Ultralow Friction Self-Lubricating Nanocomposites with Mesoporous

Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 38146-38152

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Ultralow Friction Self-Lubricating Nanocomposites with Mesoporous Metal−Organic Frameworks as Smart Nanocontainers for Lubricants Guoliang Zhang,† Guoxin Xie,*,† Lina Si,‡ Shizhu Wen,† and Dan Guo*,† †

State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China

‡

ABSTRACT: Smart nanocontainers with stimuli-responsive property can be used to fabricate a new kind of self-lubricating nanocomposite, while the practical potential of the metal− organic frameworks (MOFs) as nanocontainers for lubricants has not been realized. In this work, mesoporous Cu-BTC MOFs storing oleylamine nanocomposites were explored from synthesis and microstructure to self-lubricating characterization. The stress stimuli-responsiveness behavior of the CuBTC storing oleylamine (Cu-BTCO) for lubrication has been investigated by subjecting it to macroscopic ball-on-disc friction tests. The steady-state coefficients of friction (COFs) of the Cu-BTC nanocomposites without lubricants were ca. 0.5. In contrast, after oleylamine as the lubricant was incorporated into the Cu-BTC container in the nanocomposite, ultralow friction (COF, ca. 0.03) was achieved. It has been demonstrated that the improved lubricating performance was associated with the lubricating film which was in situ produced by the chemical reaction between the oleylamine released from the nanocontainer and the friction pairs. Therefore, the nanocomposite with smart Cu-BTC container holds the promise of realizing extraordinary self-lubricating properties under stress stimuli. KEYWORDS: ultralow friction nanocomposite, Cu-BTC m-MOFs, smart nanocontainer, self-lubricating, oleylamine

1. INTRODUCTION

The incorporation of porous microstructures could improve the uptake and storage of liquid lubricants. Such a system could avoid the irreversible damage of the depot system, and it can also compensate, to some extent, the decreased mechanical properties as compared with the usage of microcapsules. Nevertheless, as for conventional MOFs materials, the micropores are too small to adsorb lubricant macromolecules, and the compatibility between the embedded porous microstructures and the metallic/polymeric matrix remains a challenge. Mesoporous metal−organic frameworks (m-MOFs) may offer an efficient way to solve the problems due to their large specific surface area and high interfacial interaction with the matrix.21−23 Furthermore, the synthesis of m-MOFs, e.g., Cu-BTC m-MOFs, have been demonstrated to be feasible through a template-free strategy under solvothermal conditions, and microporous Cu-BTC nanoparticles were packed to form the mesopores with sizes of 26−72 nm.24 Recently, some researchers focused on stimuli-responsive metal−organic frameworks (MOFs).25 Zhu and his coworkers26 demonstrated that the free Eu3+ ions in the channels of the europium MOFs can be reversibly captured or released in response to the temperatures stimuli. The crystal-toamorphous transition of ZIFs materials may be induced by the pressure stimuli.27 In this case, it is reasonable to expect that m-MOFs, acting as smart nanocontainers with lubricants,

Recently, great research interests have been focused on stimuliresponsive smart materials for different purposes in the fields of electronics, catalysis, biology, and tribology.1−6 The functionality of these materials could change actively and rapidly in response to external stimuli, e.g., temperature, stress, electric field, pH, or ionic strength in local environments.7−9 Representatively, liquid lubricants or corrosion inhibitors initially embedded in composite materials could be controllably released in response to temperature or stress changes,10 and it is in contrast to the traditional way that lubricants or corrosion inhibitors are directly added into the contacting interface. For these smart composite materials, one of the most important factors is to develop depot systems incorporated into the matrix for providing immediate or sustained release of lubricants or corrosion inhibitors on demand. For instance, polymeric microcapsules for encapsulation and controlled release of lubricant have been widely investigated for the design of such depot systems.11−15 However, compromise between the improved lubrication or anticorrosion properties and the decreased mechanical strength of polymeric composites as the soft microcapsule content is increased in the hard matrix usually needs to be made.16 In addition, the wall materials of microcapsules would be irreversibly destroyed upon the threshold being reached triggered by external stimuli, resulting in relatively large damage. Some comparisons of containers for lubricants used as stimuli-responsive smart materials have been summarized in Table 1. © 2017 American Chemical Society

Received: August 22, 2017 Accepted: October 10, 2017 Published: October 10, 2017 38146

DOI: 10.1021/acsami.7b12591 ACS Appl. Mater. Interfaces 2017, 9, 38146−38152

Research Article

ACS Applied Materials & Interfaces Table 1. Containers for Lubricants Used as Stimuli-Responsive Smart Materials samples

COF

kinds of stimuli

application

porous polyimide (PI) materials containing lubricating oil17 polysulfone microcapsules containing tung oil18 ionic liquid core/silica gel shell microcapsules19 Fe3O4@PNIPAM nanogels20

0.2−0.4 0.37−0.46 0.09 0.03−0.25

stress, temperature stress, temperature stress, temperature temperature, magnetism, near-infrared light

bearing materials in aerospace industry epoxy coatings polyurethane coatings biotribology

Figure 1. Synthesis process for the EP/Cu-BTCO nanocomposite of (a) oleylamine stored in Cu-BTC particles, (b) the dispersion of Cu-BTCO in EP liquid, and (c) the curing and formation of EP/Cu-BTCO nanocomposite. a Bruker D8 Advance X-ray diffractometer and a Cu target (λ = 0.1540 nm Å). The surface morphologies of Cu-BTC MOFs were characterized by FEI Quanta 450 scanning electron microscopy (SEM) at an operating voltage of 5 kV. The pores structure of CuBTC-n and its dispersion in the EP matrix were recorded using JEOL JEM-2010F transmission electron microscopy (TEM) with an accelerating voltage of 200 kV. The elements of the generated lubricating film were investigated using PHI Quantera II X-ray photoelectron spectroscopy (XPS). The friction test was carried out using the CETR ultrafunctional attrition testing machine at a reciprocating speed of 12 mm/s and an applied load of 5 N for 30 min, and a steel ball (diameter of 4 mm) was used as the friction counterpart.

could facilitate dispersing them into the polymer matrix and can return to the initial state after deformation due to interfacial mechanical stresses. Despite it, very few works have been conducted for investigating MOFs as smart depot media of lubricants for feedback active nanocomposites. In this work, lubricants entrapped in Cu-BTC m-MOFs as smart nanocontainers were synthesized and incorporated into an epoxy resin (EP) matrix. Ultralow friction (COF, ca. 0.03) was achieved for the composite with Cu-BTC storing oleylamine28,29 as the lubricant.

2. EXPERIMENTAL SECTION 2.1. Materials. For preparing Cu-BTC mesoporous MOFs, Cu(NO3)2·3H2O (99.9%), acetic acid (≥99.8%), triethylamine (99.0%), and benzene-1,3,5-tricarboxylic acid (H3BTC, 98%) were purchased from Aladdin Chemicals Co., Ltd., (Shanghai, China). For the synthesis of epoxy/Cu-BTC storing oleylamine (EP/Cu-BTCO) nanocomposites, an epoxy resin and its curing agent were obtained from the Struers International Trade Co., Ltd., (Shanghai, China), and oleylamine (C18: 80−90%) was purchased from Aladdin Chemicals. Other reagents, such as methanol and ethanol, were of analytical grade and purchased from the China National Pharmaceutical Group Corp. 2.2. Preparation of EP/Cu-BTCO Nanocomposite. Cu-BTC mMOFs were prepared according to a previously published work.24 Certain amounts of Cu(NO3)2·3H2O, acetic acid, and triethylamine with the mass ratio of 1:1.5:0.8 were added into 10 mL of methanol and stirred for 1 h; then, 0.210 g of H3BTC was added followed by stirring for another 2 h. The obtained homogeneous solution was transferred into a Teflon-lined autoclave and heated at the designed temperatures for 24 h. The blue solid was collected by centrifugation and washed by ethanol to obtain the products, denoted as Cu-BTC-n (n represents the synthesis temperature). The scheme for the preparation of EP/Cu-BTCO nanocomposites is shown in Figure 1. Cu-BTC was immersed in oleylamine at 25 °C for 12 h (Figure 1a). After centrifugation at 6000 rpm for 15 min, the resultant Cu-BTCO was washed with normal hexane at 10 °C, removing the surface residual oleylamine. A certain mass of Cu-BTCO was added into 5 g of EP liquid under stirring at 10 °C for 30 min to form a good suspension (Figure 1b). Then, 1 g of curing agent was added into the above solution under stirring for 5 min and poured into a rubber mold. All of the specimens were cured at 25 °C for 24 h (Figure 1c), producing a series of EP/Cu-BTCO nanocomposites. 2.3. MOFs and EP/MOF-CO Composites Characterization. Xray diffraction (XRD) patterns of Cu-BTC-n MOFs were studied using

3. RESULTS AND DISCUSSION 3.1. Characterization of the Cu-BTC Hierarchical Porous MOFs Material. The XRD patterns of Cu-BTC-n m-MOFs are shown in Figure 2. All of the Cu-BTC-n samples exhibit broad X-ray diffraction (XRD) peaks, and all are in good agreement with the published literature data of HKUST-1 XRD patterns.30 The peak intensities are enhanced as the synthesizing temperature was increased to 125 °C, indicating that the

Figure 2. XRD patterns of the Cu-BTC-n hierarchical porous MOFs. 38147


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As the synthesizing temperature was increased, the pore sizes of Cu-BTC-n were 33.2, 52.7, and 68.4 nm for 45, 85, and 125 °C, respectively. The result indicated that the mesopores size of Cu-BTC-n was big enough to adsorb oleylamine macromolecules with the length of 2 nm,33 producing the nanocontainer for lubricant (Cu-BTC-n storing oleylamine). Therefore, in the next friction experiment, the Cu-BTC-45 with proper mesopores was selected as a nanocontainer for storing oleylamine. 3.2. Dispersion of Cu-BTCO. The dispersion of Cu-BTCO in the EP matrix was recorded using the TEM characterization, and the results are shown in Figure 5a−d, respectively. The

crystallinity of the Cu-BTC-n increased due to a difference in degree of hydration.31 The porous structure of Cu-BTC-n was investigated by N2 adsorption−desorption. As shown in Figure 3, N2 adsorption−desorption isotherms of Cu-BTC-n are a

Figure 3. N2 adsorption−desorption isotherms measured at 77 K of Cu-BTC-45, Cu-BTC-85, and Cu-BTC-125. The inset shows the corresponding pore size distribution curves calculated by the BJH method.

combination of type IV,32 which is according to the characteristic of mesoporous materials. The hysteresis loop in the range of 0.85 < P/P0 < 1.0 is further indicative of mesopores existing in Cu-BTC-n material. With the synthesizing temperature increasing, the pore volume and the pore size increase to 1.1 cm3·g−1 and 68.4 nm, respectively, which is also supported by the TEM characterization. TEM images of the mesoporous Cu-BTC-n in Figure 4a−c show that the obtained Cu-BTC-n contained a large number of agglomerates, which were assembled by nanoparticles. As for the lower synthesizing temperature, the sizes of nanoparticles and mesopores were small, as shown in Figure 4a. In addition, the sizes of nanoparticles and mesopores increased as the synthesis temperature was increased (Figure 4b,c). As the synthesis temperature was increased to 125 °C, the pores became larger and some macropores occurred. Cu-BTC-n with large mesopores was also characterized by SEM (Figure 4d−f).

Figure 5. TEM images of (a) EP/1 wt % Cu-BTCO, (b) EP/3 wt % Cu-BTCO, (c) EP/5 wt % Cu-BTCO, and (d) EP/10 wt % CuBTCO samples.

organic frameworks were composed of C, H, and O, which were the same as those of the EP matrix, and thereby they could not be observed in the TEM micrographs.34 Cu-BTC is assembled by worm-like disorder particles (diameters ∼ 40 nm), and these particles are composed of smaller nanoparticles (diameters = 2−3 nm)35 (as shown in Figure 4a). Also, the dispersion of smaller nanoparticles was examined to confirm

Figure 4. TEM images of the Cu-BTC-n prepared at (a) 45, (b) 85, and (c) 125 °C and SEM images of (d) Cu-BTC-45, (e) Cu-BTC-85, and (f) Cu-BTC-125. 38148


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Figure 6. Variation curves for COF as a function of the test time for the pure EP resin, EP with different contents of Cu-BTC, and EP with different contents of Cu-BTCO.

the dispersion of Cu-BTCO. The nanoparticles exhibit better dispersion in the EP matrix when the loadings of Cu-BTCO are 1, 3, and 5 wt % (Figure 5a−c). By increasing the percent of Cu-BTCO to 10 wt % (Figure 5d), the agglomeration of nanoparticles occurs, which reveals that a poor dispersion of Cu-BTCO in EP matrix is obtained. 3.3. Friction Measurement. Figure 6 shows the comparison of the variations of the COFs in 30 min for EP, EP with different Cu-BTC loadings, and EP with different CuBTCO contents. It indicates that the final COFs of the neat EP and its nanocomposites all reach approximately 0.5, which is attributed to the soft organic framework of Cu-BTC not bearing external forces. However, for EP/1 wt % Cu-BTC, the COF starts at 0.12 and transitions to about 0.5 after 390 s. When the Cu-BTC loading is 3 and 5 wt %, the transition times of the nanocomposites are approximately 1488 and 1703 s, respectively. This result suggests that the bonding strength between Cu-BTC and EP matrix is enhanced as the Cu-BTC contents increased, avoiding the EP matrix slipping and abrasion. As a result, the transition times of the EP composites increased. The EP with 10 wt % Cu-BTC exhibits the lowest transition time, which is attributed to its poor dispersion in the EP matrix, resulting in a porous surface with poor strength (as shown in Figure 7). Figure 6 also presents the evolution of the COF for EP with different Cu-BTCO, plotted as a function of time. It is noted that the variation in COF starts with a reduction period followed by a steady-state period except for the EP/10 wt % Cu-BTCO sample, which is partially due to the exudation of the oleylamine existing in the pores of the Cu-BTCO under external forces. Then, the oleylamine reacted with EP, leading to a thin lubricating film on the worn surface of EP during the steady-state period. The COFs of Cu-BTCO filled nanocomposites are lower than that of unfilled EP and decrease with increasing Cu-BTCO content. A lowest value of COF (0.03) appears at the Cu-BTCO content of 5 wt %, which is partially due to a great number of the oleylamine on the worn surface produced the continuous lubricating film between the composites and the friction ball, resulting in a better antifriction property. However, the composite prepared with 10 wt % CuBTCO exhibits a higher COF. This phenomenon indicates that

Figure 7. Wear rate of the pure EP and its nanocomposites.

a higher number of nancontainers with oleylamine resulted in the soft surface of the EP matrix. Therefore, the real contacting area of the friction pair could be increased under external forces, leading to the higher COF.36 To gain insights into the friction mechanisms, the surface morphology of the worn EP and its nanocomposites were examined using white light interferometer to extract detailed 2D surface topographies. The values of wear rate are also presented in Figure 7. The amount of small holes on the surface of composites increased linked to the addition of Cu-BTC increasing because the air existed in the pores of Cu-BTC escaped during the curing process of EP,37 resulting in the poor surface strength of the composites. Therefore, compared to that of neat EP, a larger wear rate was obtained as the addition of Cu-BTC increased. When the Cu-BTC content is 10 wt % of the EP, the wear rate of the composites is 13.1 × 10−5 mm3·(N m)−1, which is 2.82 times that of the neat EP (4.6 × 10−5 mm3· (N m)−1). However, for the addition of Cu-BTC ranging from 1 to 5 wt %, the wear rate of the composites reduced as the CuBTC loading increased. In light of this finding, it is postulated that the soft organic framework of Cu-BTC may slip and repair the worn area, keeping the rubbing interfaces smooth (as shown in Figure 8) to aid in further increase of the running-in period. 38149


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Figure 8. SEM images of the worn surfaces of (a) neat EP, (b) EP/1 wt % Cu-BTC, (c) EP/3 wt % Cu-BTC, (d) EP/5 wt % Cu-BTC, and (e) EP/ 10 wt % Cu-BTC samples.

The worn surface morphologies of neat EP and its composites filled with Cu-BTC were investigated by SEM in Figure 8. The morphologies of worn scar offered insight into how a material might behave in the process of friction. The area located between the two dotted yellow lines appears to be rather smoother than that of other worn areas. In Figure 8a, the most notable features are the layer detachment on the top surface of unfilled EP. Clearly, it is indicative for adhesivedelamination generated under the external load during reciprocating motion.38 Some cracks and small adhesion blocks on the surface were induced by the abrasion of the asperities of the ball counterpart (Figure 8b). Only some shallow grooves and small cracks can be seen on the worn surface in Figure 8c, and the smooth area extends compared to that of EP/1 wt % Cu-BTC in Figure 8b, indicating slight abrasive wear existed. Only slight scuffing appeared on the surface of the EP with 5 wt % Cu-BTC (Figure 8d). These results reveal that the soft organic framework of the Cu-BTC filler might pull out of the EP matrix and transform into wear debris, which adhered to the surface of the composites and developed the patches of compacted material.39 However, there are cracks exiting on the worn surface of the composites with 10 wt % Cu-BTC (Figure 8e) because the poor surface strength led to deterioration of friction property, causing a larger wear volume and a higher COF (as shown in Figure 7 and Figure 6, respectively). The worn surface morphology of EP with Cu-BTCO nanocomposites is shown in Figure 9. The surface appears smooth, and no cracks are seen except for Figure 9d. The yellow arrow lines indicate the lubricating film (in Figure 9a− c), which was characterized by several discontinuous pieces on the surface of composites. These lubricating films are mainly located in the frictional contact area, acting as lubricant and resulting in the ultralow friction. As the Cu-BTCO content is increased, the lubricating film became continuous due to the exudation amount of oleylamine increased. In Figure 9d, severe damage occurred on the composite surface; the frictional contact area was about 1.8 mm, and the worn area was bigger. It is because the Cu-BTC containing liquid lubricant weakened the whole strength of the composites, resulting in a softer

Figure 9. SEM images of the worn surfaces of (a) EP/1 wt % CuBTCO, (b) EP/3 wt % Cu-BTCO, (c) EP/5 wt % Cu-BTCO, and (d) EP/10 wt % Cu-BTCO samples.

surface. The ball was press into the composite matrix, leading to a larger contact area, and the wear was severe. The process of the in situ produced lubricating films is shown in Figure 10. The passive EP matrix is incorporated by feedback active nanocontainers of Cu-BTCO. The nanocontainers storing lubricant can provide fast response to the changes caused by external stress. When the composite experienced the loading stimuli, the stored oleylamine in the pores of CuBTCO would be squeezed out (Figure 10a). During the reciprocating motion, the released oleylamine from the CuBTC nanocontainer effectively lubricated the rubbing surfaces, reducing the COF dramatically. As the sliding proceeded, the liquid lubricant of oleylamine reacted with the surface of EP and transferred to some pieces of lubricating films (Figure 10b). Figure 10d shows X-ray photoelectron spectroscopy (XPS) analysis of the lubricating film on the worn surface. It is seen that the N 1s peak is fitted into two bands: sp2 CN 38150


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Figure 10. Lubrication mechanism diagram of (a) oleylamine release, (b) the formation of lubricating film, (c) tribochemical reaction, and (d) XPS analysis results of N 1s of the lubricating film.

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(400.6 eV) and the pyridine-like C−NC bond (398.6 eV).40 The C−NH2 bonds in oleylamine have been transformed into CNH or C−NC bonds, indicating that the polymerization reaction between EP and oleylamine successfully happened, producing the lubricating film on the composite surface. This evidence shows that additional reaction happened between oleylamine and EP under the mechanical and thermal stimuli during sliding (Figure 10c).41 The generated lubricating film lessened the direct solid−solid contact, resulting in an ultralow and stable COF.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.X.). *E-mail: [email protected] (D.G.). ORCID

Guoxin Xie: 0000-0001-7406-3432 Dan Guo: 0000-0002-7681-2377 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51475256, Grant No. 51705277, and Grant No. 51775044), and the China Postdoctoral Science Foundation (Grant No. 2016M601011).

4. CONCLUSIONS In summary, the concept of novel lubricating material based on the implementation of Cu-BTC m-MOFs nanocontainers stored with oleylamine into the EP matrix has been proposed and successfully realized. The important performance of the new lubricating materials is the quick self-response to changes of external factors. In response to the mechanical and thermal stimuli during the sliding process, the oleylamine entrapped in Cu-BTC was released and improved the lubrication of the nanocomposite. Based on SEM characterization, there are some lubricating films mainly located in the frictional contact area acting as lubricant, which was in situ produced by the reaction between the released oleylamine and the friction pair, leading to an ultralow friction (COF, 0.03). Also, EP/5.0 wt % Cu-BTCO nanocomposite exhibited excellent self-lubricating properties compared with the pure EP. Therefore, Cu-BTC m-MOFs nanocontainers are able to store liquid lubricant and immediately release it to reduce COF in the friction process.

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REFERENCES

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Ultralow Friction Self-Lubricating Nanocomposites with Mesoporous

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