Superior Lubricity and Antiwear Performances Enabled by Porous

Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 12527−12535

pubs.acs.org/journal/ascecg

Superior Lubricity and Antiwear Performances Enabled by Porous Carbon Nanospheres with Different Shell Microstructures Qian Ye,†,‡,# Sha Liu,†,# Jin Zhang,† Fei Xu,*,† Feng Zhou,*,†,§ and Weimin Liu†,§ †

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State Key Laboratory of Solidification Processing, Center of Advanced Lubrication and Seal Materials, School of Materials Science and Engineering, Northwestern Polytechnical University, No.127, Youyi West Road, Xi’an, 710072, P. R. China ‡ Research & Development Institute of Northwestern Polytechnical University in Shenzhen, Shenzhen, 518057, P. R. China § State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, No. 18, Tianshui Middle Road, Lanzhou, 730000, China S Supporting Information *

ABSTRACT: Good dispersibility and proper structural strength are both critical to the use of nanomaterials as lubricant additives to improve antiwear and friction-reducing ability. Herein we report the synthesis of porous carbon nanospheres (PCNs) with distinct shell properties and demonstrate their significance and effectiveness in enhancing the tribological performances when used as lubricant additives. PCNs with a carbonaceous resin-based shell and an amorphous disordered/graphite phase hybridized carbon shell have been successfully prepared based on a facile template-free interfacial polymerization and carefully selected carbonization temperatures. A low carbonization temperature (≤700 °C) leads to PCNs with a carbonaceous resin shell, which helps to improve dispersibility in the base oil PAO-10. Therefore, the oil film effect is dominant and the coefficient of friction can be reduced from 0.198 to 0.085 after adding 0.4 wt % carbonaceous resin. Amorphous disordered/graphite phase hybridized carbon shells, obtained at higher temperatures (>700 °C), possess good structural strength and robustness, thus resulting in excellent friction-reducing performance (friction coefficient of 0.086), high load capacity (950 N), and reduced wear volume (80%) at the additive content of 0.4 wt %. These findings will open up new possibilities for achieving good tribological performance by using polymer-derived carbon materials. KEYWORDS: porous carbon nanospheres, carbonaceous resin, amorphous carbon, tribological performances

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INTRODUCTION Improving the lubricity and antiwear performances in moving mechanical systems has sparked much attention during recent years because friction and wear are still the main reason for mechanical energy consumption. The oil viscosity and the additive are two key parameters for controlling the friction and wear of lubricants. Nanoparticle additives exhibit superior tribological properties compared with traditional lubricant additives due to size effect, colloidal effect, protective film, and third body effect.1,2 However, many lubricating oil additives are susceptible to catastrophic failure due to poor dispersibility and inappropriate structural strength.3,4 Nanoadditives easily aggregate in most base oils owing to the well-known nano effect (i.e. high surface energy leads to the aggregation of nanoparticles, and van der Waals interactions). The method of maintaining a good dispersion is generally achieved via chemical modification.5−7 The carbon materials generally exhibit relatively low friction and good wear resistance under most conditions, making them suitable for various tribological applications in both dry lubricants and oil lubricants.8−11 The tribological properties © 2019 American Chemical Society

of these carbon materials are nothing short of astonishing; for example, multilayer graphene nanoflakes exhibit friction coefficients as low as 0.001.12 Nanocarbon materials including fullerenes, carbon nanotubes (CNTs), graphene, and nanodiamonds are strong candidates for possible application as lubricant additives in nanotribology due to their good lubricity and self-lubricating properties.13 As known, graphene is a good solid lubricant which has been used in lubricating oil additives to achieve a very low coefficient of friction or even superlubricity under lower load conditions.14,15 CNT additives easily fail because the excessively long CNTs tend to entangle during the friction process.16 For nanocarbon materials as lubricant additives, 3D-diamonds (sp3 carbon) display good antiwear effect and high load capacity, while 2D-graphite (sp2 carbon) has excellent friction reduction effect under low load.17−20 Many researchers combine graphite with diamonds Received: April 24, 2019 Revised: May 28, 2019 Published: June 11, 2019 12527

DOI: 10.1021/acssuschemeng.9b02288 ACS Sustainable Chem. Eng. 2019, 7, 12527−12535

Research Article

ACS Sustainable Chemistry & Engineering

Scheme 1. Preparation of Porous Carbon Nanospheres as Lubricant Additives and the Evaluation of Their Antifriction and Antiwear Performances

Figure 1. (a) SEM image, (b) TEM image, (c) size distribution histogram, and (d) TG curve of PACP.

techniques have been developed to generate PCNs.35−37 Lu et al. have used starch as a carbon precursor and ammonium ferrous sulfate as a porogen to obtain porous carbon spheres via hydrothermal reaction.36 Osman et al. have gained porous micron carbon spheres from a zinc-based metal organic structure via carbonization and HNO3 treatment.37 Stable porous nanostructures with adjustable porosity can be obtained via the aforementioned method; however, most approaches rely on high-cost, tedious templates, multistep

to generate sp2-sp3 carbon composites to achieve an overall tribological effect and even superlubricity.21−23 Porous carbon nanospheres (PCNs), as a class of innovative advanced nanocarbon material, have been widely used in applications including adsorption−separation,24−26 energy storage,27,28 catalysis,29−31 and drug loading32 due to the controllable structure, excellent physicochemical stability, low density, and good conductivity. However, the production of uniformly monodispersed porous carbon spheres with small size remains a great challenge.33,34 Up to now, several 12528


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Figure 2. (a) SEM image, (b) TEM image, and (c) size distribution histogram of PCN-500-5. (d) SEM image, (e) TEM image, and (f) size distribution histogram of PCN-900-3.

Figure 3. (a) N2 adsorption−desorption isotherms. (b) XPS survey spectrum and N 1s scan spectrum of PCN-500-5. (c) XPS survey spectrum and N 1s scan spectrum of PCN-900-3. (d) XRD patterns. (e) Raman spectra of PCN-500-5 and PCN-900-3 with Lorentzian−Gaussian fit multipeak analysis of PCN-900-3. (f) FT-IR spectra.

resin shells were obtained, whereas PCNs with amorphous carbon shells were prepared at higher temperature. Owing to incomplete carbonization at low temperature, the organic phase that remained in the carbonaceous resins will help to form well-dispersed nanospheres in organic lubricating oils. At an elevated annealing temperature, the organic phase transfers into amorphous carbon, which contains the hybrid of disordered/graphite carbon microstructure. Consequently, these as-prepared carbon materials derived from organic

synthetic procedures, and sometimes even costly equipment, which will preclude their further application. In this work, we propose a template-free strategy and simple carbonization process to prepare highly monodisperse porous hollow carbon nanospheres with high surface area and low nanodiameter for lubricant technology. The as-obtained PCNs are classified into carbonaceous resins and amorphous carbon depending on the carbonization temperature. With a carbonization temperature below 700 °C, PCNs with carbonaceous 12529


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ACS Sustainable Chemistry & Engineering Table 1. Parameters Determined by Raman Spectra of PCNs peak area

content (%)

sample

D

A

G

D

A

G

IG/ID

La (nm)

PCN-800-3 PCN-900-3 PCN-1000-3 PCN-1100-3 PCN-1200-3

446975 734216 366889 634021 339058

58362 56479 19468 31701 22451

93588 187614 98884 185567 107312

75.50 75.10 75.60 74.50 72.30

8.80 5.80 4.00 3.70 4.80

15.70 19.20 20.40 21.80 22.90

0.21 0.26 0.27 0.29 0.32

0.91 1.11 1.17 1.27 1.38

nitrogen uptake at relative pressure P/P 0 below 0.1 demonstrates the existence of numerous micropores. A smaller adsorption at P/P0 ranging from 0.4 to 0.9 is attributed to the capillary condensation of nitrogen in the large mesopores. The BET surface area (SBET) of PACP is only 25 m2 g−1. PCN-5005 and PCN-900-3 exhibit relatively high specific surface area of 335 and 588 m2 g−1, respectively (Table S1), owing to the existence of micropores and mesopores. The total pore volume (Vt) of PCNs increases with an increase in carbonization temperature. The main elements in PCNs were C, N, and O (taking PCN-500-5 and PCN-900-3 samples as examples; see Figure 3b,c). A single N 1s peak at 397 eV is detected in the precursor PACP, and the content of N in the PCNs decreases significantly with an increase in carbonization temperature (Figure S2a). The change of N 1s spectrum with the elevated carbonization temperature is shown in Figure S2b. The N 1s spectrum of PCN-500-5 can be deconvoluted into two peaks at 398.4 and 400.3 eV (Figure 3b, inset), which can probably be assigned to −NH2 and NH derived from the skeleton of aniline and pyrrole copolymers, respectively. For PCN-900-3 (Figure 3c, inset), the peak at 398.2 eV is associated with pyridine-type N atoms (N-6) existing at the edge of the graphene sheets, and the peak at 400.9 eV can be ascribed to quaternary N atoms (N-Q) incorporated in the graphene sheets.38 The N-Q peak gradually increase while the N-6 peak decreases slowly at elevated carbonization temperature (Table S2). At about 1100 °C (PCN-1100-3), the N 1s in the material exists only in the form of N-Q (Figure S2b). The structure and phase transformation process of the obtained PCNs were studied by X-ray diffraction (XRD). The standard XRD pattern of amorphous carbon displays two peaks at around 25° and 44°, which are the corresponding (002) and (100) crystal planes, respectively.41 For all the samples, the (002) peaks are detected (Figure 3d). The (002) crystal plane gradually approaches 25° in the carbon resin due to the better ordered structure with an increase in carbonization temperature from 300 °C to 700 °C. Further increase in the carbonization temperature above 700 °C basically stabilized the (002) plane at 25° and the (100) diffraction peaks appear (Figure S3b). The weak and wide (002) peaks in the XRD curves indicate a low concentration of parallel monolayers in the resulting PCN carbon frameworks,38,42 demonstrating the presence of disordered-graphitic composite carbon. From Bragg’s Law, the spacing of the (002) crystal plane (Table S3) can be calculated, which approaches the standard graphite D(002) 0.335 nm (Table S3).43 To further explore the microstructure of PCNs, Raman analysis of the as-prepared PCNs was carried out. The two peaks located at 1350 and 1580 cm−1 match the band expected for the D (disordered) mode and G (graphitic) mode.38,44 As the carbonization temperature increases, these two peaks gradually separate and become more pronounced (Figure 3e).

polymers exhibit improved antiwear and lubricating properties as PAO-10 additives.

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EXPERIMENTAL SECTION

A description of preparation procedures, material characterizations, and tribology performance measurement of the porous carbon nanosphere-based organic polymer are provided in Supporting Information.

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RESULTS AND DISCUSSION As shown in Scheme 1, poly(aniline-co-pyrrole) (PACP) was fabricated by using the strategy of confined interfacial copolymerization of aniline and pyrrole in the presence of Triton X-100 micelles.38,39 Subsequently, the obtained precursor was transferred to an inert atmosphere for carbonization to obtain PCNs with microporosity in the shell. The hollow nanospherical structure can be well preserved after high temperature carbonization, due to the robust conjugation of the precursor pyrrole and aniline copolymer. The obtained PACP and PCNs exhibit a good nanospherical morphology (Figures 1a,b, 2a,d, and S1a−c). The size of the precursor PACP ranges from 120 to 180 nm, with a peak centered at 151 nm (Figure 1c). As shown in Figure 1d, the decomposition of PACP started at about 256 °C, and a rapid weight loss was observed before 618 °C. This is mainly due to the occurrence of organic polymer decomposition at this temperature range. Above this threshold temperature, the organic polymeric portion was decomposed and further carbonized, as reflected by the weight loss caused by carbonization with partial graphitization. Therefore, the PCNs obtained by carbonization below 700 °C contain carbonaceous resin shells. The PCNs carbonized above 700 °C are denoted as amorphous carbon which bear the hybrid of disordered/graphite phase. With the elevated temperature carbonization, a significant narrowing of the particle size range as well as the average particle size (Figure 2c,f) can be observed, suggesting that more uniformly distributed carbon spheres are obtained. External diameters of PCN-500-5 and PCN-900-3 are approximately 122 and 107 nm, respectively. The size of PCNs gradually decreases with increasing carbonization temperature. PCNs show good hollow nanosphere structure with a shell thickness of about 30−35 nm and an interior diameter of about 60 and 43 nm for PCN-500-5 and PCN-900-3, respectively (Figures 2b,e). N2 adsorption−desorption isotherms were recorded to examine the pore structures of PCNs. According to classification of the International Union of Pure and Applied Chemistry, the porous material possesses type I/IV isotherms, which are characteristic of monolayer adsorption on microporous solids, and a hysteresis loop due to the existence of hierarchical pores including micropores (50 nm).40 As shown in Figure 3a, among samples of PCN-500-5 and PCN-900-3, a very high 12530


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Figure 4. (a, b) Load capacity (temperature, 200 °C, stroke, 1 mm; frequency, 25 Hz). (c, d) Coefficient of friction (load 100 N; temperature, 25 °C; stroke, 1 mm; frequency, 25 Hz). (e, f) Wear volume. ((A) PCN-300-5, (B) PCN-400-5, (C) PCN-500-5, (D) PCN-600-5, (E) PCN-700-5); (A1) PCN-800-3, (B1) PCN-900-3, (C1) PCN-1000-3, (D1) PCN-1100-3, (E1) PCN-1200-3). Curve-fitted XPS spectra of (g) C 1s, (h) O 1s, and (i) Fe 2p of the worn surface lubricated with 0.4 wt % PCN-500-5.

Although no peak for A (amorphous) mode is detected, the peak asymmetric broadening and merging of the two peaks give clear indication for the coexistence of D, A, and G modes. In the case of PCN-900-3 as an example (Figure 3e), three bands located at around 1580, 1500, and 1350 cm−1 can be deconvoluted, corresponding to the G mode, A mode, and D mode, respectively. The Lorentzian−Gaussian fit analysis of the Raman spectra was also performed (Figures S4). A good agreement between the experimental results and the fitted curves can be achieved, confirming the coexistence of disordered, graphitic, and amorphous carbon. The microcrystalline planar crystal size La can be calculated using the empirical formula found by Tuinstra and Koenig: La = 4.35IG/ ID (nm),38 where IG and ID are the area of G band and D band. The degree of graphitization was calculated according the content of graphitic carbon (IG band intensity) shown in Table 1. Tribology Performances of PCNs. From Figure S5, according to the result of the contact angles of PAO-10 on the PCN surface (CA = 25.7°), the prepared porous carbon sphere was a lipophilic material; thus, the as-prepared PCN additive

can exhibit a good dispersibility in PAO-10. Digital pictures shown in Figure S6 are the suspensions of PACP and resulting PCNs additives with a content of 0.2 wt % dispersed in the base oil PAO-10. The precursor PACP was well dispersed in PAO-10, and little precipitation was observed even after 30 days of standing. A good dispersibility was also observed for PCN-500-5, in which the solution started to precipitate after 25 days. In contrast, PCN-900-3 showed relatively poor dispersibility, in which the dispersion began to stratify after 20 days. Under low annealing temperature, the organic content is maintained, thus giving rise to better dispersibility. This can be proved by FT-IR analysis. Figure 3f shows the FT-IR spectra of PCNs carbonized at different temperatures. All samples contain several common functional groups from the PACP framework, including N−H (at 3430 cm−1), CC (at 1625 cm−1), and C−N (at 1250−1340 cm−1). However, more peaks can be observed in PACP, PCN-500-5, and PCN-700-5 in comparison with the spectrum of PCN-900-3. The bands between 2950 and 2840 cm−1 correspond to the C−H stretching vibrations, while N−H is also clearly observed at 753 cm−1. Over a range from 1500 to 500 cm−1 in the PACP 12531


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PCNs. The COF of 0.2 wt % PCN-900-3 as additive is 0.092, and its wear resistance is 850 N at 200 °C. Amorphous carbon combined with disordered and graphitic carbon has suitable structural strength and eventually results in an improved antifriction and antiwear effect. Figure S8 shows the morphology of a typical wear scar, in which a wide and deep mark can be detected when there are no additives. After addition of PCNs to the base oil, the wear scar become much narrower and lighter. The as-obtained PCN-500-5 exhibits a good antiwear effect, and the wear volume decreases from 4.1 × 105 μm3 to 1.2 × 105 μm3 after adding 0.4 wt % carbonaceous resins. When 0.2 wt % of PCN-900-3 is added to the base oil, the wear volume can be dramatically reduced to 5.2 × 104μm3 (Figure 4e,f). To further explore the formation of the oil film on the worn surface, we probed the worn surface lubricated with PAO-10 containing 0.4 wt % PCN-500-5 by XPS analysis. As shown in Figure 4g, C−C and CC signals are derived from PCN-5005 or the residual base oil, while CO and C−O are attributed to the reaction with air during friction. The peak at 399.8 eV is mainly ascribed to the residual of N in PCN-500-5 or the formation of nitrogen-containing groups (Figure S9). The XPS spectrum of O 1s in Figure 4h contains three peaks corresponding to iron oxide (529.2 eV), ferrous carbonate (530.9 eV), and organic CO (531.8 eV).51,52 As shown in Figure 4i, the Fe peaks are located at 712.0 and 714.4 eV, which are attributed to Fe2O3 and FeCO3, respectively.52,53 In addition, FeO (at 709.8 eV)54 is also detected in the Fe 2p spectra. The above results reveal that the oil film was formed on the friction surface, probably due to physicochemical reaction. From some residues (C and N) detected by XPS, a physical film such as carbon film or nitrogen film was formed by mechanical action such as extrusion or friction. In addition, tribochemical reactions occurred on the worn surface during the friction and wearing process, leading to the formation of a boundary lubrication film composed of FeO, Fe2O3, FeCO3, and some nitrogen-containing compounds. Tribofilm which adhered to the friction surfaces of steel helps to achieve excellent tribological properties and protect the sliding interface under certain harsh conditions. To further study the tribological performance of the asprepared PCNs materials in detail, the typical materials PCN500 and PCN-900 delivering high load capacity were selected as additives. The effect of carbonization time for preparing PCN-500 and PCN-900 as well as their additive amount is investigated in detail. Carbonaceous resin PCN-500-5 carbonized at 500 °C for 5 h can achieve good friction reduction properties, in which load capacity can reach 750 N and the COF as low as 0.085 (Figure S10a−c). For amorphous carbon PCN-900-3, the load capacity is up to 950 N, and the COF is close to 0.086 with 0.4 wt % of PCN-900-3 in the base oil; meanwhile, the corresponding wear volume can be reduced by 80% (Figure S10d−f). The COF and antiwear of the asprepared PCNs as nanoadditives are impressive compared to that of commercial CNTs, graphite, and carbon black.55,56 In our test, as shown in Figure S11, the as-prepared PCNs show significant advantages over commercial carbon materials (include carbon black, carbon nanotubes, and graphite) under the same test conditions (addition amount, 0.4 wt %; temperature, 25 °C; stroke, 1 mm; frequency, 25 Hz).

spectrum, many other peaks show up. As the carbonization temperature increases, some functional groups gradually disappear. Above the temperature of 700 °C, these organic functional groups in the PCNs no longer exist. These results can explain why carbonaceous resins show good dispersibility in base oil PAO-10. The FT-IR analysis is also consistent with the results of the TG curve and XPS spectra. PAO-10 had a load capacity of 450 N at 200 °C. Figure 4a shows the load capacity of PAO-10 oil when adding carbonaceous resins as an additive. The carbonaceous resins show an irregular contribution to the load capacity, and a maximum of 750 N can be reached when using PCN-500-5 as an additive (0.4 wt %). Figure 4b depicts the load capacity of PAO-10 oil when using amorphous carbon as an additive. All amorphous carbon samples significantly improve the load capacity of the base oil, and the antiwear effect is also better than that of the carbonaceous resins as a whole. This can be understood as follows: amorphous carbon generated at high temperature leads to a more robust structure, which is advantageous in the antiwear test. The highest load capacity was found when the content of amorphous carbon was 0.4 wt %, which is the same as that of carbonaceous resins. When the content is higher than 0.4 wt %, the load capacity of the base oil will eventually drop to about 550 N. The excessive additive in base oil decreases the load capacity. This was attributed to the occurrence of the adhesion effect and the furrow effect45−47 (that is forming agglomerate blocks in the friction interface reduces the stuck load value) during the friction process. The maximum load could reach up to 850 N when the base oil contained 0.4 wt % PCN-900-3 or 0.2 wt % PCN1200-3. The coefficient of friction (COF) of all PCN-based nanolubricants including base oil was measured at a load of 100 N at room temperature (25 °C). From Figure S7a, the base oil PAO-10 has a COF of about 0.093 at the beginning; subsequently, the COF suffers an abrupt increase and then fluctuates greatly around 0.198 after several minutes. The carbonaceous resin PCN additive can extend the effective working time of the base oil to some extent after adding 0.2 wt %. However, the COF decreases slowly to an extremely low value after adding 0.4 wt % carbonaceous resins in base oil. The carbonaceous resin-based nanolubricants experienced a running-in period from 8 to 12 min (COF is about 0.108), and then the friction coefficient dropped to a minimum value of 0.082 (Figures 4c and S7b), which can be considered as the polishing effect.48 As shown in Scheme 1, because of the uneven surface of the steel block, the surface roughness peak is first polished during the running-in period and then a smooth friction period starts. On the other hand, the oil film effect49,50 is dominant. When carbonaceous resins were used as additives, the poor dispersibility induced by higher carbonization temperatures led to an increased COF. However, the minimum COFs for different carbonaceous resin additives are very close. The COF of the base oil eventually decreases to 0.082, 0.084, 0.085, 0.087, and 0.096 after adding 0.4 wt % of PCNs-300-5, PCN-400-5, PCN-500-5, PCN-600-5, and PCN-700-5, respectively (inset in Figure 4c). When using amorphous carbon as a lubricant additive (Figures 4d and S7c), there is almost no running-in period and the friction curves are relatively more stable than that of carbonaceous resins. Besides the oil film forming effect, the more dominant mechanism responsible for friction reduction switches to the hybridization of disordered/graphitic carbon in 12532


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Author Contributions

CONCLUSIONS AND OUTLOOK In summary, this work demonstrates the possibility of fabricating two types of PCNs: carbonaceous resins and amorphous carbon, through a facile template-free method by changing the carbonization temperature. The as-prepared PCNs exhibit controllable and uniform nanospherical morphology. As revealed by XRD, FT-IR, XPS, and Raman measurements, the carbonaceous resins render good dispersibility owing to the existence of the organic phase, while the amorphous carbon possesses good structural strength due to the formation of disordered/graphite phase hybridized carbon under higher carbonization temperatures. These as-obtained PCNs could both improve the tribological performance of PAO-10 when used as additives. The mechanisms governing the tribological performance are different: The dominant effect for carbonaceous resins is the oil film effect, while the structural robustness coming from disordered/graphite hybridized carbon is decisive for amorphous carbon. The carbonaceous resin additives can significantly reduce the COF of the base oil, and the COF can be reduced from 0.198 to 0.085 after adding 0.4 wt % PCN-500-5. The amorphous carbons obtained by controlling carbonization conditions (temperature and duration) achieved excellent friction-reducing performance and high load capacity. When the concentration was 0.4 wt %, the load capacity of PCN-900-3 was 950 N and the COF was as low as 0.086, and the corresponding wear volume could be reduced by 80%. Our work opens a new avenue toward the preparation of nanocarbon materials as lubricant additives and expands the application area of carbon material based on organic polymers. Furthermore, the porous hollow spaces in these PCNs could be used to load or absorb appropriate antioxidants, corrosion inhibitors, and self-healing materials to achieve multifunctional lubricant additives, and this will be the subject of our future studies.

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key Research and Development Program of China (2018YFB0703802), Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20170306153027078), NSFC (51702262), the China and Shaanxi Province Postdoctoral Science Foundation (2018T111094, 2018M643734, 2018BSHYDZZ57), and the Fundamental Research Funds for the Central Universities (3102019gx001, 31020195C001, 3102019JC005).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b02288. SEM images of PACP, PCN-500-5, and PCN-900-3, XPS spectra, N 1s spectra, and XRD spectra of asprepared PCNs, the dispersion pictures of PACP, PCN500-5, and PCN-900-3 in PAO-10, the friction coefficient and wear volume of as-prepared PCNs, the friction coefficient of commercial carbon materials, the report of pore parameters for typical PCNs, and the diffraction angle and D(002) of amorphous PCNs (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Fei Xu: 0000-0003-2446-8903 Feng Zhou: 0000-0001-7136-9233 Author Contributions #

These authors contributed equally to this work. 12533


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Superior Lubricity and Antiwear Performances Enabled by Porous

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