1 Introduction
Among the commercially available thermoset plastics, epoxy resin (EP) is versatile with good tensile strength and stiffness, high adhesive strength, high electrical insulation, low creep, and good chemical and temperature resistance [1]. Additionally, EP is attractive in academic institutions and industrial sectors because of its curing characteristics, such as almost exhibiting no byproducts and volatiles during the curing reactions, limited curing shrinkage, and being performed within a relatively wide temperature interval [2]. EP-based products have been widely used as engineering parts, function materials, adhesives, and coatings in various fields. It is also characterized by the high three-dimensional (3D) network construction and crosslinking density, making it possess relatively poor tribological features especially under dry sliding [3]. This drawback restricts EP to the broadened applications where fierce rubbing actions exist. Thus, it is essential to improve the dry-friction property of EP.
The introduction of nanoscale inorganic fillers is still a straightforward and efficient strategy to impart superior comprehensive performance of polymers because they can present remarkably improved properties at a relatively low content compared to that of conventional micro or submicro reinforcements. Among different nanofillers, the ones characterized by lamellar structures, i.e., layered nanoparticles, are the most promising candidates. Hence, several attempts have been made, and various kinds of lamellar nanomaterials [4–9], such as niobium diselenide, graphene (oxide), boron nitride, nanoclay, molybdenum disulfide, and transition metal carbides, have been adopted to improve the tribological responses without any lubricants of EP nanocomposites. These available reports have reached a consensus that the presence of layered nanoparticles can facilitate the formation of high-quality transfer films between the sample and friction counterpart, improving the tribological properties of EP under dry sliding considerably [4–8].
Nickel phyllosilicate (NiPS) is a hydrous silicate, which fundamentally contains the basic elements of Si, Ni, O, and –OH groups. The metal cations and –OH groups are usually organized into two-dimensional (2D) structures as sheets in NiPS, which can be tailored precisely to be various layered structures depending on the different stacked Si−O tetrahedral and Ni−O octahedral sheets. NiPS has attracted significant attention in many fields because of its facile synthetic routes, variable morphologies, controllable microstructures, and versatile properties [10]. Additionally, NiPS is one of the most ideal nanoscale reinforcements in the preparation of polymeric nanocomposites because of its high intercalation chemistry, high aspect ratio, ease of availability and low cost. In our previous study, we have aimed to synthesize NiPS nanoparticles using different methods and investigated the effect on the tribological response of EP. We have then concluded that the added NiPS nanoparticles can enhance the tribological response under dry sliding, thermal and mechanical properties of EP differently, depending on their varied morphological structures [11–13], of which the generally lamellar nanofillers decrease the friction coefficient [11], whereas the flower-like nanoparticles are better at improving the antiwear and mechanical properties [13]. The NiPS nanoparticles with varied morphologies affect the macroscopy properties of EP differently. Nevertheless, to the best of our knowledge, there are no more reports on this issue that can be available publicly.
Metal−organic framework (MOF) is an emerging subclass of coordination polymer constructed by metal ions/clusters and organic linkers. Because of the cushy designability and adjustable functionalities, abundant MOFs have been developed and widely used as functional materials or promising additives in polymeric composites [14,15]. Moreover, MOF can be adopted as a template or precursor to developing other functional materials by substituting organic linkers with a specific anion. In this study, the rod-like MOFs with Ni-containing units were first prepared readily, which were adopted as the templates to synthesize the NiPS whiskers using a facile hydrothermal method. Then, we introduced the obtained NiPS whiskers into the EP matrix and investigated the effects on the mechanical property, thermal stability, tribological response under dry sliding and flammable performance. This study aimed to provide a promising pathway to produce the high-quality EP-based tribomaterials reinforced by the whisker-like phyllosilicates.
2 Experimental
2.1 Materials
Ni(NO3)2·6H2O, NaOH, Na2SiO3·9H2O and NH3·H2O were purchased from SINOPHARM. Benzene-1,3,5-tricarboxylic acid (BTC) and 4,4′-diamino diphenylmethane were purchased from Aladdin with typical chemical structures in Figs. S1 and S2 (cf. Electronic Supplementary Material, ESM), respectively. Absolute alcohol and acetone were obtained from Shanghai Titan scientific Company. All chemicals were A.R. grade and used without previous purification. The diglycidyl ether of bisphenol-A (DGEBA, E-51, industrial) was purchased from Blue star New Materials Co., Ltd., with the epoxide value of 0.48–0.53 mol·100 g–1. Deionized water was obtained from a water purification system in our laboratory.
2.2 Sample preparation
The rod-like Ni-MOFs with Ni units were synthesized according to the reported route demonstrated by Zhang et al. [16]. To synthesize NiPS whiskers, 1.14 g Na2SiO3·9H2O and 0.8 g Ni-MOFs were added into 120 mL absolute alcohol-water solution with half and half in volume, mixing strongly for at least 30 min. Subsequently, 6 mL NaOH with 1 mol·L–1 was dropwise introduced into the above mixture under continuous intense agitation and ultrasonically treated for 30 min to prevent aggregation. The resulted suspension was transferred to a 400 mL Teflon-lined stainless autoclave, maintaining the reaction at 160 °C for 15 h. After repeated centrifugation, washing with deionized water and absolute alcohol, the light green powders of NiPS whiskers were collected and dried completely in a vacuum at 70 °C overnight. The EP nanocomposites containing the varied mass fraction of NiPS whiskers (0, 1, 3 and 5 phr) were prepared according to our previously reported procedure [17]. depicts the entire process for obtaining the samples.
2.3 Instrumental
The crystal phase was conducted using a powder X-ray diffraction instrument (XRD-6000) with a Cu Kα target. We applied an infrared spectrometer (Nicolet 380) to record the Fourier transform infrared (FTIR) spectrum using the KBr pellet pressing method. The surface topographies of samples were characterized by using scanning electron microscopes (SEM, SIGMA-500 and FlexSEM1000), transmission electron microscope (TEM, JEOL-2010) and energy-dispersive X-ray spectrometry mapping. The elemental composition was detected using an X-ray photoelectron spectroscope (XPS, ESCALAB Xi+) with a monochromatic Al Kα excitation source. Thermogravimetric analysis (TGA/SDTA851e) under an N2 atmosphere (50 mL·min–1) was conducted to determine the thermal stability, and limiting oxygen index (HC-2) and UL-94 vertical burning (CFZ-2-type) following the Chinese Standards of GB/T 2406.2-2009 and GB/T 2408-2008, respectively, were used to evaluate the flammable properties of samples. The sample dimensions were 100 mm × 6.5 mm × 3 mm and 100 mm × 13 mm × 3 mm, respectively. The uniaxial tensile test was conducted on a universal testing machine according to GB/T 1040.1-2006 under a crosshead speed of 2 mm·min–1, of which the specimen was 1BA type and a gauge length of 25 mm. At least five specimens were tested to obtain the average value for each sample. The tribological property under dry sliding was examined using a block-on-ring friction-testing machine (M-200) according to our previous investigation [11].
3 Results and discussion
3.1 Characterizations of NiPS whiskers
The phase structure and chemical composition for the synthesized NiPS whiskers are determined carefully and the results are exposed in Fig.1. Fig.1(a) shows the representative XRD patterns of NiPS whiskers and Ni-MOF. The XRD pattern of Ni-MOF is consistent with the previously reported data by Yaghi et al. [18]. As shown in , the rod-like green crystals of Ni-MOF [Ni3(BTC)2·12H2O] can be achieved from the reaction between Ni(II) ions and BTC under hydrothermal conditions. It comprises zigzag chains of tetra-aqua Ni(II) benzenetricarboxylate that are hydrogen-bonded to yield a tightly held 3D network [19]. Such crystal is considered to be monoclinic, of which the lattice parameters are a = 1.75 nm, b = 1.30 nm,c = 0.66 nm, α = γ = 90°, and β = 112.04° [18]. The light green powders of NiPS whisker are synthesized after another hydrothermal reaction of Ni-MOF with [SiO3]2–, showing a completely different XRD pattern from that of Ni-MOF. It shows the characteristic pattern of a 2D structure with (hk0) and (00l) lines [20]. The diffraction band located at 2θ = 19.87° corresponding to the (002/011) reflection indicates the presence of phyllosilicate structure not the nickel hydroxides [20]. Meanwhile, the peaks of 2θ = 35.97° and 60.84° can be indexed to the trioctahedral type smectite structure of phyllosilicate [21]. Based on the characteristic peak of 2θ = 5.83°, i.e., (001) line, the basal space is 1.52 nm according to the well-known Bragg equation, which is much higher than the product synthesized without providing any templates [11]. This is attributed to the organic linkers between Ni–O octahedrons that prevent the NiPS nanosheets close to each other during the growth reaction, thus enlarging the basal space.
To probe the variations on the chemical structures between Ni-MOF and NiPS whisker, we conduct the FTIR spectroscopy, and the results are shown in Fig.1(b). For pristine Ni-MOF, we can observe a wide transmission peak at about 3446 cm–1, which is the stretching vibration of O–H, verifying the existence of the intramolecular hydrogen bonds [22,23]. The adjacent relatively weak band at about 3180 cm–1 corresponds to the stretching vibration of C–H of the benzene ring. A transmission peak of the asymmetric vibration of BTC is found at 1565 cm–1, whereas the symmetric vibrations of BTC are located at 1436 and 1375 cm–1. The sharp peak at 1624 cm–1 indicates the presence of water in the metal coordinate sphere [18]. Meanwhile, the strong vibration of 728 cm–1 is attributed to the out-of-plane bending vibration of C–H in the benzene ring [18]. In NiPS whiskers, the C–H bending band of the benzene ring has disappeared, and the peaks derived from BTC located at 1375–1624 cm–1 are significantly suppressed (marked by an ellipse). Nevertheless, the characteristic bands of NiPS are emerged and are dominant. The above phenomena confirm the substitution reaction of organic linkers by silicate ions during the hydrothermal process. The chemical water in the phyllosilicate structures gives the stretching signals at 3628 and 3446 cm–1, whereas the physically bonded water gives rise to the bending vibration at 1632 cm–1 [24]. The prominent sharp peak located at 1028 cm–1 corresponds to the stretching vibration of Si–O–Ni [20], and another stretching signal at 472 cm–1 is due to the Si–O groups in NiPS [25], as the characteristic vibrations of phyllosilicate structures. Moreover, the observed stretching vibration of Ni–OH has formed a doublet at 709 and 665 cm–1 [26], which is another solid proof for the successful formation of the target products.
Next, we examined the morphologies of Ni-MOF template and MOF-derived NiPS whiskers using electron microscopes, and Fig.2 shows the results. The pristine Ni-MOF shows the microsized rod-shaped products with smooth surfaces in Fig.2(a) as expected, showing a morphology similar to the reports demonstrated by Zhang [16] and Huang [27]. The NiPS whiskers inherited the rod-like morphologies from Ni-MOF template, as shown in Fig.2(b). They are characterized by loosely integrated nanosheets different from that of the Ni-MOF, leading to rough surfaces belonging to NiPS whiskers (Fig.2(c)). Fig.2(d) shows a high-resolution TEM image of the nanosheets. This figure shows that the NiPS whiskers are constructed by abundant nanosheets, confirming the lamellar feature of NiPS whiskers. These nanosheets with basal spacing ranging from 1.34 to 1.63 nm were formed during the hydrothermal synthesis through an in situ chemical reaction together with the recrystallization process. The BTC ligands are substituted by [SiO3]2– units and dissolved in the water−ethanol solution during the NiPS whiskers generation. TEM image of an individual NiPS whisker shown inFig.2(e) verifies that the resulting rod-like product is solid, while the high angle annular dark-field picture and elemental mapping results display the uniform distribution of Ni, O, and Si elements throughout the rod-shaped frame.
The chemical compositions of Ni-MOF and NiPS whiskers are further examined using XPS and the results are depicted in Fig.3. The survey spectra in Fig.3(a) display the principal composition elements of C, O, and Ni in Ni-MOF and characteristic elements of C, O, Ni, and Si in NiPS whiskers. Ni-MOF has the highest relative concentration of C elements, whereas it is declined sharply in NiPS whiskers. It means that the atom concentration of C decreases significantly from 55.6% in Ni-MOF down to 13.57% in NiPS whiskers. This is expected, and such phenomenon is closely related to the declined BTC ligands, confirming that the substitution reactions occurred between organic linkers and [SiO3]2– units. From the Ni 2p core-level spectrum (Fig.3(b)) in NiPS whisker, it is deconvoluted into two spin-orbit doublets in 874.4 and 856.8 eV indexed to Ni 2p1/2 and Ni 2p3/2, respectively. This characteristic binding energy difference (approximately 17.6 eV) suggests the existence of Ni(II) ions [28]. There are also two peaks centered at 880.3 and 862.5 eV attributed to their shakeup satellites. The Si 2p core-level spectrum (Fig.3(c)) indicates the binding energy of Si 2p centering at approximately 103.2 eV, which is a typical value for metal silicate hydroxides [29]. Fig.3(d) shows that the asymmetric peak of the O 1s spectrum can be deconvoluted into three signals. The binding energies at 530.8 and 532.1 eV correspond to the Ni–O and Si–O bonds derived from NiPS whiskers, respectively [30]. Meanwhile, another weak peak is attributed to the C–O bond, which must be derived from the residual organic linkers.
3.2 Morphological structure
The fracture surface morphologies of samples are investigated using SEM, and Fig.4 shows the results. Pure EP demonstrated in Fig.4(a) presents a featureless fracture surface, implying the typical brittle fracture behavior. Fig.4(b–d) show that NiPS whiskers are covered tightly and embedded in the matrix. The elemental mapping images displayed in Fig.4(e) and 4(f) show that the major elements of Ni and Si derived from NiPS whiskers can disperse homogenously in the field of view, showing the uniform dispersions of whiskers in the EP matrix. The rough surfaces of NiPS whiskers are essential in obtaining the well-bonded interfaces and enhancing their interactions with EP molecules. However, a certain proportion of organic linkers remaining in these whiskers also contributes to the excellent compatibility between filler and matrix. The fracture surfaces of EP nanocomposites become coarse and uneven, suggesting that the propagation path of crazes is complicated. This is different from the featureless surface of control EP, indicating that as the crazes are generated initially, they can run through the entire fracture surface quickly without any obstructions before the failure of materials [17]. This can be ascribed to the well-dispersed rigid whiskers in the EP matrix that block, deflect, and branch the crazes during the entire fracture process [31]. Hence, this is certainly beneficial to improve the mechanical and tribological properties.
3.3 Mechanical property
We evaluated the mechanical properties of control EP and nanocomposites using quasi-state tensile tests. Fig.5 shows the representative profiles of tensile stress as a function of the elevated strain. Tab.1 presents the influence of NiPS content on the tensile strength, elastic modulus and elongation at the break of EP nanocomposites. We obtained that these samples exhibit the hard and brittle stress-strain curves, showing a prominent elastic deformation, followed by the insignificant plastic period. In other words, there is no visible yielding plateau before the sample breaks abruptly. Compared with control EP, the nanocomposites have the obviously increased elastic modulus but suppressed elongation at break. The higher the concentration of the NiPS whisker, the higher the elastic modulus and lower elongation at break of EP nanocomposites. This means that the presence of NiPS whiskers can stiffen the EP and deteriorate the corresponding flexibility steadily with the increased filler content. As 5 phr of NiPS whiskers is introduced, the elastic modulus increases from 1.26 to 1.97 GPa, whereas the elongation at break decreases from 10.51% to 5.64%, indicating the improvement and declination by 56.4% and 46.3%, respectively, compared to the control EP. This is explained by the inorganic NiPS whiskers with higher modulus and aspect ratio dispersing homogeneously in the matrix; thus, strongly providing massive interface interactions with EP molecules [2,32] and considerably restraining the mobility of polymer segments during the elastic deformation under the applied quasi-state load [33]. Thus, for EP nanocomposites, the elastic modulus increases, and elongation at break decreases simultaneously.
Tab.1 Effect of NiPS whisker content on the mechanical properties of EP nanocomposites |
| Sample | Tensile strength/ MPa | Elastic modulus/ GPa | Elongation at break/ % |
|---|---|---|---|
| 0 phr | 77.3 ± 3.4 | 1.26 ± 0.12 | 10.51 ± 0.20 |
| 1 phr | 79.6 ± 2.5 | 1.51 ± 0.15 | 8.82 ± 0.38 |
| 3 phr | 87.8 ± 3.5 | 1.85 ± 0.17 | 8.11 ± 0.34 |
| 5 phr | 82.3 ± 2.3 | 1.97 ± 0.19 | 5.64 ± 0.25 |
By adding NiPS whiskers, the tensile strength increases first and then decreases. The maximum value of 87.8 MPa is obtained by introducing 3 phr whiskers, which is 13.6% higher than that of control EP (77.3 MPa). The reinforcement of added NiPS whiskers on the tensile strength is less than that on the elastic modulus; these incorporated fillers improve the tensile strength to a certain degree. As discussed above, NiPS whiskers are characterized by rough surfaces and can be dispersed homogeneously in the EP matrix. The strong interactions between fillers and matrix can effectively transfer the applied stress, making the rigid whiskers bear more load. Moreover, the existed fillers can increase the mechanical interlocking inside the nanocomposites and consume abundant energy during the failure process by prolonging the propagation path of crazes [33]. Additionally, the further added whiskers slightly decreased the tensile strength. Despite that, the value of 82.3 MPa is still higher than that of the control EP. This can be attributed to the excessive inorganic fillers tending to be aggregations and acting as the mechanical defects in the nanocomposites, weakening the reinforcing effect accordingly [34].
3.4 Thermal stability
The thermal degradation behaviors of the control EP and nanocomposites are tested under N2 atmosphere. Fig.6 shows the resulting weight and derivative weight thermograms. Tab.2 presents some characteristic parameters. Here, Ti is the initial decomposition temperature determined according to the ISO procedure; Tp represents the maximum temperature in which the maximum decomposition rate is obtained; (dW/dT)i and (dW/dT)p represent the corresponding weight loss rates at Ti and Tp, respectively. The residual char content at 700 °C is also collected simultaneously.
Tab.2 Effect of NiPS whiskers on the thermal stability of EP nanocomposites |
| Sample | Ti/°C | (dW/dT)i/(%·K–1) | Tp/°C | (dW/dT)p/(%·K–1) | Char/% |
|---|---|---|---|---|---|
| 0 phr | 362.3 | –0.86 | 375.5 | –2.28 | 14.8 |
| 1 phr | 369.1 | –0.75 | 387.5 | –1.58 | 15.2 |
| 3 phr | 366.8 | –0.66 | 386.8 | –1.43 | 18.4 |
| 5 phr | 365.8 | –0.65 | 386.3 | –1.42 | 19.1 |
The control EP quickly degrades through a one-step process, showing the residues char of 14.8% at 700 °C. Briefly speaking, EP is exceedingly inclined to pyrolyze in a large proportion under an inert atmosphere with a fast heat release. The introduction of NiPS whiskers presents a remarkable influence on the thermal decomposition of EP, increasing the decomposition temperature and decreasing the weight loss rates compared with that of control EP. Additionally, the Ti and Tp values increase from 362.3 and 375.5 °C for the control EP to 365.8–369.1 and 386.3–387.5 °C for the nanocomposites. Simultaneously, the (dW/dT)i and (dW/dT)p values decrease from –0.86 and –2.28%·K–1 to –0.65 to –0.75%·K−1 and –1.42 to –1.58%·K–1, respectively. Hence, the introduced NiPS whiskers can enhance the thermal stability of EP. As shown in Fig. S3 (cf. ESM), the weight loss of the pristine NiPS whiskers occurs during a relatively broad temperature interval with an increased temperature, showing only approximately 20% of weight loss, even when it increased to 700 °C. These inorganic whiskers show a much higher thermal resistance than that of control EP (approximately 85%), which can postpone the thermal decomposition of the EP matrix in the initial stage and make the decomposition temperature shift to a high temperature. Moreover, the well-dispersed NiPS whiskers in the matrix can construct a reinforcing network and act as physical barriers, providing the tortuous paths in the matrix to delay the diffusion and escape of low degraded products during the thermal decomposition [35]. Moreover, the char residues increase to 15.2%–19.1%. The higher the whisker content, the higher the char content for the nanocomposites. This phenomenon is closely related to the fact that inorganic NiPS whiskers cannot be totally ignited and removed along with the volatilized low molecular matters; the remaining parts will be left to participate in generating char residues.
3.5 Flammable property
The limited oxygen index (LOI) and UL-94 vertical burning tests are conducted to evaluate the influence of NiPS whiskers on the flammable properties of EP nanocomposites [36,37]. Figures S4 (cf. ESM) and 7 respectively show the corresponding results. The control EP has the lowest LOI value and shows a vigorous flame until burned to the clamp after ignition, having no vertical burning rating [38,39]. This phenomenon indicates the inherent nature of flammability of the control EP, indicating a strong tendency to spread flame away from the fire source. Although the EP nanocomposites still exhibit no UL-94 ratings, the LOI values of the nanocomposites increase slightly and steadily. The burning durations for the nanocomposites are significantly suppressed with the increased NiPS whiskers. When NiPS whiskers are added, the LOI values increase from 23.8% to 26.2%–27.1%, whereas the burning duration decreases from approximately 224 to 147–113 s. After ignition, the obtained char residue of the control EP becomes too weak to maintain the integrity of the entire process and falls apart immediately as it is removed from the clamp. The difficulties in burning the nanocomposites are improved significantly, and there are no drippings after burning for a certain time, leaving the mass residual chars with sufficient strength to maintain complete length. The burned length gradually decreases from 65 mm for the 1 phr case to 25 mm as the NiPS whiskers increase to 5 phr. This indicates that the addition of NiPS whiskers can improve the flame retardancy of EP to some extent. The well-dispersed inorganic whiskers in the matrix play a significant role of reinforcements and barriers and effectively elevate the viscosity, endowing the nanocomposites more “stiffer” than the control EP at high temperature due to the attractive polymer-filler interactions [40,41]. Hence, the suppressed motion of polymer chains restricts and eliminates the melt drops during the tests accordingly [42].
The morphologies of char residues after UL-94 tests are examined using SEM to explore the flame retardancy mechanism of NiPS whiskers. Fig.7(e) and 7(f) show the SEM images of the control EP and sample containing 5 phr NiPS whiskers. The char residue derived from control EP shows loose, with many irregular holes and caves emerging in the char layer. Such poor-quality residual char cannot to isolate the heat and flame to protect the underlying materials, showing the notably flammable feature of control EP. By contrast, the EP nanocomposites exhibit a much more compact and dense char layer. The char residue is tightly held due to the existence of NiPS whisker, showing the appearance of some proportion of micropores. Such improvements in the char residues explain why the EP nanocomposites are hard to burn, shortening the burning durations remarkably than that of the control EP. It is well-known that the compact and dense char layer plays a significant role in inhibiting the flame spread and heat transfer; it also protects the materials from being further destroyed.
3.6 Tribological response
We also investigated the influence of NiPS whiskers on the tribological properties under dry sliding, and the results are depicted in Fig.8. The friction coefficient variations and the increased sliding time shown in Fig.8(a) show that the entire friction process can be briefly divided into two distinguished stages. The initial region is the running-in stage (regime I). It is characterized by the plot fluctuating greatly, which is caused by the rapid growth of the real contact area and failure of the generation of transfer film in the contact area [9]. The other region is called the steady-state stage (regime II), showing the relatively flattened curve, as it appeared accompanied by the growing proportion of well-formed transfer film [9,43]. As the NiPS whiskers are added, the running-in stages for the nanocomposites become more complicated, showing a relatively higher friction coefficient and prolonged durations than that of the control EP. Although the duration of the running-in stage decreases steadily with increased inorganic fillers, it still maintains the longer friction time for the control sample. Additionally, all nanocomposite samples show higher average values of friction coefficient (χ). This implies that NiPS whiskers can increase the friction coefficient because of the presence of rigid inorganic fillers that roughen the surface of the sample in contact with the friction ring.
The enhanced friction coefficient endows the EP nanocomposites with a more excellent antisliding property. However, the weight loss of materials during the entire friction process is also essential because it is closely related to the service life for the friction materials. Fig.8(b) shows the variations of wear rates for the investigated samples as a function of NiPS whisker content. The control EP has the highest wear rate as expected, and the addition of NiPS whiskers significantly decreases the wear rate for the nanocomposites at low content. When 1 phr NiPS whiskers are added, the wear rate drops sharply from 7.03 × 10–5 mm3·N–1·m–1 for the pure matrix resin to the lowest value of 1.40 × 10–5 mm3·N–1·m–1 for EP nanocomposites. Although the further addition of NiPS whiskers increases the wear rate to some extent again, the values are still much lower than that of the control EP. This phenomenon suggests that the presence of such inorganic whiskers can effectively improve the antiwear properties of EP. This is expected because the well-dispersed abundant inorganic NiPS whiskers in the nanocomposites cause strong filler-polymer interactions between different phases and strengthen EP nanocomposites, thus improving the wear resistance to the compression-shearing actions during the entire friction process.
To further demonstrate the positive effect of NiPS whiskers on the wear resistance of EP and explore the wear mechanism, the worn surfaces of investigated samples are examined using SEM and the results are shown in Fig.9. Fig.9(a) shows that the control EP exhibits severe adhesive wear, showing the surface characterized by greatly coarse morphology accompanied by many visible scratches and deciduous pits, leaving much abrasive dust. This phenomenon suggests that the control EP has been greatly damaged as rubbing with the steel counterpart, and such poor wear resistance dominantly mechanism should be adhesive wear and fatigue spalling [44]. Nevertheless, the situations are different when the NiPS whiskers are added, showing more compact and flattened worn surfaces for those nanocomposites. Fig.9(b) shows a homogeneous scale-like morphology with 1 phr whiskers for the sample. It shows that the suppressed detachments peeled off from the bulk materials without obvious brittle torn debris in the friction zone. The absence of cracks and pits indicates that the NiPS whiskers can effectively strengthen the bulk materials to bear the friction force, making the surface less easily damaged than the control EP. The rigid whiskers tend to embrittle the EP to generate elevated resin debris as the filler concentration increases. However, the detached resin debris is not peeled off the substrate completely but it is most apt to be remained in the friction zone due to the strengthening and inhibition of well-dispersed NiPS whiskers. The captured debris is melted and smeared onto the worn surface under the strong compression and shearing action of the steel ring, indicating the presence of delamination marks on the wear track, as shown in Fig.9(c) and 9(d). Since the delamination is generated by the re-melt of the solid EP resin, it is not strong enough but prone to be broken and peeled off the substrate when rubbing with the counterpart. Therefore, the antiwear properties of the nanocomposites are weakened when the NiPS whiskers increase.
4 Conclusions
In this work, we applied the rod-like Ni-MOFs as the hard template and Ni source to synthesize NiPS whiskers using a facile hydrothermal technology. The as-prepared inorganic whiskers were carefully characterized using various measurements. Then, the effects on the mechanical, flammable, and tribological properties of EP nanocomposites were investigated. Some useful conclusions have been drawn as follows.
It was found that NiPS whiskers could be successfully prepared through the chemical reactions between the solid nickel sources and silicate ions under the structure direction of Ni-MOF rods. The loosely integrated nanosheets endowed these whiskers with greatly rough surfaces and strong interactions with the matrix resin, promoting disperse homogeneously in the nanocomposites. Consequently, the mechanical properties of EP nanocomposites are improved, showing the significantly improved tensile strength and elastic modulus. The thermal stabilities of EP nanocomposites were also improved by adding the NiPS whiskers, showing the increased decomposition temperature and suppressed mass loss rate. These inorganic whiskers increased the LOI values, declined the burning durations, and made it hard to burn for the EP nanocomposites; nevertheless, no ratings could be achieved for the investigated samples. Tribological evaluations confirmed that NiPS whiskers could not improve the antifriction performances of EP nanocomposites. Meanwhile, they played a significant role in enhancing antiwear properties. The introduction of only 1 phr fillers led to the declination of the wear rate by 80% compared with that of the control EP. This study aimed to provide a facile method for preparing high-performance EP-based nanocomposites.