Improved Lubricating Performance by Combining Oil-Soluble Hairy Silica Nanoparticles and an Ionic Liquid as an Additive for a Synthetic Base Oil

Pulsed Laser Manufactured Heteroatom Doped Carbon Dots via Heterocyclic Aromatic Hydrocarbons for Improved Tribology Performance

Traditional laser-assisted method (top-down synthesis strategy) is applied in the preparation of carbon dots (CDs) by cutting larger carbon materials, which requires harsh conditions, and the size distribution of the CDs is seldom monodisperse. In this work, heteroatom-doped CDs, represented by N,S co-doped CDs (N,S-CDs), can be prepared successfully by pulsed laser irradiation of heterocyclic aromatic hydrocarbons-based small molecule compound solution. The friction coefficient (COF) of base oil PAO decreases from 0.650 to 0.093, and the wear volume reduces by 92.0% accompanied by 1 wt.% N,S-CDs addition, while the load-bearing capacity is improved from 100 to 950 N. The excellent lubrication performance is mainly attributed to the formation of a robust tribofilm via a tribochemical reaction between N,S-CDs and friction pairs, and the N,S-CDs can play a mending effect and polishing effect for worn surfaces. Furthermore, the lubricant containing heteroatom doped CDs are capable of being prepared in situ via pulsed laser irradiation of heterocyclic aromatic hydrocarbons in base oil, which can avoid the redispersed problem of nano-additive in base oil to maintain long-term dispersion, with COF of 0.103 and low wear volume ≈1.99 × 105 µm3 (76.9% reduction) even after standing for 9 months.

Effect of polymer concentration on the morphology of the PHPMAA-g-PLA graft copolymer nanoparticles produced by microfluidics nanoprecipitation

Successful generation of micelles, vesicles, and/or worms with controllable sizes was achieved through the self-assembly process of the poly[N-(2-hydroxypropyl)]methacrylamide-g-polylactide (PHPMAA-g-PLA) graft copolymer within a microfluidic channel. A product diagram was created to illustrate various morphologies associated with different polymer concentrations, all while maintaining a constant flow velocity ratio between water and the polymer solution.

Synthesis of Polymer-Grafted Carbon Dots Serving as Multifunctional Lubricant Additives with Excellent Tribological Performance and Additional Rust Resistance Function for Steel/Steel Contact

Poly(bis(2-ethylhexyl) phosphoric) methacrylic anhydride-grafted carbon dots (CD-g-PEPMA) have been controllably synthesized and used as multifunctional lubricant additives of poly(ethylene glycol) (PEG200). The polymers on the surfaces of CD-g-PEPMA not only endow them with particularly excellent dispersion stability but also enhance their adsorbability on the steel surface and embedded stability between the rubbing surfaces. Hence, CD-g-PEPMA as an additive showed outstanding rust resistance and tribological performance. Among CD-g-PEPMA-X (X represents the polymerization time), CD-g-PEPMA-2 (X = 2 h) exhibited the best tribological performance. At the optimum c of 2.0 wt %, CD-g-PEPMA-2 reduced the coefficient of friction and wear volume of PEG200 by 60.1 and 74.0%, respectively. The striking friction-reducing and antiwear functions of CD-g-PEPMA as additives can be attributed to the synergistic lubrication effect of carbon cores and polymers. The rolling, polishing, and mending effects of carbon cores combined with the favorable adsorbability and reactivity of polymers on steel surfaces synergistically induced the formation of boundary lubrication films on wear track surfaces. The films are composed of tribochemical reaction products such as Fe2(CO3)3, Fe3C, and FePO4 embedded with carbon cores, tremendously reducing the friction and wear of the friction pair. The research results can provide substantial theoretical guidance and data support for designing and developing high-efficiency polymer-CD integrative multifunctional nanoadditives toward PEG.

Double-template-regulated biomimetic construction and tribological properties of superdispersed calcium borate@polydopamine/cellulose acetate-laurate nanocomposite

Herein, a novel superdispersed calcium borate@polydopamine/cellulose acetate-laurate nanocomposite (CTAB-CB@PDA/CAL) is successfully synthesized by a double-template-regulated biomimetic mineralization strategy using PDA/CAL as a hard template and cetyltrimethylammonium bromide (CTAB) as a soft template and surface hydrophobic modifier. The results show that CB can grow uniformly on the CAL surface, and CTAB can improve the hydrophobicity of CTAB-CB@PDA/CAL due to the synergistic effect of the double templates, which contributes to the enhanced dispersibility and long-term dispersion stability of CTAB-CB@PDA/CAL in poly-alpha-olefin (PAO) base oil. Furthermore, CB can rapidly enter the friction interface due to the long substituents of CTAB and CAL, so CTAB-CB@PDA/CAL used as a lubricant additive in PAO base oil exhibits superior tribological performance compared to CB, CB/CAL, and CB@PDA/CAL.

Macroscale superlubricity induced by film-forming polymer brush-grafted colloidal additives

HYPOTHESIS

Modifying surfaces with concentrated polymer brushes (CPBs) is an effective way to reduce friction of tribo-pairs lubricated with liquids. We investigate the hypothesis that colloids grafted with CPBs (hybrid colloids) can deposit onto tribo-substrates by varying the solvent quality with respect to the polymer, in order to obtain ultra-low coefficients of friction (CoFs), so-called superlubricity.

EXPERIMENTS

Hybrid colloids are synthesized and characterized, and a dynamic light scattering compares their swellings in aqueous solutions of glycerol or polyethylene glycol. A mini-traction machine with viscoelastic tribo-pairs is used for lubrication experiments. Adsorption of colloids and film structures are tested using a quartz crystal microbalance and an atomic force microscope.

FINDINGS

The solvent controls whether hybrid colloids spontaneously adsorb to the substrate under quiescent conditions or require contact forces to enable (tribo-)deposition. In both cases, the friction in the boundary-mixed lubrication regimes is lower upon increasing the degree of swelling of CPBs and upon increasing coverage of deposited colloids. The greatest lubrication enhancement and surface coverage occur for the spontaneously adsorbed colloids, with ultra-low CoFs of order 10^-3 over a large range of speeds. The results demonstrate the potential for hybrid colloids to be used as solvent dispersible "friction modifier additives".

Spreading Dynamics of a Precursor Film of Ionic Liquid or Water on a Micropatterned Polyelectrolyte Brush Surface

Time evolution of the microscopic wetting velocity of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI) or water on a micrometer-scale line-patterned surface with a poly(3-sulfopropyl methacrylate) brush and a hydrophobic perfluoroalkyl monolayer was precisely measured by direct observation using optical microscopy and a selective dyeing method over a long period (178 days). When a liquid droplet was placed on the dyed line-patterned brush surface, the liquid penetrated and spread into the polymer brush layer, forming a precursor thin film that extended beyond the macroscopic contact line. The elongation proceeded in two stages by an adiabatic process followed by a diffusive process. The elongation distance X increased with time in proportion to t^2.6 for water and t^0.81 for EMI-TFSI during the adiabatic process. In a diffusive process, the advancing velocity of the precursor film was markedly reduced to be expressed as X ∝ t^0.66 for water and X ∝ t^0.21 for EMI-TFSI, indicating that the diffusive process was affected by the energy dissipation of the wetting system. The high viscosity and the strong molecular interaction of EMI-TFSI with the polymer brush gave a large entropy change during the wetting process to result in a slower spreading velocity.

Competitive Adsorption of Ionic Liquids Versus Friction Modifier and Anti-Wear Additive at Solid/Lubricant Interface—Speciation with Vibrational Sum Frequency Generation Spectroscopy

A modern lubricant contains various additives with different functionalities and the interactions or reactions between these additives could induce synergistic or antagonistic effects in tribological performance. In this study, sum frequency generation (SFG) spectroscopy was used to investigate competitive adsorption of lubricant additives at a solid/base oil interface. A silica substrate was used as a model solid surface. The lubricant additives studied here included two oil-soluble ionic liquids (ILs, [N888H][DEHP] and [P8888][DEHP]), an antiwear additive (secondary ZDDP), an organic friction modifier (OFM), and a dispersant (PIBSI). Our results showed that for mixtures of ZDDP and IL in a base oil (PAO4), the silica surface is dominated by the IL molecules. In the cases of base oils containing OFM and IL, the silica/lubricant interface is dominated by OFM over [N888H][DEHP], while it is preferentially occupied by [P8888][DEHP] over OFM. The presence of PIBSI in the mixture of PAO4 and IL leads to the formation of a mixed surface layer at the silica surface with PIBSI as a major component. The SFG results in this investigation provide fundamental insights that are helpful to design the formulation of new lubricant additives of desired properties.

Competitive Adsorption of Lubricant Base Oil and Ionic Liquid Additives at Air/Liquid and Solid/Liquid Interfaces

Oil-soluble ionic liquids (ILs) have been proved as effective additives in lubricant oils through tribological experiments and post-test analytical analyses. In this study, surface structures of lubricant base oil, oil-soluble ILs, and their mixtures at the air/liquid and solid/liquid interfaces have been studied using sum frequency generation (SFG) vibrational spectroscopy. At the air/base oil and air/IL interfaces, the alkyl chains of the studied compounds were shown to be conformationally disordered and their terminal methyl groups point outward at the liquid surface. The base oil dominates the air/(base oil + IL) interface due to its higher surface excess propensity and larger bulk concentration. At the solid (silica) surface, ILs adopt a structure with their charged headgroups in contact with the silica surface, while their alkyl chains are more conformationally ordered or packed compared to the air/IL interface. At the interface between silica and (base oil + IL) mixtures, ILs also preferentially adsorb to the silica surface with their layer structures somewhat different from those of ILs alone. These results showed that ILs can adsorb onto the solid surface even before tribological contacts are made. The insights obtained from this SFG study provide a better understanding of the role of ionic liquids in lubrication.

Ionic liquid lubricants: when chemistry meets tribology

Ionic liquids (ILs) have emerged as potential lubricants in 2001. Subsequently, there has been tremendous research interest in ILs from the tribology society since their discovery as novel synthetic lubricating materials. This also expands the research area of ILs. Consistent with the requirement of searching for alternative and eco-friendly lubricants, IL lubrication will experience further development in the coming years. Herein, we review the research progress of IL lubricants. Generally, the tribological properties of IL lubricants as lubricating oils, additives and thin films are reviewed in detail and their lubrication mechanisms discussed. Considering their actual applications, the flexible design of ILs allows the synthesis of task-specific and tribologically interesting ILs to overcome the drawbacks of the application of ILs, such as high cost, poor compatibility with traditional oils, thermal oxidization and corrosion. Nowadays, increasing research is focused on halogen-free ILs, green ILs, synthesis-free ILs and functional ILs. In addition to their macroscopic properties, the nanoscopic performance of ILs on a small scale and in small gaps is also important in revealing their tribological mechanisms. It has been shown that when sliding surfaces are compressed, in comparison with a less polar molecular lubricant, ion pairs resist "squeeze out" due to the strong interaction between the ions of ILs and oppositely charged surfaces, resulting in a film that remains in place at higher shear forces. Thus, the lubricity of ILs can be externally controlled in situ by applying electric potentials. In summary, ILs demonstrate sufficient design versatility as a type of model lubricant for meeting the requirements of mechanical engineering. Accordingly, their perspectives and future development are discussed in this review.

On the ionic liquid films 'pinned' by core-shell structured Fe3O4@carbon nanoparticles and their tribological properties

A strongly 'pinned' ionic liquid (IL, [BMIM][PF6]) film on a silicon (Si) surface via carbon capsuled Fe3O4 core-shell (Fe3O4@C) nanoparticles is achieved, revealing excellent friction-reducing ability at a high load. The adhesion force is measured to be ∼198 nN at the Fe3O4@C-Si interface by the Fe3O4@C colloidal AFM tip, which is stronger than that at both Fe3O4@C-Fe3O4@C (∼60 nN) and IL-Si (∼10 nN) interfaces, indicating a strong 'normal pin-force' towards the Si substrate. The resulting strengthened force enables the formation of lateral IL networks via the dipole-dipole attractions among Fe3O4 cores. The observed blue shift of the characteristic band related to the IL anion in the ATR-FTIR spectra confirmed the enhanced interaction. The N-Si, P-O chemical bonds formed as a result of the IL interactions with the Si substrate confirmed by XPS spectroscopy suggested that the IL lay on the Si plane. This orientation is favorable for Fe3O4@C nanoparticles to exert 'normal pin-force' and press the IL film strongly onto surfaces. The IL ions/clusters are thus anchored by these Fe3O4@C 'pins' onto the substrate to form a dense film, resulting in a smaller interaction size parameter, which is responsible for the reduced friction coefficient μ.

Characterizing Polymer-Grafted Nanoparticles: From Basic Defining Parameters to Behavior in Solvents and Self-Assembled Structures

Polymer-grafted nanoparticles, often called hairy nanoparticles (HNPs), are an intriguing class of nanostructured hybrid materials with great potential in a variety of applications, including advanced polymer nanocomposite fabrication, drug delivery, imaging, and lubrication. This Feature provides an introduction to characterization of various aspects of HNPs, from basic defining parameters to behavior of HNPs in solvents and self-assembled structures of multicomponent brush nanoparticles, by using a broad range of analytical tools.

Practical use of polymer brushes in sustainable energy applications: interfacial nanoarchitectonics for high-efficiency devices

The discovery and development of novel approaches, materials and manufacturing processes in the field of energy are compelling increasing recognition as a major challenge for contemporary societies. The performance and lifetime of energy devices are critically dependent on nanoscale interfacial phenomena. From the viewpoint of materials design, the improvement of current technologies inevitably relies on gaining control over the complex interface between dissimilar materials. In this sense, interfacial nanoarchitectonics with polymer brushes has seen growing interest due to its potential to overcome many of the limitations of energy storage and conversion devices. Polymer brushes offer a broad variety of resources to manipulate interfacial properties and gain molecular control over the synergistic combination of materials. Many recent examples show that the rational integration of polymer brushes in hybrid nanoarchitectures greatly improves the performance of energy devices in terms of power density, lifetime and stability. Seen in this light, polymer brushes provide a new perspective from which to consider the development of hybrid materials and devices with improved functionalities. The aim of this review is therefore to focus on what polymer brush-based solutions can offer and to show how the practical use of surface-grafted polymer layers can improve the performance and efficiency of fuel cells, lithium-ion batteries, organic radical batteries, supercapacitors, photoelectrochemical cells and photovoltaic devices.

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.