Two-dimensional nanomaterials as lubricant additives: the state-of-the-art and future prospects

Abstract

Two-dimensional (2D) nanomaterials, such as graphene, transition metal dichalcogenides (TMDs), Ti₃C₂T_x MXene and g-C₃N₄, have shown outstanding potential as lubricant additives, due to their nanoscale thickness, ultra-low interlayer shear strength, large surface area, and good stability. This review summarizes the recent progresses in the applications of 2D nanomaterials as water- and oil-based lubricant additives and their lubrication mechanisms such as tribofilm formation, interlayer sliding, rolling effect, surface repairing effect, and polishing effect. Additionally, it discusses the challenges faced in their practical application, such as compatibility and stability issues, and suggests future research directions to foster innovation in 2D material-based nano-lubricants, aiming to advance their practical implementation in various lubrication scenarios.

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Zhengquan Jiang

Zhengquan Jiang. He received his PhD degree in Henan University, Kaifeng, China, in 2019. He subsequently joined the School of Materials Science and Engineering at North China University of Water Resources and Electric Power, where he serves as the director of the Institute of Special Friction and Lubricating Materials. His main research interests are the preparation and application of lubricant nano additives and coating materials.

Jiahao Wu

Jiahao Wu. He received his bachelor's degree from North China University of Water Resources and Electric Power, Zhengzhou, China, in 2018. After then, he became a master's student at the School of Materials Science and Engineering at North China University of Water Resources and Electric Power, Zhengzhou, China. His research interests include preparation, application and lubrication mechanism of nano-additives.

Laigui Yu

Laigui Yu. He received his PhD degree in the State key Laboratory of Solid Lubrication from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, China, in 1997, and joined the Nanomaterials Engineering Research Center of Henan University in 2007. His current position is a professor and doctoral supervisor. He is currently primarily engaged in research on tribology and materials chemistry.

1. Introduction

Tribology is an interdisciplinary field of study concerned with the scientific analysis of the frictional behaviour of surfaces. Its principal areas of focus are surface friction, lubrication and tribological wear, which encompasses the interactions between surfaces in relative motion or with a tendency towards relative motion. It encompasses the fundamental theories and technologies that explore the interrelationships among friction, lubrication, and wear.^1–3 Since ancient times, people have been aware of the phenomenon of friction. Historically, methods such as the use of wheels, rollers and balls to transform sliding friction into rolling friction with the objective of reducing friction were commonplace. Furthermore, historical records indicate the utilisation of diverse lubricants, including animal fats and mineral oils, for the purpose of reducing friction. Currently, approximately one-third to one-half of global energy consumption is attributed to frictional losses.^4–6 Lubrication is the primary method for addressing friction and wear. In both daily life and industrial production, lubricants are commonly used to enhance the long-term service life and operational efficiency of mechanical components.

At present, with the development of industry, aerospace and navigation, traditional lubricants can no longer meet the requirements of use. Therefore, lubrication additives are often used to give the lubricant new properties, making up for the lack of basic lubricants and greatly improving the tribological properties.^7–9 The complex lubricants available today usually consist of a base lubricant and some lubricant additives as the main components of the lubricant to determine the comprehensive properties. Additives constitute a minor component of the formulation, yet they are instrumental in conferring novel properties upon the lubricant or in rectifying deficiencies inherent to the base stock.^10–13 All formulated lubricants are composed of a principal lubricant and a set of additives that facilitate the fulfilment of the criteria necessary for the base lubricant to be designated a premium lubricant.¹⁴ Various substances, such as nanomaterials,^15–18 ionic liquids (ILs),¹⁹ and organic compounds,²⁰ have been used as additives to enhance lubricant performance. Nanomaterials, due to their nanoscale size and favorable surface properties, are particularly important for their self-repairing capabilities on worn surfaces and their ability to form lubricant films on contact areas.^17,21 These materials are commonly employed as lubricant additives to notably decrease friction and wear while enhancing the anti-wear and load-bearing properties of the base lubricant. Additionally, some nanomaterials can adsorb physically or chemically onto contact surfaces, effectively improving the lubricant's tribological performance.²²

Two-dimensional (2D) nanomaterials, recognized for their ultra-low shear strength, have been investigated across various fields since the advent of graphene, often referred to as a ‘wonder material’. Researchers are increasingly focused on exploring their remarkable properties. 2D materials offer several distinct advantages over conventional lubricant additives like zinc dialkyldithiophosphate (ZDDP). Conventional additives often release environmentally harmful substances such as phosphorus and sulphur during operation, which can cause environmental pollution. In contrast, 2D materials such as graphene and transition metal disulfides (TMDS) offer eco-friendly alternatives due to their chemical stability and minimal emissions. Besides, the layered atomic structure of 2D materials has an ultra-low shear strength, allowing for excellent friction reduction and wear resistance under high loads and extreme conditions. 2D materials also offer excellent thermal and oxidative stability compared to conventional additives. While conventional additives (e.g., ZDDP) degrade at elevated temperatures, resulting in reduced performance and residue formation, 2D materials maintain structural integrity and ensure consistent lubrication. In addition, 2D materials can be chemically modified to improve dispersion stability in base oils, thereby enhancing their compatibility with polar and non-polar lubricants. Moreover, the ability of 2D materials to form protective films and self-heal worn surfaces further enhances their effectiveness as additives.^23–26 As illustrated in Fig. 1, the annual number of publications on 2D materials used as lubrication additives is on the rise.

Fig. 1 Number of publications on two-dimensional materials as lubricant additives retrieved from web of science from 2006 to September 11, 2024.

This review mainly introduces the recent research progresses and potential challenges facing 2D nano-lubricating additives including graphene materials,^27,28 TMDS,²⁹ novel 2D material MXene,³⁰ and other commonly used 2D materials. Research involving 2D materials in liquid lubricants is covered, including their respective tribological properties in base lubricants, surface modification methods to improve dispersion stability, and lubrication mechanisms; challenges and prospects for 2D materials as lubricant additives are also presented.

2. Graphene

Graphene consists of a single atomic layer of carbon arranged in a hexagonal lattice formed by sp² hybridized carbon atoms. Its unique honeycomb structure grants it outstanding physical, chemical, and mechanical properties,³¹ making it an effective additive for improving the tribological performance of lubricants²⁵ and reducing friction and wear between mechanical components, ultimately enhancing their durability and reliability.³² In contrast to graphene, graphene oxide (GO) particles exhibit high hydrophilicity, enabling the formation of stable suspensions in water and certain organic solvents. Reduced graphene oxide (RGO), produced by the reduction of oxygen functional groups in graphene oxide, exhibits a hydrophobic character, enhanced physical strength, an expanded specific surface area, and augmented electrical and thermal conductivities.^33,34 Derivatives of graphene, including GO and RGO, exhibit several analogous physical and chemical properties, which position them as optimal base materials for the synthesis of additives that enhance dispersion and lubrication characteristics.^35,36

This section reviews the advancements in research on graphene materials as lubricant additives, covering the tribological properties of graphene and its derivatives, as well as the performance of graphene-based materials when combined with other substances. Additionally, it discusses potential engineering applications of these materials as lubricant additives, aiming to highlight unresolved challenges and prospects in this area.

2.1. Oil lubrication

2.1.1. Graphene materials as lubricating oil additive. Graphene is frequently employed as an additive in lubricating oils, demonstrating remarkable performance characteristics.³⁷ Some recent representative researches on graphene nanoparticles as lubricant additives are given in Table 1. Guo et al.³⁸ demonstrated that adding 0.05 wt% graphene to polyalphaolefin (PAO2) oil reduced the coefficient of friction by 78% and wear by 16%, attributed to the formation of a protective tribo-layer. Similarly, graphene nanosheets have shown excellent performance in other oils. In a study of the lubricating properties of graphene oxide, carbon nanotubes (CNT) and graphene nanosheets (GN) in vegetable oils, it was found that the incorporation of graphene or its derivatives reduced the coefficient of friction, thereby improving the tribological properties of the oils as compared to base oils without additives.³⁹ Francesca Curà et al.⁴⁰ confirmed that adding graphene nanosheets to commercial lubricants significantly improved friction-reducing and antiwear properties, further highlighting graphene's potential in industrial applications. Bader Alqahtani et al.⁴¹ investigated graphene nanoplatelets (GNs) in 5 W-30 oil and observed the lowest coefficient of friction and wear scar diameter (WSD), emphasizing the role of graphene's two-dimensional structure and mechanical properties in forming a robust tribo-film. These findings underscore graphene's versatility and effectiveness across different oils and load conditions, making it a promising additive for practical applications.

Table 1 Some researches on graphene nanoparticles as lubricant additives

Additives	Size	Base	Concentration	Dispersion stability	Friction test condition	Reduced friction coefficient (%)	Reduced wear (%)	Ref.
Graphene	10 μm	PAO2	0.05 wt%	A few weeks	Four ball; 120 N, 250 rev per min	78	16	38
Graphene		VO	0.025 mg mL^−1	45 days	Four balls test	13	9.7	42
Graphene	100–600 nm	Hydraulic oil	0.3 wt%	—	UMT-3; steel/steel friction pair, 3 N	32.4	96.9	43
Graphene	100–600 nm	Hydraulic oil	0.3 wt%	—	UMT-3; steel/copper friction pair, 3 N	74.8	35.8	43
Graphene	0–4 μm	PAO4	0.1 mg mL^−1	—	High frequency friction machine (PLINTTE77), 40 N	37	47	44
Graphene	10–12 nm	PAO4	0.8 mg mL^−1	—	High frequency friction machine (PLINT TE77), 40 N	33	35	44

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In practical applications, lubricants are exposed to application conditions with different loads, which means that the study of the frictional properties of graphene lubricant additives under different loads is important for their practical applications. MAO et al.⁴³ investigated the tribological behavior of graphene as a lubricant additive for steel/steel and steel/copper friction pairs. They revealed that graphene flakes significantly reduced the coefficient of friction and wear scar depth under low loads due to the formation of a protective tribo-film. Notably, graphene remained excellent tribological properties even under high loads, highlighting its adaptability to diverse operating conditions. This provides valuable insights into optimizing graphene-based lubricants in practical applications, emphasizing the need to tailor additives to specific load requirements.

The efficacy of lubricant additives is markedly contingent upon their particle size and thickness.⁴⁵ Kong et al.⁴⁴ systematically analyzed the tribological properties of few-layer graphene (G2) and multilayer graphene (G10) when incorporated into PAO4 base oil. Particularly, G10 was separated into large-particle (G10L) and small-particle (G10S) fractions through centrifugation to investigate the role of particle size. The results showed that G10S achieved the most significant improvements, reducing friction by 37% and wear by 47%. As shown in Fig. 2d, these enhancements are attributed to the smaller particle size of G10S, which provides a larger surface area for interaction and promotes the formation of a continuous, robust tribo-film on the contact surface. Additionally, the multilayer structure of G10S contributed to its superior load-bearing capacity, further reducing friction and wear. These findings underline the importance of optimizing both particle size and layer configuration when designing graphene-based lubricant additives.

Fig. 2 (a) Structure of the experimental setup of the frictional pair under reciprocating sliding conditions, (b) microscopic morphology and (c) microstructural features of GO,⁴³ (Reproduced from ref. 43 with permission from Springer Nature, Copyright 2020) and (d) Schematic diagrams of the friction mechanisms of few-layer, multi-layer, large-size, and small-size graphene nanosheets as well as the sliding pair.⁴⁴ (Reproduced from ref. 44 with permission from Springer Nature, Copyright 2020).

Patel et al.⁴⁶ extended the study of graphene derivatives by examining three forms of reduced graphene oxide (rGO) as lubricant additives. They said that rGO can significantly enhance lubrication and antiwear performance without altering the physicochemical properties of the base oil. The improved tribological performance was attributed to the rGO's ability to form a low-shear-strength interface, reducing friction, while its structural stability minimized wear. This highlights the importance of tailoring graphene additives to specific requirements, such as optimizing particle size and thickness, to maximize tribological performance. In one word, the cost-effectiveness and customizability of graphene derivatives make them promising candidates for diverse industrial applications, paving the way for innovations in lubrication technology.

2.1.2. Modified graphene as lubricating oil additive. The limitation of dispersion represents a pivotal factor influencing the tribological performance of lubricant additives in liquids. Most members of the graphene family display robust π–π interaction characteristics and are inherently insoluble in most solvents. This phenomenon results in pronounced aggregation and sedimentation of these materials in lubricating oils, thereby impeding their extensive utilization as lubricant additives.^47,48 Preparing stable nano-lubricants is a crucial step for their industrial applications in gearboxes or bearings of wind turbines or hybrid or electric vehicles which require that the lubricants exhibit long-term stability. In this sense, the attractive interactions between particles must be overcome to afford stable nanofluids, which requires compensation with other types of forces. One potential avenue for resolution is the incorporation of surfactants, which operate through two distinct mechanisms. The first is the utilization of ionic surfactants, which employ electrostatic stabilization to prevent agglomeration in the presence of a double layer of charges surrounding the nanoparticles. The second is the deployment of surfactant molecules with extended hydrocarbon chains, which provide steric stabilization. Nevertheless, the utilization of surfactants may impede the efficacy of the additives. Another strategy to enhance the stability of nanoparticle dispersions involves their surface chemical modification, which represents a new and evolving strategy to enhance the dispersion stability of nanoparticles in base oils. To achieve this, organic compounds with polar groups are frequently employed as surface modifiers. These include compounds comprising long hydrocarbon chains and a variety of polar functional groups, such as carboxylic acids, amines, silanes, thiols and alcohols. Furthermore, alternative forms of chemical modification have been demonstrated to enhance anti-settlement stability.

The objective of this section is to examine the surface modification of nanoparticles through the formation of covalent bonds with a range of molecular species, including organic acids, silanes, amines, organophosphates, alcohols, polymers, and others.

2.1.2.1. Organic acid modification. Organic acid molecules often contain long-chain alkyl or aromatic rings, and these lipophilic groups can effectively reduce the hydrophilicity of graphene's surface and enhance its solubility and dispersion in oil. Namely, the organic acid molecules are grafted onto graphene surface through chemical bonding or physical adsorption to form the modified layer, thereby improving the lipophilicity of graphene and enhancing its dispersion stability in base oil.

La et al.⁴⁹ employed a chemical exfoliation process to prepare graphene nanosheets, which were then subjected to further modification in the presence of an oleic acid surfactant and a sodium dodecyl sulfate (SDS) detergent. The infrared (IR) spectroscopic analysis revealed that all the absorption peaks of the target product were either significantly weaker or almost absent as compared with those of the unmodified counterpart, which indicates that the graphene nanosheets were uniformly wrapped by oleic acid and the bonding between them and oleic acid was ascribed to π–π interactions. The resultant oleic acid-modified graphene nanosheets exhibited good stability in the lubricating oil and were free of sedimentation for more than 30 days therein. Liang et al.⁵⁰ employed ultrasonic-assisted ball milling to disperse chemically modified multilayer graphene (GR) in the base oil. The modification involved the introduction of oleic acid. The hydrophilic group (–COOH) of oleic acid was grafted onto the surface of GR, and the lipophilic group (long-chain hydrocarbons) was deeply embedded in the base oil. This surface-capped graphene additive demonstrated exceptional tribological performance, including friction reduction, anti-wear properties, and improved anti-corrosion capabilities. What should be pointed out is that, although organic acids as surface modifiers can improve the dispersion stability of graphene nanoparticles in lubricating oils to a certain extent and show good anti-wear and friction reduction properties, most of current studies on organic acid-modified graphene additives focus on short-term laboratory tests and lack of systematic researches on their performances and stability in long-term use.

2.1.2.2. Organic amine modification. It has been established that organic amine molecules can be immobilized on the external layer of graphene through the formation of hydrogen bonds or the application of electrostatic forces. Since organic amine molecules have positive charges, they can neutralize the negative charges on the surface of graphene and reduce the electrostatic repulsion between graphene flakes, thus improving its dispersion stability. Wang et al.⁵¹ employed a straightforward phase-transfer method to organically modify graphene oxide. Oleylamine was added to the graphene oxide dispersion in the form of drops, resulting in electrostatic adsorption on the surface of the GO nanosheets. The polar GO nanosheets modified by oleylamine could be well dispersed in cetane base oil to improve the tribological properties. The ease with which the lubricant containing graphene oxide nanosheets can enter the area of contact and the effectiveness with which it can protect the surface of the sample from wear are the reasons for this. In a study conducted by Han et al.,⁵² an imidazolylenedicarbonitrile amine salt was employed as a high-quality alternative intercalator and modifier with the objective of achieving the exfoliation of graphite into modified graphene. This process was facilitated by the application of low voltage assistance. The modified graphene, especially thinner and smaller modified one, could be stably dispersed in PEG200 to enhance lubrication properties (reducing friction and wear by about 16% and 26%).

The amino (–NH₂) or other reactive groups in organic amines can form covalent bonds with oxygen-containing functional groups (e.g., hydroxyl, carboxyl) on the graphene surface to enhance its functionalization. For example, octadecyl ethylamine (ODA) was attached to the surface of graphene oxide through a straightforward amination process. The flake ODA–RGO, obtained through the method, proved to be an effective lubricant additive, significantly enhancing the friction-reducing and antiwear capabilities of the lubricant.⁵³ Wu et al.⁵⁴ were therefore able to attain considerable dispersion stability of graphene in base oils by combining octadecyl amine and bis (cyclohexyl carbodiimide)-chemically modified graphene with an effective dispersant. The dispersant used in the experiments consisted mainly of polyisobutylene succinimide. The modified graphene (0.5 wt%) exhibited a stable dispersion time in PAO6 of approximately 120 days. Furthermore, the combination of modified graphene (0.5 wt%) and the dispersant (1 wt%) resulted in a reduction of approximately 40% in the coefficient of friction and 90% in the depth of wear, in comparison to the base oil.

2.1.2.3. Other modification methods. The alkylation of graphene nanosheets represents a highly efficacious method for the synthesis of graphene-based hybrids. Many studies have shown that alkylation functionalization enhances the ability of graphene to disperse in organic solvents. Wang et al.⁵⁵ used cetyltrimethylammonium bromide organic molecule to modify graphene by surface alkylation to obtain modified reduced graphene oxide (MRGO) powder. They found that the surface alkylation of graphene improved its dispersion as well deposition or adsorption on a frictional surface, thereby facilitating the generation of a more comprehensive lubrication film and achieving more stable anti-wear and friction-reduction outcomes. Zhu et al.⁵⁶ described a straightforward and efficacious approach to modifying graphene-based nanomaterials through the reduction of particle size (Fig. 3); the as-prepared long-chain alkyl-functionalized ultrafine reduced graphene oxide (RGO-g-OA; OA referring to oleic acid) had high dispersibility and excellent tribological properties. They utilized XRD and Raman spectroscopy to assess the structural integrity and analyze the phase structure of RGO-g-OA, GO-g-OA and GO materials. The results demonstrated that RGO-g-OA has a distinctive hydrophobic/highly lipophilic structure, comprising a substantial hydrophobic graphite-like structural domain within its interstitial zone. Moreover, the long-chain alkane functional units are firmly attached to the graphene oxide. The combination of a significant increase in lipophilicity obtained by grafting and partial chemical reduction of long-chain alkyl groups at the edges of the lamellae and the small size effect results in remarkable prolonged dispersion stability (up to one months) of RGO-g-OA when dispersed in refined oils.

Fig. 3 (a) Schematic of the preparation process of RGO-g-OA, (b) Raman spectra and (c) XRD patterns of GO-g-OA and RGO-g-OA.⁵⁶ (Reproduced from ref. 56 with permission from Royal Society of Chemistry, Copyright 2019).

In addition to the alkylation of graphene surfaces, some researchers have developed alternative types of surface modifiers with the objective of enhancing the dispersion stability of graphene nanoparticles in lubricants. The use of ionic liquids (ILs) as environmentally friendly improvers and high-performance lubricant additives for graphene modification and the preparation of high-performance graphene lubricants is a promising avenue of research. ILs are considered to possess non-toxicity, good physicochemical and anti-wear properties, which makes them an attractive option for potential applications in this field. Similarly, organophosphorus–phosphorus ionic liquids are miscible in non-polar oils and could be a potential lubricating oil additive due to their three-dimensional quadraternal structure comprising long-chain alkane chains.⁵⁷ Gan et al.⁵⁸ devised a method for the synthesis of trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate modified graphene gel ([P66614] [DEHP]-G) utilizing a straightforward mechanical grinding approach, for the purpose of producing an efficacious high-performance additive. The graphene material, prepared in accordance with the established methodology, demonstrated exceptional thermal stability, dispersibility, and wear resistance even when subjected to elevated loads and temperatures. This material demonstrated exceptional thermal stability, wear resistance, and dispersibility under high loads and temperatures, providing a practical solution for applications in harsh environments. Moreover, Fan et al.⁵⁹ configured a novel oil-based lubricant additive (DES-GOs) by intercalating graphene oxide with deep eutectic solvents (DES) composed of ethylene glycol and choline chloride. The DES forms strong hydrogen bonds with the GO nanosheets. Precipitation experiments and UV-visible absorption spectroscopy indicated that DES-GOs with a mass ratio of 1 : 3 had high dispersion stability in PEG 200, showing no precipitation even after 120 days; and it exhibited better lubricating performance than PEG 200, due to the high dispersion stability, the formation of a dense tribofilm, and the combined effect of shear stress at the interlayer, are key factors in this process. In view of the considerable variety of DES components and two-dimensional materials, the development of novel DES-functionalised two-dimensional materials as lubricant additives for various base oils under varying working conditions might have significant industrial application potential.

Table 2 lists the modification methods for graphene. A critical evaluation of the above-mentioned approaches demonstrates that ionic liquid modification offers several advantages over alkylation in specific scenarios. Unlike alkyl-functionalized graphene, which relies primarily on long-chain hydrocarbons for stability, IL-modified graphene benefits from strong ionic interactions that enhance both dispersion stability and compatibility with a broader range of lubricating oils, including high-viscosity and polar oils. Additionally, IL-modified graphene often exhibits superior thermal and chemical stability, making it more suitable for extreme operating conditions. For example, Gan et al.⁵⁸ found that ionic liquid-modified graphene gel exhibited better performance under high-load and high-temperature environments than alkylated graphene. However, alkylation remains advantageous for simpler systems or low-polarity oils whose hydrophobic and lipophilic properties are more critical.

Despite the above-mentioned advances, some challenges still remain for modified graphene. Firstly, the compatibility of modified graphene with various lubricating oils possessing different polarity and viscosity needs to be systematically studied. Current studies often use low-viscosity oils such as PAO6 and PAO4, limiting broader industrial applications. Secondly, the lack of standardization in preparation processes, differences in graphite sources, and variability in surface functional groups contribute to the difficulty in achieving effective and controllable modifications. Further research is required to address these challenges and improve the integration of graphene-based additives into diverse lubrication systems.

Table 2 Modification methods of graphene

	Material	Surface bonding	Base	Concentration	Stability	Ref.
Organic acid	Graphene	The occurrence of CH2 and CH3 groups with functional groups C=O, C–H group indicates that the modified graphene surface is coated with oleic acid	PAO9	0.02%	Excellent dispersion of oleic acid modified graphene in PAO9	60
	Graphene	Bonding between GNPs and oleic acid is a π–π interaction	HD50	0.01 wt%	30 days	49
	Multi-layer graphene	The carboxyl functional group of oleic acid undergoes esterification with the hydroxyl group on the surface of GR	PAO6	0.2 wt%	Under centrifugation, the concentration of the suspension to which MGR was added was relatively stable	50
Organic amine	GO	Oleylamine on the surface of graphene oxide nanosheets due to electrostatic adsorption	16C	10 mg L^−1		51
	RGO	Octadecyl amine (ODA) grafted onto graphene oxide by facile amination	GTL8	0.075 mg mL^−1	2 weeks	53
	GO	Octadecyl amine and bis (cyclohexyl carbodiimide), branched onto graphene via long alkyl links	PAO6	0.5 wt%	120 days	54
Other modification	RGO	Cetyltrimethylammonium bromide organic molecules are wrapped around the graphene surface, and some of the carboxyl groups on the surface are connected by carbon branches	PAO6	0.2 wt%	The addition of MRGO particles resulted in a good dispersion stability and a slow settling rate, with a relative concentration of approximately 83%/120 min	55
	RGO	Long-chain alkyl (–CH2 and –CH3) grafts	Synthetic oil	0.005 wt%	1 month	56
	Graphene	The modifier molecules [P66614] [DEHP] and graphene are effectively attached to the graphene surface through van der Waals force interactions.	150 N	0.75 wt%	30 days	58

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Graphene modification strategies play a key role in improving its dispersion stability and lubrication performance, and different methods have their own characteristics and application scenarios. Organic acid and alkylation modifications enhance hydrophobicity and lubricity by introducing long-chain molecules, which are suitable for low-viscosity and low-polarity lubricants with low cost and simple preparation process. Organic amine modification provides better compatibility with polar oils by virtue of its amphoteric molecular structure, allowing the formation of self-healing lubricant film during friction process. Ionic liquid modification has wider applicability, due to its strong ionic interactions and excellent thermal stability, which can provide excellent antiwear and lubrication performance under high temperature and pressure environments and is applicable to a wide range of polar and non-polar lubricant systems. Nevertheless, these approaches still face challenges such as long-term dispersion stability, cost control, and environmental friendliness. In future, the optimization of modifier combinations (e.g., combining ionic liquid and organic molecule modification strategies), the development of green preparation processes, and the exploration of the compatibility among modified graphene and lubricants with high viscosity and high polarity should be focused on. This would help to promote the industrialization of graphene-modified materials in a variety of fields.

2.1.3. Graphene nanocomposites as lubricating oil additive. To enhance the functionality of lubricants, researchers have synthesised a series of graphene-based nanohybrids, including those comprising metal sulphides/graphene metal nanoparticles/graphene, and so forth^61–63 and added them to base oils for improving tribological properties. The integration of nanoparticles and graphene not only preserves the distinctive lubrication properties of graphene but also produces a synergistic effect;^64,65 and the self-repairing effect and rolling effect of the additive package can enhance the lubrication efficacy and bearing capacity of the film, which is subjected to a load. Furthermore, the incorporation of nanoparticles alters the degree of graphene exfoliation and the interlayer distance, thereby facilitating interlayer shear. Table 3 summarizes graphene complexed with different materials as lubricant additives.

Table 3 Graphene nanocomposites as lubricant additives

	Composites	Base	Concentration (wt%)	Test conditions	Tribological properties	Ref.
Graphene–metal	Ag	Liquid paraffin	0.1%	Four balls	Reduction in COF and wear spot diameter by 40% and 36%	62
				Load: 392 N
				Speed: 1200 rpm
				Time: 30 min
	Au	PAO6	0.1%	Ball-on-disk	Reduction in COF and wear rate by 33.6% and 72.8% compared to those of pure PAO6 oil	66
				Load: 10 N
				Sliding velocity: 0.1 m s^−1
				Temperature: 25 °C
				Time: 30 min
	Cu	PEG	0.08%	Reciprocating sliding	COF reduced by 40.1%, wear reduced by 47%	67
Graphene–metal oxide	ZrO2	Liquid paraffin	0.05%	Ball-on-plate	COF and wear rate reduced by 20.7% and 21.5%	63
				Load: 50 N
				Speed: 400 rpm
				Time: 30 min
	CeO2	PAO40	0.15%	Ball-on-disk	FG-based lubricant reduces wear by 40%	68
				Load: 100–600 N
				Frequency: 25 Hz
				Sliding velocity: 0.05 m s^−1
				Temperature: 25 °C
	ZnO	Ester base oil	0.5%	Four balls	COF further reduced by about 20% relative to ZnO NPs	69
				Load: 392 N
				Speed: 1200 rpm
				Time: 1 h
	Mn3O4	L-XBCEA 0 lithium grease	0.1%	Ball-on-plate	COF and wear depth reduced by 43.5% and 86.1%	70
				Load: 700 N
				Temperature: 25 °C
				Time: 1800 s
Graphene–metal sulfide	RGO/FeS2	Liquid paraffin	7%	Ball-on-plate	—	71
				Load: 6–30 N
				Speed: 100–400 rpm
				Room temperature
				Time: 10 min
Graphene–non-metal and its compounds	GO/h-BNS	Liquid paraffin	0.5%	UMT-2	COF reduction of 41.18% for composite nano-lubricants	61
				Load: 10 N, 40 N
				Sliding velocity: 5 mm s^−1 or 0.5 mm s^−1
				Time: 1 h

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2.1.3.1. Graphene-metal and their composites. Among various nanoparticles, metal nanoparticles have low shear strength, unique ductility and good thermal stability. Additionally, they demonstrate noteworthy friction-reducing and anti-wear properties.⁷² Wang et al.⁶² presented a straightforward and efficient one-step laser irradiation technique for the synthesis of silver/graphene nanocomposites with excellent dispersion stability. Monodisperse silver (Ag) nanospheres were grown in a uniform manner on laminated graphene sheets, and the regular laminate structure thus formed provided further assurance of enhanced lubrication. The as-prepared L-Ag@rGO was adsorbed on the tribological surface, forming a tribological lubricant film that prevented direct contact between the contacting surfaces. Moreover, the highly laminar structure of L-Ag@rGO demonstrated remarkable self-lubricating capabilities. The spherical morphology of the Ag particles enabled the transformation of sliding friction into rolling friction, which contributed to the effective reduction of friction and wear. This approach proved to be advantageous in the practical application of graphene-based nanocomposites in tribological applications. Meng et al.,⁶⁶ with the help of supercritical carbon dioxide (ScCO₂) fluid, successfully introduced gold nanoparticles on graphene oxide flakes. They found that Sc-Au/GO as the lubricant additive in PAO6 oil had much better lubrication performance than Au nanoparticles, GO and Au/GO. Gan et al.⁶⁷ synthesized graphene oxide/copper nanocomposites (see Fig. 4c). The incorporation of 0.08 wt% of the modified nanomaterials in PEG200 led to a notable reduction in friction and wear, by 40.1% and 47%, respectively. This finding has important implications for the tribological practical applications of graphene-based nanocomposites.

Fig. 4 (a) Schematic diagram of the preparation process of FG/CeO₂ nanocomposites, (b) digital photographs of corresponding oil samples two weeks before and after precipitation;⁶⁸ (Reproduced from ref. 68 with permission from Elsevier, Copyright 2019). (c) Schematic diagram of the preparation of IL-GO/Cu composites; and transmission electron microscopic (TEM) images of (d) GO, (e) IL-GO, (h) Cu, (i) GO/Cu, and (j) and (k) IL-GO/Cu.⁶⁷ (Reproduced from ref. 67 with permission from Elsevier, Copyright 2021).

Graphene-metal (e.g., Ag, Au, Cu) composites utilize the spherical morphology of metal nanoparticles to convert sliding friction into rolling friction, while the layered structure of graphene provides self-lubricating performance. In this way, graphene and metal act synergistically to form stable lubrication films under high load, high temperature industrial lubrication environments (e.g., heavy machinery and automotive lubrication oils) where friction reduction and anti-wear are highly required. However, the preparation process (e.g., laser irradiation technology) is often complex and costly while the metal particles are prone to oxidation thereby affecting long-term stability.

Apart from metal nanoparticles, oxides are the most common and abundant inorganic substances. Metal oxide nanoparticles (e.g., Fe₂O₃, CuO, TiO₂, Mo₃O₄, and ZnO) are frequently employed as additives due to their high wear and hardness resistance. This enables the enhancement of lubrication characteristics, as well as extreme pressure and antiwear properties. Graphene oxide and its derivatives are frequently employed as the precursors for the synthesis of graphene–metal oxide nanocomposites.

Zhou and colleagues⁶³ described the preparation of graphene–loaded ZrO₂ nanocomposites (ZrO₂@GO) by the method of electrostatic self-assembly and investigated their dispersibility and wear-resistant properties as additives to paraffin oil lubricants. The ZrO₂@GO nanocomposites, prepared according to the established methodology, exhibited favourable dispersion stability and tribological properties in paraffin oil. This can be attributed to the synergistic effect of the frictional loading of ZrO₂ nanoparticles and the formation of a graphene oxide transfer film on the surface of the worn metal. To improve the dispersibility and lubricating capacity of perfluorinated graphene (FG) without affecting its fluorine content, Ma et al.⁶⁸ prepared hierarchically structured FG/CeO₂ nanocomposites using a simple ultrasound-assisted solvent-heat methodology. Under this combined effect of ultrasound and solvent-heat treatment, the CeO₂ nanoparticles were observed to be uniformly dispersed on the FG nanosheets, and the relative fluorine content was found to remain largely unaltered following the treatment. The as-prepared nanocomposites demonstrated enhanced antiwear capability and comparatively low oil distribution stability (see Fig. 4a and b). The incorporation of CeO₂ nanoparticles did not affect the original extreme pressure properties of FG; however, it did improve the oil distribution stability of FG and significantly enhance its friction-reducing performance under extreme operating loads. The research team led by Ren⁶⁹ developed a ZnO@graphene core–shell nano-additive. Their findings indicated that the ZnO@graphene hard-core soft-shell nanostructure demonstrated enhanced performance in dynamic friction environments. The synergistic effect between the two components was identified as a key factor contributing to the effective maintenance of the lubricant film's load-bearing capacity and stability. Jin and colleagues⁷⁰ developed Mn₃O₄/graphene nanocomposites (Mn₃O₄/G) as potential lubricant additives using an environmentally friendly hydrothermal method. The nanocomposites were added to L-XBCEA 0 lithium grease and subjected to comparison with a composite grease containing a commercial graphene additive. It was observed that the addition of Mn₃O₄/G in trace amounts (up to 0.02 wt%) resulted in a notable reduction in the friction coefficient of the grease.

Graphene–metal oxide composites (e.g., CuO, TiO₂, ZnO) complement each other through the high thermal conductivity of graphene and the high hardness and wear resistance of metal oxides. Metal oxides form protective lubricant films at the friction interface, which, in synergism with graphene's self-lubricating film, contributes to significantly improving antiwear ability under extreme pressure and high temperature environments. Therefore, graphene–metal oxide composites are suitable for extreme pressure and high temperature lubrication applications, such as the lubrication of aerospace, heavy-duty industrial equipment, and deep-sea equipment.

The importance of metal sulphides as solid lubricants and lubricant additives has been recognised for some time. For example, molybdenum disulphide has been employed extensively as a lubricant additive due to its chemical stability exemplary, mechanical properties and layered structure. Zhang and colleagues⁷¹ FeS₂/rGO heterojunction nanocomposites were prepared via a hydrothermal method and subsequently investigated for their tribological properties as additives in liquid paraffinic base oils. The authors reported that the FeS₂/rGO heterojunction nanocomposites enhanced the tribological properties of paraffinic oil, primarily due to the distinctive laminar structure of FeS₂ and rGO in conjunction with the formation of a continuous tribofilm on rubbed metal surfaces.

Graphene–metal sulphide composites (e.g., MoS₂ and FeS₂) combine the interlayer shear behavior of sulphides with the high strength and lubrication performance of graphene to form a stable solid lubrication film, which is particularly suitable for high temperature and low humidity environments. However, sulphide composites suffer from process complexity and potential sulphur contamination during their preparation.

2.1.3.2. Graphene/non-metal composites. The oxygen-containing functionality present on the surface or periphery of graphene oxide can facilitate the formation of chemical bonds with non-metallic nanomaterials through the application of van der Waals forces and other interactions, such as electrostatic forces. This process introduces novel properties to graphene-based nanomaterials, including enhanced dispersion, stability, and friction properties. Shang et al.⁷³ prepared carbon hybrids containing carbon quantum dots (CQDs) and graphene oxide as lubricants for poly ethylene glycol synthetic base oils using one-pot pyrolysis of citric acid. The hybrids prepared according to the specified methodology exhibited favourable solubility and tribological characteristics when tested in poly ethylene glycol base oils, particularly under boundary lubrication conditions.

Notably, as the load increased, the capabilities for reducing friction and preventing wear showed marked improvement. Likewise, hexagonal boron nitride (h-BN), characterized by its ultra-smooth surface, serves as an effective additive in various lubricants, attributed to its weak van der Waals bonds between layers. Samanta et al.⁶¹ synthesized h-BNAS@GO nanocomposites by covalently attaching h-BN nanosheets onto the edges or surfaces of graphene oxide nanosheets. When added to paraffin oil, these h-BNAS@GO composites formed a cushioning lubricative barrier at the interface, which diminished interlayer interactions and facilitated the lamellar shearing of contacting surfaces. This resulted in outstanding tribological performance across various loads and sliding speeds (Fig. 5). Remarkable, the h-BNAS@GO composites achieved a reduction in the coefficient of friction by 50.7% at a 10 N load and by 41.18% at a 40 N load.

Fig. 5 (a) Step-by-step schematic of covalent grafting of graphene oxide on h-BN nanosheets and variation in coefficient of friction (COF) with sliding time from ball loading macrotribometric studies of 0.5wt% particles in paraffin oil suspension for (b) h-BN, (c) h-BNOH, (d) h-BNAS, and (e) h-BNAS@GO and (f) comparable COF values of the nano lubricants at normal loads of 30, 50, and 80 N with a sliding speed of 0.4 m s⁻¹.⁶¹ (Reproduced from ref. 61 with permission from American Chemical Society, Copyright 2020).

Metal oxide composites (e.g., h-BN and SiO₂) enhance dispersion stability and lubricant film properties through structural advantages, being particularly suitable for low to medium load and environmentally friendly lubrication applications. However, at extreme temperatures or in corrosive environments, non-metallic oxides may experience performance degradation, which requires to improve their chemical stability thereunder.

Graphene composites show great application potential in the lubrication field, and different composite strategies have their own advantages in friction reduction, antiwear and thermal stability, being suitable for different working conditions. In the future, graphene composites will play an increasingly important role in lubrication. Multi-functional, multi-scale synergistic composites, such as core–shell structures and heterojunction designs, will be developed through material structure design and function optimization. These composites will achieve a comprehensive improvement in tribological properties, dispersion stability and resistance to extreme working conditions. At the same time, the use of green surface modifying agents and new dispersants to enhance the compatibility of materials with different lubricants, and the development of low-cost, low-pollution scale preparation processes will further promote their industrial applications. In addition, it is imperative to explore adaptive and smart response materials, expand their applications in extreme environments and high-end equipment, and provide efficient and sustainable lubrication solutions for aerospace, heavy machinery and green engineering. With the deepening of interdisciplinary cooperation and the advancement of technology standardization, graphene composites are expected to achieve large-scale applications in the field of multidisciplinary lubrication engineering, and help the lubricant industry's innovation and sustainable development.

2.2. Water lubrication

The poor biodegradability of lubricants poses various environmental challenges and potential risks to human health. As pollution and energy use continue to rise, there is an urgent need to identify lubricant materials that are both eco-friendly and highly effective.⁷⁴ Recently, water-based lubricants have garnered significant interest due to their broad availability, affordability, and environmental benefits. Despite these advantages, the inherently low viscosity and surface tension of pure water restrict its effectiveness as a lubricant. Consequently, developing additives for water-based lubricants to enhance their tribological properties has become a priority. In this context, water-based nano-lubricants hold promise as eco-friendly options, as they could integrate water's exceptional cooling ability with the superior lubrication provided by nano-additives.^75–77

2.2.1. Graphene materials as lubrication additives in water. Graphene oxide, an important derivative of graphene, is rich in oxygen-containing functional groups (–O, –COOH, –OH, etc.) on the basal surface and edges, and exhibits good dispersion in water and organic solvents. Therefore, graphene oxide is often used as a water lubrication additive.^78,79 In aqueous dispersions, materials based on graphene oxide can adsorb onto surfaces, creating an efficient lubricating film through ligand interactions and hydrogen bonding between oxygen-containing groups and the contact surface. Unlike the lubrication film formed by graphene oxide, the film produced by water has reduced slip resistance, attributed to its lower viscosity and thinner film structure.

Xie et al.⁸⁰ investigated the lubricating effects of graphene and graphene oxide in aqueous solutions, discovering that the optimal lubrication was achieved with 0.5 wt% graphene oxide (Fig. 6), and corresponding COF and wear rate in graphene oxide aqueous solution decreased by 77.5% and 90%, respectively, being superior to graphene nanofluid. Singh et al.⁸¹ explored the lubrication potential of graphene oxide aqueous dispersions for self-mated stainless steel tribo-pairs and observed that, at an optimal concentration of 0.1 wt%, the coefficient of friction dropped from 0.56 (with pure water) to 0.12, while the wear rate decreased by 68%. However, when the concentration of graphene oxide in water was too high, the particles tended to agglomerate on the friction surface, decreasing its smoothness and leading to abrasive wear, thereby diminishing the lubrication effectiveness.⁸² In practice, the concentration of graphene oxide used as an additive in aqueous lubricants rarely exceeds 1 wt%, as higher concentrations cause the graphene oxide solution to lose its homogeneous dispersion over extended storage. This non-uniform dispersion results in particle accumulation on the wear track, contributing to abrasive wear.

Fig. 6 (a) Effect of nanoparticle concentration as a water lubrication additive on the average coefficient of friction, (b) effect of nanoparticle concentration on the average wear rate, (c) sliding to lubrication film rupture time and (d) Average time to lubrication film failure.⁸⁰ (Reproduced from ref. 80 with permission from American MDPI, Copyright 2018).

The high concentration of oxygen-containing functional groups in graphene oxide makes its structure highly sensitive to the protonation effect, and thus to pH levels. This sensitivity impacts its macroscopic properties, as pH variations significantly influence the factors that determine GO's lubrication performance, including dispersibility, lateral size, wettability, and interfacial characteristics. Prior investigation⁸³ has shown that the oxygen-containing functional groups on graphene oxide, particularly carboxyl groups, display varying levels of hydrophilicity depending on the pH environment. An alkaline environment promotes better dispersion of graphene oxide in water, as deprotonation of these functional groups increases electrostatic repulsion between graphene oxide sheets, reducing their tendency to aggregate. This even distribution of graphene oxide on metal surfaces enhances lubrication efficiency. Additionally, alkaline graphene oxide dispersions allow for a higher concentration limit, which directly correlates with improved lubrication performance. Meng et al.⁸⁴ observed that an alkaline graphene oxide suspension (pH = 9), adjusted with triethanolamine, exhibited superior lubrication on strip surfaces during cold rolling. Compared to a graphene oxide suspension at pH = 2.8, the pH = 9 solution reduced the coefficient of friction, wear scar diameter, and minimum rolled thickness of the strip by 28.6%, 21.6%, and 10.84%, respectively. The researchers attributed this improvement to the enhanced dispersion and smaller particle size of graphene oxide in the alkaline suspension, which allowed the flakes to penetrate the contact area between roll bands, forming a thin, efficient adsorption film that minimized friction and wear in the sliding interface. Whatever, the actual lubrication properties need to be considered in detail for the operating conditions. In addition to tribological properties, other effects, such as corrosion and safety, also need to be considered.

2.2.2. Modified graphene as lubrication additive in water. Modifying the surface of graphene enhances the functionality of graphene oxide-based lubricant additives. Such modifications retain the superior physical properties of graphene oxide while further improving its lubricating performance, dispersion stability, and interfacial activity. Zhang et al.⁸⁵ developed reduced graphene oxide modified with hyperbranched polyurethane ester (RGO-HBPE) and evaluated its effectiveness as an aqueous lubrication additive. They reported that RGO-HBPE demonstrated strong dispersibility in distilled water and outstanding lubricating performance, even without the addition of any base oil. Min et al.⁸⁶ induced many oxygen-containing functional groups in fluorinated graphene oxide (FGO; Fig. 7(a)). The FGO-based water lubricant additives demonstrated remarkable dispersibility in water and strong anti-wear capabilities, indicating their promise for application in wear-resistant materials, particularly within water-based lubricants. Ma et al.⁸⁷ successfully synthesized hydroxyl-modified, highly fluorinated graphene (HOFG). Fig. 7(d) presents a schematic of the microwave-assisted mechanism by which hydroxyl groups are substituted on FG; the resultant FG is said to exhibit excellent water dispersibility as well as notably enhanced friction-reducing and anti-wear characteristics in water.

Fig. 7 (a) Preparation of fluorinated graphene oxide, (b) FTIR spectra of GO, Fr-GO and FGO, (c) their Raman spectra,⁸⁶ (Reproduced from ref. 86 with permission from Elsevier, Copyright 2019). (d) Schematic diagram of the microwave-assisted substitution mechanism of FG with hydroxyl groups, (e) TEM and corresponding HRTEM images of FG as well as (f) TEM images and corresponding EDX spectrum of HOFG-4.⁸⁷ (Reproduced from ref. 87 with permission from Elsevier, Copyright 2021).

The interfacial activity and lubrication performance of graphene oxide vary depending on the introduced structure. Yang et al.⁸⁸ explored the effects of the branched chain lengths of graphene on its microstructure, physicochemical properties, and lubrication performance. They adopted alkylamines with different branching lengths (C = 0, 4, and 8) to functionalize the edges of graphene (GO-0, GO-4, and GO-8), obtaining various graphene oxide composites. A comprehensive study on the correlation between graphene's molecular structure and its physicochemical behaviors revealed that wrinkling in graphene oxide composites occurs due to a reduction in intermolecular hydrogen bonding and an increase in the length of branched chains. It was found that a moderate branched chain length (C = 4) optimized lubrication performance in modified graphene oxide, while the composite with the shortest branch chain (C = 0) demonstrated superior interfacial properties.

Despite of the excellent lubricating performance of graphene oxide, its carboxyl groups often may result in the formation of dispersions at a low pH at which corrosion must be considered for graphene oxide as the lubricant additive.⁸⁹ Sun et al.⁹⁰ synthesized triethanolamine-modified graphene oxide (TMGO) through a high-temperature reaction between triethanolamine and GO. Their findings showed that 0.1 wt% TMGO was more effective than 0.1 wt% GO in reducing both the average coefficient of friction and wear scar diameter, while also avoiding corrosion on the metal surface.

2.2.3. Graphene nanocomposites as lubrication additives in water. Graphene oxide composites retain the outstanding physical properties of graphene oxide while offering improved lubrication, dispersibility, and interfacial activity. This is mainly because the synergistic effects among various ingredients of the composites may produce some unexpected features. In addition, graphene oxide composites can be tailored to respond to different application scenarios. Furthermore, graphene oxide composites can be customized to suit specific application needs, making them a practical and effective choice as lubricant additives.

The use of inorganic nanoparticles in water-based lubrication systems has been widely researched, with a range of nanoparticles—such as different oxides and carbon-based materials—employed as lubricants. Beyond their exceptional friction-reducing and anti-wear capabilities, these inorganic nanoparticles adhere effectively to tribofilms, contributing to self-repair of worn surfaces and enhancing heat transfer functions. Table 4 summarizes some studies on graphene composites as water lubrication additives.

Table 4 Graphene nanocomposites as water lubrication additives

		Composites	Lubrication system	Concentration	Test conditions	COF or WSD	Ref.
Graphene composites	Graphene–carbon nanoparticles	GO/ND	Aqueous solution	0.1 wt%, 0.5 wt%	Ball on plate	COF = 0.03	91
					Load: 0.005–0.08 N
					Velocity: 0.0004 m s^−1
					Temperature: 25 °C
		g-C3N4/GO	Aqueous solution	0.06 wt%; g-C3N4: GO ratio = 1 : 1	Ball on plate	37% reduction in COF (from 0.39 to 0.25); 19.6% reduction in WSD	92
					Load: 10–35 N
					Velocity: 0.0025–0.0125 m s^−1
					Temperature: 25 °C
	Graphene–Nonmetal Oxide	GO/SiO2	Mixed aqueous solution (70 wt% H2O)	0.16 wt%	Block on ring	50% reduction in COF (from 0.52 to 0.26); 38% reduction in Ra of lapped surface	93
					Load: 20 N
					Velocity: 0.02 m s^−1
					Temperature: RT
		GO/SiO2	Aqueous solution	0.02 wt%, 0.50 wt%	Four balls	Almost 30% reduction in COF (from 0.11 to 0.07); almost 15% reduction in wear scar diameter	94
					Load: 294 N
					Velocity: 1200 rpm
					Temperature: 25 °C
	Graphene–metal and its compounds	GO–Ag	Aqueous solution	0.2 wt%	Ball-on-plate	Improvements in friction reduction and wear resistance by 52.2% and 54.2%	95
					Load: 3 N
					Velocity: 150 rpm
		GO–Al2O3	Aqueous solution	0.06 wt%; GO/Al2O3 ratio = 1 : 1	Block on ring	66% reduction in COF (from 0.53 to 0.19); 64% improvement in surface finish	96
					Load: 10–20 N
					Velocity: 0.01 m s^−1
					Temperature: 20–25 °C
		GO–Fe3O4	Aqueous solution	0.7 wt%	Four balls	The coefficient of friction and wear scar diameter were reduced by 33.6% and 32.3% compared with base fluid	97
					Load: 392 N
					Velocity: 1200 rpm
					Temperature: 20 °C
	Graphene-Others	MBS–GO	Aqueous solution	0.5 wt%; mass ratio of GO to MBS = 60 : 40	Ball-on-plate	Reductions in coefficient of friction (69.7%) and wear volume (60.5%) than in water	98
					Load: 10–40 N
					Sliding speed: 50 mm s^−1
					Frequency: 5 Hz
		GO-PTFE	Aqueous solution	0.6 wt%	Ball-on-disk	The coefficient of friction and wear rate were reduced by 77% and 2 orders of magnitude	99
					Load: (1–20 N)
					Temperature 25 ± 2 °C

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2.2.3.1. Graphene–carbon nanoparticle composite. Carbon-based nanoparticles come in several forms, including spherical fullerenes (C60), one-dimensional carbon nanotubes, two-dimensional graphene oxide and its derivatives, layered graphite, and three-dimensional diamond structures. In a previous study,¹⁰⁰ researchers examined the tribological behavior of carbon-based nanoparticles of varying sizes in an aqueous environment and discovered that graphene oxide notably reduced the break-in period compared to other carbon-based materials, including graphite, carbon nanotubes, and fullerenes (C60). Nanodiamonds are nanoparticles composed of pure carbon with very high hardness and excellent wear resistance. Nanodiamonds can be used as lubricant additives and modifiers, thereby significantly enhance the friction-reducing and anti-wear properties of lubricants. Wu et al.⁹¹ developed a novel lubricant by using graphene oxide and nanodiamond (ND) together as lubricant additives. The lowest coefficient of friction (about 0.03) was obtained when the graphene oxide content in water was 0.1 wt% and the ND content was 0.5 wt%. The researchers proposed that the tribomechanisms behind the superior friction reduction and anti-wear properties included the formation of a sliding tribofilm between graphene flakes, low shear resistance among these flakes, a potential ball-bearing effect from nanodiamonds, and the combined synergistic effects of graphene oxide and nanodiamonds. Graphite has a longstanding history as a traditional lubricant. He et al.⁹² prepared hybrid suspensions of graphitic carbon nitride and graphene oxide to enhance the lubrication stability of pure suspensions, which tend to be less effective at higher loads and speeds. They observed that these hybrid suspensions exhibited excellent tribological performance across all testing conditions. This improvement may be attributed to the graphene oxide nanosheets' smoother, less folded structure and reduced accumulation, which facilitates the formation of a more robust composite tribofilm.

The combination of graphene oxide–carbon nanoparticles has the potential to be an environmentally friendly lubricant, and its superior lubricating performance and non-toxic and environmentally friendly characteristics make it promising for a wide range of applications.

2.2.3.2. Graphene–nonmetal oxide composite. SiO₂ is widely used in lubrication as the main non-metallic oxide additive, recognized for its eco-friendliness, accessibility and excellent tribological properties. Its excellent lubrication effect is mainly attributed to the formation of a protective film and the filling of microscopic grooves during sliding. In addition, due to the high hardness of SiO₂, its composites with graphene oxide may exhibit significantly increased load-bearing capacity. Huang et al.⁹³ synthesized aqueous-based slurries with graphene oxide/silica hybrid nanostructures as additives. They said that the formation of graphene oxide/silica hybrid nanostructures reduced the aggregation of silica particles in water and the accumulation of graphene oxide flakes, thereby enhancing the slurry's wetting ability, reducing friction, and lowering the surface concavity at the abrasive-substrate interface. Lv et al.⁹⁴ developed a graphene oxide/silica (GO/SiO₂) composite as a water-based lubricant. Using a four-ball machine and a milling test rig, they found that the GO/SiO₂ hybrid readily penetrated the contact area of the rubbed metal surfaces to achieve easier shear slip of graphene oxide and rolling effect of SiO₂, thereby accounting for the reduced friction and wear of the sliding pair.
2.2.3.3. Graphene–metal and its oxide composite. Wu et al.⁹⁵ prepared silver/graphene nanocomposites (SGN) by electrostatic adsorption and in situ reduction and found that the tribological properties of ceramic elements were significantly improved by the addition of SGN in water. The combination of metal oxides and graphene is more commonly used in lubrication, since the composites have good lubrication performance and thermal conductivity in water-lubricated environments and can provide effective friction reduction and heat transfer at high temperature, pressure and humidity. Huang et al.⁹⁶ developed a water-based lubricant incorporating graphene oxide nanosheets and alumina (Al₂O₃) nanoparticles as additives. Their findings indicated that Al₂O₃ nanoparticles could efficiently penetrate the contact interface, forming a composite protective layer with graphene oxide nanosheets. This protective layer substantially reduced friction and wear while also decreasing the roughness of the worn surface. Sun and colleagues⁹⁷ employed a solvothermal process to synthesise graphene oxide–Fe₃O₄ (GO–Fe₃O₄) nanocomposites. These GO–Fe₃O₄ nanocomposites demonstrated excellent dispersion stability in water, attributed to the synergistic lubrication effects between Fe₃O₄ nanorods and GO nanosheets. The potential and prospect of the combination of metal oxides and graphene in water lubrication is enormous, and further research and practice await to promote their application therein.
2.2.3.4. Graphene-based composites. Li et al.⁹⁸ investigated the tribological properties of modified biodiesel soot (MBS) nanoparticles, graphene oxide nanosheets, and their combined mixture (MBS–GO) as water-based lubricant additives. Thanks to the synergistic interaction between the MBS nanoparticles and graphene oxide nanosheets, the MBS–GO aqueous samples demonstrated superior lubrication performance compared to both waters alone and the individual additives of MBS and graphene oxide. By electrostatically encapsulating polytetrafluroethelene (PTFE) with GO nanosheets, Yang et al.⁹⁹ developed a novel aqueous lubricant additive, namely GO@PTFE composite, with the objective of enhancing the dispersion stability and wettability of GO in water (Fig. 8). The friction and wear tests of zirconia balls and brass blocks (typical bearing materials) showed that GO@PTFE added in water contributed to reducing the coefficient of friction and wear rate by 77% and 2 orders of magnitude, being superior to PTFE and GO alone therein.

Fig. 8 (a) Schematic diagram of positively charged modified graphene oxide sheets, (b) schematic diagram of electrostatic encapsulation of polytetrafluoroethylene, (c) schematic diagram of tribological tests, (d) COF versus sliding time, (e) wear rate under lubrication of pure water as well as water with PTFE, GO and GO@PTFE lubricant at a concentration of 0.6 wt% at 10 N and 2 cm s⁻¹, as well as COF and wear rate under different additive (f), load (g) and sliding velocity (h).⁹⁹ (Reproduced from ref. 99 with permission from Elsevier, Copyright 2022).

2.3. Summary

Graphene, characterized by its single layer of carbon atoms arranged in a 2D structure, possesses remarkable thermal conductivity and superior mechanical properties. Whether as an oil lubrication additive or water lubrication additive, it can form an efficient lubrication film to reduce friction and wear, reducing the coefficient of friction and improving lubrication. However, graphene has poor dispersion stability. Although the dispersion stability of graphene can be improved to a certain extent by surface modification, it is difficult to achieve effective and controllable modification, due to the uncertainty of the chemical structure and component distribution of graphene surface. Moving forward, efforts should focus on the controlled customization of surface functional groups on graphene materials. This approach would allow for precise regulation of the surface chemical structure and adjustment of graphene's physicochemical properties, ultimately enhancing self-dispersion stability and achieving improved lubrication performance.

Currently, graphene has gradually entered the industrial lubricant market due to its excellent friction reduction and anti-wear properties, especially in high-end applications (e.g. aerospace, automotive engines). However, pristine graphene is highly chemically stable, and its two-dimensional layered structure and strong C–C bonds make it significantly more resistant to oxidation and chemical degradation. In other words, graphene is difficult to be degraded under natural conditions, and undegraded graphene may cause contamination in soils and water bodies. Therefore, it is important to research and develop graphene degradation accelerators (e.g., bio-enzymatic or photochemical-based methods) to shorten the time of its environmental degradation. In this aspect, making use of recycling and reutilization technologies (e.g. centrifugal recycling or filtration separation) is particularly important, while reducing the unit cost of graphene through large-scale production will enable its use more widely in mid-market applications (e.g., industrial lubrication systems).

Graphene oxide is an ideal lubricant in water lubrication, because of its good water dispersibility and reactivity ascribed to abundant oxygen. At present, there are relatively few reports on the adsorption and shear dynamic processes of graphene oxide in water lubrication. Most of the studies focused on pure graphene or graphene–lubricant composite systems in oil lubrication condition, largely because of the complexity of graphene oxide-water system and the interaction between graphene and water making it more challenging to model and simulate. In this sense, graphene composites used in water-lubricated condition should be the focus of future research; and the synergistic effect of graphene with other materials should be considered to develop more efficient graphene lubrication additives. In summary, despite existing challenges, graphene-based materials hold a promising future in tribology. As insights into the lubrication mechanisms and structure of graphene oxide deepen, its application will become increasingly efficient.

3. Layered transition group metal disulphides (TMDS)

Transition metal disulfides, represented by the formula MX₂ (where M is typically a transition metal like Mo or W, and X is S, Se, or Te), have a structure in which metal atoms are sandwiched between layers of S, Se, or Te.^101,102 The adjacent layers in MX₂ are held together by weak van der Waals forces, enabling easy sliding under low shear forces. Consequently, layered transition metal disulfides such as MoS₂ and WS₂ are widely used as lubricant additives to improve lubrication performance;^7,103,104 with MoS₂ particularly noted for its outstanding effectiveness as a lubricant additive.¹⁰⁵

3.1. Oil lubrication

3.1.1. TMDS as oil lubrication additive. M. Gulzar and others¹⁰⁶ evaluated the enhancement of anti-wear (AW) and extreme pressure (EP) properties of chemically modified palm oil (CMPO) when supplemented with CuO and MoS₂ nanoparticles. Findings indicated that MoS₂ nanoparticles offered superior AW/EP properties compared to CuO. Additionally, Guo et al.⁷ reported that monodispersed MoS₂ quantum dots maintained stable dispersion in paraffin oil without settling over a 10-day period. During friction, these nano-MoS₂ quantum dots oxidized to MoO₃, forming a composite tribofilm composed of MoS₂, MoO₃, FeS, and FeSO₄, which contributed to reduced friction and wear.

The morphology of MoS₂ nanoparticles affects their anti-wear and friction-reduction properties. As shown in Fig. 9, there are some differences in the tribological properties of flower-like, microsphere-like and nanosheet-like MoS₂ nanoparticles dispersed in liquid paraffin. Ball-and-disk friction and wear tests demonstrated that all samples effectively improved the tribological properties of liquid paraffin, with MoS₂ nanosheets performing particularly well under heavy loads. This performance is attributed to the ability of two-dimensional MoS₂ to exfoliate into ultrathin sheets and nanofragments. Similarly, ultrathin MoS₂ (5 nm thick), synthesized through solid-phase reaction, showed remarkable friction-reducing and anti-wear effects in base oils. These benefits result from the diffusion of MoS₂ nanosheets into the friction zone and the development of a continuous tribofilm on the contact surface.¹⁰⁷

Fig. 9 Scanning electron microscopy (SEM) and TEM images of as-prepared MoS₂ nanoparticles with floral (a)–(d), micro-spherical (e)–(h), and sheet-like (i)–(l) morphologies.¹⁰⁷ (Reproduced from ref. 107 with permission from Elsevier, Copyright 2017).

Structural defects have been reported to be more important in friction film formation and friction reduction than size and morphology, as they promote the shedding of molybdenum disulfide nanoparticles.^108,109 Researchers have investigated the behaviour of individual molybdenum disulphide nanoparticle under pressure and shear by in situ TEM characterization, providing insights into the atomic mechanisms of interlayer tribology in layered materials.¹¹⁰

Compared to MoS₂ additives, WS₂ nano-lubricants have higher thermal stability and endurance, providing better improvement in tribological properties. In the recent past, researchers have investigated the lubricating performance of WS₂ as an additive and found that it sustains excellent lubrication across a broad temperature range, from −273 °C to 425 °C, outperforming many traditional lubricants.^111–113 The tribological performance of WS₂ additives in base oils is also influenced by their particle size and morphological characteristics. Hu et al.¹¹⁴ explored the tribological characteristics of both laminar and spherical WS₂ when used as lubricant additives in POA6, examining their behavior under different loads and sliding speeds. The laminar form of WS₂, due to its layered structure, allows layers to slip easily across each other, which contributes to a reduction in the coefficient of friction. In contrast, the spherical WS₂ particles act similarly to “micro-bearings,” transforming sliding friction on the contact surface into rolling friction. This shift not only decreases the COF but also enhances the tribological performance of the base oil.

3.1.2. Modified TMDS as oil lubrication additive. TMDS is often recognized for its excellent tribological properties as a lubricant additive, largely attributed to its distinctive exfoliation characteristics. However, a significant challenge arises from its tendency to aggregate when dispersed in base oils. The poor dispersion stability of MX₂ nanoparticles, coupled with inconsistent performance under extreme conditions like high temperatures and pressures, poses a substantial limitation to its broader application as a lubricant additive.¹¹⁵ Consequently, enhancing the dispersion stability of MX₂ in lubricants and improving its frictional performance in demanding environments should be prioritized in future research efforts.

In terms of various methods for preparing stable nano-lubricants, the surface modification of MX₂ with oleylamine or oleic acid is the most common one. Yi et al.¹¹⁶ synthesized two-dimensional MoS₂ nanosheets using a simple solvothermal method with oleylamine as the reaction solvent. The as-growing MoS₂ grains were encapsulated by oleylamine molecules, and the resultant oleylamine-modified MoS₂ nanosheets demonstrated good dispersion stability in paraffin oil and exhibited enhanced friction-reducing and anti-wear properties. At 100 °C, the ultrathin MoS₂ nanosheets, used as additives in SAE10W40 engine lubricating oil, significantly reduced the coefficient of friction by 41.9% and wear rate by 18.2%.¹¹⁷

In our group's previous studies,¹¹⁸ a liquid-phase technique was employed to encapsulate tungsten disulfide (WS₂) nanosheets with oleic acid at a moderately low temperature of 200 °C, enhancing their solubility in organic lubricating base oils. As illustrated in Fig. 10, the incorporation of OA-coated WS₂ nanosheets into PAO6 base oil resulted in a marked improvement in the lubricant's tribological performance at elevated temperatures. The OA-modified WS₂ nanosheets, when used as lubricant additives, exhibited superior tribological properties compared to ZDDP, performing effectively at both room temperature and elevated temperatures of up to 300 °C. To further enhance the tribological characteristics of WS₂ nanoparticles, we conducted dual-surface modification with oleic acid and dodecyl maleic anhydride ester.¹¹⁹ The dual-surface modification substantially enhanced the dispersion stability of WS₂ nanoparticles in synthetic ester base oil, improving their overall compatibility. When added to diisooctyl sebacate base oil, these modified WS₂ nanoparticles demonstrated outstanding friction-reducing and anti-wear properties across a temperature range from room temperature up to 150 °C.

Fig. 10 (a) Preparation of oleic acid modified WS₂ nanosheets; (b) TEM image of modified WS₂ nanosheets; (c) coefficient of friction of steel ball contact under lubrication of PAO6 and PAO6 with 1.0 wt% of OA-modified WS₂ nanosheets at different temperatures and (d) wear scar diameter.¹¹⁸ (Reproduced from ref. 118 with permission from Elsevier, Copyright 2019).

Wu et al.¹²⁰ employed oleic acid diethanolamine borate (ODAB) to modify MoS₂ nanosheets, resulting in ODAB–MoS₂ nanosheets with enhanced dispersion and stability within the base oil and superior tribological properties compared to unmodified MoS₂. Similarly, Guo et al.¹²¹ investigated the effects of oleic acid and ionic liquids as surfactants in ZnO and WS₂ nanofluids. Their findings indicated that phosphate-based ionic liquids provided long-term stability as surfactants. However, adding phosphonate ionic liquids alone did not enhance the tribological properties of the nanofluids. In contrast, the inclusion of oleic acid as a surfactant effectively reduced friction and wear in both ZnO and WS₂ nanofluids. SEM analysis further revealed that the oleic acid surfactant formed a protective coating on the disk surface, minimizing friction and wear.

3.1.3. TMDS nanocomposites as oil lubrication additive. Optimal combinations of MoS₂ with other materials contribute to lowering the coefficient of friction and minimizing wear in frictional systems.
3.1.3.1. TMDS–metals and their composites. Xu et al.¹²² applied an eco-friendly biomimetic approach to synthesize tungsten disulfide–polydopamine–copper (WS₂–PDA–Cu) nanocomposites for use as lubricant additives in poly alkylene glycol. These WS₂–PDA–Cu nanocomposites demonstrated excellent dispersion stability and significantly outperformed Cu nanoparticles WS₂, and WS₂–Cu mixtures in PAO base oils, providing superior friction reduction and anti-wear capabilities. Li et al.¹²³ prepared ZnFe₂O₄ nanospheres with different shell thicknesses, flaky MoS₂ and three ZnFe₂O₄@MoS₂ core–shell composites and added them to base oils to simulate the wear of hybrid bearings. As shown in Fig. 11, ZnFe₂O₄@MoS₂ core–shell composites exhibited better lubrication performance than ZnFe₂O₄ and MoS₂, especially for the ZnFe₂O₄@MoS₂ sample with a concentration of 0.5 wt%. This improvement may result from the synergistic interaction between the hard and soft layers in the core–shell composites, which offers notable benefits. These composites are also more inclined to adsorb at the sliding interface, forming a stable adsorption film that enhances tribological properties.

Fig. 11 (a) Schematic of synthesis of WS₂–PDA–Cu as well as (b) coefficient of friction and wear volume (c) of the disks lubricated by PAG containing different additives (load, 100 N; temperature, 150 °C; stroke, 1 mm; frequency, 25 Hz).¹²² (Reproduced from ref¹²² with permission from Royal Society of Chemistry, Copyright 2019).

3.1.3.2. TMDS–carbon materials and their composite derivatives. Theoretical analyses indicate that friction in sliding layered materials is largely influenced by the commensurability of their lattice structures. When lattice alignment is precise, interfacial contacts create a substantial energy barrier, resulting in high friction. However, an angular misalignment or structural mismatch between the lattices of layered materials lowers this energy barrier, enhancing lubrication properties. Consequently, heterostructures composed of stacked, distinct 2D materials are gaining attention for their potential in low-friction applications. The weak interfacial interactions between mismatched lattice structures in two different 2D layered materials are especially promising for developing a new generation of advanced lubricant additives.

Zuzanna Bojarska et al.¹²⁴ utilized a jet reactor to deposit MoS₂ nanoparticles onto carbon-based graphene oxide surfaces, creating hybrid materials that significantly improved the rheological and tribological characteristics of engine oils. At temperatures above 0 °C, the incorporation of MoS₂-based nanomaterials resulted in up to a 55% reduction in the friction coefficient of the engine oil. Gong et al.¹²⁵ applied MoS₂ nanoparticles onto graphene sheets, forming MoS₂/GO nanocomposites that enhanced the thermal conductivity and viscoelasticity of PAO base oil. The addition of these nanocomposites significantly boosted the base oil's friction-reducing and anti-wear properties at temperatures ranging from 50 °C to 100 °C and under loads of 25–100 N. This approach offers a promising pathway for leveraging MoS₂ as a lubricant additive in high-temperature applications. Xin et al.¹²⁶ prepared graphene oxide/nanometer molybdenum disulfide (GMS) composite with self-lubricating and anti-wear properties. GMS hybrids had the high mechanical strength of graphene oxide and the excellent toughness of nanoscale molybdenum disulfide. The graphene oxide/molybdenum disulfide nanocomposite could be stably and uniformly dispersed in gear oil. As a gear oil additive, it exhibited excellent tribological properties, reducing the coefficient of friction by ∼16.7% and wear rate by 80%. Zheng et al.¹²⁷ prepared a novel composite material of graphene anchored with WS₂ nanoparticles (WS₂/GP). Using a UMT-2 ball-on-plate tribometer, they found that the oil with WS₂/GP (0.02%-0.04 wt%) exhibited significant lubricating performance and was able to reduce the coefficient of friction and wear rate by 70.2% and 65.8%, being superior to single nanomaterials GP or WS₂ nanoparticles and the base oil with physically mixed nano-WS₂ and GP. Ajay Chouhan et al.¹²⁸ introduced an efficient synthesis method for ZnO-modified reduced graphene oxide/MoS₂ (Gr–MS–Zn) nanosheets, which improved the stable dispersion of Gr–MS–Zn within fully formulated engine oil. Incorporating a small amount of Gr–MS–Zn nanomaterial as an additive in engine oil notably enhanced the tribological performance of steel-steel friction pairs.

The standout in the MoS₂ family, fullerene-like MoS₂ (IF-MoS₂), featuring a uniquely curved hexagonal S–Mo–S plane, has been attracting growing interest due to its exceptionally low friction coefficient compared to traditional hexagonal MoS₂ structures. IF-MoS₂ exhibits a hollow morphology with a shell thickness of approximately 15 nm, where the MoS₂ layers maintain an ideal layered arrangement. This hollow, fullerene-like architecture is anticipated to provide outstanding tribological performance.¹²⁹ Inspired by the innovative hollow structure of IF-MoS₂ and the favorable weak interaction between MoS₂ and graphene for reducing friction and enhancing anti-wear performance, Wu et al.¹³⁰ synthesized hollow IF-MoS2/reduced graphene oxide (HIF-MoS₂/RGO) nanocomposites via a hydrothermal method. The tribological properties of these HIF-MoS₂/RGO nanocomposites were investigated by incorporating them into ionic lubricating grease. As illustrated in Fig. 12(c) and (d), the HIF-MoS₂/RGO nanocomposites demonstrated superior anti-wear performance compared to commercial MoS₂ nanoparticles, RGO, and both the individual and physically blended forms of HIF-MoS₂, particularly under high-load conditions. Under pressures of 3 GPa, the wear reduction achieved by the HIF-MoS₂/RGO-enhanced grease reached an impressive 96%. Furthermore, the functionalized grease maintained a lower and more stable coefficient of friction at 3 GPa.

Fig. 12 (a) Schematic diagram of the formation of HIF-MoS₂/RGO heterostructure, (a)–(f) TEM images of HIF-MoS₂/0.46RGO at different magnifications; (g)–(k) EDS element distributions, (l) and (m) wear volume of the disk and wear scar diameter of the ball under the lubrication of grease with different additives at 3 GPa load.¹³⁰ (Reproduced from ref. 130 with permission from Elsevier, Copyright 2018).

Currently available researches on two-dimensional heterostructures as lubricant additives provide a new direction for liquid lubricant applications. Continued research is needed to refine the compatibility of 2D materials with different lubricating oils and to develop methods for controlled and effective surface modifications.

3.1.3.3. TMDS-other materials’ composites. Hu et al.¹³¹ deposited MoS₂ on the surface of activated fly ash (FA), encapsulating the FA particles to form MoS₂/FA composites. Friction tests revealed that the MoS₂/FA composites exhibited better lubrication performance than nano-MoS₂. This suggests a synergistic interaction between nano-MoS₂ and FA, enhancing their combined performance. The primary component of serpentine minerals, magnesium silicate hydroxide (MSH), holds promise as a lubricant additive due to its notable wear resistance and self-healing properties. To integrate the friction-reducing capability of MoS₂ with MSH's superior anti-wear and self-healing characteristics, Guan et al.¹³² synthesized MSH-enhanced MoS₂ hybrid nanomaterials via hydrothermal synthesis. As depicted in Fig. 13, at a reaction temperature of 220 °C (MM2), an edge-pinning effect emerged from the structural incoherence between the MoS₂ nanosheets and MSH nanorods, securely binding them together. This hybrid additive displayed excellent dispersion stability in PAO base oil, providing superior anti-wear and friction-reducing performance.

Fig. 13 (a) TEM image and (b) Elements distribution detected by TEM; (c) High-resolution TEM image of MM2; (d) and (e) Corresponding IFFT images and profiles inside the rectangular area of the HR-TEM image; (f) COF curves as a function of friction time, (g) average COF and wear scar diameter under the lubrication of different oil samples, and (h) COF curves with the lubrication of various oil samples under different loads; 2D, 3D, and roughness profiles of the worn surfaces lubricated with different additives in PAO oil (60 min, 392 N): (i) PAO, (j) PAO-MSH, (k) PAO-MM1, (l) PAO-MM2, (m) PAO-MM3, and (n) PAO-MM4.¹³² (Reproduced from ref. 132 with permission from Elsevier, Copyright 2022).

3.2. Summary

In summary, both molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) exhibit excellent lubrication performance, due to their layered structures. They provide effective lubrication on metal surfaces and are widely used to reduce friction and wear between metals, especially under high-temperature and high-pressure conditions. However, the dispersion of TMDS in water is challenging, corresponding to limited research on their use as water-based lubricants. Enhancing the dispersion of TMDS in water-based lubricants should be a focus of future research. For two-dimensional (2D) layered materials, the number of layers also affects their lubrication performance. Precise control of the layer number during preparation to optimize lubrication properties will be a key research focus. Composite 2D materials (nano-hybrids/heterojunctions) offer potential for synergistic effects in lubrication. Combining different 2D layered nanomaterials could be a promising approach to further enhance friction and wear reduction. This offers valuable insights for lubricant applications under extreme conditions. Future work should emphasize the development and utilization of composite TMDS, as well as the exploration of the combinations of different 2D layered nanomaterials as additives to improve lubricant performance. Research on the effects of 2D material heterojunctions on lubricant performance is currently limited and should be a focus of future studies.

TMDSs are typically used in lubricants at low concentrations (0.1%-1.0 wt%) and exhibit excellent friction reduction and antiwear properties under high loads and high temperatures, significantly reducing equipment wear and energy consumption. TMDSs are synthesized by mechanical stripping, pyrolysis and chemical vapour deposition (CVD), which are mature, low-cost and industrially available, and suitable for a wide range of lubrication scenarios, including high temperature, high pressure and extreme conditions. However, in acidic or reducing environments, TMDSs may release trace metal ions (e.g., Mo⁶⁺, W⁶⁺) which are potential threats to ecosystem and human health, and high-performance applications (e.g., functionalized TMDSs) may increase costs. In this sense, it is important to develop technologies for recyclable TMDSs so as to reduce the accumulation of material residues in the environment. Besides, it is imperative to carry out prolonged testing of TMDSs in a variety of industrial lubrication scenarios (e.g. high temperature and high-pressure conditions) so as to optimize their formulations and validate their economics, achieving a better balance between environmental impact and economics.

Although there is increasing interest in utilizing 2D materials for achieving macro-scale super-lubrication, research on the use of MX₂ materials in this area remains limited. Consequently, prioritizing the exploration of MX₂ materials for super-lubrication applications is essential for future research.

4. Mxenes

In 2011, researchers at Drexel University, under the leadership of Gogotsi and Barsoum, identified a novel class of two-dimensional materials, now referred to as MXenes.¹³³ MXenes represent a class of two-dimensional materials consisting of transition metal carbides, carbonitrides, and nitrides, generally characterized by the chemical formula M_n+1X_n. These compounds are synthesized by selectively removing the A layers from MAX phases (M_n+1AX_n), where MMM denotes an early transition metal, AAA is an element from group 13 or 14, and XXX stands for carbon and/or nitrogen atoms, with n taking values of 1, 2, or 3.¹³⁴ Ti₃C₂T_x is a representative example of a 2D transition metal carbide/nitride MXenes, obtained by etching the Al layer from the Ti₃AlC₂ MAX phase. The stripping schematic is shown in Fig. 14,^135–137 where T_x denotes surface functional groups (–OH, –F, –O). The reaction that occurs when Ti₃AlC₂ is immersed in HF is as follows:

Ti₃AlC₂ + 3HF = AlF₃ + 3/2H₂ + Ti₃C₂ (1)

Ti₃C₂ + 2H₂O = Ti₃C₂(OH)₂ + H₂ (2)

Ti₃C₂ + 2HF = Ti₃C₂F₂ + H₂ (3)

Fig. 14 Schematic diagram of the stripping process of Ti₃AlC₂.¹³⁵ (Reproduced from ref. 135 with permission from MDPI, Copyright 2021).

The shear strength between Ti₃C₂T_x MXene sheets is relatively low, allowing the atomic layers to easily slide. Additionally, MXenes tend to adsorb onto the surfaces of friction pairs, reducing wear. Therefore, Ti₃C₂T_x MXenes have potential applications in the field of tribology.^138,139

4.1. Oil lubrication

4.1.1. Mxenes as lubricating oil additive. The mass fraction of MXene is an important parameter for the lubrication system, as lower mass fractions may not have sufficient impact, while too many MXene nanoflakes may deteriorate the lubrication performance, due to severe agglomeration and particle fragmentation hindering MXene flakes to enter narrow contact areas. Yang et al.¹⁴⁰ was the first to investigate the use of Ti₃C₂T_x as a lubricant additive. The researchers directly mixed Ti₃C₂T_x nanosheets into paraffin oil at different weight concentrations and assessed the frictional behavior at a fixed rotational speed and varying load. Results indicated that adding 1.0 wt% of Ti₃C₂T_x nearly halved the coefficient of friction, highlighting its potential as a highly effective friction-reducing agent under suitable compressive conditions. At this concentration, Ti₃C₂T_x nanoparticles were deposited on worn metal surfaces to act as a polishing and repairing agent. However, as the concentration of Ti₃C₂T_x increased, aggregation due to poor dispersion led to decreased friction-reducing performance. Liu et al.¹⁴¹ conducted frictional experiments with a mixture of PAO and Ti₃C₂T_x nanosheets and found that Ti₃C₂T_x added at a concentration of 0.8 wt% reduced both friction and wear. They suggested that the highly exfoliated Ti₃C₂T_x nanosheets at the optimal concentration facilitated the adhesion of Ti₃C₂T_x nanosheets on worn surface and the formation of a cohesive tribofilm thereon, significantly enhancing the tribological properties.

Most of the research activities related to the application of synthesized hydrophilic MXene nanosheets in hydrophobic base oils have focused on exploring the optimal MXene concentration. Boidi G et al.¹⁴² conducted ball-and-disk testing of multilayer Ti₃C₂T_x MXene nanosheets as PAO additives at different speeds, temperatures, loads, slip-roll ratios, and MXene concentrations (0.5 and 3 wt%) to evaluate their performance under a wide range of experimental conditions. In most cases, the multilayer Ti₃C₂T_x MXenes (3 wt%) significantly reduced the coefficient of friction. Under severe conditions (higher loads, higher sliding ratios and higher temperatures), which lead to reduced film thickness and harsher contact, MXenes significantly reduced friction by about 30%. Therefore, multilayer Ti₃C₂T_x MXene nanosheets show good potential as lubricant additives for components operating under harsh and changing conditions.

Existing studies have shown that MXenes (Ti₃C₂T_x) as lubricant additives in different base oils have significant friction reduction and antiwear properties under varying operation conditions. In terms of MXene-based tribological applications, such as degradation over time, particle fragmentation, agglomeration, oxidation in different mediums, variations in specific temperatures or pressures, the possible reactions between MXene flakes and the influences of structural defects have not been well characterized yet. The degree of exfoliation, the number of layers and the distribution of surface groups of MXenes vary in different preparation processes, which directly affects their lubrication performance. To address this issue, researchers need to develop advanced material characterization facilities and techniques; and they also need to establish standardized methods for the preparation of MXenes in order to ensure the reproducibility of research results. At the same time, it is critical to conduct larger scale tests under operating conditions, especially for application evaluation on industrial grade equipment, and to study the stability and wear control effects of MXenes lubrication films under long-term operation. Based on standardized experiments, optimized dispersion technology and industrial validation, MXenes would be expected to become the next generation of high-performance lubricant additives, providing a reliable solution for industrial friction and wear control.

4.1.2. Modified MXene as lubricating oil additive. The dispersion of nano-additives within lubricant base oils plays a crucial role in the lubricants' tribological performance. Earlier research has shown that incorporating pure MXenes into base oils significantly enhances the lubricants' friction-reduction and wear-resistance characteristics.^140,143 However, the natural hydrophilic surface termination of MXenes, due to groups like –OH, leads to poor dispersion stability in hydrophobic base oils. Fortunately, these surface functional groups provide numerous active sites, enabling extensive possibilities for chemical modification. Employing targeted functionalization methods, such as grafting small molecules or polymers, can adjust the interlayer spacing and modify the hydrophobicity of MXenes. These adjustments enhance their dispersion stability in base oils, thereby improving the lubricants' tribological properties.

Feng et al.¹⁴⁴ developed a method for chemically modifying 2D Ti₃C₂ MXene materials using tetrabutyl phosphoric acid (TDPA). The modification involves a condensation reaction between the phosphoric groups on the Ti₃C₂ surface and hydroxyl groups, represented by the reaction of Ti₃C₂–OH + R–P–OH → Ti₃C₂–O–P + H₂O (Fig. 15(a)) resulting in modified Ti₃C₂ nanosheets. Fig. 15(b) demonstrates that the modified material exhibits good dispersion stability in castor oil. During lubrication, TDPA–Ti₃C₂ was deposited and adsorbed on the friction surfaces to form a physical transfer film, thereby preventing the direct contact between friction pairs and effectively enhancing the lubricant's performance (Fig. 15(c) and (d)). In terms of the lubrication mechanism, Ti₃C₂ directly added as a solid lubricant additive in castor oil tends to clump or deposit, resulting in poor lubrication effect (Fig. 15(e)). Following TDPA grafting, the long-chain alkyl phosphoric acid can integrate into the oil molecules, establishing effective binding with them. Moreover, phosphoric acid molecules intercalate within the 2D layers, leading to an expansion of the nanosheets' interlayer spacing, which further enhances interactions between the oil molecules and nanosheets. The uniform dispersion of TDPA-modified within castor oil facilitates the lubricant's access to the frictional interface, creating a consistent lubrication film over the contact area. This film prevents direct contact between frictional surfaces, significantly reducing wear and friction between them.

Fig. 15 (a) Schematic diagram of TDPA grafted Ti₃C₂, (b) photographs of castor oil with different mass concentrations of nano-additives at different time and the Tyndall effect of castor oil with 0.1% TDPA–Ti₃C₂, (c) mean friction coefficient of castor oil, castor oil with 0.1 wt% Ti₃C₂ and castor oil with 0.1 wt% TDPA–Ti₃C₂, (d) average wear rate of castor oil, castor oil containing 0.1 wt% Ti₃C₂ and castor oil containing 0.1 wt% TDPA–Ti₃C₂, and (e) lubrication mechanism diagram.¹⁴⁴ (Reproduced from ref¹⁴⁴ with permission from Elsevier, Copyright 2021).

Gang et al.¹⁴⁵ proposed a multi-layer MXene exfoliation strategy based on water-assisted intercalation. MXenes and TDPA@MXene sheets were prepared using chemical etching and TDPA as lubricant oil additive. To assess the impact of the additive on the lubricants' physicochemical properties, the particle size distribution and zeta potential of the prepared samples were analyzed. Findings revealed that TDPA grafting improved the compatibility of MXenes with PAO8, attributed to the presence of long-chain alkyl phosphoric acid on the MXene surface. Consequently, the stability of the PAO8-TDPA@MXene mixture was enhanced, indicating substantial potential for improving the tribological performance of base oils. Functionalizing Ti₃C₂T_x nanosheets with TDPA expanded the interlayer spacing and enhanced their dispersion in oil, facilitating the formation of a lubricant film within the contact area. As a lubricant additive, these delaminated MXene layers, with fewer stacked sheets, are more capable of establishing a continuous tribofilm, thereby improving tribological performance. The creation of durable MXene tribofilms with sustained anti-wear properties is vital for enabling the energy-efficient functioning of mechanical systems.¹⁴⁶

MXenes can undergo functionalization with various molecular structures. For instance, Guo et al.¹⁴⁷ developed a fluorinated MXene by modifying its surface with a fluoropolymer. This tailored MXene exhibited excellent dispersion in a fluorinated lubricant, PFPE, and effectively enhanced the anti-wear performance of PFPE in steel/steel contact applications.

A comparative evaluation of Mexeen material properties and modification strategies is presented in Table 5. Through targeted modification strategies (e.g., TDPA functionalization, fluoropolymer modification, and water-assisted stripping), the dispersibility, lubrication performance, and environmental adaptability of MXene were significantly enhanced. At the same time, these modification strategies can effectively solve the problems of agglomeration and sedimentation by enlarging the MXene layer spacing, enhancing its compatibility with oil molecules, and promoting the formation of lubricant film.

In practical use, the modification strategies are selected for different working conditions. For example, TDPA functionalization can be preferred in normal lubricants, while fluoropolymer-modified MXene is more advantageous under extreme conditions such as high temperature and high pressure. For promoting the large-scale application of modified MXene in real industry, it is essential to optimize the path to industrialization by simplifying the stripping and modification process and reducing the material cost. In terms of long-term performance research of modified MXene, it is imperative to evaluate its long-term stability, heat resistance and anti-wear performance under complex lubrication conditions so as to ensure its reliability. Furthermore, with a view to green lubrication technology development, it needs to explore more environmentally friendly modification methods to reduce the potential impact of MXene lubricant materials on the environment and promote their sustainable application.

Table 5 Comparative evaluation of material properties and modification strategies

Materials	Modification method	Dispersion stability	Lubricating properties	Conditions of application	limitations	Ref.
Ti3C2	No modification	Poor, easily agglomerated or settled	Limited friction reduction and anti-wear properties	General lubricant environment	Poor lubrication due to agglomeration	140 and 143
TDPA modified Ti3C2	TDPA condensation reaction	High, long-chain alkyls enhance dispersion	Excellent, significantly reduced coefficient of friction and wear	Castor oil, low temperature, low load	Extreme environmental stability needs further study	144
Stripping + TDPA modification	Water-assisted stripping + TDPA modification	High, stripping reduces sheet stacking	Excellent, layer spacing expanded to form a lubricating film	PAO8, low temperature, medium load	Complex process, long-term stability needs to be verified	145
Fluorinated MXene	Very high, suitable for perfluorinated lubrication systems	Excellent, high chemical and high temperature resistance	Excellent, high chemical and high temperature resistance	High temperature, high load, extreme working conditions	High cost and environmental impact issues	147

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4.1.3. MXene nanocomposites as lubricating oil additives. Combining MXene with other materials is also a valuable approach to improve its dispersion stability and lubrication performance. For example, spherical nanoparticles can enhance the load-bearing capacity of base oils. Cui et al.¹⁴⁸ developed MXene@Cu nanocomposites via electrostatic adsorption of Cu²⁺ ions followed by in situ reduction. The Cu nanoparticles not only expanded the interlayer spacing and promoted interlayer slip but also lowered the surface energy of MXene, enhancing its dispersibility in organic solvents. During lubrication, MXene and Cu exhibited a synergistic effect, which significantly boosted lubrication performance, especially under extreme conditions. The synthesis process of MXenes is more complicated and costly, which limits large-scale commercial applications. A recyclable oil-soluble magnetic MXene was synthesized by in situ loading of Fe₃O₄ nanoparticles on MXene nanosheets and modifying them with dodecylphosphonic acid, and it was easily recovered by applying a magnetic field.¹⁴⁹ The addition of 0.5 wt% MXene@Fe₃O₄ reduced the average coefficient of friction and wear volume by 14.8% and 95.0%, respectively, compared to the base oil. The friction film formed on the wear track was analyzed using focused ion beam (FIB) cross-section cutting to explore the friction film composition and lubrication mechanism of C12PA-MXene@Fe₃O₄, as shown in Fig. 16. The excellent tribological performance of MXene@Fe₃O₄ is attributed to its good dispersion and the synergistic effect of MXene nanosheets and Fe₃O₄ nanoparticles. Namely, the MXene nanosheets act as the carriers of Fe₃O₄ nanoparticles to prevent their aggregation, while the Fe₃O₄ nanoparticles loaded on the MXene nanosheets promote the sliding of the MXene layer by enlarging the layer spacing. After tribological testing, a continuous and relatively smooth tribofilm with a thickness between 38–80 nm, consisting of phosphate ions, titanium oxide, and iron oxide, formed on the rubbed metal surface to prevent the direct contact between the rubbing surfaces and contributes to excellent anti-wear performance.

Fig. 16 FIB-TEM analysis of the formed tribofilm on the worn surface lubricated by the lubricant containing C12PA-MXene@Fe₃O₄. (a) Cross-sectional TEM image of the tribofilm, (b) high angle annual dark field (HAADF) image with corresponding element mapping images of region 1 indicated in a, (c) EDS line-scan (along the direction marked in a) analyses of line 1, (d) and (e) cross-sectional HR-TEM images of the tribofilm, and (f) lubrication mechanism of the lubricant containing C12PA-MXene@Fe₃O₄.¹⁴⁹ (Reproduced from ref. 149 with permission from Elsevier, Copyright 2025).

MXene@Fe₃O₄ composites achieve recyclable functionality while maintaining excellent antiwear and friction reduction properties. The Fe₃O₄ nanoparticles not only extend the MXene layer spacing but also provide added value through magnetism. However, its improvement in friction-reducing ability is slightly lower than that of Cu particles. In practice, MXene@Cu is more suitable for high thermal conductivity and high load lubrication scenarios, while MXene@Fe₃O₄ is more sustainable and economically viable for industrial recirculating lubrication systems due to its good recyclability.

Nitrogen-doped carbon quantum dots (NCQDs) serve as lubricant additives with a singular tribological mechanism, and their surface is challenging to functionalize. Thus, combining NCQDs with two-dimensional layered materials presents a promising strategy to enhance lubrication performance and improve interfacial bonding behavior. Wang et al.¹⁵⁰ took advantage of the affinity of the active groups on the surface of Ti₃C₂T_x to assemble NCQDs onto the layered structure of Ti₃C₂T_x. The NCQDs@Ti₃C₂T_x was dispersed in base oils sunflower oil and PAO10 and their tribological properties were evaluated. As shown in Fig. 17, NCQDs@Ti₃C₂T_x dispersed in the base oils had significant friction-reduction and antiwear properties. Compared with NCQDs as lubricant additives, NCQDs@Ti₃C₂T_x composites can exert synergistic lubrication effect and participate in tribochemical reactions, resulting in the formation of transfer films between surfaces, which provides the composites of zero-dimensional and two-dimensional materials with good prospect in engineering. Moreover, optimizing the microstructure of MXene in the base oil is also an effective strategy to improve its stable dispersion in lubricant base oil. For example, compared to multilayer MXenes, few-layer MXenes have better dispersion, due to enlarged interlayer spacing, weakened interlayer interactions, improved dispersive stability, and enhanced formation of protective film. Cui et al.¹⁵¹ successfully synthesized carbon dots functionalized with catechol moieties by using a pulsed laser for irradiation of tannic acid in acetone. By leveraging chemical bonding between the catechol groups in carbon dots and Ti–OH groups grafted onto the MXene surface, researchers synthesized MXene@CDs hybrids. These carbon dots expanded the layer spacing of MXene nanosheets, reducing self-stacking and facilitating smoother interlayer sliding. As an oil-based lubrication additive, MXene@CDs demonstrated remarkable performance by lowering the coefficient of friction from 0.58 (for PAO10) to 0.1 and reducing wear volume by 91.3%. The unique intercalated structure of MXene@CDs, driven by shear-inducing and self-healing mechanisms, enabled the formation of a protective film during friction. This film minimized the contact area of the frictional surfaces and eliminated the substrate's polishing effect, thereby achieving a synergistic lubrication effect.

Fig. 17 (a) Friction curves and (d) average coefficient of friction and wear track widths at 0.010–0.125% additive concentration, (b) friction curves and (e) average coefficient of friction and wear track widths under different loads (1–3 N), (c) friction curves and (f) average coefficient of friction and wear track width at 100–400 rpm, (g) friction curves and (h) average coefficient of friction and wear track width at 0.010–0.125% additive concentration, (j) friction curves and (k) average coefficient of friction and wear track width under different loads (0.5–2.5 N), (i) friction curves and (l) average coefficient of friction and wear track width at 100–400 rpm.¹⁵⁰ (Reproduced from ref. 150 with permission from Elsevier, Copyright 2023).

Polytetrafluoroethylene (PTFE) is regarded as a highly promising material for lubricant additives due to its exceptional hardness and thermal stability. Cui et al.¹⁵² prepared MXene@mPTFE composites by loading hydrogen-bonded phenolic resin-modified PTFE (mPTFE) onto MXene nanosheets. The synthesized MXene@mPTFE composites, composed of layered MXene combined with spherical PTFE particles, represent a multi-scale material integration, as illustrated in Fig. 18(a). The unique multi-component structure of MXene@mPTFE minimizes MXene's self-aggregation while achieves a lower COF as well as good antiwear ability and high load-bearing capacity (Fig. 18((b) and (c)), attributed to multiple lubrication mechanisms. The high hardness and high thermal stability of mPTFE play a key role in enhancing the overall material properties. Multi-scale designs (e.g., MXene@mPTFE) are more flexible than single-scale (e.g., MXene@Cu) composites, adapting to a wide range of operating conditions. However, such complex designs also bring increased preparation costs and technical difficulties, limiting large-scale applications.

Fig. 18 (a) Preparation process of MXene@mPTFE, (b) COF variation curves of PAO6, mPTFE, MXene and MXene@mPTFE at 150 N, 50 °C, 25 Hz (1.2 wt%) and (c) average COF.¹⁵² (Reproduced from ref. 152 with permission from Elsevier, Copyright 2024).

Heterojunctions formed with 2D materials exhibit superior tribological properties and long-term stability over spherical nanoparticles in lubrication applications due to their face-to-face contact structure, high thermal conductivity and good dispersion stability. Feng et al.¹⁵³ successfully synthesised few-layer Ti₃C₂T_x/MoS₂ composites by using hydrothermal technique to achieve vertical growth of MoS₂ nanosheets on few-layer Ti₃C₂T_x substrates to form a stable face-to-face contact heterostructure. These composites exhibited excellent dispersion stability in the lubricant, ensuring consistent properties. With the synergistic lubrication of Ti₃C₂T_x and MoS₂, the addition of few-layer Ti₃C₂T_x/MoS₂ composites with a concentration of 0.3 wt% resulted in a reduction of the coefficient of friction by 38.9% and a reduction of wear by 83.5%. The high thermal conductivity and structural stability of the heterojunction of two-dimensional materials make them more advantageous under high load and high temperature working conditions.

Guo et al.¹⁵⁴ introduced an innovative solution to enhance the dispersibility of MXene as an oil lubricant additive by functionalizing it with ionic liquid, resulting in MX@IL. The polymer attached to the MXene increased the spacing between layers, effectively preventing the layers from aggregating, thereby enhancing both dispersion and stability within the 500SN base oil. Friction testing indicated that the MX@IL additives lowered the friction coefficient from 0.188 to 0.112, achieving a wear reduction of 81.7% in comparison with traditional base oils. Furthermore, the material's load-bearing capacity was elevated to 600 N, while maintaining a stable friction coefficient across temperatures ranging from 40 to 120 °C. Zhou et al.¹⁵⁵ achieved structural modification of MXenes to form heterogeneous structures via in situ growth of hydroxyl salts (HS). Compared to pure MXenes, MXene/HS composites demonstrated significantly improved dispersion stability in PAO-10 base oil and greatly enhanced lubrication performance.

Table 6 lists the comparison of different composites, covering material types, composite methods, advantages, challenges and application scenarios. Although composites are usually found to have significant friction reduction and antiwear properties under specific conditions, the results need to be verified under a wider range of practical application conditions, and the processes used in some of the studies (e.g., Cu²⁺ electrostatic adsorption, electrochemical deposition, etc.) work well in the laboratory but may not be reproducible under industrial conditions, due to reaction homogeneity and production efficiency issues. Therefore, there is a need to further clarify the lubrication mechanisms and synergistic effects of different composites, optimize the material design (e.g., combining the advantages of 2D–2D structures and multi-scale composites), conduct a comprehensive performance evaluation under actual working conditions (e.g., high load, high temperature, and long-term operation), ensure the reproducibility of the results, and strengthen the research on industrial applications.

Table 6 Comparison of different composite materials

Material	Composite method	Dominance	Applicable scenarios	Challenge	Ref.
MXene@Cu	In situ reduction after electrostatic adsorption of Cu^2+	Increase MXene layer spacing, promote interlayer slip, enhance thermal conductivity and anti-friction properties, improve dispersion, and reduce surface energy	Lubrication of machinery and equipment under high load and high temperature working conditions	Complicated preparation process, difficult to control the uniformity of Cu particle distribution	148
MXene@Fe3O4	In situ loading of Fe3O4 nanoparticles	Excellent anti-wear properties, 95% wear reduction, magnetic particles are recyclable and environmentally friendly	Recoverable Lubricant Additives for Industrial Lubrication Systems	Magnetic design adds complexity to the process and requires verification of stability at high loads and over long periods of use	149
NCQDs@MXene	Binding of carbon quantum dots (CQDs) to MXene surface functional groups	Zero-dimensional and two-dimensional materials work in synergy to significantly reduce friction and wear.	Lubrication applications under low load, low temperature conditions	Lubricating performance affected by temperature and oil variation, uniformity of carbon quantum dot distribution and reproducibility; chemical reactions non-verified	150
MXene@CDs	Carbon quantum dots prepared by laser and bound to MXene	Carbon dots extend the layer spacing and promote lubricant layer slip, reducing the coefficient of friction from 0.58 to 0.1 in PAO-10 and reducing wear by 91.3 percent	Highly effective anti-wear additives for lubrication systems (e.g., engine lubrication)	Complex synthesis of carbon dots and high cost of laser processing	151
MXene@mPTFE	Formation of multiscale composite structures by combining MXene with modified PTFE particles	PTFE hardness and thermal stability enhance material properties, multi-scale design reduces MXene aggregation problems	High-load, long-life industrial lubrication applications	Higher preparation cost and high dispersion uniformity required for multi-component composite design	152
MXene@MoS2	Hydrothermal vertical growth of MoS2 nanosheets on MXene surface	2D–2D face-to-face contact, large contact area, high thermal conductivity and high-pressure resistance, excellent performance under high temperature and high pressure	Lubrication applications in high temperature, high pressure environments (e.g. engines, aerospace equipment)	Higher preparation costs, process not yet optimized for large-scale production	153
MXene@IL	Ionic liquids bound by functionalized MXene surfaces	Improved load capacity and stability at high temperatures (600 N load, stable coefficient of friction in the range 40–120 °C)	High temperature, high pressure machinery lubricant additives	High cost of ionic liquids, environmental friendliness needs to be evaluated	154
MXene@HS	MXene grows and hydroxyl salts (HS) in situ form heterojunctions	Modified to outperform pure MXene in PAO10 base oils	Efficient lubrication applications in a wide range of base stocks	Difficulty in industrializing the preparation process	155

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4.2. Water lubrication

Water-based lubricants are extensively applied in machinery lubrication, cutting fluids, and hydraulic fluids due to their superior cooling and fire-resistant characteristics. Additionally, they offer greater safety and reduced environmental impact compared to oil-based lubricants, leading to lower contamination levels.^156–158 Traditional water-only lubricants have increasingly fallen short of meeting the demands of certain specialized applications. To address this limitation, researchers frequently incorporate nano-additives into water-based lubricants to improve their tribological performance.^159,160

Chen et al.¹⁶¹ introduced a novel approach to synthesize Ti₃C₂T_x nanosheets and investigated their tribological performance as nano-additives in water-based lubrication. The hydrophilic functional groups on the Ti₃C₂T_x surface enhance its compatibility as a water lubricant additive, significantly contributing to friction reduction and wear resistance during sliding. When added to the lubricant at a low concentration of 0.8 mg mL⁻¹, the as-prepared AP-Ti₃C₂T_x nanosheets (with a thickness of 1.9–2.3 nm) achieved a 34.74% reduction in the average coefficient of friction and a 45.58% decrease in wear rate compared to pure water alone. Cheng et al.¹⁶² prepared novel two-dimensional Nb₂C nanosheets and investigated the tribological properties with different oxidation levels as water lubrication additives. According to the UMT-3 friction and wear tests, moderately oxidized Nb₂C nanosheets, comprising Nb₂C/Nb₂O₅/C, demonstrated outstanding tribological performance, achieving a 90.3% decrease in the coefficient of friction and a 73.1% reduction in wear rate relative to pure water. Analyses using transmission electron microscopy and Raman spectroscopy revealed that Nb₂O₅ nanoparticles filled the worn area, while layered Nb₂C and C adhered to the friction sub-surface, creating a protective lubricating film. This synergistic effect led to superior lubrication properties, highlighting the growing interest in enhancing water's tribological properties through the design of nanomaterial-based lubricant additives. Ti₃C₂T_x has the advantage of being hydrophilic, making it suitable for use in water-based lubricants, and the presence of functional groups on the surface enhances its friction-reducing effect, whereas Nb₂C shows greater resistance to wear when moderately oxidized, and forms a stronger protective film for high friction applications.

Investigations seem to indicate that MXene has the best lubricity when used with other additives.¹⁶³ To further elucidate the possible synergistic lubrication mechanism of MXene/GO and to test the potential of MXene/GO as an aqueous boundary lubricant additive, SUN et al.¹⁶⁴ designed tribological experiments to compare the lubricity of Ti₃C₂T_x MXene, GO and aqueous dispersions of MXene/GO. Based on FIB microscopy, in situ contact observation, and atomic force microscopy (AFM) studies of the stability and structure of the tribofilm, they found that the lubricating performance of aqueous GO dispersions was superior to the one of aqueous MXene dispersions at the same concentration. The lubrication performance of mixed graphene oxide/MXene was significantly better than that of monodisperse graphene oxide or MXene, with the lowest coefficient of friction being ∼0.021. High-resolution characterization and modelling experiments showed that aqueous MXene/GO dispersions synergistically combined the adhesion and cohesion capabilities of large surface area graphene oxide flakes and the mechanical properties of MXene additives to form lamellar adsorption and protective tribofilms near the friction interface, resulting in excellent lubrication functions.

MXene nanosheets contain an abundance of surface termini such as –F, –OH and –O which largely determine their hydrophilicity. Wang et al.¹⁶⁵ prepared Ag@Ti₃C₂T_x by adsorption self-reduction route and evaluated its tribological properties as a lubricant additive. They said that Ag@Ti₃C₂T_x nanocomposites with an addition of 1.5 wt% in distilled water could reduce the COF and wear volume by 22.2% and 81.4%, respectively. Besides, Ag@Ti₃C₂T_x had good compatibility with benzotriazole (BTA), a commonly used rust preventive agent, and could improve the friction-reducing and antiwear abilities of water containing BTA. Cui et al.¹⁶⁶ employed free radical polymerization to graft poly(n-isopropylacrylamide) microgels onto Fe₃O₄ nanoparticles, creating Fe₃O₄@PNA. Through hydrogen bonding interactions, the Fe₃O₄@PNA was then anchored to MXene nanosheets, resulting in MXene@Fe₃O₄@PNA composites. The synthesized composites, when used as water-based lubricant additives, demonstrated effective friction and wear reduction. In addition, they exhibited excellent photothermal conversion performance in both underwater environment and dry state, making it feasible to efficiently and rapidly collect, convert, store, and utilize infrared light. Therefore, the multifunctional MXene@Fe₃O₄@PNA has a broad application prospect as a water-based smart lubrication additive. Moreover, Ag@Ti₃C₂T_x provides additional effects of anti-wear and anti-corrosion in addition to improved lubrication through the incorporation of silver. Fe₃O₄@PNA, on the other hand, enhances the stability and dispersion of the composites through hydrogen bonding to MXene, which makes it suitable for a long period of stable lubrication. According to its respective characteristics, Ag@Ti₃C₂T_x is suitable for lubrication conditions that require anti-wear and anti-corrosion, while Fe₃O₄@PNA composites are suitable for maintaining better lubrication over a longer period. In contrast, single materials such as Ti₃C₂T_x and Nb₂C have limitations in their performance under different loads and friction conditions, and it often needs to enhance their tribological properties through surface functionalization and composite design. MXene/GO composites exhibit the best performance in terms of friction reduction and anti-wear, due to their synergistic effects, and are suitable for use in a variety of lubrication applications. Therefore, suitable nanomaterials and composite strategies need to be selected according to specific working conditions in practical applications.

4.3. Summary

MXenes are applicable as lubricant additives in both water- and oil-based systems. Functionalized MXene-based nanomaterials exhibit improved dispersion and compatibility, along with enhanced adhesion and mechanical strength. These properties enable the formation of dense, durable lubricating films that provide outstanding anti-wear and friction-reducing performance. Although MXs-based lubricant materials can achieve excellent lubrication or even super-lubrication, there are still some challenges that require further researches. Firstly, short antiwear life and fragility are the biggest challenges for 2D materials (e.g., graphene and MoS₂); and it often needs to make use of the high mechanical strength and effective formation of beneficial tribofilms of MXenes to achieve longer sliding antiwear life.²⁴ Secondly, MXenes do not exhibit layer-dependent lubrication compared to other 2D materials; and hence it is meaningful to consider the structural and chemical diversity of MXenes for allowing different interlayer/interfacial strategies, creating additional potential for further friction reductions and prolongation of the effective antiwear life.¹⁴³ In addition, the development of MXenes with larger flake sizes could enhance the use of lubricant additives in low surface energy base oils, expanding their application therein.

Wet etching techniques are applicable to preparing MXene nanosheets, but they are costly, energy-intensive, and require the use of highly corrosive chemicals (e.g., HF and LiF), which not only increases carbon emissions but may also pose a threat to the environment and human health. Studies have shown that functionalized MXene consumes more energy than conventional lubricant additives (e.g., MoS₂ and graphene), and industrial production may generate large quantities of fluorinated waste, exacerbating ecological risks. Although MXene added at lower concentrations (0.1–0.5 wt%) can reduce lubricant usage and equipment wear, modified MXene (e.g., TDPA-MXene) may be toxic to the ecosystem, and it is difficult to degrade and may accumulate over time. For reducing environmental risks, priority should be given to the development of MXene recycling technologies, such as efficient recycling through fabricating magnetic composites (MXene@Fe₃O₄), thereby reducing the environmental burden and lowering economic pressure. In one word, the research and development of MXene recycling technology should be strengthened to optimize lubrication formulations and enhance the sustainability and reliability of industrial applications.

The lubrication mechanism of MXenes shares similarities with graphene and MoS₂, yet the specific intrinsic factors and interactions driving the synergistic effects in MXene-based composites require further clarification. Additionally, computational modeling and simulations could be valuable tools for uncovering these internal mechanisms. The super-lubrication capabilities of MXenes are still in the early stages, highlighting the need for deeper research to advance fast super-lubrication (like graphene oxide) essential for industrial applications.

5. G-C₃N₄

Carbon nitride (g-C₃N₄) is a non-metallic, polymeric nanomaterial with a graphite-like, two-dimensional layered structure. This material is known for its porosity, hardness, chemical inertness, and biocompatibility. The layered configuration of g-C₃N₄, characterized by weak van der Waals forces between layers and low shear strength, enhances its potential as a lubricant. The stable triazine and heptazine rings, composed of C–N bonds, contribute to g-C₃N₄'s durability in both aqueous and oxygenated conditions, while also ensuring it does not release harmful substances, even under high-temperature wear conditions.^167,168 The raw materials used in the preparation are cheap and readily available, and the preparation process is simple (e.g., direct pyrolysis), which ensures a relatively low preparation cost.

G-C₃N₄ exhibits good lubrication performance and can effectively reduce the coefficient of friction and improve the antiwear ability of lubricants. It gave significant friction reduction at metal–metal and metal-ceramic interfaces and performed well at high temperatures. It also exhibits some antioxidant capability, which helps to slow down the oxidation process of the lubricant during the lubrication process and protect the metal surface from the effect of oxidation.

5.1. Oil lubrication

The morphology and size of nanoparticles significantly influence their performance in lubricants. Zhai et al.¹⁶⁹ synthesized three distinct forms of carbon nitride (g-C₃N₄)—bulk (CNB), sheet-like (CNS), and tubular (CNNT)—to examine their effectiveness as grease additives for improving tribological properties. Among these, CNNT exhibited the most significant enhancement in lubrication performance compared to the conventional CNB and CNS additives. This improvement is attributed to CNNT's stable dispersion within the grease, promoting the efficient formation of a lubricating film on friction surfaces. Furthermore, CNNT's moderate structural strength and relatively low adsorption to metal surfaces support a sliding-rolling motion, enabling smoother relative movement.

Liu et al.¹⁷⁰ integrated g-C₃N₄ nanosheets as an additive with Ti-DLC films to develop a solid–liquid composite lubrication system, resulting in an environmentally friendly lubricant with outstanding anti-wear and friction-reducing capabilities. Using a ball-on-disk sliding test rig, they assessed the tribological performance of this g-C₃N₄ composite system across various concentrations and applied loads. Findings showed that as g-C₃N₄ concentration rose, both friction and wear rates initially decreased but later increased. At normal loads of 200 N, 100 N, and 50 N, the lubricant containing 1 wt% g-C3N4 achieved optimal performance, reducing friction by approximately 12%, 20%, and 24%, respectively, compared to the base oil.

The dispersion stability of g-C₃N₄ in base lubricating oils is poor, so g-C₃N₄ is often modified or surface functionalized. Nisha Ramjan et al.¹⁷¹ investigated the tribological modification of vegetable oils by TiO₂/g-C₃N₄ nanocomposites (as a nano-additive). The nano-additives dispersed in vegetable oils without any surfactants showed good oil dispersion stability; and the tribological properties of the base oils were significantly improved by the addition of the lubricant additives. The synergistic interaction between various solid particles in liquid lubricants has garnered significant attention. Base oil containing both g-C₃N₄ and h-BN exhibited optimal lubrication properties, achieving a 12.3% reduction in friction coefficient and a 68.6% decrease in wear rate compared to white oil with 0.5 wt% of g-C₃N₄ and h-BN (in a 1 : 1 weight ratio). Additionally, the combination of g-C₃N₄ and h-BN enhanced the high-temperature lubrication performance of white oil; although the friction coefficient and wear rate increased with rising temperature, the large contact area of g-C₃N₄ with the sliding surface, along with h-BN's strong adhesion to the sliding interface, promoted efficient film formation. This synergy significantly improved tribological properties in oil-lubricated environments.¹⁷²

5.2. Water lubrication

He et al.⁹² investigated the tribological properties of graphene oxide, graphitic carbon nitride (g-C₃N₄) and their mixed aqueous suspensions (g-C₃N₄/GO). The COF of 0.06 wt% GO, 0.06 wt% g-C₃N₄ and 0.06 wt% g-C₃N₄/GO (mass ratio 1 : 1) suspensions were reduced by 37%, 26% and 37%, respectively, and the radius of the wear tracks by 19.1%, 16.0% and 19.6%, respectively, as compared to water. Although g-C₃N₄ nanosheets possess many of the outstanding properties of efficient water-based lubricant additives, its poor dispersion in water greatly limits its prospects for application in water lubrication. Tang et al.¹⁷³ synthesized well-defined, water-dispersible C₃N₄ nanosheets through an efficient, low-cost hydrothermal method. These C₃N₄ nanosheets demonstrated excellent dispersion stability in water, allowing direct use as additives due to surface modifications with various covalently bonded hydrophilic groups. Supported by molecular dynamics (MD) simulations and wear track analysis, the C₃N₄ nanosheets fulfilled multiple tribological roles between friction surfaces, showing a prolonged service life even under demanding conditions. The high-water diffusivity of these nanosheets enabled the initial formation of a water-based lubrication film, physically separating frictional surfaces and enhancing the lubricant's performance. As testing continued, this thin water-based film was gradually replaced by a durable tribochemical film composed of iron oxides, metal carbonates, metal nitrides, organic compounds, and C₃N₄ nanosheets, adapting to the complex friction environment. What should be pointed out is that, although g-C₃N₄ as a water-based lubricant additive has been found to exhibit good performance, more insights into its triboeffect as well as its integration with other lubricating materials still await further studies.

5.3. Summary

Since g-C₃N₄ is mainly composed of carbon and nitrogen, it has a relatively low environmental impact and is eco-friendly compared to some traditional lubricant additives. Besides, g-C₃N₄'s surface properties can be modulated by synthetic methods, and its lubricating properties can also be altered by surface modification. This opens possibilities for its customization in different application scenarios.

Despite the above advantages of g-C₃N₄ as a lubricant additive, it has the same problems common to 2D materials, such as poor dispersion in lubricating oils and easy aggregation. In this sense, improving the dispersion stability of g-C₃N₄ lubricant additive to enhance its performance and reliability should be the focus of future researches.

6. Other types of 2D materials

6.1. Black phosphorus (BP)

Black phosphorus, a novel 2D material, has garnered significant interest in recent years owing to its anisotropic layered structure, adjustable bandgap, high carrier mobility, and excellent thermal stability. Recent research on BP's micro-friction characteristics indicates that variations in the bending deformation of BP nanosheets result in pronounced anisotropic super-lubrication during atomic-scale sliding. In this state, friction between the contact surfaces nearly vanishes, demonstrating BP's remarkable lubrication potential.

Wang et al.¹⁷⁴ achieved strong super lubrication by adding NaOH (BP-oh) modified BP nanosheets in water with Si₃N₄ balls/SiO₂ disks as the friction vice. In the ball-on-plate friction experiment, the lowest COF value recorded was 0.0006, occurring on the roughest friction surface (see Fig. 19). The BP-oh nanosheets, generated within the silica layer via a hydrolysis reaction during sliding, were crucial to achieving strong super-lubrication. This effect is largely due to the highly effective lubrication provided by the water layer bound to BP-oh nanosheets through hydrogen bonding. Based on these findings, a straightforward super-lubrication model centered on boundary lubrication was proposed. Similarly, to reveal the lubrication mechanism of titanium alloy/steel pair under BP lubrication, some researchers evaluated the tribological properties of BP nanosheets as water-based lubrication additive (BP-wl) for Ti6Al4V (TC4)/GCr15 contact. The results showed that water containing 70 mg L⁻¹ BP-wl had excellent lubrication performance, reducing the COF and ball wear rate by 32.4% and 61.1%, respectively, as compared with pure water. However, with the increase of load, the tribological properties of BP-wl gradually decreased, due to the agglomeration of BP nanosheets. The main reason for the reduced friction is the low interlayer shear and the adsorption of BP nanosheets. In addition, the friction chemically reactive film consisting of TiO₂, Al₂O₃ and Fe₂O₃ effectively protected the surface of titanium alloy/steel from wear. This new water-based lubrication additive can be used for processing titanium alloys.¹⁷⁵

Fig. 19 (a) Variation of COF with time when 7 wt % BP-OH solution is used as lubricant, the inset shows a magnified view of the COFs after break-in; (b) comparison of COFs for four different lubricants; (c) variation of COF with sliding speed after 5 wt % BP-OH solution is used as a lubricant to enter the super-lubricated condition; and (d) COFs at different contact pressures for 5 wt% BP-OH solution as lubricant with super-lubrication.¹⁷⁴ (Reproduced from ref. 174 with permission from American Chemical Society, Copyright 2018).

In recent years, studies on the lubrication performance of layered BP nanosheets as lubricating additives or fillers for self-lubricating materials have shown that BP has a promising application in lubrication practice. From a microscopic point of view, previous studies on the atomic-scale frictional behaviour of BP using AFM have shown that BP flakes exhibit a thickness-dependent behaviour like that of other two-dimensional materials.¹⁷⁶ For example, significant anisotropy of friction along the armchair and zigzag directions of BP sheets was observed both experimentally and theoretically.¹⁷⁷

Although BP's distinctive tribological properties have drawn significant interest, ensuring long-term stable lubrication and preventing BP's degradation in ultrapure water remain substantial challenges.

Dong et al.¹⁷⁸ designed and synthesized BPQDs@SiO₂ core–shell materials. Thanks to the uniform distribution of BP in the SiO₂ shell layer, the as-prepared BPQDs@SiO₂ showed remarkable lubricity, wettability and stability as a water-based lubrication additive. This approach could help to establish a simple, convenient and feasible method for fabricating black phosphorus materials as water-based lubrication additives and shed light on the development of high-performance lubricating nanomaterials. Wang et al.¹⁷⁹ prepared BP/MoS₂ composites as water-based lubrication additives. For GCr15 steel ball/TC4 titanium alloy disk contact at different additive concentrations as well as loads and rotational speeds, 0.01 wt% BP and 0.04 wt.% MoS₂ had the best synergistic lubrication effect as the water-based lubrication additives. The lubrication mechanism of BP/MoS₂ composites, when used as water-based lubricant additives, functions as follows: under contact pressure, BP/MoS₂ nanosheets migrate to the frictional interface, particularly within areas experiencing traction and compressive stresses. Here, they penetrate surface irregularities, effectively filling depressions and forming a continuous, smooth film that enhances lubricative performance.

Researchers not only investigated the role of BP nanosheets in water lubrication, but also tested their tribological properties as lubricant additives. Tang et al.¹⁸⁰ employed a chemical reduction method to synthesize BP dotted with silver nanoparticles and investigated the tribological performance of Ag/BP nanocomposites as lubricant additives in PAO6 base oils. Even with minimal amounts, the Ag/BP nano-additives considerably improved lubrication for steel-on-steel contact. In particular, the base oil containing 0.075 wt% Ag/BP nano-additives demonstrated a 73.4% decrease in the coefficient of friction and a 92.0% reduction in wear rate compared to pure PAO6 base oil.

Violet phosphorus (VP), an allotrope of black phosphorus and an emerging 2D material, has gained significant interest for its properties, including high carrier mobility, anisotropic characteristics, broad bandgap, strong stability, and ease of exfoliation. Li et al.¹⁸¹ systematically investigated the micro friction properties of partially oxidized VP (oVP) in oleic acid oil and its mechanism for reducing friction and wear of a steel-to-steel contact. The addition of oVP to oleic acid reduced the coefficient of friction from 0.084 to 0.014 and wear rate by 53.9%, due to the formation of an ultra-low shear strength tribofilm consisting of amorphous carbon and phosphorus oxides.

In metal cutting, lower concentrations of phosphorus-based additives can significantly improve the tribological properties of the lubricants used. Phosphorus in the base oil provides nutrients that favour microbial colonization and improve the biodegradability of the lubricant. Wu et al.¹⁸² demonstrated that the environmental degradation of BP significantly favours its lubrication behaviour, with a reduction in friction of about 50% in the region of degradation of BP nanosheets clearly observed by AFM. Therefore, in addition to the lamellar structure of BP, its environmental degradation also significantly contributes to its lubrication behaviour.

Black phosphorus has significant performance advantages as a new lubricant material, but its application faces challenges such as environmental stability, aggregation effect, and preparation cost. These problems can be partially solved by surface modification, composite material design, and novel material exploration. In terms of bio-environmental friendliness, BP materials exhibit good biodegradability and eco-friendliness, providing a new way for the development of environmentally friendly lubricants. However, further optimization of the preparation process, cost reduction, and extension of their application testing in complex working conditions are required to achieve their industrial feasibility.

6.2. Layered silicate materials

Silicon-based lubricants, particularly those made from layered silicate materials, exhibit excellent anti-wear and friction-reducing characteristics.^183,184 Consequently, designing layered Si–P compounds presents a promising approach for developing new high-performance lubricant additives. Yu et al.¹⁸⁵ prepared two-dimensional SiP single crystals and systematically investigated their lubrication performance as cetyl additives. The ultimate pressure of cetyl lubricants with SiP nanosheets reached 3265 MPa, surpassing the 2709 MPa achieved with MoS₂ nanosheets.

6.3. Layered alpha-phosphate

In earlier studies, researchers examined the frictional characteristics of phosphorus-containing lubricants and discovered that adding phosphorus enhances lubrication performance. This improvement is achieved by boosting the lubricant's extreme pressure resistance and its antioxidant properties.^186,187

Laminated α-zirconium phosphate (Zr(HPO₄)₂-H₂O, α-ZrP) nanosheets are extensively utilized across various fields.^188,189 Their distinctive two-dimensional layered structure, high purity, and adaptability for surface modification have attracted significant research interest, particularly for lubrication applications. Jiang et al.¹⁹⁰ synthesized 2D α-ZrP nanosheets intercalated with various amines and examined their dispersion stability and tribological performance as lubricant additives, primarily demonstrating how enhancing dispersion stability can improve the tribological properties of nanomaterials in oil-based lubricants. Chen et al.¹⁹¹ studied the tribological characteristics of Cu-α-ZrP andα-ZrP in lithium grease under reciprocating motion. Their findings indicated that α-ZrP exhibited superior friction reduction and anti-wear properties, suggesting its potential suitability for practical applications. By studying amphiphilic ZrP nanoparticles as lubricant additives, Chen et al.¹⁹² also found that the tribofilm produced by the nano-lubricant reduced friction by 40% and wear by 90%. Zhang et al.¹⁹³ applied a modified, eco-friendly hydrothermal synthesis method to control particle size and morphology, producing layered magnesium phosphate (MgHPO₄·1.2H₂O) as a sustainable lubricant additive. The tribological performance of this layered magnesium phosphate in PAO8 calcium-based grease was assessed, with macroscopic and microscopic analysis of worn surfaces revealing a protective effect on the grease matrix and the formation of a distinct lubricant film on friction surfaces. This research supports the broader use of laminated materials as green lubrication additives in industries such as food, cosmetics, agricultural engineering, and pharmaceutical machinery.

6.4. Boron nitride

Hexagonal boron nitride (h-BN) is gaining considerable attention among 2D nanomaterial-based lubricants.¹⁹⁴ Its layered structure features ultra-flat surfaces, strong covalent bonds within layers of boron and nitrogen, and weak van der Waals forces between layers, allowing for low-strength atomic shear. These properties endow h-BN with exceptional anti-wear and friction-reducing capabilities, making it suitable for both high- and low-temperature applications due to its minimal thermal expansion, high thermal conductivity, and low dielectric constant. Additionally, h-BN is an affordable, biodegradable lubricant that demonstrates excellent tribological performance when combined with vegetable oils traditional engine oils, or water.

Abdollah et al.¹⁹⁵ added 0.1–5.0 vol% of h-BN as a nano-additive to distilled water and used a four-ball tribometer to determine the friction characteristics and lubrication performance. The experimental results showed that h-BN as a lubricant additive exhibited the best lubrication effect at the optimum concentration of 1.0 vol%. Inorganic boron nitride nanosheets generally exhibit excellent solubility in oils, making them a valuable alternative to traditional sulfur- and phosphate-based additives. However, their tendency to agglomerate without adequate surface modification poses a challenge to their use as environmentally friendly lubricant additives. Wu et al.¹⁹⁶ developed a straightforward and effective approach to produce highly oil-dispersible, ultrathin alkyl-functionalized boron nitride (BN) nanosheets through a ball milling process combined with surface modification. This chemical-assisted exfoliation process gave the BN nanosheets excellent dispersibility in non-polar lubricants. Their ultrathin structure and strong compatibility enabled these BN nanosheets to serve as effective oil additives, significantly reducing friction, wear, and surface pitting. The superior lubrication performance is largely due to the continuous delivery of BN nanosheets to the contact interface, along with the formation of a protective boron-based tribofilm.

In practice, the relatively high cost of boron nitride may limit its use in some large-scale applications. Overall, boron nitride has potential advantages as a lubricant additive in some special applications such as high temperature, high pressure, and electrical equipment, but lubrication requirements, cost, and environmental protection still need to be considered when selecting lubricant additives.

The tribological performance of 2D nanolubricant additives under high load and high temperature conditions is critical for their industrial applications. In the following section, the operating limits of several typical 2D nano-additives, especially the lubrication performance of different 2D nanomaterials under maximum load (MPa) and maximum temperature (°C) conditions, are systematically analyzed. As shown in Table 7, there are significant differences in the operating limits of different 2D nano-lubricant additives under high load and high temperature conditions, and their performances are affected by the structural characteristics of the materials, environmental factors and lubrication systems. Graphene, MoS₂, and black phosphorus show excellent friction reduction and anti-wear properties under medium to high load and high temperature environments, but their oxidation sensitivity and dispersion issues still need to be addressed.^{106,182,197,198} Strategies such as standardized testing, composite modification and antioxidant treatment can further enhance the operating limits of these materials and provide more targeted solutions for industrial applications. These studies not only deepen the understanding of 2D nanomaterials in tribology, but also lay an important foundation for the development of high-performance lubrication systems.

Table 7 Operational limits of common 2D nano-additives

Material type	Maximum temperature (°C)	Maximum load (MPa)	Applicable scenarios	Failure mechanisms	Ref.
Graphene	300	500–800	Medium load, medium and low temperature machinery lubrication	Shear slip, lubrication film failure	197 and 198
MoS2	350–400	800–1000	High load, high temperature scenarios	Layer destruction, oxidative failure	106
WS2	400–500	1000–1500	High temperature, high pressure lubrication (e.g., aerospace)	Oxidation to WO3	111–113
MXenes	200–300	800–1000	Medium temperature equipment	Oxidation failure	142
BP	350–400	700–1000	High temperature and high load	Oxidation failure	182

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7. Lubrication mechanism

7.1. Lubrication mechanisms of 2D materials as lubrication additives

2D materials have received much attention as lubricant additives, due to their layered structure, easy shear, and high strength. Chemical or physical modification of 2D materials and the construction of composite structures with other nanomaterials are considered as effective strategies to improve their tribological properties. The lubrication mechanism of 2D nanomaterials as lubrication additives can be mainly described from the following aspects: interlayer shear mechanism, formation of tribofilm, polishing effect, self-repairing effect and rolling bearing effect, and synergistic lubrication.

7.2. Interlayer slip mechanism

The tribological properties of lubricants are largely influenced by the state of dispersion of nanosheets in the oil. Two-dimensional materials have flakes and small dimensions that allow them to easily enter the frictional contact. The introduction of nanoparticles into 2D materials further enhances their penetration into the contact surface gap, which makes lubrication possible.¹⁴⁸ When two contact surfaces rub against each other under normal force, the 2D nanomaterials within the contact surfaces are also subjected to normal pressure. The relative motion of the contact surfaces produces shear stresses on these materials. Thus, in multilayered 2D materials, the easy shear and frictional contact between layers forms a sliding system.¹⁹⁹ Specifically, the COF for OA-α-ZrP nanosheets in mineral oil consistently remained lower than that of pure mineral oil. This reduction in COF occurs because the dispersed OA-α-ZrP nanosheets tend to detach under shear stress due to their expanded layer spacing, leading to a significant decrease in COF.¹⁹⁰

A similar lubrication mechanism is observed in other two-dimensional materials. For instance, in the presence of frictional shear stress, NC-MXene/PFW@PDA (with NC representing nanocellulose) quickly enters the metal contact surfaces, forming a mixed lubrication film. In this process, MXene/PFW@PDA particles remain bound to the NC surface, dispersing evenly as the NC is rapidly consumed (Fig. 20). The combined effect of these components effectively reduces surface wear on the steel disk. Additionally, the PFW@PDA coating on the MXene surface plays a crucial role in the sliding process, creating an MXene-PFW@PDA-MXene (M-P-M) sliding layer that significantly lowers sliding resistance.²⁰⁰

Fig. 20 (a) Effect of state of dispersion on lubrication performance and (b) exfoliation mechanism of well-dispersed nanosheets,¹⁹⁰ (Reproduced from ref. 190 with permission from Springer Nature, Copyright 2019); (c) lubrication mechanism of NC-MXene/PFW@PDA lubricant on engineering steel surfaces.²⁰⁰ (Reproduced from ref. 200 with permission from Elsevier, Copyright 2023).

7.3. Lubrication film

2D materials as lubricant additives can play an important role in reducing friction and wear reduction, and they often can form lubricating protective films to avoid direct contact of the friction surfaces, thereby reducing friction. The formation of lubricant film often involves two stages. Firstly, due to the good dispersion of 2D material in the base lubricant, it can continuously enter the contact area of the friction pair to form a physical film under the shear stress. Secondly, 2D materials with quite weak interlayer coupling are liable to multilayer stacking as well as delamination. The delamination and orientation of the layers contributes to the transformation of the solid–solid contact into tribofilm–tribofilm contact, leading to easy shearing while the as-formed tribofilm protects the sliding surfaces from abrasion and reduces friction. The formation of a lubricant film is generally influenced by the molecular polarity of the nanoparticles and the base lubricant. When the nano-additives share similar polarity with the base lubricant, they can distribute uniformly over the metal contact surfaces, aiding in the development of a consistent tribofilm. Additionally, spillover electrons generated at the friction interface create an intensified interfacial electric field, enhancing the adsorption of polar molecules, particularly magnetic nanoparticles, onto the surface. Consequently, polar compounds as lubricant additives typically exhibit strong adsorption on metal surfaces, readily forming protective oil films. With the onset of the friction motion and the deposition of the specimen, the heat generated from the rupture of the physical film will lead to tribochemical reactions between the lubricant and the substrate material, which results in the formation of a continuous and stable chemically protective and physically adsorptive film, greatly improving the tribological properties. For example, MoS₂ nanoparticles on the rubbed surface of steel substrate were able to form a composite tribofilm consisting of MoS₂, MoO₃, FeS, and FeSO₄, leading to reduced friction and wear.⁷ Graphene-based lubricants similarly demonstrate the ability to form tribofilms within contact areas, thereby enhancing the lubricant's tribological properties. Wu et al.⁵⁴ for instance, examined the tribological behavior of graphene oxide nanosheets as additives in water-based lubricants and observed that a tribofilm formed on the friction surface, contributing to improved tribological performance, as illustrated in Fig. 21.

Fig. 21 (a) TEM images and (b)–(d) HR-TEM images of typical areas within tribofilm formed by PAO-6 containing MG (0.5 wt%) on a steel plate as well as (e) friction mechanism diagrams.⁵⁴ (Reproduced from ref. 54 with permission from Springer Nature, Copyright 2020).

In general, when a sliding system operates under boundary lubrication conditions, the thickness of the oil film can be approximated using the Hamrock–Dowson equation:

where α is the pressure–viscosity coefficient and H_C is the centre film thickness, U is the entrainment velocity E′ is the effective elastic modulus of the contacting solid, k is the contact ellipticity parameter, R_e is the combined radius of curvature of the two surfaces in the direction of entrainment, and F is the normal load.^201,202

7.4. Polishing effects

Polishing effect, also known as smoothing effect, refers to the phenomenon that nanoparticles adsorbed on the friction surface reduce its roughness. Defects such as depressions can exist on the surface of the friction substrate, and rough substrate surfaces are often subject to greater friction and wear. Two-dimensional nanomaterials, especially those anchored with nanoparticles, can fill in the concave areas of the friction surface and reduce its roughness. A smoother surface with fewer concavities can reduce local contact pressures, and lower pressures can reduce the potential of plastic deformation.¹⁵² As reported by Wu et al., the silver/graphene nanocomposite had fewer layers with a monolayer thickness of about 0.35 nm as well as Ag nanoparticles with a size of about 7 nm. Due to the small size and nanostructure, silver/graphene nanocomposite was able to easily enter the contact interface and release Ag nanoparticles to fill into valleys thereon, thereby reducing friciton and wear of the sliding pair. Fig. 22 gives a schematic diagram of the polishing effect of silver/graphene nanocomposite, where the disk surface is polished under high Hertzian contact stress.⁹⁵

Fig. 22 Schematic diagram showing the polishing effect of silver/graphene nanocomposite.⁹⁵ (Reproduced from ref. 95 with permission from American Chemical Society, Copyright 2023).

7.5. Rolling effects

The conventional mechanism of rolling effect usually refers to the phenomenon of spherical nanoparticles’ reducing friction by geometrical rolling between frictional interfaces. For 2D nanomaterials, although morphologically different from spherical particles, they can exhibit a pseudo-rolling effect, whereby the lamellae can roll between the contact surfaces like small balls, thus reducing the frictional resistance. This so-called rolling effect depends to some extent on the shape and size of the lamellae.

In a lubricant environment, 2D nanosheets may flip, slide or even slightly curl, a dynamic behaviour that allows them to act like rolling bodies, constantly adjusting their attitude to dissipate stresses in a frictional interface. Such a dynamic behavior has been supported by molecular dynamics simulations and experimental observations. For example, graphite nanosheets were found to suffer from edge self-curling nanodeformation at low temperatures (77 K). When the temperature was varied from 300 K to 77 K (liquid nitrogen temperature), the overlapping GNSs were separated and the horizontal (001) crystalline plane transformed into a vertical (002) basal plane. This, in combination with the formation of many ordered nanocoils at the friction interface under 50 K, suggests that the low temperature led to the edge self-coiling especially a high degree of nanocoiling at the friction interface of the GNSs. The enlarged view of the nanocoil shows that it is formed by rolling three layers of GNSs from end to end, with the lattice spacing (0.34 nm) coinciding with the graphite (002) basal plane spacing (Fig. 23). In addition, the SEM morphology of the friction interface shows that the nanocoils are distributed in parallel rather than in a disordered manner, which suggests that the nanocoils are indeed nano-coils that act as molecular bearings for rolling rather than sliding during the friction process. After the friction process, the curled GNSs were transformed into intact nanocoils, and the pressure of curling increased with the number of layers, and only 1–3 layers could undergo self-curling, also as verified by molecular simulation experiments that graphite nanorolls can achieve more effective rolling lubrication as molecular bearings.²⁰³

Fig. 23 The illustration of the molecular rolling lubrication of graphite at cryogenic temperature: (a) schematic of layer-layer sliding and molecular rolling lubrication as well as the structural evolution, (b) microstructure of the friction interface at 300 K, (c) microstructure of the friction interface at 50 K, (d) the enlarged view of the formed nanoscroll, (e) and (f) TEM images of the axial view of the formed graphite nanoroller, and (g) SEM morphology of the friction interface at 50 K.²⁰³ (Reproduced from ref. 203 with permission from Springer Nature, Copyright 2024).

7.6. Self-healing

2D nanomaterials in surface recesses aid in forming a continuous oil film, which further improves wetting. The transient high temperatures generated during sliding can cause these nanoparticles to melt, allowing them to fill surface imperfections, effectively repairing defects and thereby reducing friction and wear. The self-healing mechanism, in contrast to tribofilm formation, involves nanoparticles in the base lubricant moving into gaps within the frictional interface. These particles facilitate the formation of a deposition or adsorption layer on the friction surface, effectively preventing direct contact between the sliding components. Fig. 24 shows the schematic diagram for MXene@CDs composites to improve the tribological properties of the lubricants. Namely, MXene@CDs with good dispersion stability in PAO10 ensures its continuous access to the contact area of the friction partner, which is favorable for them to exert lubrication effect. Under high contact pressure, MXene@CDs break into smaller fragments that embed themselves within the grooves of worn areas, effectively repairing the rubbed interface. This process reduces surface roughness, enhancing the lubricant's friction-reducing and anti-wear properties.¹⁵¹

Fig. 24 Schematic diagram showing the self-repair mechanism of MXene@CDs as nano-additives.¹⁵¹ (Reproduced from ref. 151 with permission from Elsevier, Copyright 2023).

7.7. Synergistic lubrication

The lubrication mechanism of two-dimensional materials often may vary with varying synthesis methods and modification routes. This is especially true for some composites who's excellent tribological properties often refer to increased complexity in their lubrication mechanism.^62,148 As depicted in Fig. 25, the TEM images and element mappings of the cross-sections of worn metal surfaces lubricated with MXene@Fe₃O₄@PNA reveal the formation of a tribofilm, characterized by intermediate bright zones and a topmost protective Pt/C layer. This protective film is dense and uniform, being approximately 5 nm in thickness. The detection of Ti, Fe, O, and C elements within the tribofilm indicates that MXene@Fe₃O₄@PNA was physically adsorbed onto the contact area, creating an adsorption layer. Additionally, the friction-heat induced tribochemical reactions resulted in a tribochemical film containing titanium oxide and iron oxide. The lubrication mechanism of MXene@Fe₃O₄@PNA is facilitated by its excellent dispersion in water, enabling it to consistently reach the contact zone of the friction interface. During operation, the relative movement between friction surfaces generates shear stress on the MXene, allowing the 2D MXene@Fe₃O₄@PNA to slide between layers and remain on the friction surface. Fragmented MXene@Fe₃O₄@PNA and released Fe₃O₄@PNA particles can fill up depressions on the steel disk, smoothing the surface and reducing both friction and wear. Additionally, Fe₃O₄@PNA particles serve as rolling elements, promoting rolling friction. The hydrophilic groups on the MXene@Fe₃O₄@PNA surface adsorb water molecules, creating a thicker hydration layer that prevents nanosheet agglomeration and functions as a boundary lubrication film.¹⁶⁶ As a result, the synergistic triboeffects of various components of the composite lubricant additives contribute to improving the tribological properties of the lubricants.

Fig. 25 (a)–(c) TEM images of cross sections of worn metal surfaces lubricated by MXene@Fe₃O₄@PNA, (d) HAADF maps, (e) and (f) element mappings, and (g)–(i) schematic illustration of lubrication mechanism of MXene@Fe₃O₄@PNA as well as (k) environmental recycling.¹⁶⁶ (Reproduced from ref. 151 with permission from Elsevier, Copyright 2023).

As mentioned above, a variety of nanomaterials (including inorganic and organic nanomaterials) can be readily combined with two-dimensional materials through different synthesis strategies. This allows rational structural design and manipulation of 2D nanomaterials through dopant loading, wrapping, grafting, etc. In essence, the physicochemical properties of 2D nano-lubricant additives, including dispersibility and structural stability, can be optimized by selecting suitable synthesis methods and adjusting synthesis parameters. Nanomaterials enhance the exfoliation degree and interlayer spacing of nanosheets, which facilitates their interlayer sliding and improves dispersion in the lubricant. As a result, synergistic interactions between 2D materials and can produce excellent tribological properties such as friction reduction, wear resistance and resistance to extreme pressure. It is worth noting that multiple lubrication mechanisms are often present simultaneously during friction, and their interactions can add to the complexity of studying these mechanisms.

In tribological studies, especially when applying two-dimensional materials as lubricant additives, various lubrication mechanisms play different roles depending on the actual operating conditions. Understanding how these mechanisms interact and how they dominate under real-world conditions is critical to designing more efficient lubricants. Interlayer slip is the dominant lubrication mechanism on smoother surfaces and under lower pressure conditions. For example, interlayer slip can play an important role in providing stable lubrication in vacuum environments or under low to medium load conditions.¹⁹⁰ The polishing effect is particularly important when the initial roughness of the mechanical surface is high. In this case the nanoparticles in the lubricant will microscopically alter the surface roughness, thereby reducing surface friction. Through prolonged friction, the nanoparticles flatten areas of higher roughness, which results in more uniform contact between the friction partners and contributes to other lubrication mechanisms.⁹⁵ In high-pressure, high-speed machinery, the lubricant film acts as a first line of defence to reduce surface wear, not only by reducing direct contact between metal surfaces, but also by adapting to pressure variations under different operating conditions through dynamic reorganization and flow.^148,152 In addition, in high temperature, high load environments, the stability of the lubricant film is critical to preventing surface contact. When the lubricant film fails due to wear or high temperatures, the self-healing mechanism can function to repair the damaged area in time to maintain lubrication performance.¹⁵¹ This synergy is especially critical in aerospace or high-performance machinery.

8. Summary and outlook

2D nanomaterials have made significant strides as lubricants and lubricant additives, benefiting from their exceptional physical, chemical, and mechanical properties. However, certain limitations still restrict their full potential in industrial applications. To realize broader adoption, key bottlenecks must be identified and addressed, with targeted solutions developed to overcome these challenges.

The size, number of layers and surface properties of 2D materials significantly affect their lubricating performance, so the precise control of parameters during preparation is required for the development of functionalized nanoadditives. However, the dispersion of such materials in lubricating oils is poor. For example, graphene and WS₂ are prone to agglomeration due to strong van der Waals forces and large specific surface area, while MXenes are poorly dispersed in non-polar oils, due to strong hydrophilicity. To solve the problem of dispersion stability, we may utilize surface functionalization (e.g., introduction of lipophilic or polar groups), surfactant assistance, and nanoemulsions, as well as optimized design for the chemical properties (e.g. polarity, viscosity) of different base oils. Meanwhile, the compatibility of 2D nanomaterials with different base oils is also crucial, because the chemical properties (e.g., polarity, viscosity index) of different oils significantly affect the dispersion stability of nanomaterials and their lubrication performance. For example, in non-polar mineral oils, nanoparticles are prone to flocculation and agglomeration due to the lack of synthetic additives, leading to higher coefficient of friction and wear rate. In more polar synthetic oils, whereas, the interaction between the additives and the base oils is usually more pronounced, which helps to achieve better dispersion and lubrication.

Although a variety of new two-dimensional materials and their nanocomposites have been developed as lubricant additives in recent years and found to have excellent overall performance, there are still fewer basic studies on their dispersion stability, compatibility with different base oils, and their effect on tribological performance. To address this issue, we need to introduce advanced characterization techniques for quantitative comparison, such as dynamic light scattering (DLS), SEM, and zeta potential measurements, to characterize the particle size, distribution, and dispersion stability of the particles. We also need to conduct contact angle and interfacial tension measurements of the lubricants in different base oils so as to quantitatively analyze their affinity to the base oils. Besides, we may need to conduct solubility parameter comparison using Hansen solubility parameters in order to quantitatively assess the compatibility of the lubricants with the oil phase, and we ought to select the most suitable modification strategy while compare the compatibility of different types of base oils with lubrication additives.

In terms of lubrication mechanisms, the formation of a lubricating film is crucial, and the chemical reactions occurring during the formation of chemical films cannot be precisely determined. Further exploration and in-depth characterizations are required in this respect. Modern advanced characterization techniques offer the possibility of in-depth studies of lubricant films. For example, on the one hand, AFM can be used to characterize the nanoscale roughness, friction distribution and surface defects of lubricant films, providing direct evidence for understanding the microscopic interactions between lubricant films and substrates. Raman spectroscopy and in situ transmission electron microscopy (in situ TEM), on the other hand, can be used to study the chemical composition and formation process of lubrication films, especially the interlayer slip and self-healing behaviour of the lubrication films of 2D materials (e.g., MoS₂, MXenes). In combination with XPS, the chemical bonding changes and valence distribution on the surface of the lubricant film can be analyzed in depth, revealing the structure of the chemical film generated by the reaction between the lubricant and the interface. In addition, the study of the mechanical properties of lubricant film is of paramount importance. For example, nanoindentation can be done to measure the hardness and elastic modulus of lubricant films to further assess their stability under high loading conditions. Techniques such as small angle X-ray scattering (SAXS) and optoelectronic energy loss spectroscopy (EELS), at the same time, provide important tools for the study of lubricant film's lamellar structure and dynamic chemical reactions. In one word, the systematic study of the lubrication film from its structure and chemical properties to its dynamic behaviour through the combination of various characterization techniques can significantly improve the understanding of the mechanisms of lubrication film formation and evolution, thus laying the foundation for the design of more efficient and durable lubrication systems.

Regarding practical applications, most studies on 2D nanomaterial additives have been conducted at room temperature and within laboratory settings. Moving forward, it will be essential to develop new additives that demonstrate robust tribological performance under extreme conditions. Equally important are cost-effective, scalable production methods and performance evaluations under real-world application conditions, which are critical for enabling these additives' industrial use. For example, MXenes and boron nitride are expensive to prepare, and laboratory preparation methods do not meet industrial demand in terms of yield and consistency. Currently, these high-performance lubricants are more often used in high-end equipment or extreme working conditions (e.g., high-temperature and high-pressure environments) but are frequently found to be less less competitive with traditional lubricant additives. Therefore, future research should focus on developing more environmentally friendly and economical preparation methods, such as replacing traditional chemical etching process with electrochemical etching, or finding cheaper precursor materials to reduce production costs. The other topics in future researches involve the exploration of continuous production processes (e.g., large-scale synthesis using controlled fluid reactors), the optimization of by-product treatment processes to reduce the environmental burden, and the clarification of the full life cycle environmental impact of the lubricating materials through life cycle assessment to ensure true sustainability.

In advanced mechanical components of spacecraft (e.g., axles, bearings, and bushings), the friction substitutes are usually exposed to extreme operating conditions, which requires lubricants not only to withstand harsh environments such as high temperatures, vacuum, solar wind irradiation, and cosmic radiation, but also to be super-lubricating, thermally stable, and extremely oxidation-resistant. Existing 2D materials, such as MoS₂, WS₂, and MXenes, although showing excellent performance under certain extreme conditions, still face the problems of high temperature oxidation and inadequate stability under high pressure environment. For example, MoS₂ exhibits good lubricity in vacuum environments, especially at low and medium temperatures, and the interlayer slip of MoS₂ can effectively reduce friction. However, at very high temperatures or high loads, the lubricating effect of MoS₂ is limited and its lubricating performance may be affected by excessive heat generated by friction. MXenes materials have good thermal conductivity and mechanical strength, but are relatively unstable at high temperatures. In high temperature environments, surface modification is often required to improve the thermal stability of lubricating materials. For example, the introduction of antioxidant functional groups (e.g., phosphate esters, nitrogen-based compounds) on the surface of 2D materials can significantly improve their high-temperature resistance. In addition, the integration of lubricants with high-temperature stabilizing materials (e.g., boron nitride, graphene, etc.) can help to further enhance the oxidation resistance. Through the composite design, the lubricant can more effectively resist oxidative degradation at high temperatures and in vacuum environments. In the meantime, next-generation lubricants should have enhanced self-healing capabilities to cope with frictional damage that may occur during the lubrication process. For example, the composites with reversible chemical bonds or incorporating nanoparticles (e.g., carbon nanotubes, ferrite particles, etc.) can automatically repair the lubricant film during friction, thus ensuring long-lasting lubrication performance. In vacuum environments, these composites can provide longer-lasting lubrication and respond effectively to rupture or wear of the lubricant film, thus prolonging the lubricant's service life and ensuring the reliable operation of spacecraft components under extreme operating conditions. In addition, lubrication research needs to focus on experimental validation and industrial applications, especially performance testing under complex operating conditions such as high loads, high temperatures and long operating times, to ensure the stability and reliability of new lubricants in practical applications. Collaboration with industry will contribute to the development of mass production technologies and cost reductions of high-performance materials, thus promoting the commercial application of new lubricants. Interdisciplinary cooperation will be key in this process; for example, the cross-fertilization of the fields of chemistry, physics, materials science and computer science will provide new ideas and solutions for the innovation and application of tribological materials. By building an open research platform and promoting collaborative innovation between academia and industry, we will accelerate the transformation of theoretical research results on lubrication materials into practical applications, and provide more efficient and reliable lubrication systems for spacecraft and other high-end mechanical equipment.

In summary, next-generation lubricants will need to improve performance in extreme environments by combining multiple functions, such as high-temperature stability, oxidation resistance and self-healing capabilities. These performance requirements need to be achieved through material innovation and surface engineering optimization. To address these challenges, we may make use of artificial intelligence (AI) and machine learning technologies to accelerate the development of new lubricants by analyzing material characterization results and performance data through big data, building predictive models and quickly screening the most suitable material combinations. Meanwhile, by combining methods such as MD, first-principle calculations and finite element analysis, we may establish a multi-scale simulation system from the nanoscale to the macroscale in order to provide theoretical guidance for material design.

It is noteworthy that the research on 2D nano-oil additives not only promotes technological advances in the field of tribology, but also provides a wealth of research opportunities for beginners in tribology. The unique properties and diverse applications of these materials may greatly enhance the research capabilities of beginners, especially in understanding and designing high-performance lubricants. Beginners can explore the relationship from nanoscale frictional behaviour to macroscopic tribological properties with the help of experimental and theoretical studies of 2D nanomaterials, opening a broader perspective for them. When faced with challenges such as surface functionalization, dispersion optimization, and performance evaluation, beginners will be able to gain a deep understanding of the combination of fundamental material properties, lubrication mechanisms and related technologies, which will in turn propel them to carry out innovative research work. In addition, the interdisciplinary applications of 2D nanomaterials, such as composites with other nanomaterials, integration of surface modification techniques, and combination of tribological experiments and simulation techniques, can provide beginners in tribology with rich opportunities to explore new theories, develop new materials, and broaden their research. In summary, the research on 2D nano-oil additives not only promotes the development of the lubrication discipline, but also provides a challenging and innovative research platform for the new generation of researchers, which would be significant for them to achieve breakthroughs in the field of tribology.

Author contributions

Zhengquan Jiang: funding acquisition, writing – original draft, writing – review & editing. Jiahao Wu: writing – original draft, writing – review & editing. Laigui Yu: writing – review & editing. Jinglei Bi: investigation. Yadong Wang: methodology, funding acquisition. Xiaoyi Hu: visualization. Yujuan Zhang: writing – review & editing. Weihua Li: conceptualization, supervision.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the support of the National Natural Science Foundation of China (52305191), the Postdoctoral Research Grant Program of Henan Province (346032), the Key Scientific Research Program of Higher Education Institutions in Henan Province (25A430016), and the Henan Key Laboratory of Infrastructure Corrosion and Protection.

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