English

• 1. Introduction

  Since the beginning of the 21^st century, the continuous consumption of fossil fuels has exacerbated global energy shortages and environmental degradation, which have become major factors hindering sustainable development [1-4]. In this context, friction and wear not only lead to significant energy consumption, but they are also among the main factors contributing to the failure of equipment, devices, and materials [5-8]. Statistical data indicate that energy losses due to friction account for approximately 30% of the world's primary energy consumption, while wear is the main cause of 80% of equipment damage or failure [9-12]. During friction, the unevenness and roughness of the contact surfaces must not be overlooked [13]. In fact, the interaction between microbumps induces plastic deformation of the peaks and mutual shearing, resulting in stress concentration at the friction interface, which further exacerbates friction and wear, potentially leading to material failure in severe cases [14-16]. To address this challenge, oil-based lubrication technology is widely recognized as an effective strategy for reducing friction and controlling wear, with the goal of pushing the limits of current material lubrication performance [8, 17-19]. In general, this technology primarily relies on the formation of an ultrathin and stable tribofilm at the contact interface of the friction pair, which not only reduces direct contact and engagement of asperities but also significantly decreases the friction coefficient by specific molecular arrangements and interactions [3, 20-24]. In certain cases, this technology can even achieve near-superlubricity, where the sliding friction coefficient is reduced to the magnitude of 0.001 [25]. At this point, friction and wear are almost negligible, reaching an ideal state of ultra-low friction and near-zero wear [2, 14, 26-28]. Nowadays, extreme operating conditions such as heavy loads, high temperatures, high speeds, vibrations, and impacts have become the major challenges restricting the performance, lifespan, and reliability of critical mechanical components. Traditional lubricating materials can no longer meet the demands for complexity and diversity brought about by the development of advanced technological equipment and nanomechanical systems [29-32]. Therefore, developing high-performance oil-based lubricant materials and new lubrication systems to meet the demands of engineering applications is crucial to improving the energy efficiency and lubrication performance of major mechanical equipment.

  Today, the rapid advancement of nanotechnology has offered new perspectives for two-dimensional (2D) nanomaterials [2, 33-36]. Their ultrathin, atomic-level thickness endows them with unique properties not possessed by traditional lubricants, such as small-molecule zinc dialkyl dithiophosphate (ZDDP) and molybdenum dialkyl dithiocarbamate (MoDTC), and greatly expands the design ideas of oil-based lubricant materials [4, 37-45]. Specifically, (1) when 2D nanomaterials are subjected to shear stress at the sliding interface, their low interlayer shear strength combined with extremely high in-plane mechanical strength, facilitates the formation of easily sheared layered sliding systems, resulting in excellent tribological performance [46-49] (2) Their high surface energy promotes the rapid diffusion of the lubricant at the friction interface, effectively enhancing the self-repair effect [12, 50-52] (3) Their high surface area causes it to be quickly adsorbed to the contact surface of the friction pair, effectively preventing direct contact between friction pairs and reducing friction and wear [14, 19, 42, 53]; (4) Based on the designable structure, functional groups, and abundant active sites of 2D nanomaterials, researchers can purposefully achieve precise structural regulation, thereby effectively modulating the interfacial lubrication state [54-59]. These properties make 2D nanomaterials an ideal platform for studying the structure-performance relationship between chemical structure and lubrication performance, providing a valuable reference for their widespread application in the lubrication field [33]. However, due to the stronger van der Waals interactions and high specific surface area, 2D nanomaterials are prone to agglomeration in the lubrication system, which affects their dispersion stability and lubrication effect, so how to optimize their dispersion by structural regulation is still an important research direction to promote their practical application [60, 61].

  In fact, building on previous research, 2D nanomaterials composed of modules such as the graphene family [55, 61, 62], transition metal dichalcogenides (TMDs) [39, 40, 63], hexagonal boron nitride (h-BN) [64-66], black phosphorus (BP) [67-69], and transition metal carbides and nitrides (MXenes) [70-72] were demonstrated to be useful for the construction of high-performance lubricating materials. It is worth noting that our group [73] previously developed a novel water-based lubricant based on Laponite nanoclay, which significantly enhanced lubrication performance in high-temperature hot rolling of steel by introducing zwitterionic surfactants in its intercalation. This study not only demonstrated the structural advantages of Laponite nanoclay but also proved the effectiveness of 2D nanomaterials under extreme working conditions. Based on the findings of this work, our group [63] further studied the friction process of nanofluid lubrication using molecular dynamics simulations. By constructing simulation models containing MoS₂ nanoparticles and MoS₂ combined with nitrogen-doped carbon quantum dots (N—CQDs), we systematically analyzed the dynamic distribution of C, N, Mo, S, and O elements at the friction interface, uncovering the critical role of N—CQDs in lubrication and further improving interfacial lubrication performance. This study not only explained the mechanism of enhanced friction performance of N—CQDs from a microscopic perspective but also established a strong connection with previous work, showing the potential of 2D nanomaterials in various lubrication systems. In conclusion, 2D nanomaterials, with their unique physicochemical properties, offer significant advantages in enhancing the lubrication performance of mechanical systems, playing a crucial role in improving the reliability and service life of mechanical transmission and motion systems.

  Although 2D nanomaterials show great potential in the field of oil-based lubrication, their implementation in practical engineering applications still faces numerous challenges, particularly their poor dispersion. To address these challenges, researchers are striving to develop more advanced structural regulation strategies and have made significant progress in studying the relationship between chemical structure and lubrication performance. However, a comprehensive review that systematically elucidates the structure-performance relationship between the chemical structure and lubrication properties of 2D nanomaterials, while also providing guidance for their future design, remains lacking. This paper systematically reviews research on 2D nanomaterials in the field of oil-based lubrication (Fig. 1) [74], with a focus on analyzing the structure-performance relationship between their chemical structure and lubrication properties. It also explores the relevant mechanisms and structural regulation strategies while leveraging computational simulations to further uncover their underlying principles. Based on these analyses, this paper summarizes the major challenges and future development directions, aiming to provide a theoretical foundation and practical guidance for the design of high-performance oil-based lubrication materials, while promoting the broader application of 2D nanomaterials in the field of oil-based lubrication.

  Figure 1

  

  Figure 1.  Analysis of the progress of 2D nanomaterials in oil-based lubrication. Reproduced with permission [74]. Copyright 2023, Elsevier. Reproduced with permission [63]. Copyright 2022, The authors.

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  2. Mechanisms and bottlenecks of 2D nanomaterials in oil-based lubrication

  Bottlenecks and mechanisms can be found in Supporting information.

  3. Structural regulation strategies for 2D nanomaterials

  Structural regulation strategies can be found in Supporting information.

  4. Structure-performance relationship and computational simulation

  Although 2D nanomaterials have made initial progress in this field, there remains a significant gap in the comprehensive understanding of their structure-performance relationships, limiting the ability to control lubrication performance through structural design. Currently, methods for achieving efficient lubrication with 2D nanomaterials heavily depend on factors such as thickness, number of layers, sheet diameter, interlayer spacing, Moiré patterns, wettability, and functional groups [4, 11, 48, 75-82]. Among them, the concentration, interfacial compatibility, and dispersion behavior not only affect the arrangement and distribution of the material at the friction interface but also determine its interaction with the lubricant medium and the substrate, thus having an impact on the overall tribological performance [34, 60, 83]. In addition, density functional theory (DFT) calculations and molecular dynamics (MD) simulations, as emerging computational methods, can predict the macroscopic performance of lubrication systems from the molecular microstructure and characteristics [84]. At the same time, they can also reproduce complex physicochemical phenomena in the friction and wear process on the atomic and molecular scales, which becomes an important tool for revealing the mechanism of microscopic interactions between different components in lubrication systems [63, 85].

  4.1 Structural parameters and lubrication performance

  4.1.1   Thickness and number of layers

  The mechanism underlying the effects of sheet thickness and the number of layers on tribological performance remains somewhat controversial. Some scholars argue that multilayered 2D nanomaterials are less susceptible to structural defects and folds, which contribute to a gradual reduction in friction during continuous sliding, resulting in lower coefficients of friction and superior wear resistance [80, 86-89]. This phenomenon is commonly attributed to the fact that the multilayer structure reduces layer flexibility, and the pinning forces between interface atoms are relatively weak [90]. However, some scholars propose the viewpoint that "the thinner, the more wear-resistant" [48, 91]. This newly discovered phenomenon can be explained by the increased surface energy of the friction pair, which enhances the interaction of 2D nanomaterials with the interface. Additionally, the relationship between the structural evolution of 2D nanomaterials during friction and their performance is of particular interest. For example, Zhao et al. [86] highlighted that the initial degree of graphene exfoliation significantly influences its friction-induced structural evolution (Fig. 2a). During friction, highly exfoliated graphene tends to transform into a more ordered structure and undergo graphitization, significantly enhancing its lubrication performance (Fig. 2b). In contrast, graphene with low exfoliation exhibits clear structural defects and a higher coefficient of friction (Fig. 2c).

  Figure 2

  

  Figure 2.  (a) Specific surface area curve for different additives, (b) friction coefficient versus time curves, and (c) average friction coefficient under different concentration conditions. Reproduced with permission [86]. Copyright 2017, Elsevier. (d) Particle size distribution of h-BN and (e) average friction coefficient under different testing conditions. Reproduced with permission [92]. Copyright 2023, Elsevier.

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  4.1.2   Sheet diameter

  The sheet diameter of 2D nanomaterials directly influences their dispersion stability, friction reduction, and anti-wear performance during the lubrication process [26, 92, 93]. Specifically, 2D nanomaterials with smaller sheet diameters demonstrate excellent dispersibility in oil-based media. Their surface polar groups effectively interact with oil molecules and friction surfaces, promoting the rapid formation of an oil film and improving the surface quality of the friction sub-surfaces [83]. This is due to the fact that the smaller particle size provides a larger specific surface area, which allows the material to be in close contact with the friction surface and reduces the friction caused by the direct contact of the friction pair [94, 95]. However, 2D nanomaterials with larger sheet diameters are poorly dispersed in oil-based lubrication environments and tend to aggregate or precipitate, making it difficult to achieve the desired lubrication effect [96]. Additionally, 2D nanomaterials with larger sheet diameters often fail to adequately fill microscopic pits on the friction surface, disrupting the continuity of the oil film and triggering localized excessive wear [97]. The study by Huo et al. [81] confirmed this trend, noting that 2D nanomaterials with larger sheet diameters offer limited improvements in lubrication performance. In light of this, the study of Zhou et al. [92] further revealed the effect of particle diameter on oil film action (Figs. 2d and e). Research showed that the larger particle diameters of h-BN (1 and 5 µm) primarily exert micro-carrying and interlayer slip effects, which are more significant under thicker oil film conditions. On the other hand, the smaller particle diameters of h-BN (0.1 and 0.5 µm) showed better surface repair and impact modulation effects when the oil film was thin, which were especially effective in reducing the local stresses on the friction surface and enhancing lubrication performance. It is evident that boundary lubrication occurs when the oil film is thin, and the lubrication advantage is particularly prominent for small particle diameter 2D nanomaterials.

  Similarly, Li et al. [96] investigated the influence of agglomerate size on tribofilm formation. They found that when the agglomerates were decomposed into smaller particles under shear and the particle size was smaller than the base oil film thickness, the growth pressure of the tribofilm decreased, leading to a significant reduction in the tribofilm thickness. Therefore, both the advantages of 2D nanomaterials with smaller sheet size in boundary lubrication and the impact of agglomerate size on tribofilm growth emphasize the importance of optimizing the particle size of 2D nanomaterials for achieving efficient oil-based lubrication.

  4.1.3   Interlayer spacing

  It is widely accepted that increasing the layer spacing can effectively reduce van der Waals forces, π-π interactions, and electrostatic attraction between the layers of 2D nanomaterials, thereby reducing frictional resistance and enhancing interlayer slip [98]. This principle provides a theoretical foundation for optimizing tribological performance. Cui et al. [74] successfully expanded the interlayer spacing of MXene by intercalating functionalized CDs, forming a stable physical barrier and significantly suppressing the interlayer stacking. The diffraction peak angle on the crystalline surface decreased after intercalation (002), indicating a significant increase in interlayer spacing (Fig. 3a). The integrity of the lattice structure of MXene and CDs after intercalation was further verified. The results showed that the friction coefficient was significantly reduced from 0.58 to 0.1, the wear volume was reduced by 91.3%, and the load capacity was significantly increased to 650 N (Figs. 3b-d). Additionally, Fan et al. [99] discovered that the hydrogen bonding between the deep eutectic solvent (DES), a mixture of choline chloride and ethylene glycol, and graphene oxide (GO) nanosheets plays a crucial role in the intercalation process. The DES molecules were able to penetrate between the GO layers to form an interlayer structure with enhanced lubrication performance, resulting in a reduction of the coefficient of friction and wear volume by 38.3% and 91.2%, respectively (Figs. 3e-g).

  Figure 3

  

  Figure 3.  (a) X-ray diffraction pattern for different additives, (b) friction coefficient versus time curve, (c) wear volume, and (d) extreme pressure test. Reproduced with permission [74]. Copyright 2023, Elsevier. (e) X-ray diffraction pattern of GOs and DES-GOs, (f) friction coefficient versus time curve, and (g) average friction coefficient and wear volume. Reproduced with permission [99]. Copyright 2023, Elsevier. Average friction coefficient variation with (h) Moiré size, (i) normal force, and (j) sliding speed. Reproduced with permission [106]. Copyright 2024, Royal Society of Chemistry.

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  4.1.4   Moiré patterns

  2D nanomaterials primarily form interlayer bonds through van der Waals forces, which are relatively weak non-covalent interactions [62]. Due to the mismatch of lattice constants and angles, the interlayer structure can freely stack or twist in various directions, forming the so-called Moiré superstructure [100, 101]. These Moiré patterns are significantly affected by fluctuations in the potential energy surface in the rotating regime during interlayer slip, leading to changes in their structure and morphology [75, 102]. This non-metric structure helps counteract the fluctuations in the potential energy surface, resulting in efficient interlayer lubrication [3, 5, 36]. Therefore, an in-depth study of the effect of Moiré patterns on lubrication behavior is crucial for the development of high-performance nano-lubricants.

  From previous studies, it is widely recognized in academia that the variation of the Moiré pattern directly affects the lubrication performance, especially in the interlayer rotating system, where the fluctuation of the potential energy surface triggered by irregular truncation is often regarded as the main source of friction [100]. However, as research on the Moiré structure deepens, scholars are gradually recognizing its potential complexity in structural regulation [103]. To further explore the complexity of the Moiré structure, several researchers have proposed new insights. For example, the study by Ruan et al. presented a new perspective by showing that an increase in the size of the Moiré dot matrix can lead to a decrease in the coefficient of friction, a phenomenon that is closely related to the interfacial energy, the charge density, and the atomic arrangement [104]. These findings challenge the previous singular understanding of friction sources, highlighting the complexity of the Moiré structure and its critical role in microscopic lubrication. More specifically, a larger Moiré size helps to smooth the interfacial energy and reduce potential energy fluctuations, thus effectively reducing friction. In addition, the presence of a large-size Moiré lattice implies lower interlayer charge transfer and deformation charge densities, weakening the interlayer adhesion. This dispersed and homogeneous interatomic contact effectively avoids high friction caused by localized over-contact.

  It is worth noting that the theoretical calculation method also corroborates this phenomenon at the microscopic level. Recent first-principles calculations show [105] that the Moiré patterns formed by twisting and heterostructure can significantly reduce the energy barriers of the potential surface, thereby achieving efficient lubrication. The reduction of the potential energy barrier also facilitates the transition from stick-slip to continuous sliding, confirming the key influence of the Moiré pattern on friction behavior.

  Among the factors influencing interfacial friction, the size of the Moiré pattern, normal force, temperature, and sliding speed are considered the main factors [106]. Li et al. [36], in quantifying the relationship between Moiré size and the coefficient of friction, found that decreasing the Moiré size reduces the sliding energy barrier, thereby reducing the coefficient of friction. This result revealed that the frictional properties of material surfaces can be effectively regulated by precisely controlling the structural dimensions of the Moiré superlattice, further realizing efficient lubrication. Wang et al. [106] further showed that the coefficient of friction increases significantly when the Moiré size exceeds 20 Å, the normal force is greater than 1.0 nN per atom, and the sliding velocity is high (Figs. 3h-j). However, 2D nanomaterials can still maintain superlubricity under certain conditions. In contrast, Liao et al. [5] showed that large mismatch heterojunctions (e.g., MoS₂/graphene) are virtually orientation-independent in the construction of non-metric interfaces, with friction close to zero, mainly originating from the edge pinning effect. Small mismatch heterostructures (e.g., graphene/h-BN), with better lattice matching, experience friction mainly from interface sliding resistance, showing a certain degree of directional dependence. This finding underscores the importance of lattice mismatch in regulating lubrication behavior.

  In conclusion, in exploring the method of regulating the lubrication behavior by Moiré pattern of 2D nanomaterials, the precise control of the Moiré superlattice size is not only expected to achieve the optimization of the interlayer structure but also provides a new way to reduce the coefficient of friction and achieve an efficient lubrication state with a promising practical application. In addition, existing studies employed a variety of computational techniques, each with different applicability and limitations, and these methods played different roles in revealing the evolution of the Moiré on lubrication behavior. Therefore, future research should focus on the integration of different research methods to enhance the understanding of the role of Moiré patterns in microscale lubrication.

  4.1.5   Functional groups and wettability

  When a droplet comes into contact with a solid surface, the liquid displaces the gas covering the solid and spreads out, a phenomenon known as wetting [107, 108]. The surface functional groups and wettability of 2D nanomaterials are linked to their lubrication performance as they determines the distribution, adsorption, and interactions between oil molecules in the interfaces of the friction pair [75, 108, 109]. In nanoscale contact spaces, Wang et al. [39] introduced oxygen-containing functional groups onto the MoS₂ surface via acid oxidation treatment, significantly enhancing its surface chemical reactivity. After modification via alkylamine grafting, the oxidized MoS₂ shifted from hydrophilic to hydrophobic, with its wettability regulated by both graft density and chain length. Tribological experiments showed that functional group-modified MoS₂ nanosheets played a key role in the uniform formation of the oil film and facilitating interfacial slip (Figs. 4a-c). Additionally, Kumari et al. [75] surface-modified the h-BN/MoS₂ heterostructure with cetyltrimethylammonium bromide, significantly improving its hydrophobicity of the material (contact angle increased from 91° to 116°) and enhanced its dispersion stability in lubricating oils, resulting in an effective reduction of the coefficient of friction by 44% and the wear volume by 96% (Figs. 4d-f).

  Figure 4

  

  Figure 4.  (a) Water contact angle, (b) thermogravimetric analysis curve, and (c) average friction coefficient and wear scar diameter for different additives. Reproduced with permission [39]. Copyright 2023, American Chemical Society. (d) Water contact angle, (e) friction coefficient versus time curve, and (f) average friction coefficient and wear scar diameter for different additives. Reproduced with permission [75]. Copyright 2021, Elsevier. (g) Contact angle of PAO8 on the surface of MnP additive, (h) effect of MnP additive in PAO8 on coefficient of friction and wear rate, and (i) coefficient of friction of MnP additive with dispersant in PAO8. Reproduced with permission [112]. Copyright 2023, Elsevier.

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  It is important to note that the type and distribution of surface functional groups can significantly alter the intermolecular forces at the solid-liquid interface under shear stress and load [19]. Specifically, 2D nanomaterials with different functional groups promote the formation of adsorption interactions between lubricating molecules and surfaces of varying strengths, thereby influencing tribological performance [110, 111]. For example, Duan et al. [112] found that the PO₄ ^3- ions in Mn₃(PO₄)₂·3H₂O form a stable lubricating film on the friction surface through dipole-dipole interactions, and its porous structure and excellent wettability enable efficient adsorption of lubricant molecules, further enhancing its lubrication performance (Fig. 4g). However, when the introduction of dispersants disrupts the Coulomb repulsive force at the liquid-solid interface, the friction-reducing effect of the Mn₃(PO₄)₂·3H₂O additive significantly decreases (Figs. 4h and i). Therefore, the influence of surface functional groups and wettability on lubrication performance primarily arises from their chemical polarity and interactions with oil molecules.

  Indeed, the surface functional groups and wettability of 2D nanomaterials are the key factors affecting their lubrication properties. On the one hand, surface functional groups can influence the interfacial intermolecular forces by modulating the adsorption strength and distribution pattern of lubricating molecules. On the other hand, wettability further modulates lubrication behavior by altering interfacial chemical reactivity and the efficiency of lubrication film formation.

  4.2 Key factors affecting structure-performance relationships

  4.2.1   Concentration

  In lubrication systems, the additive concentration is one of the factors determining the structure-performance relationship [32, 34, 39, 113-116]. Studies have shown that either too low or too high additive concentration can negatively impact lubrication performance. Therefore, finding the right additive concentration is the key to achieve efficient lubrication in oil-based lubrication [32, 61, 117]. When the additive concentration is insufficient, the lubricant is difficult to form a stable lubrication film at the friction interface, resulting in a decline in boundary lubrication performance, so that the surface of the friction sub-surface is in direct contact, thus leading to lubrication failure [67, 118]. On the contrary, too high a concentration of additives may trigger the adhesion effect and furrow effect, destroying the uniformity of the lubricant film, and the agglomeration effect triggers a local stress concentration, which further destroys the lubrication state [27, 74].

  The study by Fan et al. [32] confirmed the above observations. At an additive concentration of 0.04 wt%, the friction coefficient decreased by 16.7%, while the wear scar diameter was reduced by 22.6%, demonstrating optimal friction reduction and anti-wear performance. This suggests that at this concentration, additives are fully dispersed, forming a stable and uniform lubricant film at the friction interface, thereby significantly enhancing lubrication performance. Specifically, at low concentrations, the lubrication performance is limited by insufficient lubricant film coverage. At high concentrations, the aggregation weakens the effectiveness of the lubricating film and does not further improve the lubrication performance (Figs. S8a and b in Supporting information). The study by Gong et al. [109] further pointed out that optimized additive concentration not only helps to enhance rolling lubrication performance but also improves the surface quality of the rolled plates by refining the grain structure and increasing the hardness of the rolled plates (Figs. S8c-e in Supporting information). Therefore, there is a significant structure-performance relationship between additive concentration and tribological performance. Balancing the dispersion stability and lubricant film formation ability of additives by regulating the concentration is the key to achieving efficient oil-based lubrication.

  4.2.2   Interfacial compatibility and dispersion behavior

  When 2D nanomaterials are used as lubrication additives, they are highly susceptible to aggregation and precipitation due to van der Waals forces between the lamellae, which significantly limits their ability to form tribofilms in oil-based lubrication applications [32, 99, 119]. Good interfacial compatibility and dispersion stability are essential prerequisites to achieve low-friction performance or even superlubrication effect of 2D nanomaterials [120, 121]. In addition, the differences in the morphology of different 2D nanomaterials (e.g., sheet diameter, thickness, edge structure, and surface defects) have an important impact on their dispersion stability and tribological performance. For example, surface defects and edge roughness reduce their dispersion in the oil medium and induce localized stress concentrations during friction, thus altering their tribological behavior [39, 122]. The lack of understanding of the chemical structure-dispersion stability-lubrication performance relationship of 2D nanomaterials in previous years made the design of early lubrication experiments rely heavily on trial-and-error methods and empirical parameters [123, 124]. In recent years, with the deepening of research, many teams have focused on exploring the control of interfacial compatibility and dispersion of 2D nanomaterials in oil-based systems, gradually revealing their key structure-performance relationships [33, 41, 61, 83, 99, 123, 125-129]. For example, Zhang et al. [129] functionalized ZnMgAl layered double hydroxide (LDH) by oleic acid (OA) and stearic acid (SA), and the long-chain molecules of OA and SA extended in the oil molecule, which enhanced the dispersion of LDH in the oil molecule. The results of the dispersion test showed that OA-ZnMgAl LDH and SA-ZnMgAl LDH remained stably dispersed after 3 days with stable transmittance values, which were significantly better than the unmodified LDH (Fig. S9a in Supporting information). Tribological tests showed that 0.15 wt% OA-ZnMgAl LDH and SA-ZnMgAl LDH significantly reduced the coefficient of friction by ~65% and wear volume by ~99% (Figs. S9b and c in Supporting information). Wang et al. [128] found that laser-treated l-Ag@rGO exhibited significantly better dispersion in the base oil than untreated Ag@GO, which began to precipitate within 10 days and completely settled after 60 days, while l-Ag@rGO remained stably dispersed for 60 days (Fig. S9d in Supporting information). Tribological tests showed that the addition of 0.1 wt% l-Ag@rGO reduced the friction coefficient by 40% and the wear scar diameter by 36% (Figs. S9e and f in Supporting information). This phenomenon suggests that good interfacial compatibility and dispersion enable 2D nanomaterials to diffuse more easily into the contact interface and form synergistic interactions with oil molecules. Therefore, the good dispersion of 2D nanomaterials ensures their continuous supply and effective penetration in the contact area, which becomes a key prerequisite for fully utilizing their oil-based efficient lubrication.

  4.3 Computational simulation

  In parallel with experimental studies, with the rapid development of efficient computational techniques, high-throughput experimental methods and even big data analytics in the field of oil-based lubrication [11, 63, 130-134], DFT calculations and MD simulations have become the core tools for predicting or reproducing 2D nanomaterials in oil-based lubrication processes [63, 98, 135-140]. These advanced computational methods not only provide an effective means to analyze the structure-performance relationship between the structure of nanomaterials and their lubrication performance from a microscopic perspective but also lay a solid theoretical foundation for the in-depth exploration of lubrication mechanisms [69, 135, 136, 141]. Specifically, DFT calculations accurately describe the electronic properties and chemical reactivity of materials from a quantum mechanical perspective, while MD simulations model the dynamic response of materials at the microscopic scale. Together, they reveal the atomic-level dynamic behaviors and interaction mechanisms of 2D nanomaterials during the lubrication process [85, 142-145].

  DFT calculations can deeply analyze the bonding modes between atoms, electron gains and losses, and the strength of chemical bonds, which are crucial for understanding the reactivity of atoms within molecules and the adsorption characteristics between materials [146]. For example, in a 2022 study, Kumari et al. [147] used DFT calculations to examine the covalent interactions between tungsten disulfide oxide (WS₂—O) nanosheets and organosilanes, revealing their impact on the dispersion stability of WS₂—O nanosheets in oil molecules and proposed corresponding structural regulation strategies (Figs. S10a and b in Supporting information). In addition, the thermal movement of different molecules in the medium and their interactions lead to differences in transport properties and diffusion behavior in different systems [40, 115, 148]. For example, Man et al. [84] demonstrated the lubrication effect of MoS₂-Al₂O₃ nanofluids during hot-rolled steel, revealing that Mo and S atoms enter the iron lattice through substitution and gap diffusion mechanisms to form a stable diffusion layer (Figs. S10c-f in Supporting information).

  Further, computational studies based on DFT reveal the rules of how structure affects the tribological performance of 2D nanomaterials, providing fundamental theoretical support for achieving efficient lubrication [133]. However, from the perspective of the tribological interface derivative model, accurately capturing the structural and chemical changes at the sliding interface still requires tracking the motion patterns and trajectories of particles at the atomic scale, clearly defining their characteristics and interrelationships. This contributes to the development of more effective structural regulation strategies for obtaining high-performance 2D nanomaterials.

  Modelling the interaction between 2D nanomaterials, lubricant molecules, and atoms or molecules on the surface of the friction pair using MD simulation allows the lubrication mechanism of nanomaterials to be understood at the atomic scale and to predict their specific performance under real operating conditions [137, 142, 143, 149]. For example, in 2021, Tang's group [69] used reactive force field molecular dynamics simulations in Lammps software to study the chemical reaction pathways between OA and BP under high contact pressure and shear stress conditions (Fig. S11a in Supporting information). Under boundary lubrication conditions, the OA molecules are sandwiched between the layers, and as the sliding action proceeds, the OA molecules begin to degrade and release organic functional groups. Meanwhile, the oxygen atoms in OA rapidly adsorb onto the BP surface, forming stable O‒P (or O=P) chemical bonds. These chemical reactions contribute to the formation of a solid lubrication film and a passive liquid layer at the contact interface, which significantly reduces friction and wear, revealing a molecular-level mechanism for achieving ultralow friction and wear. These preliminary findings stimulate more in-depth studies on the structure-performance relationships and mechanisms of 2D nanomaterials.

  By 2023, Tang's group [135] further explored the adsorption properties of gold nanoparticle-modified BP as a lubricant additive at the friction interface and its atomic-level friction behavior (Fig. S11b in Supporting information). The simulation results revealed how gold nanoparticles enhance the adsorption of oil molecules on the phosphorus sheet surface and induce non-conjugate stacking between the sheets, thereby effectively achieving an ultralow friction state.

  However, 2D nanomaterials are prone to self-stacking due to strong van der Waals forces between neighbouring layers, limiting their large specific surface area and the effective use of structural defects (vacancies, dislocations, dangling bonds), which affects the manifestation of diffusion dynamics during the friction process [150, 151]. To address this issue, Xiong et al. [85] in 2024 revealed how nanoparticles form a stable friction film on the steel/steel sliding interface by analyzing the microscopic motion of nanoparticles, the dynamic distribution of atoms at the interface, and diffusion behavior during the friction process (Fig. S11c in Supporting information).

  Meanwhile, the interfacial evolution of 2D nanomaterials under different working conditions such as dispersant concentration, pressure, temperature, humidity, and so on, was also reported [40, 137, 138, 152]. For example, in 2022, Shi et al. [137] found that increasing the concentration of polyisobutene succinimide polyamine (PIBSI) under static conditions effectively dispersed the soot. Although the effect of external pressure on soot dispersion was not significant under pressure-limited conditions, soot dispersion was still better under these conditions than under static conditions. Under shear conditions, especially under high pressure and different temperature (300 and 373 K) environments, 10 wt% PIBSI exhibits the best dispersibility. This study not only highlights the importance of a rational choice of dispersant proportions but also demonstrates the great potential of MD simulation in predicting and optimising the lubrication performance of 2D nanomaterials.

  The realization of oil-based lubrication technology is not achieved overnight and requires comprehensive consideration of multiple structural parameters and factors, such as the flake thickness, number of layers, sheet diameter, wettability, functional groups, Moiré patterns, concentration, and interface compatibility and dispersion behavior of 2D nanomaterials. To compensate for the limitations of experimental methods, DFT calculations and MD simulations are considered as effective tools. These techniques enable the study of the reactive activity of 2D nanomaterials at the electronic scale and facilitate the optimization of the structural regulation process. At the same time, they can efficiently capture the complex evolution process between 2D nanomaterials, lubricants, and the friction pair surfaces at the atomic and molecular scales. By revealing the formation and breaking of bonds during the friction process, these techniques help to understand the microscopic mechanisms of the lubrication system. Advanced theoretical simulation technologies greatly facilitate the rapid analysis and prediction of the environmental service behavior and structure-performance relationship of 2D nanomaterials, providing a key scientific basis and data support for the design, screening, and structural regulation of novel lubricant materials.

  5. Conclusions and outlook

  This review comprehensively summarizes the research progress of 2D nanomaterials in the field of oil-based lubrication, detailing the bottlenecks they face and their mechanisms of action. The mechanisms include the formation of adsorption protective films, charge adsorption effects, tribochemical reaction films, interlayer slip, and synergistic effects. Based on these findings, this paper further explores the structural regulation strategies of 2D nanomaterials in recent years, covering doping engineering, surface modification, structural optimization, and interfacial mixing engineering methods. By combining experiments and simulations, the intrinsic relationship between the chemical structure and lubrication performance of 2D nanomaterials has been elucidated. However, despite significant progress in the field of 2D nanomaterials for oil-based lubrication, several key challenges remain in practical engineering applications. First, the load-bearing capacity and durability of 2D nanomaterials are limited under high contact pressure conditions, and most existing studies focus on simpler frictional contact modes (e.g., point-surface, line-surface, and surface-surface contact), which do not fully capture the tribological behavior under complex engineering conditions. Second, in engineering applications, the scale-up preparation of functionalized 2D nanomaterials faces poor reproducibility, necessitating the development of efficient and controllable production processes. Third, the mechanisms of 2D nanomaterials in oil-based lubrication still need further study, requiring a combination of theoretical analyses and experimental validation at both microscopic and macroscopic scales.

  To address these challenges, future research should focus on the following directions, aiming to provide new development opportunities for the engineering applications of 2D nanomaterials in oil-based lubrication.

  First, leveraging the deep integration of advanced technologies such as efficient computational simulations, high-throughput experiments, databases, machine learning, and artificial intelligence, future efforts will focus on constructing a predictive model database for the chemical structure-macroscopic lubrication behavior of 2D nanomaterials based on quantitative structure-performance models. The construction of this database aims to enable a systematic bottom-up reverse design and development of 2D nanomaterials, spanning from the nanoscale to the macroscale and from microscopic properties to macroscopic performance. Recently, Zhou's research team proposed the concept of a "Lubrication Brain", utilizing multiscale simulation methods to design novel lubricant molecules and extending this approach to macroscopic tribological system design [153]. This research direction is expected to yield significant progress in the lubrication mechanisms, stability, macroscopic performance, and oil-based lubrication engineering applications of 2D nanomaterials. It holds substantial scientific significance and engineering application value, making it a key development focus for future studies.

  Second, with the widespread adoption of new energy vehicles, there is a steadily increasing demand for efficient heat dissipation and long-lasting lubrication in their transmission, battery, and braking systems. Among existing studies, 2D nanomaterials in oil-based lubrication have demonstrated significant lubrication effects. However, future developments should focus more on the creation of oil-based lubrication systems that provide dual functions of lubrication and cooling. To achieve this, future research should focus on improving the dispersibility, thermal conductivity, and thermal stability of 2D nanomaterials in oil-based lubrication systems. This will ensure long-lasting lubrication and effective heat dissipation under complex conditions such as electromagnetic fields, temperature fields, frictional thermal stress fields, and multi-field coupling, thereby meeting the urgent demands of new energy vehicles in terms of operational efficiency, range, and service life.

  Third, the stable and efficient lubrication provided by 2D nanomaterials primarily depends on the formation of tribochemical reaction films. However, achieving highly efficient lubrication often necessitates a prolonged running-in period, during which friction pairs may undergo significant wear before reaching the optimal lubrication state. Therefore, reducing the break-in period and improving initial lubrication performance are key to advancing the use of 2D nanomaterials in engineering applications. Combining surface weaving, coating technology, surface boron penetration treatment, and oil-based nanofluids to create a composite lubrication system could provide an effective way to solve this problem.

  Fourth, in developing oil-based lubrication materials, attention must be given not only to their initial lubrication performance but also to the accumulation of contaminants over long-term use, such as wear debris, carbon deposits, and oil oxidation products. The accumulation of these contaminants can lead to abnormal wear of friction pairs and lubrication failure, thereby negatively impacting lubrication performance. Therefore, to effectively prevent the accumulation of contaminants and ensure the stability of lubrication systems, future research should focus on self-repairing materials, intelligent lubrication regulation, contaminant adsorption and dispersion technologies, and integrating green lubrication technologies with biodegradable materials. These measures are not only effective in extending the service life of oil-based lubrication systems but also crucial for realizing their sustainable development.

  In conclusion, 2D nanomaterials hold significant potential as oil-based lubrication materials. Despite existing challenges, with ongoing optimization of structural regulation strategies and lubrication system designs, oil-based lubrication technology is anticipated to see widespread application in mechanical systems, including wind power generation, steel rolling, rail transportation, and construction machinery.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Changxing Yang: Writing – review & editing, Writing – original draft, Methodology, Conceptualization. Guxia Wang: Writing – review & editing, Resources. Shengwei Guo: Project administration, Funding acquisition, Formal analysis. Jianlin Sun: Writing – review & editing, Supervision, Project administration, Funding acquisition.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (No. 51874036) and the Natural Science Foundation of Ningxia (No. 2024AAC02034).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111178.

  
  
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  Figure 1  Analysis of the progress of 2D nanomaterials in oil-based lubrication. Reproduced with permission [74]. Copyright 2023, Elsevier. Reproduced with permission [63]. Copyright 2022, The authors.

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  Figure 2  (a) Specific surface area curve for different additives, (b) friction coefficient versus time curves, and (c) average friction coefficient under different concentration conditions. Reproduced with permission [86]. Copyright 2017, Elsevier. (d) Particle size distribution of h-BN and (e) average friction coefficient under different testing conditions. Reproduced with permission [92]. Copyright 2023, Elsevier.

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  Figure 3  (a) X-ray diffraction pattern for different additives, (b) friction coefficient versus time curve, (c) wear volume, and (d) extreme pressure test. Reproduced with permission [74]. Copyright 2023, Elsevier. (e) X-ray diffraction pattern of GOs and DES-GOs, (f) friction coefficient versus time curve, and (g) average friction coefficient and wear volume. Reproduced with permission [99]. Copyright 2023, Elsevier. Average friction coefficient variation with (h) Moiré size, (i) normal force, and (j) sliding speed. Reproduced with permission [106]. Copyright 2024, Royal Society of Chemistry.

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  Figure 4  (a) Water contact angle, (b) thermogravimetric analysis curve, and (c) average friction coefficient and wear scar diameter for different additives. Reproduced with permission [39]. Copyright 2023, American Chemical Society. (d) Water contact angle, (e) friction coefficient versus time curve, and (f) average friction coefficient and wear scar diameter for different additives. Reproduced with permission [75]. Copyright 2021, Elsevier. (g) Contact angle of PAO8 on the surface of MnP additive, (h) effect of MnP additive in PAO8 on coefficient of friction and wear rate, and (i) coefficient of friction of MnP additive with dispersant in PAO8. Reproduced with permission [112]. Copyright 2023, Elsevier.

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