Organic-Modified Silver Nanoparticles as Lubricant Additives

Construction and Tribological Performance of Ag Nanowire-Dotted 2D Ni-BDC Nanocomposites in Oil

The collaborative lubrication effect based on different lubricating nanostructures could not only overcome the respective drawbacks of different nanostructures as nanoadditives but also comprehensively improve friction performance. For this, we first developed a novel collaborative lubrication nanostructure based on Ag nanowires (NWs) and two-dimensional (2D) Ni-BDC nanosheets (Ag NWs/2D Ni-BDC) by the in situ chemical reduction strategy. The structural characterizations corroborated that Ag NWs with a width of approximately 20-30 nm grew on the surface of 2D Ni-BDC nanosheets, which presented the chemical interface between both. As the nanoadditive applied in oils, the tribological finding confirmed that the Ag NWs/2D Ni-BDC enabled the friction coefficient and wear scar diameter to reduce by 38 and 37% at 0.04 wt % additive content, respectively. The worn surface was characterized by a series of analytical techniques, which basically revealed the friction-reducing and antiwear mechanism based on the layered structure and self-repair effect. Consequently, the nanocomposite structure incorporating the nanowire and 2D nanosheets should be taken into account as a new potential lubricating material.

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.

Electroless Deposition of Silver onto Silica Nanoparticles to Produce Lipophilic Core-Shell Nanoparticles

HYPOTHESIS

The colloidal stability of noble metal nanoparticles can be tuned for solvents of varying hydrophobicity by modifying the surface chemistry of the particles with different capping agent architectures. Challenges arise when attempting to separately control multiple nanoparticle properties due to the interdependence of this adsorption process on the surface chemistry and metal architecture. A surfactant-mediated, templated synthesis strategy should decouple control over size and stability to produce lipophilic nanoparticles from aqueous reagents.

EXPERIMENTS

A modified electroless plating process that produces oil-dispersible core-shell silver-silica nanoparticles is presented. Amine-terminated alkanes are utilized as the capping agents to generate lipophilic surface coatings and the particles are temporarily stabilized during the synthesis by adding a Pluronic surfactant that enhances dispersibility in the aqueous reaction medium. The evolution of shell morphology, composition, and colloidal stability was analyzed against capping agent architecture and concentration. The role of particle shape was also tested by interchanging the template geometry.

FINDINGS

The capping agents installed on the silver shell surface displayed both colloidal stability enhancements and a minimum effective capping concentration that is a function of molecular weight without influencing the shell composition. Particle geometry can be controlled by interchanging the silica template size and shape.

Organic-modified ZnS nanoparticles as a high-performance lubricant additive

Lubricants are essential in transportation vehicles and industrial machinery to improve the lifetime of moving components. Antiwear additives in lubricants significantly minimize wear and material removal due to friction. While a wide range of modified and unmodified nanoparticles (NPs) have been extensively studied as lubricant additives, fully oil-miscible and oil-transparent NPs are essential to improve performance and oil visibility. Here, we report dodecanethiol-modified oil-suspendable and optical-transparent ZnS nanoparticles (NPs) with a nominal diameter of 4 nm as antiwear additives to a non-polar base oil. The ZnS NPs formed a transparent and long-term stable suspension in a synthetic polyalphaolefin (PAO) lubricating oil. The ZnS NPs in PAO oil at 0.5 or 1.0 wt% concentration demonstrated excellent friction and wear protection. The synthesized ZnS NPs showed 98% wear reduction compared to the neat PAO4 base oil. For the first time, this report showed the outstanding tribological performance of the ZnS NPs benchmarked to the commercial antiwear additive zinc dialkyldithiophosphate (ZDDP) with an additional 40-70% wear reduction. Surface characterization revealed a ZnS-derived self-healing polycrystalline tribofilm (<250 nm), which is key to superior lubricating performance. Our results indicate the potential of ZnS NPs as a high-performance and competitive antiwear additive to ZDDP, which has broad transportation and industrial applications.

Synthesis of Eco-Friendly Carbon Dots as Self-Repairing Additives of Polyalphaolefin by Means of a Green Solvation Effect

The eco-friendly menthol-modified carbon dots (CDs-Menth) were synthesized for the first time and exhibited the particularly promising application potential as additives of polyalphaolefin (PAO4). On the one hand, the CDs-Menth could be well dispersed into PAO4 with excellent and long-term dispersion stability via a convenient and green mean, that is, the solvation effect of petroleum ether. This mean was far more advanced to current strategies such as chemical modifications and adding dispersants. On the other hand, the CDs-Menth as additives possessed not only the duty-bound merits such as the distinguished friction-reducing, anti-wear, and load-carrying functions, but also an amazing ability of self-repairing effect. The repairing rate of lower disc in the ball-on-disc friction pair lubricated with CDs-Menth/PAO4 lubricant (2.5 wt %) was about 19.3% if the friction duration was prolonged from 20 to 120 min. Meanwhile, the wear volume reduction for PAO4 caused by CDs-Menth remarkably increased from 43.5 to 74.6%. By virtue of the self-repairing effect, the CDs-Menth could form the tough and tensile boundary lubrication films on the rubbing surfaces, not only tremendously reducing the friction and wear of friction pair, but also hopefully protecting the friction interfaces from the potential oxidation and corrosion.

A Review of Nanomaterials with Different Dimensions as Lubricant Additives

Lubricant additives can effectively enhance the performance and environmental adaptability of lubricants and reduce the energy loss and machine wear caused by friction. Nanomaterials, as important additive materials, have an essential role in the research and development of new lubricants, whose lubrication performances and mechanisms are not only related to their physical and chemical properties, but also influenced by the geometric shape. In this paper, the friction reduction and antiwear performances of nanomaterials as lubricant additives are first reviewed according to the classification of the dimensions, and their lubrication mechanisms and influence rules are revealed. Second, the recent research progress of composite nanomaterials as lubrication additives is introduced, focusing on their synergistic mechanism to improve the lubrication performance further. Finally, we briefly discuss the challenges faced by nanoadditives and provide an outlook on future research. The review expects to provide new ideas for the selection and development of lubricant additives to expand the application of nanoadditives.

Development of Doped Carbon Quantum Dot-Based Nanomaterials for Lubricant Additive Applications

The development of advanced lubricants is essential for the pursuit of energy efficiency and sustainable development. In order to improve the properties of lubricating fluids, high-performance lubricating additives are required. In recent research studies, carbon nanomaterials such as fullerenes, carbon nanotubes, and graphene have been examined as lubricating additives to water or oil. Lubricating oils are well known for the presence of additives, especially friction-reducers and anti-wear additives. As part of this work, we have studied the advancement in the research and development of carbon dot (CD)-based lubricant additives by presenting a number of several applications of CD-based additives. We have also highlighted the friction-reducing properties and anti-wear properties of CDs and their lubrication mechanism along with some challenges and future perspectives of CDs as an additive. CDs are carbon nanomaterials that are synthesized from single-atom-thick sheets containing a large number of oxygen-containing functional groups; they have gained increasing attention as friction-reducing and antiwear additives. CDs have gradually been revealed to have exceptional tribological properties, particularly acting as additives to lubricating base oils. In our final section, we discuss the main challenges, future research directions, and a number of suggestions for a complete functionalized or hybrid doped CD-based material.

Multifunctional TiO2 coatings developed by plasma electrolytic oxidation technique on a Ti20Nb20Zr4Ta alloy for dental applications

A newly developed β-Ti alloy based on the Ti-Nb-Zr-Ta system (Ti20Nb20Zr4Ta) has been subjected to Plasma Electrolytic Oxidation (PEO) treatment to obtain a multifunctional ceramic-like (TiO₂) coating with superior tribocorrosion (wear and corrosion) resistance and improved biocompatibility. For this aim, elements such as Ca, P, and Ag NPs have been incorporated into the oxide film to obtain bioactive and biocide properties. The chemical composition and morphology of the TiO₂-PEO coating was characterized, and its multifunctionality was addressed by several means, including antibacterial activity assessment, formation of bone-like apatite, metallic ion release evaluation, in vitro cellular response analysis, and corrosion and tribocorrosion tests in artificial saliva. The developed coatings enhanced the corrosion and tribocorrosion resistance of the bare alloy and exhibited antibacterial ability with low cytotoxicity and negligible ion release. Furthermore, they were able to sustain MC3T3-E1 preosteoblast viability/proliferation and osteogenic differentiation. Altogether, the results obtained demonstrate the potential of the TiO₂ coating incorporating Ca, P, and Ag NPs to be used for dental applications.

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.

Integration of Functionalized Polyelectrolytes onto Carbon Dots for Synergistically Improving the Tribological Properties of Polyethylene Glycol

In this work, we carefully designed and synthesized a series of novel polyelectrolyte-functionalized carbon dots (CDs-PEI-X) by a facile and reversible phase transfer method based on the protonation reaction and anion exchange process executed on the surface of polyethylenimine-grafted CDs (CDs-PEI), where X denotes the anionic moieties of polyelectrolyte shells including hexafluorophosphate (PF₆^-), bis(trifluoromethane)sulfonimide (NTf₂^-), oleate (OL^-), and bis(salicylato)borate (BScB^-), respectively. Attributed to the favorable compatibility of these anions and polyethylene glycol (PEG) molecules, the hydrophobic CDs-PEI-X displayed excellent dispersibility and long-term stability in PEG200 base oil. Subsequently, the tribological behaviors of CDs-PEI-X as the lubricant additives of PEG200 were systematically investigated. It was proved that the anionic moieties of the polyelectrolyte shells of CDs-PEI-X played a crucial role in regulating their tribological behaviors. Particularly, CDs-PEI-OL was confirmed as an optimal additive, exhibiting the best lubricity, outstanding load-bearing capacity, long service life, and remarkable operational stability under boundary lubrication regime. Based on the tribological evaluations and worn surface analyses, the lubrication mechanism of CDs-PEI-OL was mainly attributed to the formation of the organic-inorganic hybrid adsorption film, the protective tribofilm, and its nanolubrication functions as scrollable "ball-bearing", i.e., the synergistic lubrication effects of surface polyelectrolyte shells and carbon cores. This study provides a feasible and versatile strategy to rapidly and effectively tailor the surface chemistry of CDs and discloses the essential contribution of carbon cores and surface groups on the lubrication process, which facilitates the development of advanced CDs-based nanolubricant additives.

Polymer-Based Graphene Derivatives and Microwave-Assisted Silver Nanoparticles Decoration as a Potential Antibacterial Agent

Nanocomposites obtained by the decoration of graphene-based materials with silver nanoparticles (AgNPs) have received increasing attention owing to their antimicrobial activity. However, the complex synthetic methods for their preparation have limited practical applications. This study aims to synthesize novel NanoHybrid Systems based on graphene, polymer, and AgNPs (namely, NanoHy-GPS) through an easy microwave irradiation approach free of reductants and surfactants. The polymer plays a crucial role, as it assures the coating layer/substrate compatibility making the platform easily adaptable for a specific substrate. AgNPs' loading (from 5% to 87%) can be tuned by the amount of Silver salt used during the microwave-assisted reaction, obtaining spherical AgNPs with average sizes of 5-12 nm homogeneously distributed on a polymer-graphene nanosystem. Interestingly, microwave irradiation partially restored the graphene sp2 network without damage of ester bonds. The structure, morphology, and chemical composition of NanoHy-GPS and its subunits were characterized by means of UV-vis spectroscopy, thermal analysis, differential light scattering (DLS), Field Emission Scanning Electron Microscopy (FE-SEM), Energy Dispersive X-ray analysis (EDX), Atomic Force Microscopy (AFM), and High-Resolution Transmission Electron Microscopy (HRTEM) techniques. A preliminary qualitative empirical assay against the typical bacterial load on common hand-contacted surfaces has been performed to assess the antibacterial properties of NanoHy-GPS, evidencing a significative reduction of bacterial colonies spreading.

Constructing a novel and high-performance liquid nanoparticle additive from a Ga-based liquid metal

The development of a high-performance nanoparticle (NP) additive for lubricating oil is a research hotspot for the tribology and engineering areas. In this study, the concept of a novel liquid nano-additive has been proposed based on the emergence of Ga-based liquid metals (GLMs), which display excellent extreme-pressure and high-temperature lubricity. Herein, the liquid NPs (designated as GLM-NP/C12) were prepared from a GLM droplet through the ultrasonic method, modified with 1-dodecanethiol, and are mainly distributed at 286 ± 21 nm. They have a core-shell structure with liquid-state GLM on the inside, and gallium oxide and a self-assembled alkylthiolate monolayer on the outside. In terms of the tribological performance, GLM-NP/C12s have a wonderful dispersion-stability in PAO10 oil, and provide excellent anti-adhesion, friction-reducing, and wear-resistance properties. When the additive concentration was 0.17 wt% in PAO10, the friction coefficient was reduced by 39% and the wear rate was reduced by 93% compared to those lubricated by the neat PAO10. This kind of liquid nano-additive has the advantages of easy preparation, internal composition regulation and recyclability, compared to conventional solid NPs. In addition, the liquid NPs were readily introduced into the frictional interfaces. More generally, the optimal additive concentration of the liquid NPs was much lower than that of the solid NPs. This observation has important implications for understanding the differences of the lubrication mechanisms between the solid and liquid nano-additives, and may provide a new design method and strategy of nano-additives for lubricating oil.

Study on Tribological Properties and Mechanisms of Different Morphology WS2 as Lubricant Additives

In the present work, the relationship curve of the coefficient of friction (COF) with varying loads of different morphology WS₂ lubricating additives in the friction process at various sliding speeds was studied. On this basis, wear marks and elements on the wear surfaces after friction were analyzed, and then the anti-wear and mechanism effects of WS₂ of different forms in the lubrication process were discussed. Meanwhile, the Stribeck curve was used to study the lubrication state of the lubricating oil in the friction process. It was revealed that the COF of lubricating oil containing lamellar WS₂ decreased by 29.35% at optimum condition and the minimum COF was concentrated at around 100 N. The COF of lubricating oil containing spherical WS₂ decreased by 30.24% and the minimum coefficient was concentrated at 120 N. The extreme pressure property of spherical WS₂ was better than that of lamellar WS₂, and the wear resistance of spherical WS₂ was more stable when the load was over 80 N. The different morphology of WS₂ additives can play anti-wear and anti-friction roles within a wide range of sliding speeds.

Zinc Oxide- and Magnesium-Doped Zinc Oxide-Decorated Nanocomposites of Reduced Graphene Oxide as Friction and Wear Modifiers

Zinc oxide (ZnO) and magnesium-doped zinc oxide nanoparticles, Zn_0.88Mg_0.12O (ZMO), were prepared by autocombustion method. Further, nanocomposites of the as-prepared nanoparticles with microwave-synthesized reduced graphene oxide (rGO) nanosheets, ZnO-rGO and ZMO-rGO, have also been prepared with a view to see the effect of doping of magnesium in zinc oxide on the tribological properties of the nanocomposite. Morphologies of nanoparticles/nanosheets and their nanohybrids have been studied by employing scanning electron microscopy (SEM)/high-resolution (HR) SEM with energy-dispersive X-ray (EDX), transmission electron microscopy (TEM)/HR-TEM, X-ray diffraction, Fourier transform infrared, UV-visible, Raman, and X-ray photoelectron spectroscopy (XPS) techniques. Triboactivity of the additives in paraffin oil has been interpreted considering the parameters mean wear scar diameter, coefficient of friction, load-carrying capacity, and wear rates obtained from ASTM D4172 and ASTM D5183 tests using a four-ball lubricant tester at optimized concentration (0.125% w/v). The performance of base lube and its admixtures has been found to lie in the order ZMO-rGO > ZnO-rGO > ZMO > ZnO > rGO > paraffin oil. Outstanding enhancement in triboactivity of nanocomposites, particularly that of ZMO-rGO indicates that nanoparticles are irrefutably instrumental in reinforcement of rGO, and on the other hand, rGO is associated with abatement of agglomeration of the nanoparticles. Thus, interactions between rGO and nanoparticles are vehemently synergic in nature. It is noteworthy that the best results were obtained with the following optimized concentrations: ZnO/ZMO 0.25%; rGO 0.15% and composites 0.125% w/v. Morphological studies of the wear track lubricated with different additives have been performed using SEM and contact mode atomic force microscopy. Results are in conformity with the order given above. The EDX analysis of ZMO-rGO exhibits the presence of zinc and magnesium on the worn surface, supporting their role in the formation of in situ tribofilm. Their role is further corroborated by XPS studies. Owing to their excellent tribological behavior, these sulfur- and phosphorus-free composites may be recommended as potential wear and friction modifiers.

Dispersion of Nanoparticles in Lubricating Oil: A Critical Review

Nanolubricants have attracted great interest due to the promise of friction and wear reduction by introducing nanoparticles. To date, the foremost challenge for developing a new nanolubricant is particle suspension. To understand the mechanisms of nanoparticle dispersion and identify bottlenecks, we conducted a comprehensive review of published literature and carried out an analysis of dispersion based on available data from the past 20 years. This research has led to three findings. First, there are two primary methods in dispersion: formulation with dispersant and surface modification. Second, surfactant and alkoxysilanes are primary chemical groups used for surface modification. Third, functionalization using surfactant is found to be suitable for nanoparticles smaller than 50 nm. For larger particles (>50 nm), alkoxysilanes are the best. The existence of a critical size has not been previously known. To better understand these three findings, we conducted an analysis using a numerical calculation based on colloidal theory. It revealed that a minimal thickness of the grafted layer in surfactant-modified nanoparticles was responsible for suspending small nanoparticles. For larger nanoparticles (>50 nm), they were suitable for silanization of alkoxysilane due to increased grafting density. This research provides new understanding and guidelines to disperse nanoparticle in a lubricating oil.

Palladium Nanoparticle-Enabled Ultrathick Tribofilm with Unique Composition

There is a consensus that savings of 1.0-1.4% of a country's gross domestic product may be achieved through lubrication R&D. Recent studies have shown great potential for using surface-functionalized nanoparticles (NPs) as lubricant additives to enhance lubricating performance. NPs were reported with ability of producing a low-friction antiwear tribofilm, usually 20-200 nm in thickness, on the contact surface. In contrast, this study reports an unexpected 10 times thicker (2-3 μm) tribofilm formed by dodecanethiol-modified palladium NPs (core size: 2-4 nm) in boundary lubrication of a steel-cast iron contact. Adding 0.5-1.0 wt % such NPs to a lubricating oil resulted in significant reductions in friction and wear by up to 40 and 97%, respectively. Further investigation suggested that the PdNP core primarily was responsible for the improvement in both friction and wear, whereas the thiolate ligand only contributed to the wear protection but had little impact on the friction behavior. In addition, unlike most previously reported tribofilms that contain a substantial amount of metal oxides, this PdNP-induced tribofilm is clearly dominated by Pd/S compounds, as revealed by nanostructural examination and chemical analysis. Such a ultrathick tribofilm with unique composition is believed to be responsible for the superior lubricating behavior.

Improvement in lubricating properties of TritonX-100/n-C10H21OH/H2O lamellar liquid crystals with the amphiphilic ionic liquid 1-alkyl-3-methylimidazolium hexafluorophosphate

The applications of ionic liquids (ILs)/lamellar liquid crystals (LLCs) have great potential in nanotribology because they could be used where conventional oils could not work. To clarify the lubricating mechanism, herein, ILs/LLCs lubricants were prepared by addition of amphiphilic 1-alkyl-3-methylimidazolium hexafluorophosphate (C_nmimPF₆, n = 8, 12) into TritonX-100/n-C₁₀H₂₁OH/H₂O LLCs with different concentration. The influence of alkyl chain lengths of ILs on the microstructures and the tribological properties of LLCs were investigated. The phase structure parameters and the tribological properties of the LLCs in the presence of C_nmimPF₆ were analyzed via freeze-fracture transmission electron microscopy (FF-TEM), the small-angle X-ray scattering (SAXS) technique, oscillating reciprocating friction and wear tester. Compared with the LLCs without C_nmimPF₆, 4.5 wt% C_nmimPF₆ /LLCs can reduce the friction and wear of sliding pairs. The better lubricating property and antiwear capability of the C_nmimPF₆/LLCs may be attributed to the increasing of the interlayer thickness d and the decreasing of the bilayer thickness d₀ in microstructures. This work provides a better understanding of the relationship between the microstructures and friction wear performances of ILs/LLCs.

Facile synthesis of core–shell Ag@C nanospheres with improved tribological properties for water-based additives

The core–shell Ag@C nanospheres were obtained from glucose solution by coupling reduction AgNO₃ and catalytic carbonation. The tribological performance of Ag@C nanospheres as a water-based additive can be improved.