiO₂–NH₂/EP composites (Fig. 6e and f) and 0.5 wt% SiO₂@GO/EP composites (Fig. 6g and h) shows a good dispersion and homogeneity on the fracture surface. For 0.5 wt% SiO₂–NH₂/EP composites, the fracture morphology is rough with some obviously crack grain, the SiO₂ particles are well-embedded in the matrix with stronger interface. On the other side, the 0.5 wt% SiO₂@GO/EP composites show a much rougher fracture surface, where the ribbons become irregular and tortuous, the SiO₂ nanoparticles aggregation effect decreased significantly with strong interface interaction from the magnification fracture image. It can be attributed to the abundant functional groups of GO, surface wrapping of GO could improve the dispersion of the SiO₂ particles effectively. From the pulling-out of the epoxy composites, the rough fracture graphs originate indicating the suppression ability of the fillers on the propagation and advancing of cracks in the composite interface.³¹

Fig. 6 FESEM images of fractured surface (a and b) neat EP, (c and d) 0.5 wt% SiO₂/EP, (e and f) 0.5 wt% SiO₂–NH₂/EP, (g and h) 0.5 wt% SiO₂@GO/EP, at low and high magnifications, respectively.

3.5 Tribological effect of SiO₂@GO/EP composites

Generally speaking, adding nanoparticles to improve the wear resistance of polymers have some advantages, such as:^1,32 (1) the mechanical behavior of the EP matrix can be emulated while the abrasiveness of the hard nanoparticles decreases remarkably as a result of a reduction in their angularity; (2) due to the nanofillers have the same size as the segments of the surrounding polymer chains, the wear volume of composites modified by nanoparticles would be much slighter than that of conventional composites; (3) some light sheet layer of nanometer materials are easy to form transfer film in the process of tribology test, the transferred film can be strengthened by the nanoparticles that would blend well with wear particles.

Core–shell SiO₂@GO derives the attributes of low friction coefficient from modified nano-SiO₂ and high wear resistance from GO, and appears to be an anti-wear filler for polymer composites. To investigate tribological effectiveness of core–shell SiO₂@GO addition, the friction coefficient and wear volume of SiO₂@GO/EP specimens were measured under a load of 300 g at a reciprocating frequency 100 times min⁻¹. Fig. 7A illustrates friction coefficient values for SiO₂@GO/EP specimens as s function of sliding time. The incorporation of SiO₂@GO significantly reduces friction coefficient of SiO₂@GO/EP composites. Moreover, the fluctuation phenomenon in the friction coefficient curves obviously diminishes and tends to be stable with SiO₂@GO addition. For SiO₂@GO/EP composites, when SiO₂@GO content reaches 0.5 wt%, the friction coefficient is the lowest and most stable. At the same time, compared with the neat EP, the run-in period of the SiO₂@GO/EP specimens are reduced significantly. When the SiO₂@GO content is more than 0.5 wt%, the friction coefficient of composites show a rising trend with increasing filler content. However, the friction coefficient of the specimens was sufficiently close while the SiO₂@GO content in the range between 0.8 wt% and 3 wt%. We can speculate that the transfer film in the steady stage of SiO₂@GO/EP composites mainly consists of the polymer wear debris which in the present of spherical silica and is enwrapped by GO nanosheet. Owing to their transfer film between the friction sample and counterpart pain have the same composition, the polymer particles were wrapped by lubricating effect of GO nanosheet,³³ the friction coefficient of the composites was quite close with different SiO₂@GO contents in the steady stage. At the same time, the more nanoparticles content of the composites the more friction coefficient gradually increases of the composites after the optimal content, indicating that the lubricating effects is gradually reducing. However, the friction coefficient value is still much lower than that of neat EP. The average friction coefficient values were extracted from the plots of friction coefficient as a function of sliding time.³⁴ Fig. 7B shows the volume wear rate of the composites with different SiO₂@GO contents. Compared with the neat EP, volume wear rate decreases with the addition of SiO₂@GO. When the SiO₂@GO content is 0.5 wt%, the wear rate of the composites reaches 3.472 × 10⁻⁹ mm³ N⁻¹ m⁻¹, and decreases a couple of orders of magnitude compared to the neat EP. However, the changes in the wear rate are slow when the SiO₂@GO content is more than 0.5 wt%, which can attributed to the nanoparticles agglomerates in the matrix.

Fig. 7 (A) Friction coefficient values for the SiO₂@GO/EP composites as a function of sliding time. (B) Relationship of wear rate of SiO₂@GO/EP specimens with different SiO₂@GO content (reciprocating frequency: 100 times min⁻¹, load: 300 g).

In order to investigate the influence of the tribological properties of the composites by wrapping silica with GO shells, we compare the tribological properties of neat EP, 0.5 wt% SiO₂/EP, 0.5 wt% SiO₂–NH₂/EP and 0.5 wt% SiO₂@GO/EP specimens. Fig. 8 shows the lowest values both in friction coefficient and wear rate of composites by SiO₂@GO addition. Compared with the unmodified silica/epoxy composites, both the friction coefficient and wear rate values are lower of 0.5 wt% SiO₂–NH₂/EP composites, which silica surfaces were modified with coupling agent APTES. Moreover, make comparison with 0.5 wt% SiO₂–NH₂/EP composites, the torque reduction in friction coefficient of 0.5 wt% SiO₂@GO/EP specimens.

Fig. 8 Comparisons tribological properties of neat EP, 0.5 wt% SiO₂/EP, 0.5 wt% SiO₂–NH₂/EP and 0.5 wt% SiO₂@GO/EP specimens (reciprocating frequency: 100 times min⁻¹, load: 300 g).

In order to further confirm the conclusion above and can be beneficial to explore the related friction and wear mechanism, the morphologies of the worn surfaces of neat EP and its composites filled with nanofillers were investigated to determine the wearing mechanisms, shown in Fig. 9. As is well-known, the trend in wear rate is consistent with that of wear width. As shown in Fig. 9a and b, the average wear width for neat EP is 583 um, it can be seen that plastic deformation and ploughing predominantly constitute the worn surface; there is also created deep grooves and produced wear debris. For the 0.5 wt% SiO₂@GO/EP composite (Fig. 9c and d), the wear width is narrowest (160 μm) in the all specimens, reduced about 73% in comparison with that neat EP. At the same time, it also indirectly proves that the minimum wear rate of 0.5 wt% SiO₂@GO/EP composite. To confirm the influence of the tribological properties of the composites by wrapping silica with GO shells, we compare the wear width of 0.5 wt% SiO₂/EP, 0.5 wt% SiO₂–NH₂/EP and 0.5 wt% SiO₂@GO/EP specimens. With the incorporation of uniform sized nanoparticles into the EP matrix, the propagation of the cracks into the EP matrix was hindered to a certain degree by the particles on or near the surface layer,³² which can be seen from Fig. 9c–h. It can be observed that, the wear width (160 μm) of 0.5 wt% SiO₂@GO/EP is lower than two others (Fig. 9e–h). And the wear width for 0.5 wt% SiO₂–NH₂/EP (228 μm, Fig. 9e and f) is narrower than 0.5 wt% SiO₂/EP (329 μm, Fig. 9g and h). Although composites loaded with SiO₂@GO comparison to two others exhibit a rough worn surface and deep furrows seen from the high magnification image, the wear width is still narrower. We suspect that it can be attributed to the self-lubricating of GO coated on the silica surface. Moreover, compared with relatively flat of neat EP wear surface and displays signs of adhesive and abrasive wear, the wear surfaces of composites filled with nanofillers show some typical fatigue deformations and deep furrows. It can be indicated that the type of wear changed from adhesion and abrasive wear into fatigue wear.

Fig. 9 SEM images of worn surfaces of (a and b) neat EP, (c and d) 0.5 wt% SiO₂/EP, (e and f) 0.5 wt% SiO₂–NH₂/EP, and (g and h) 0.5 wt% SiO₂@GO/EP, at low and high magnifications, respectively.

A simplified schematic diagram provides an explanation for the improved wear resistance imparted by tribological effects of SiO₂@GO (Fig. 10a and b). As shown in Fig. 10b, the bands at 1357 and 1595 cm⁻¹ arise in Raman spectra of worn surfaces of 0.5 wt% SiO₂@GO/EP specimen. From these two characteristic bands of GO, the difference demonstrate that SiO₂@GO exists on the worn surface of 0.5 wt% SiO₂@GO/EP specimen. So, the high specific area and multi-dimensional core–shell SiO₂@GO on the worn surface also absorb the energy generated by compression and shear during friction process. Moreover, in the present of SiO₂@GO on the worn surface provides direct evidence of the protective effect against the external force associated with sliding, thus contributing to the increased wear resistance. When the ball does reciprocating sliding, spherical nanometer SiO₂ particles coated by GO layers can act as bearing between moving parts, so that changing sliding friction into both rolling and sliding friction to reduce friction. In addition, under contact stress the tightly coated soft GO shells over the silica surface cover up the nanogaps of the rubbing surfaces are easy to form a transfer film between two contact surfaces, thus can also significantly reduce abrasive wear on the worn surface by greatly reducing direct contact and scraping.³⁵ Moreover, the ultrathin lamellar structure of GO composites provides low resistance to shear, resulting in reduction in wear and friction. From the above, core–shell SiO₂@GO nanoparticles can improve the tribological properties of the composites by changing the friction forms.

Fig. 10 (a) Typical proposed model for the friction and wear mechanism of neat EP and SiO₂@GO/EP specimens. (b) Raman spectra of worn surface of neat EP and 0.5 wt% SiO₂@GO/EP specimens.

The tribological properties of polymer composites depend on numerous factors.²⁰ Thus, the anti-wear and antifriction of SiO₂@GO/EP composites may possibly be attributed to the presence of ultrathin laminated structure of GO,³⁶ which the good mechanical interlocking arising from the wrinkled rough surface of GO would lead to an excellent interfacial adhesion between GO and epoxy¹² and the hybrid also creates a network with lower cross-link density at the interface region, which may also cause an additional increase in toughness due to absorb energy more efficiently than a highly cross-link network.^15,37,38

4. Conclusions

In this work, core–shell structured SiO₂@GO hybrids were prepared by electrostatic assembly, and their epoxy composites were fabricated by a solution-free curing process. The tribological properties and mechanical as well as thermal properties of the aligned SiO₂@GO/EP composites can be improved by suitable addition of SiO₂@GO. And the synergy effect of nano-SiO₂ and GO was achieved in the EP matrices. When the addition amount is 0.5 wt%, the friction coefficient and wear rate of composites reduce nearly 88.4% and more than several orders of magnitude, respectively. Moreover, the incorporation of 0.5 wt% SiO₂@GO produced the polymer mechanical properties and the thermal stability optimum. We can speculate the case of superior wear resistance was that the uniform distribution of SiO₂@GO, the increase in mechanical properties and thermal stability, the transfer film and protective effect of SiO₂@GO against the friction force. After the optimum fillers content, the friction coefficient reduced with the nanoparticles aggregation effect increased and unevenly disperses in the matrix, it could be attributed to the nanoparticles formation hard spots spun off to cause part of transfer film damage during the friction sliding. Under the same content nanofillers (0.5 wt% SiO₂, 0.5 wt% SiO₂–NH₂ and 0.5 wt% SiO₂@GO) addition, the lowest values both in friction coefficient and wear rate of composites by SiO₂@GO addition. It can be also attributed the self-lubricating of GO shells coated on the silica surface. The homogeneous SiO₂@GO dispersion and strong interface, a certain mechanical strength, transfer film and tribological effect of SiO₂@GO can further improve wear resistance.

Acknowledgements

This work was financially supported by National High Technology Research and Development (863) Program, MOST of China (2015AA034602), Fundamental Research Funds for the Central Universities (Self-Determined and Innovative Research Funds of WUT, 2014-‖-009), Project of Basic Science and Advanced Technology Research, Chongqing Science and Technology Commission (cstc2016jcyjA0796), and Project of Applied Basic Research, Wuhan Science and Technology Bureau (2015010101010015).

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