Superlubricity of Polyalkylene Glycol Aqueous Solutions Enabled by Ultrathin Layered Double Hydroxide Nanosheets

Thermosensitive Triblock Copolymer for Slow-Release Lubricants under Ocular Conditions

The ocular environment is crucial for a biological lubrication system. An unstable condition of tear film may cause a series of ocular diseases due to serious friction, such as dry eye syndrome, which has drawn extensive attention nowadays. In this study, an in vitro biocompatible superlubricity system, containing thermogelling copolymers (PCGA-PEG-PCGA) and slow-release lubricant (PEG 300/Tween 80), was constructed. First, the sol-gel transition temperature and gel strength of PCGA-PEG-PCGA were adjusted based on the ocular environment by regulating the length of PCGA blocks. Furthermore, the copolymer hydrogel exhibited a reliable slow-release property within 10 days and showed low cytotoxicity. Then, the superlubricity (coefficient of friction of approximately 0.005) was achieved with its released PEG 300/Tween 80 aqueous solution at the sliding velocity range of 1-100 mm s-1 and pressure range of 10-22 kPa. However, the lubrication behaviors varied, while PEG 300 chains and Tween 80 micelles were demonstrated to form a multilayer and a single layer adsorption structure on the sliding surface, respectively. On the whole, the composite lubrication systems, especially the one composed of Tween 80, showed excellent tribological properties owing to the stable slow-release and full hydration effects under ocular conditions, which hold great potential for improving ocular lubrication and maintaining human visual health.

Superlubricity of Materials: Progress, Potential, and Challenges

This review paper provides a comprehensive overview of the phenomenon of superlubricity, its associated material characteristics, and its potential applications. Superlubricity, the state of near-zero friction between two surfaces, presents significant potential for enhancing the efficiency of mechanical systems, thus attracting significant attention in both academic and industrial realms. We explore the atomic/molecular structures that enable this characteristic and discuss notable superlubric materials, including graphite, diamond-like carbon, and advanced engineering composites. The review further elaborates on the methods of achieving superlubricity at both nanoscale and macroscale levels, highlighting the influence of environmental conditions. We also discuss superlubricity's applications, ranging from mechanical systems to energy conservation and biomedical applications. Despite the promising potential, the realization of superlubricity is laden with challenges. We address these technical difficulties, specifically those related to achieving and maintaining superlubricity, and the issues encountered in scaling up for industrial applications. The paper also underscores the sustainability concerns associated with superlubricity and proposes potential solutions. We conclude with a discussion of the possible future research directions and the impact of technological innovations in this field. This review thus provides a valuable resource for researchers and industry professionals engaged in the development and application of superlubric materials.

Macroscale Superlubricity with Ultralow Wear and Ultrashort Running-In Period (∼1 s) through Phytic Acid-Based Complex Green Liquid Lubricants

Liquid superlubricity has attracted much attention, due to its ability to significantly reduce friction on the macroscale. However, the severe wear caused by the long running-in period is still one of the bottlenecks restricting the practical application of liquid superlubricating materials. In this work, the obtained polyethylene glycol-phytic acid (PEG-PA) composite liquid lubricants showed outstanding superlubricating properties (μ ≈ 0.006) for Si3N4/glass friction pairs with an ultrashort running-in period (∼1 s) under high Hertzian contact pressure of ∼758 MPa. More importantly, even after up to 12 h (∼700 m of travel), only about 100 nm deep wear scars were found on the surface of the glass sheet (wear rate = 2.51× 10-9 mm3 N-1 m-1). From the molecular point of view, the water molecules anchored between the two friction pairs have extremely low shear force during the friction process, and the strong hydrogen bond interaction between PEG and PA greatly improves the bearing capacity of the lubricant. This work addresses the challenge of liquid superlubricant simultaneously exhibiting low shear force and high load-carrying capacity and makes it possible to obtain liquid superlubrication performance with an extremely short running-in time.

Insight into the Nanotribological Mechanism of Two-Dimensional Covalent Organic Frameworks

Two-dimensional (2D) materials are promising in reducing friction-induced energy loss and wear in automotive and electronics industries because of their superior tribological performance. As a kind of organic 2D materials, the structure and functionality of covalent organic frameworks (COFs) are much easier to tailor compared to other inorganic 2D materials, which expand their potential application in a Micro-Electro-Mechanical System (MEMS). In this manuscript, several kinds of COFs are synthesized and characterized on the surface of highly oriented pyrolytic graphite (HOPG) to investigate the nanotribological mechanism of organic 2D materials. It is surprisingly revealed that the friction coefficients of surface COFs are positively correlated with the pore sizes of honeycomb networks. The COFs with smaller pores would have a smoother potential energy surface and exhibit a lower friction coefficient. Besides, the porous structures of surface COFs make them good candidates to be host templates. The host-guest assembly structures are successfully constructed after introducing coronene molecules, and these host-guest systems display higher friction coefficients because the assembly structure of the guest molecules would be perturbed during the friction process and bring additional slip energy barriers, but the capacity of COFs to form composite assembly with functional guest molecules greatly promotes their further application in the MEMS.

Macroscale Superlubricity of Black Phosphorus Quantum Dots

In the present work, Black Phosphorus Quantum Dots (BPQDs) were synthesized via sonication-assisted liquid-phase exfoliation. The average size of the BPQDs was 3.3 ± 0.85 nm. The BPQDs exhibited excellent dispersion stability in ultrapure water. Macroscale superlubricity was realized with the unmodified BPQDs on rough Si3N4/SiO2 interfaces. A minimum coefficient of friction (COF) of 0.0022 was achieved at the concentration of 0.015 wt%. In addition, the glycerol was introduced to promote the stability of the superlubricity state. The COF of the BPQDs-Glycerol aqueous solution (BGaq) was 83.75% lower than that of the Glycerol aqueous solution (Gaq). Based on the above analysis, the lubrication model was presented. The hydrogen-bonded network and silica gel layer were formed on the friction interface, which played a major role in the realization of macroscale superlubricity. In addition, the adsorption water layer could also prevent the worn surfaces from making contact with each other. Moreover, the synergistic effect between BPQDs and glycerol could significantly decrease the COF and maintain the superlubricity state. The findings theoretically support the realization of macroscale superlubricity with unmodified BPQDs as a water-based lubrication additive.

Regulation Mechanism for Friction Coefficient of Poly(vinylphosphoric acid) (PVPA) Superlubricity System Based on Ionic Properties

Adjustable lubrication aims to achieve active control of the relative motion of the friction interface, providing a new idea for intelligent operation. A new phenomenon of sudden changes of friction coefficient (COF) in the poly(vinylphosphoric acid) (PVPA) superlubricity system by mixing different lubricants, was found in this study. It was found that anions were the critical factor for the COF change. The change degrees of the COF were investigated by a universal micro tribometer (UMT). A quartz crystal microbalance (QCM)-D was used to analyze the adsorption quantity of anions on the PVPA surface. The hydratability of the PVPA interface was controlled by changing the anionic properties (the amount of charge and structure), thus regulating the COF. The adsorption difference of anions is an important reasoning of how anionic properties can regulate the hydratability. It was analyzed by molecular dynamics simulation. For anions carrying different numbers of charges or double bonds, the adsorption quantity of anions was mainly affected by the adsorption degree on the PVPA surface, while the adsorption quantity of anions with different molecular configuration was synergistically regulated by the adsorption degree and adsorption area of anions on the PVPA surface. This work can be used to develop smart surfaces for applications.

Macroscale Robust Superlubricity on Metallic NbB2

Robust superlubricity (RSL), defined by concurrent superlow friction and wear, holds great promise for reducing material and energy loss in vast industrial and technological operations. Despite recent advances, challenges remain in finding materials that exhibit RSL on macrolength and time scales and possess vigorous electrical conduction ability. Here, the discovery of RSL is reported on hydrated NbB₂ films that exhibit vanishingly small coefficient of friction (0.001-0.006) and superlow wear rate (≈10^-17 m³ N^-1 m^-1 ) on large length scales reaching millimeter range and prolonged time scales lasting through extensive loading durations. Moreover, the measured low resistivity (≈10^-6 Ω m) of the synthesized NbB₂ film indicates ample capability for electrical conduction, extending macroscale RSL to hitherto largely untapped metallic materials. Pertinent microscopic mechanisms are elucidated by deciphering the intricate load-driven chemical reactions that generate and sustain the observed superlubricating state and assessing the strong stress responses under diverse strains that produce the superior durability.

A Comprehensive Review of Water-Based Nanolubricants

Applying nanomaterials and nanotechnology in lubrication has become increasingly popular and important to further reduce the friction and wear in engineering applications. To achieve green manufacturing and its sustainable development, water-based nanolubricants are emerging as promising alternatives to the traditional oil-containing lubricants that inevitably pose environmental issues when burnt and discharged. This review presents an overview of recent advances in water-based nanolubricants, starting from the preparation of the lubricants using different types of nanoadditives, followed by the techniques to evaluate and enhance their dispersion stability, and the commonly used tribo-testing methods. The lubrication mechanisms and models are discussed with special attention given to the roles of the nanoadditives. Finally, the applications of water-based nanolubricants in metal rolling are summarised, and the outlook for future research directions is proposed.

Water-Based Lubricants: Development, Properties, and Performances

Water-based lubricants (WBLs) have been at the forefront of recent research, due to the abundant availability of water at a low cost. However, in metallic tribo-systems, WBLs often exhibit poor performance compared to petroleum-based lubricants. Research and development indicate that nano-additives improve the lubrication performance of water. Some of these additives could be categorized as solid nanoparticles, ionic liquids, and bio-based oils. These additives improve the tribological properties and help to reduce friction, wear, and corrosion. This review explored different water-based lubricant additives and summarized their properties and performances. Viscosity, density, wettability, and solubility are discussed to determine the viability of using water-based nano-lubricants compared to petroleum-based lubricants for reducing friction and wear in machining. Water-based liquid lubricants also have environmental benefits over petroleum-based lubricants. Further research is needed to understand and optimize water-based lubrication for tribological systems completely.

Shear-Induced Interfacial Structural Conversion Triggers Macroscale Superlubricity: From Black Phosphorus Nanoflakes to Phosphorus Oxide

As a new two-dimensional (2D) material, black phosphorus (BP) exhibits great potential for friction reduction. However, achieving macroscale superlubricity with a BP coating remains a great challenge. In this study, we designed a new lubrication system to achieve superlubricity with a BP coating at the macroscale, involving the formation of a BP coating with deposited BP nanoflakes, followed by water lubrication. Robust superlubricity with a coefficient of friction of 0.001 can be achieved on the BP coating in a pure water environment. The superlubricity mechanism is mainly attributed to the shear-induced interfacial structural conversion of BP to phosphorus oxide, leading to the formation of tribofilms on the friction pairs with extremely low shear strength. This finding provides a new strategy for achieving superlubricity of 2D material coatings at the macroscale, which has important implications for the development of novel superlubrication systems for industrial applications.

Facile Fabrication of Novel Multifunctional Lubricant-Infused Surfaces with Exceptional Tribological and Anticorrosive Properties

The large-area preparation of excellent lubricating materials with good resistance to leakage and an oxidation atmosphere and ease of replenishment has remained a challenge. Here, inspired by the Nepenthes pitcher slippery surface, we have fabricated multifunctional lubricant-infused surfaces (LISs) via a scalable technique, in which the solid lubricants and the lubricant oil are reciprocally well-combined to overcome their respective weakness. The designed LIS coating exhibits a multiple lubrication ability with a coefficient of friction of 0.022 and ball wear rate of 2.62 × 10^-18 m³·N^-1·m^-1 in air, which are 21 times and three orders of magnitude lower than those of the steel-steel contact under macroscale test conditions (10 N, 5 Hz), respectively. In addition, the outstanding water-repellent and self-cleaning LIS coating enables the resistance to the strong acid or base corrosion even after 30 days of immersion, and the excellent anticorrosion performance during the electrochemical corrosion test. With the exceptional lubrication, multifunctionality performance, and large-scale fabrication capacity, the prepared LIS coating should find potential applications in machines, pipelines, navigation, infrastructures, outdoor equipment, and so on.

Superlubricity between Graphite Layers in Ultrahigh Vacuum

Graphite has been conventionally believed to exhibit an inferior lubricating performance with significantly larger friction coefficient and wear rate in a vacuum environment than in ambient air. Dangling bonds at the edge planes of graphite, accounting for the high friction in inert atmosphere are saturated by chemisorbed vapor molecules in air, which contributes to low surface adhesion and low friction. However, there is still a lack of direct experimental evidence whether basal planes of graphite excluding the negative effects of edges or dangling bonds shows intrinsic lubricity when sliding under ultrahigh vacuum (UHV) conditions. By the interlayer friction measurement enabled by graphite flake-wrapped atomic force microscope tips in UHV, we show a record-low friction coefficient of 4 × 10^-5 (slope of friction vs normal force curve) when sliding between graphite layers, which is much lower than that in ambient air. This discrepancy manifests the intrinsic sliding frictional behavior between the graphite basal planes when the tribo-materials and experimental conditions are well-designed and strictly controlled. In addition, the temperature dependence of the kinetic friction between the graphite layers has been investigated under UHV conditions over the temperature range of 125-448 K, which is consistent with the thermally activated process.

Superlubricity of Fullerene Derivatives Induced by Host-Guest Assembly

Fullerenes have been recognized as good candidates for solid lubricants. In this study, the microscale superlubricity of fullerene derivatives was accomplished by the construction of regular host-guest assembly structures. Herein, the host-guest assembly structures of fullerene derivatives were successfully constructed on a highly oriented pyrolytic graphite (HOPG) surface by introducing the macrocycles as the templates and were explicitly revealed by scanning tunneling microscopy (STM). Meanwhile, the nanotribological properties of the host-guest assemblies were measured using atomic force microscopy (AFM), revealing ultralow friction coefficients of 0.003-0.008, which could be attributed to the restriction on removal of fullerene molecules after introducing the templates. The interaction energies were calculated by density functional theory (DFT) method, which indicates the correlation between friction coefficients and interaction strength in the host-guest assemblies. The effort on fullerene-related superlubricity could extend the solid superlubrication systems and provide a novel pathway to explore the friction mechanisms at the molecular level.

Macroscale Superlubricity Achieved on the Hydrophobic Graphene Coating with Glycerol

Introduction of graphene-family nanoflakes in liquid results in a reduction in friction and enhanced wear resistance. However, the high demand for dispersity and stability of the nanoflakes in liquid largely restricted the choice of graphene-family nanoflakes thus far. This study proposed a new strategy to overcome this limitation, involving the formation of a graphene coating with deposited graphene-family nanoflakes, followed by the lubrication of the coating with glycerol solution. Pristine graphene (PG), fluorinated graphene (FG), and graphene oxide (GO) nanoflakes were chosen to be deposited on the respective SiO₂ substrates to form graphene coatings, and then an aqueous solution of glycerol was used as lubricant. The coefficient of friction (COF) and wear rate were reduced for all deposited coatings. However, the PG coating exhibited better lubrication and antiwear performance than FG and GO coatings. A robust superlubricity with COF of approximately 0.004 can be achieved by combining glycerol with the PG coating. The superlubricity mechanism was attributed to the formation of a tribofilm, mainly composed of graphene nanoflakes in the contact zone. The extremely low friction achieved on the hydrophobic graphene coating with liquid can aid in the development of a high-performing new lubrication system for industrial applications.

Catalytically Active Oil-Based Lubricant Additives Enabled by Calcining Ni-Al Layered Double Hydroxides

Layered double hydroxides (LDHs) have lately been hailed as robust lubricant additives for improving tribological properties and as ideal catalysts for synthesizing carbon-based nanomaterials. In this paper, in situ analytical tools are used to track the evolution of the crystal structure and chemical composition of LDHs during calcination. Nickel oxide and elemental nickel can be produced by calcining NiAl-LDH in air (LDH-C-Air) and argon (LDH-C-Ar), respectively. For the base oil with 1 wt % LDH-C-Air, negligible wear can be detected even after a 2 h friction test under a severe contact pressure (∼637 MPa). A relatively thick tribofilm (∼60 nm) with a better mechanical property is formed, which protects the solid surface from severe wear. In addition, the possible formed carbon debris may also prevent the direct collision of asperities and effectively improve the wear resistance. This work provides a unique vision for the application of calcined LDHs with the combination of catalysis and tribology.