Fabrication of Two-Dimensional Functional Covalent Organic Frameworks via the Thiol-Ene "Click" Reaction as Lubricant Additives for Antiwear and Friction Reduction

The Study of the Inhibitive Behavior of COF-5 Lubricant Additives on Wear and Corrosion of the Tin Bronze Friction Pair during the Current-Carrying Process

COF-5 is an excellent choice as a lubricant additive due to its high specific surface area and regular layer stacking structure. In this paper, the porous covalent organic frameworks of the COF-5 nanomaterial were successfully synthesized as a lubricant additive via a method of sonochemical synthesis. Pure [BMIM][SCN] (BS) was used as the base oil, and the tribological properties of the COF-5-BS composite lubricant were evaluated by using a ball-on-disk wear tester. The results were demonstrated that the wear volume of BS with the addition of 0.1 wt % COF-5 was reduced by 46.64% as compared to that of the pure BS, and the corrosion on the friction interface was effectively inhibited by the addition of COF-5 nanomaterial. This phenomenon was ascribed to COF-5 with a large specific surface area, being adsorbed onto the friction interface, which could repair the wear-induced pits and prevent the direct contact between the metal substrate and the corrosive medium. The tribological properties of the COF-5-BS composite lubricant could be efficiently regulated under the applied electric current. The results of intelligent electric current-controlled friction were indicated that the average coefficients of friction (COF) under the current stimulation of 30 mA with on/off cycling were reduced by 12.05% as compared to that of a continuous current operated for 1 h. Moreover, the wear volume and depth were reduced by 11.69% and 8.75%, respectively. That is because the short duration of current regulation could enhance the rapid formation of a lubricant film composed of BS liquid and COF-5 nanosheets on the friction interface. Meanwhile, the temperature on the interface would not be high to produce the failure of lubricating film, resulting in an excellent low friction coefficient and excellent antifriction property.

A critical review of COFs-based photocatalysis for environmental remediation

Covalent organic frameworks (COFs) are highly porous crystalline polymers formed through covalent bonding of molecular building blocks. Numerous fabrication strategies have been developed, including solvothermal, ionothermal, microwave, mechanochemical, and sonochemical methods, alongside ligand substitution and post-modification techniques, which allow for precise control over the structures and properties of COFs. The exceptional physicochemical stability, large specific surface area, broad visible light absorption, and extended π-conjugated systems have sparked significant interest in photocatalytic applications. Recently, COFs have shown remarkable efficacy in environmental remediation, demonstrating the ability to degrade a wide range of organic pollutants, including dyes, antibiotics, and drugs, as well as to reduce/oxidize heavy metals such as Cr(VI), U(VI), and As(III), in addition to targeting biological pollutants. This review comprehensively explores recent advancements in COFs-based photocatalysis, covering synthetic methods, COF types, modification method, theoretical calculations, environmental applications, and underlying mechanisms. Additionally, the challenges and opportunities for COFs as a robust, cost-effective technology in practical applications was discussed, and offering valuable insights for researchers in environmental remediation, materials science, and photocatalysis.

Multielement-Doped Porous Carbon Nanosheets Derived from Metal-Organic Frameworks as Lubricating Additives for Antifriction and Antiwear Properties

The development of advanced lubricant additives is crucial for improving the tribological properties in mechanical systems. Doping has been certified as one of the effective methods for enhancing the lubricating properties of carbon nanomaterials. Herein, multielement (B, N, P)-doped porous carbon nanosheets (B, N, P@PCNs) were successfully prepared using ionic liquid-assisted pyrolysis of "copper-pyridine" coordination metal-organic frameworks (Cu-MOFs). The as-obtained B, N, P@PCNs exhibit good friction-reducing and antiwear behavior as lubricant additives. Compared to base oil 500SN, the friction coefficient was reduced to 0.104, resulting in a 77.5% decrease in wear volume, and the load-bearing capacity increased from 150 to 550 N simultaneously. Through Raman and XPS analysis, it was ascertained that the boron, nitrogen, and phosphorus elements actively participate in tribological chemical reactions, resulting in the formation of a protective film, which significantly mitigates wear and tear of friction pair by combining adsorption, restoration, and polishing mechanisms.

Impact of Postsynthetic Modification on the Covalent Organic Framework (COF) Structures

Covalent organic frameworks (COFs) have emerged as a versatile class of materials owing to their well-defined crystalline structures and inherent porosity. In the realm of COFs, their appeal lies in their customizable nature, which can be further enhanced by incorporating diverse functionalities. Postsynthetic modifications (PSMs) emerge as a potent strategy, facilitating the introduction of desired functionalities postsynthesis. A significant challenge in PSM pertains to preserving the crystallinity and porosity of the COFs. In this study, we aim to investigate the intricate interplay between PSM strategies and the resulting crystalline and porous structures of the COFs. The investigation delves into the diverse methodologies employed in PSMs, to elucidate their distinct influences on the crystallinity and porosity of the COFs. Through a comprehensive analysis of recent advancements and case studies, the study highlights the intricate relationships among PSM parameters, including reaction conditions, precursor selection, and functional groups, and their impact on the structural features of COFs. By understanding how PSM strategies can fine-tune the crystalline and porous characteristics of COFs, researchers can harness this knowledge to design COFs with tailored properties for specific applications, contributing to the advancement of functional materials in diverse fields. This work not only deepens our understanding of COFs but also provides valuable insights into the broader realm of PSM strategies for other solid materials.

Fabrication of Ag Nanoparticle-Embedded Covalent Organic Frameworks for Oil Gel with Long-Term Stability and High Lubrication Performance

In previous reports, covalent organic frameworks (COFs) have demonstrated significant potential as lubricant additives. Herein, we embedded Ag nanoparticles in the DT-COF (polycondensation polymer of 2,5-dihydroxyterephthalaldehyde and 4,4',4″-(1,3,5-triazine-2,4,6-triyl) trianiline) matrix via the ball milling method and utilized this composite (Ag@DT-COF) as an additive for supermolecule oil gel. The low molecular weight gelator effectively mitigates the dispersion challenges of COFs in lubricant oil, while the embedded Ag nanoparticles enhance the repairing effect and antipressure performance of the lubricant. The resulting Ag@DT-COF gel exhibits a reduction in the average friction coefficient and wear volume of base oil by 46.0% and 87.5%, respectively, and increases the load-carrying capacity to 750 N. The remarkable tribological properties are attributed to the easy adsorption of DT-COF, antiwear characteristic of Ag nanoparticles, and the gelator that ensures the long-term stability of oil gel.

Covalent-Organic Framework Nanomaterials: Energy Band Engineering Generating Ultrathin Lubrication Films for Excellent Lubrication

It is, in fact, inevitable for steel to be covered with a layer of iron oxides and/or peroxides on its surface. However, knowledge of its existence and functionality for tribological behaviors is usually ignored. Herein, covalent-organic framework nanomaterials (CONs) composed of three well-screened acceptors and a donor through the imide linkage were fabricated to explore their lubrication performances. The results indicate that the energy-level matching between CONs and iron oxides or peroxides leads to the formation of a Z-scheme heterojunction structure at the rubbing interface. Also, the friction produces an internal electric field in the heterojunction, which drives the negative atomic/ionic species from the sliding interface to immigrate into the pore of CONs and resettle inside to engender the pinning effects, producing a fixed lubrication layer. Synchronously, it also attracts the free CONs in the base oil to form an easy-shear lubrication layer assembling onto the fixed one, producing a lubrication film with two layered configurations. Finally, the unique lubrication film, despite its thickness of a dozen nanometers, still exhibits impressive friction reduction and antiwear. This finding will inspire the technology to utilize the intrinsic surface nature of steel materials to exploit lubricant additives or modulate tribological behaviors.

Covalent organic frameworks in tribology - A perspective

Two-dimensional covalent organic frameworks (2D COFs) are an emerging class of crystalline porous materials formed through covalent bonds between organic building blocks. COFs uniquely combine a large surface area, an excellent stability, numerous abundant active sites, and tunable functionalities, thus making them highly attractive for numerous applications. Especially, their abundant active sites and weak interlayer interaction make these materials promising candidates for tribological research. Recently, notable attention has been paid to COFs as lubricant additives due to their excellent tribological performance. Our review aims at critically summarizing the state-of-art developments of 2D COFs in tribology. We discuss their structural and functional design principles, as well as synthetic strategies with a special focus on tribology. The generation of COF thin films is also assessed in detail, which can alleviate their most challenging drawbacks for this application. Subsequently, we analyze the existing state-of-the-art regarding the usage of COFs as lubricant additives, self-lubrication composite coatings, and solid lubricants at the nanoscale. Finally, critical challenges and future trends of 2D COFs in tribology are outlined to initiate and boost new research activities in this exciting field.

Structural design of thiadiazole-based donor-acceptor COF/Fe-doped N vacancy g-C3Nx nanosheets for photocatalytic nitrogen fixation under visible light

The rational design of efficient photocatalysts to achieve artificial nitrogen fixation is an urgent challenge. Herein, we combined donor-acceptor covalent organic framework with iron-doped nitrogen vacancy graphitized carbon nitride (D-A COF/Fe-g-C3Nx) for photocatalytic nitrogen fixation. The photocatalyst exhibited good crystallinity, high porosity, and a large specific surface area. Without a sacrificial agent, the optimal 40 % D-A COF/Fe-g-C3Nx exhibited an excellent rate of ammonia production (646 μmol h-1 g-1) at 420 nm, and durable stability after successive cycling. Exhaustive experimental research and theory calculations verified that the D-A unit and Fe doping redistributed the distribution of the charge, which enhanced the visible light utilization and provided chemisorption sites for further polarization. Besides N-vacancies can serve as electron-trapping active sites to promote the directional migration of carriers. The reaction mechanism demonstrated that superoxide radical and hydrogen peroxide were formed by electron and hole, respectively, which promote the reduction of nitrogen to ammonia. This work provides a new idea for the rationalizing design of efficient catalysts for photocatalytic nitrogen fixation under mild conditions.

Improved Tribological Performance of MOF@MXene/PI under High Temperature

High-temperature-resistant and self-lubricating polymer composites with long life and high reliability are increasingly indispensable in the aerospace field. Herein, ZIF-67 grown on the MXene lamella was successfully prepared, and ZIF-67@MXene/PI composites with a regular layered structure were obtained by a hot-pressing three-dimensional network aerogel. It was revealed that incorporating ZIF-67@MXene into PI dramatically reduced the friction and abrasion with elevated temperatures. Largely, aerogel walls always paralleled the sliding direction by compressing, providing a significant antifriction effect. More notably, the presence of a vigorous tribofilm composed of a PI matrix and a diamond-type lattice MOF-modified MXene provided rolling and sliding interface friction under high temperatures, simultaneously. In addition, the uniform tribofilm with a thickness of about 200 nm can effectively avoid the direct contact of the friction pair during the sliding process. Hence, the combination of the constructed multiscale nanocomposites and nanostructured tribofilm with outstanding tribological performance endow the material potentially useful in reducing energy consumption, thus addressing the energy wastage problem caused by friction.

Supramolecular gelator functionalized liquid metal nanodroplets as lubricant additive for friction reduction and anti-wear

In this work, dopamine n-butenylamide (DBA) modified GLM nanodroplets were prepared via directional ultrasound of bulk liquid metal in ethanol aqueous solution as well as DBA self-assembly, followed by grafting with urea-based gelators via radical polymerization to obtain GLM-based supramolecular gelators (Gelator@GLM). The grafting gelators can impart their good compatibility between the GLM nanodroplets and the base oil, so that the Gelator@GLM nanodroplets can be dispersed in the base oil uniformly and stably for more than 3 weeks. Meanwhile, the tribological properties of Gelator@GLM nanodroplets was significantly enhanced, with a reduction of coefficient of friction (COF) and the wear volume of 41.18% and 92.13%, respectively, when compared with the base oil. Furthermore, Gelator@GLM additives exhibited stable lubrication performance even under variable temperature and frequency conditions. The synergistic effect of GLM nanodroplets and the gels generating a physical adsorption film and a chemically protective film (containing iron and chromium oxides, nitrides and carbides) can be credited with the improved tribological performance.

Nitrogen-Doped Carbon Dot as a Lubricant Additive in Polar and Non-polar Oils for Superior Tribological Properties via Condensation Reaction

Nitrogen-doped lubricating additives have been proved to be an effective strategy to improve the tribological properties of lubricating oil. However, the traditional preparation methods of nitrogen-doped lubricating additives have the defects including harsh preparation conditions and a time-consuming preparation process. Herein, we report a preparation method of nitrogen-doped carbon dot (NCD) lubricating additives in a short time by one-step aldehyde condensation reaction at room temperature. The small size effect and nitrogen-containing functional groups of NCD lubricating additives provide favorable conditions for their dispersion and low friction in base oil. The tribological properties of NCD lubricating additives in sunflower oil (SFO) and PAO10 were systematically evaluated. The results show that NCD lubricating additives could reduce the average friction coefficient of SFO from 0.15 to 0.06 and PAO10 oil from 0.12 to 0.06, and the wear width is also decreased by 50-60%. In particular, the friction curve is very stable, and the friction coefficient was maintained at about 0.06 even under the working time of 5 h. By analyzing the morphology and chemical properties of the worn surface, the lubrication effect of NCDs is attributed to its small size effect and adsorption, which was easy to enter the friction gap to fill and repair. Furthermore, the doping of nitrogen induces the occurrence of friction chemical reactions, forming a friction film of nitrides and metal oxides at the friction interface, which effectively reduces the friction and wear of the surface. These findings provide a possibility for the convenient and effective preparation of NCD lubricating additives.

Preparation and Tribological Properties of Bismaleimide Matrix Composites Reinforced with Covalent Organic Framework Coated Graphene Nanosheets

The poor compatibility between the polymer matrix and complex modification processes greatly affects the excellent tribological properties of graphene in the polymer matrix. In this study, a covalent organic framework (COF)-coated graphene hybrid lubricating filler (G/COFs) was synthesized in situ using a sample one-step mechanochemical synthesis process. This was used to improve the tribological properties of bismaleimide (BMI) resin. The morphology and microstructure of the G/COFs hybrid were characterized, and the effect of the added amount on the tribological properties of the G/COFs/BMI composites was studied. The results showed that the G/COFs hybrid could improve the stability of the friction coefficient and decrease the volume wear rate of BMI composites. Compared to the neat BMI, the 0.6 wt% G/COFs/BMI composites showed optimal tribological performance, with the friction coefficient and volume wear rate decreasing from 0.35 to 0.14 and from 48 × 10^-6 to 10.6 × 10^-6 mm³/(N‧m), respectively. In addition, the G/COFs/BMI composites showed lower friction coefficient fluctuations and volume wear rates than G/BMI composites. This is mainly attributed to the fact that the deposition of COFs can not only effectively prevent the aggregation of graphene nanosheets, but can also significantly improve the compatibility and interfacial bond between the graphene and BMI matrix. Moreover, the good synergistic effect between the lamellar COFs and graphene nanosheets can generate high-quality self-lubricating transfer films during the friction process. The excellent dispersibility, efficient chemical functionalization, better friction reduction and wear-resistance properties, and facile preparation method make graphene/COFs hybrid nanoparticles promising as an excellent lubricating filler.

Investigation of Tribological Behaviour on DLC Coatings for AA5051 using DC Sputtering

DLC coatings are deposited on aluminium substrates to improve the wear resistance property of the substrates using sputtering deposition in this study. DLC coatings are deposited using the graphite target onto the Al5051 substrates using DC sputtering. The deposited coatings are then analyzed for their adhesion strength, hardness, coefficient of friction, and chemical compositions. The pin-on-disk method is conducted in vacuumed conditions, dry air conditions (0% RH), and ambient air conditions (40% RH). The different testing conditions have shown different results for the same testing sample. This indicates the nature of DLC film adsorption in ambient air conditions. The chemical composition study has further revealed the adsorbing compounds and the ability of hydrogen and water molecules to get adsorbed on the thin-film surface. This study gives insight into the effect of molecules present in the ambient air on the performance of DLC coatings. It investigates the effect of three DLC coatings deposited using the graphite target onto the Al5051 substrates using DC sputtering.

Ruthenium Complex of sp2 Carbon-Conjugated Covalent Organic Frameworks as an Efficient Electrocatalyst for Hydrogen Evolution

It is still a great challenge to explore hydrogen evolution reaction (HER) electrocatalysts with both lower overpotential and higher stability in acidic electrolytes. In this work, an efficient HER catalyst, Ru@COF-1, is prepared by complexation of triazine-cored sp² carbon-conjugated covalent organic frameworks (COFs) with ruthenium ion. Ru@COF-1 possesses high crystallinity and porosity, which are beneficial for electrocatalysis. The large specific surface area and regular porous channels of Ru@COF-1 facilitate full contact between reactants and catalytic sites. The nitrogen atoms of triazines are protonated in the acidic media, which greatly improve the conductivity of Ru@COF-1. This synergistic effect makes the overpotential of Ru@COF-1 about 200 mV at 10 mA cm^-2 , which is lower than other reported COFs-based electrocatalysts. Moreover, Ru@COF-1 exhibits exceptionally electrocatalytic durability in the acidic electrolytes. It is particularly stable and remains highly active after 1000 cyclic voltammetry cycles. Density functional theory calculations demonstrate that tetracoordinated Ru-N₂ Cl₂ moieties are the major contributors to the outstanding HER performance. This work provides a new idea for developing protonated HER electrocatalysts in acidic media.

MoS2 Lubricating Film Meets Supramolecular Gel: A Novel Composite Lubricating System for Space Applications

In the field of space mechanical lubrication, to improve the reliability and life of space lubrication, solid lubricating film-liquid lubricant composite lubrication has been used in recent years. This lubrication method can improve the durability of sliding friction mating surfaces, reduce equipment wear, and extend the service life of motion mechanisms. However, due to unstable factors such as volatilization and creeping of liquid lubricants in microgravity and ultra-high-vacuum environments, the solid lubricating film wears out after long-term use and produces wear debris and other unfavorable factors. To solve the above problems, this study proposes a novel composite lubrication system constituting a MoS₂ film in combination with a supramolecular gel. The tribological performance of this lubrication system establishes an extended service life with a lower wear rate compared to the MoS₂ film, regardless of functioning in vacuum or atmospheric conditions. More importantly, the results of the irradiation experiment demonstrate that MoS₂-gel exhibits better anticreep performance as compared to MoS₂-oil when exposed to atomic oxygen and ultraviolet light for 4 h. The analysis of this composite lubrication mechanism also reveals the formation of a continuous transfer film on the surface of the friction pairs by virtue of the outstanding synergistic effect between the MoS₂ film and the gel. MoS₂ debris is present in the gel as an additive, and the gel is capable of replenishing automatically once the MoS₂ film is depleted. Moreover, the strong anticreep properties of the gel are attributable to the multialkylated cyclopentane oil being trapped by the intricate reassembling of the gelator network. It is firmly believed that this novel MoS₂-gel composite lubrication system may have good prospective applications in space and special machinery domains.