Nanomaterials for lubricating oil application: A review

Friction and wear are ubiquitous, from nano-electro-mechanical systems in biomedicine to large-scale integrated electric propulsion in aircraft carriers. Applications of nanomaterials as lubricating oil additives have achieved great advances, which are of great significance to control friction and wear. This review focuses on the applications of nanomaterials in lubricating oil and comprehensively compares their tribological characteristics as lubricating oil additives. Statistical analysis of tribology data is provided and discussed accordingly; moreover, the interaction between nanomaterials and sliding surface, lubricating oil, other additives, and synergistic lubrication in nanocomposites are systematically elaborated. Finally, suggestions for future research on nanomaterials as lubricating oil additives are proposed. Hence, this review will promote a better fundamental understanding of nanomaterials for lubricating oil application and help to achieve the superior design of nanoadditives with outstanding tribological performances.


Introduction
Friction and wear are everywhere and critical to carbon release. With great friction comes a great demand for energy to overcome it. However, at present, the energy source of industry and transportation is mainly fossil fuels, which generate a considerable part of greenhouse gases emission. On a global scale, 100 million terajoules are annually consumed as a result of tribological contacts, which is contributing to the greenhouse gas emission with 7,000 million tons annually [1][2][3]. It is a great challenge to the achievement of carbon neutrality on account of the inevitability of friction [3]. Accordingly, governing friction and reducing wear are of great significance in modern technologies and up to now, one of the most effective approaches in the industry is the lubrication by lubricating oil [4,5], which are usually stable in diverse environment.
Additives in base oil could achieve prominent friction reduction and excellent anti-wear properties in friction pairs [6,7]. The additive existing could be divided into four types: organic friction modifiers, functional polymers, oil-soluble organic additives, and nanomaterials [5]. As a novel class of materials, nanomaterials have found applicability across the tribology field, and they provide excellent lubrication performance as lubricating oil additives [8,9]. There has been growing attention to the exploration of nanomaterials as lubricating oil additives in recent years despite a great number of researches have been undertaken over the past decades [10][11][12][13]. The number of times cited and publications during 2001-2022 are shown in Fig. 1.
By reviewing several hundreds of papers published by scholars from different groups, we found that plenty of studies has shown remarkable tribological properties improvement in lubricating oil dispersed with different types of nanomaterials. However, each study is based on exclusive conditions, such as sizes and morphologies of nanomaterials, surface functionalization, compatibility, concentrations, test parameters, friction pair, base oil types, etc. For appointed working conditions and base oil, it is still difficult to decide on a suitable nanomaterial additive. In order to overcome the issue mentioned, practically all known nanomaterial additives in lubricating oil are systematically discussed in this review.
This review focuses on the applications of nanomaterials in lubricating oil and the comparisons of tribological characteristics and mechanisms of different nanomaterial additives, highlights the existing problems and prospects of nanomaterials as lubricating oil additives, and aims to provide a recommendation to choose an applicable nanomaterial additive depending on different working conditions.
The structure and contents of this review are presented in Fig. 2. Section 1 opens with the  www.Springer.com/journal/40544 | Friction introduction, presenting the background of this paper. Then followed the categories for nanomaterials design as lubricating oil additives. Nanomaterial additives with zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and composites are presented in Section 3. The main classifications of nanomaterial additives with subcategories are also shown in Fig. 2. Sections 4 and 5 are the summary and discussion based on the data of Section 3 and the lubrication mechanisms of nanomaterial additives. Sections 3-5 are the main body of this paper, in which exhaustive analysis and comparison of the entire current experimental data about implementing nanomaterial additives for friction reduction are attempted, and Tables 1-4 and Fig. 12 are given to reveal the details of tribological tests. Tribological properties and lubrication regimes of nanomaterials in different dimensions are also summarized and analysed. Finally, challenges, suggestions, and outlook of nanomaterials application in lubricating oil are briefly summarized. (Note that ionic liquids (ILs) do not fall within the scope of nanomaterial in theory; however, owing to the unique physicochemical properties, oil-soluble ILs have been developed into the new focal and promising additives in lubricating oil. Therefore, the application of ILs in lubricating oil is briefly introduced in Section 3 to facilitate the comparison with nanomaterials.)

Categories for nanomaterials design as lubricating oil additives
Undeniably, nanomaterials have become one of the most attractive branches in physics, chemistry, biology, and material science [14,15]. The evolution of nanomaterials has led to the development of lubricating oil additives. A wide variety of studies have been implemented to investigate the potential of nanomaterials and nanocomposites as lubricating oil additives [16]. What are the original intentions of using nanomaterials as lubricating oil additives? How can design nanomaterials achieve excellent tribological performances? Categories for nanomaterials design are summarized and illustrated in Fig. 3. 1) Small sizes and nanostructure. The characteristic size of nanomaterials in the nanometer range from 1 to 500 nm [5]. By Brownian motion, small-size or functionalized nanomaterials could be dispersed in oil [5,6] and pass through filters undisturbedly [5]; but even more, they can enter the contact interface easily to reduce the friction and prevent friction pairs from being worn, as a result of tribo-film formation, rolling mechanism, or surface repairing, etc. [17,18] Moreover, the nanostructure of sphere, tube, or layer is another key consideration in nanomaterials design because it is directly associated with the pressure that nanomaterials are subjected to during loading [19]. Especially, spherical nanomaterials could offer a high capacity of load and pressure.
2) Unique physical properties. It is well-known that a substance in small sizes may show specific properties that bulky ones do not. Most nanomaterials have distinguished properties including large specific surface area [20], high mechanical strength, and load capacity [21], which contribute to improving the performance of lubricating oil. Owing to the large specific surface areas, nanomaterials with strong adsorption could allow the combination of several substances to obtain good dispersion in lubricant or form nanocomposites [22]. Besides, a part of layer nanomaterials with weak interlayer interactions could offer low interfacial shear force, which is beneficial to friction reduction [23]. On the other hand, most nanomaterials have high thermal conductivity [8], which promotes the release of frictional heat and contributes to the stability of the friction systems. In comparison with organic oil additives, nanomaterials are also considered thermal stable at increasing temperatures, which is necessary for the durability of lubricating oil [24].
3) Surface functionalization. By serving as a host substrate for various functional chemical groups and species, nanomaterial offers the opportunity to graft targeted groups and develop novel types of composite nanomaterials that display enhanced or new performances in comparison to the parent counterparts [25]. Therefore, it is highly desirable to control the size, functionalization, and composition of the incorporated nanomaterials, as such ability should achieve the relevant properties of the nanomaterials to be tuned systematically [26]. Note that surface functionalization of nanomaterials can effectively enhance their dispersion stability and homogeneous distribution in base oil, and well-dispersed nanofluids have lower yield stress to reduce the starting resistance. However, most non-functionalized nanomaterials are considered prone to aggregation in nonpolar oil [27], which cannot be changed by the mechanical stirring process. The categories of surface functionalization are multifarious, and primary methods are summarized by four processes shown in Fig. 4: (1) Precursor of nanomaterial reacts with the modifying agent under appointed process, and nanomaterial preparation and surface functionalization occur simultaneously; (2) the as-prepared nanomaterials are mixed with the modifying agent; after reaction process, functionalized nanomaterials are generated; (3) coupling agent/ crosslinking agent (or others) is used to connect the as-prepared nanomaterials and modifying agent; (4) the as-prepared nanomaterials are mixed with modifying agent; after dielectric barrier discharge plasma-assisted ball milling, functionalized nanomaterials are generated. 4) Synergistic lubrication. One potential method to improve the tribological properties of nanomaterials and exploit the mentioned advantages for extensive applications would be the preparation of nanocomposites, which can exhibit the synergistic effect in thin film boundary [28]. The obtained results show that benefitting from the synergistic lubricating effect, the combination of two or more nanomaterials has better tribological behavior than individual nanomaterial. Due to the different synergistic mechanisms, nanocomposites could display superior friction reduction and anti-wear properties, antioxidation  and were more effective for friction and wear reduction when added individually [37]. However, it is worthy to note the type of oil. In nonpolar oil, Cu nanoparticles were remarkable as friction modifiers and anti-wear agents for each concentration and load, attributing to the deposition of Cu nanoparticles on the rubbing surfaces. However, the lubricating layer of polar oil on the metal-to-metal contact surface could be destroyed by Cu nanoparticles [38].
It is of interest to use Ni nanoparticles as nanoadditives [31]. Chen et al. [39] reported that Ni nanoparticles synthesized in polyalphaolefins (PAO) could be tuned to diverse nano-sizes and had good dispersibility. Owing to the release of Ni nanocores with high activity and a stable protective layer formed by smaller-size Ni nanoparticles, the as-synthesized nanolubricants mixed with Ni nanoparticles exhibited good friction and anti-wear behaviors. This is a method for preparing oil-soluble metal nanoparticles, which can achieve good dispersion of nanoparticles in oil.
The changes of tribological properties by adding metal nanoparticles have been of great attraction to researchers. An innovative and simple study is that gallium-based liquid metal (GLM) nanodroplets functionalized with DDP were prepared by polydopamine (PDA)-coating, which combined the advantages of nanoparticles and traditional additives. Dopamine is a biomolecule owning two functional groups (catechol and ethylamine), which possesses the ability to self-polymerize to form PDA [47]. The experimental conditions were mild and convenient for operating, and excellent friction reduction and anti-wear performances were obtained [45]. The preparation method of functionalization is in accord with Fig. 4(c). The related processes and results are given in Fig. 5.
On the other hand, it must be accepted that untreated nanoparticles tend to aggregate in nonpolar oil. Improving the dispersion of nanoparticles in lubricating oil by surface functionalization with organic molecules is necessary (organic molecules include DDP [48], polyethylene glycol (PEG) [49], oleylamine, oleic acid, thiolated ligands, etc.). Of great importance is that functionalized nanoparticles have better friction reduction and anti-wear properties compared to the untreated.
Friction and wear are also affected by the hardness of nanoparticle additives. When Sn, Fe, and Cu nanoparticles were used as additives that had lower hardness than friction pair, a film with low hardness could be formed on the contact area [34,40]. After 654 Friction 11(5): 647-684 (2023) | https://mc03.manuscriptcentral.com/friction comparing the tribological behaviors, it was concluded that hard nanoparticles showed obviously different tribological behaviors, and the friction mechanisms of hard (such as Mo and W) or soft (such as Pb, Sn, Ag, Cu, Ni, Fe, and Co) nanoparticles were also different [44]. For soft metallic nanoparticles, their antiwear property becomes more significant as the hardness and shear modulus increase, but has little effect on the improvement of friction reduction. Because their mechanisms of antiwear and friction-reducing are both attributed to the formation of metallic boundary films on the worn surfaces. For harder metallic nanoparticles, there are two effects that need to be considered: the promotion by nanoparticles to form a protective film or behave as rollers; their disturbance of continuity and stability of oil film. Tribological behaviors depend on which effect is dominant [44].
Another problem that needs to be considered is the negative effect of metal particles on the oxidation stability of lubricating oil. In engines, one of the most significant factors in the deterioration of lubricating oil probably is the presence of metals. Many tests were devised to evaluate the effect of different metals on lubricating oil deterioration. However, Fe, Cu, and Pb are considered well-recognized catalysts for the deterioration. Not only the copper salts are the effective catalysts, but also copper in bulk form [50].
Besides, up to now, the increasingly metal-base catalysts that could effectively catalyze the oxidative degradation of oil are reported. Therefore, not only the tribological properties but also the effect of metal particles on the oxidation stability of lubricating oil needs to be considered.

Oxides/sulfides/nitrides/fluorides
Among the study of oxides, sulfides, nitrides, and fluorides, the most widely used were oxides and sulfides. According to the number of references, oxides and sulfides nanomaterials account for 83% of Section 3.1.2.
Due to high surface activity, low cost, and loadbearing capacities, SiO 2 and TiO 2 have been widely used as additives in lubricating oil [51]. Typically, SiO 2 -and TiO 2 -grafted oil-soluble polymer brush [52] were dispersed in PAO steadily, and no change was observed after being kept at 100 °C for 55 days. Significant reductions of oil-soluble SiO 2 and TiO 2 in wear volume of flat and ball were achieved. The preparation method of functionalization is in accord with Fig. 4(c). The same dispersibility was also observed by using polytetrafluoroethylene (PTFE)@SiO 2 [53]. It was also found that the smaller-size SiO 2 exhibited outstanding tribological performances under lower frequency, larger-size SiO 2 performed better www. Springer [68], PbS [69], and CuS [70]. To obtain a good dispersion, dispersants [64] and surface functionalization were widely used [65][66][67]. The comparison of them is given in Table 1.
Sulfides have a common disadvantage because of the active element S, which can lead to oxidation corrosion of mechanical systems and environmental pollution [12]. However, because of the outstanding tribological properties, MoS 2 has been extensively discussed as an important lubricating additive for a long time. Four different types of fullerene (C 60 )-like MoS 2 were synthesized and tested, and it was revealed that they all possessed excellent friction reduction properties in severe boundary lubrication [9]. However, the tribological performance of C 60 -like MoS 2 varies with the internal structure. High load-carrying capacity is obtained by using perfectly spherical and crystallized C 60 -MoS 2 particles, attributing to a higher contact pressure needed to achieve the collapse of structure. However, outstanding tribological properties are observed when using poorly crystalline C 60 -MoS 2 nanoparticles, because they could be easily exfoliated to produce sheets and assembled layer upon layer to form a tribo-film. Similarly, the presence of point defects and hollow structures in the particles also facilitate easier exfoliation and faster tribo-film formation [71]. This result also indirectly shows the different tribological characteristics between MoS 2 sheets and particles.

Carbon
In most studies of nanomaterials in oil lubrication, carbon-based nanomaterials have been emphasized, such as nanodiamond (ND), C 60 , carbon dots (CDs), CSs, carbon nanotubes (CNTs), graphene (G) and graphene oxide (GO). In Section 3.1.3, 0D carbonbased nanomaterials are discussed, and the others will be presented in following sections.
As the nontoxicity and chemical stability of ND, their utilization in boundary lubrication and durability to extreme pressure have always been attractive during the past several years. The viscosity, rheological relation, dynamic, friction torque, secondary addition, and thermal behavior of ND-dispersed lubricant were both investigated [75][76][77][78][79][80], which provided the foundation for exploring the mechanisms of tribological performances.
Carbon sphere (CS) possessing the regular spherical structure has a diameter of more than 100 nm, in contradistinction to metal nanoparticles [81][82][83]. Ultrasmooth submicron CSs were demonstrated as an efficient lubricating oil additive, and the particle sizes ranged from 100 to 500 nm [81]. Generally, the size of CS will affect the friction performances. However, for nitrogen-phosphorus co-doped CSs, the relatively large size of CS did not affect their Nitrogen and phosphorus co-doped CSs (denoted as CN) were prepared via the carbonization of poly (aniline-co-pyrrole) (PACP), and the prepared CSs with N, P elements showed enhanced anti-wear and friction-reducing performance as 500SN additives. Reproduced with permission from Ref. [83], © American Chemical Society 2020. excellent performance [83]. The preparation method of functionalization is in accord with Fig. 4(a), and the graphic abstract is shown in Fig. 6.
Several studies have also been devoted to the application of CDs as lubricating oil additives, as a consequence of their small size (< 10 nm), which is important to solve the problem of dispersion stability. Moreover, good compatibility of CDs with base oil can be directly obtained in the synthesis process [84]. Subsequently, oleylamine [85], polyelectrolyte [86,87], and poly(ionic liquid) brush-grafted CDs [88] were studied. The preparation methods of functionalization are both in accord with Fig. 4(c). In addition, other 0D carbon materials were also demonstrated, such as C 60 [89], onion-like carbons [90], and so on [91,92].
Thermal and rheological properties of lubricants mixed with different carbon nanostructures were evaluated and compared [93]. The results showed that all the thermal conductivities of sample oils were greater than that of the base oil. CS particles exhibited the most positive influence on the thermal conductivity and flash point. But G nanosheets had the greatest improvement effect on pour point. Moreover, all the viscosity of sample oils rises with the increasing concentration of nanomaterials and decreasing temperature, but different structures had little effect.

Metal salts/natural minerals
Among the metal salts studied, the most widely used over the past decade were borates. Oil-soluble calcium borate nanoparticles modified by oleic acid and lauric acid were prepared [94,95], and the functionalization methods were described in Fig. 4(a). Modified calcium borates had good properties of anti-wear and friction reduction, and during the sliding process, a wear resistance film composed of depositions and tribo-chemical reaction products was formed. CaCO 3 nanoparticles were also studied, but their tribological properties depended on the experimental conditions. Only under lower load, higher frequency, and lower temperature, the tribo-chemical reaction product (CaO) was produced, and a film was formed on the worn surface [96].
Overbased calcium sulfonates and/or salicylates have been widely applied [97]. For example, overbased calcium sulfonate detergents are colloidal sols, in which nanosized CaCO 3 particles are stabilized in oil by calcium alkylbenzene sulfonate molecules [98]. They are widely applied in the automotive and marine engine to neutralize acidic products and prevent the build-up of deposits [97]. As oil additives, they could also improve the tribological properties of oil [99]. In addition to the mentioned above, there are also some other metal salts applied in lubricating oil [100][101][102].
Serpentine oil additives had been comprehensively studied, such as the chemical compositions, mechanical properties [103], auto-restoration mechanism [104], and thermal treated properties [105]. Especially in boundary and mixed lubrication, by adding serpentine additives, the tribo-film with excellent mechanical properties composed of SiO 2 , Al 2 O 3 , Fe 3 O 4 , and other compounds was formed, which was attributed to the crystalline structure and reinforced phase of those chemical reaction products [106]. Generally, the resource-rich natural minerals are composed of many complex components and possess a high chemically active. The chemical reactions and physical-chemical depositions could occur easily under friction conditions, which may be beneficial for tribological properties of oil. However, there may be some by-products generated in the process of tribo-chemical reaction due to the complex components in natural minerals. The effect of these by-products on oil has not been pointed out.

Metal-organic framework (MOF)
MOFs are prepared by reticular synthesis via the link of inorganic and organic units with strong chemical bonds. Robust crystalline structures can be yielded when linked to metal-containing units [107]. In particular, the geometry, size, and functionalization of MOFs can be precisely controlled and designed. Hence, there has been significant interest recently in the application of MOFs in lubricating oil. However, there are only a few studies concerning MOFs as lubricating oil additives were reported.
It was found that MOF-based oil additives mainly focused on ZIFs. However, during the friction process, the collapsed structures of ZIFs were observed, and the structure stability was also influenced by acidic environment originating from the oxidation of base oil [113]. It is of interest to investigate DDP-modified Zr-based MOFs (Zr-MOFs) by self-assembly as lubricating oil additives [113]. Because of the coordinatively unsaturated metal sites, Zr-MOFs can be easily functionalized. Combining with the advantages of traditional zinc dialkyldithiophosphate (ZDDP), reduction in both coefficient of friction (COF) and wear volume were achieved, and the oxidation induction time of oil mixed with Zr-MOFs@DDP was expanded much longer. Related functionalization processes and the results of Zr-MOFs@DDP are given in Fig. 7. As one of the most attractive functional nanomaterials, MOFs show superiority in many fields, including liquid separation, gas storage, sensors, drug delivery, supercapacitor, and catalysis [114,115] because of their design ability of molecules and frameworks. MOFs are promising as lubricating oil additives and worth being explored in tribology, and thus more interdisciplinary researches are needed to be carried out.

ILs
ILs composed of cations and anions possess unique characteristics, including great adsorption capacity, low volatility, and high thermal stability [116]. From 2001 to 2011, ILs were primarily explored as base lubricants. However, the achievement of miscibility in 2012 promoted the development of ILs as lubricating oil additives [117].
Qu et al. [118] has been devoted to the research of oil-soluble ILs as lubricant additives for a long time. In 2012, with good solubility in nonpolar oil, the IL trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate was investigated as oil additive. The wear rate was surprisingly reduced by 3 orders of magnitude, which was attributed to a protective boundary tribofilm. Similarly, the IL trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl) phosphinate was also investigated [119]. The ranking of effectiveness in anti-wear behavior for the anions of ILs was summarized [120], and the synergistic effect between phosphonium-alkylphosphate IL and ZDDP was revealed [121]. In addition, multifunctional oil-soluble ILs were synthesized and evaluated, exhibiting excellent properties of solubility, anti-corrosion, anti-rust, antiwear, and higher load-carrying capacity [122]. ILs also exhibited superior tribological performances than ZDDP [123,124]. However, the miscibility, molecular structure, ion pairing of ILs, and the polarity of base oil all have an impact on oil film thickness and tribological performance [125,126].
The viscosity of lubricating oil is also affected by the viscosity of ILs, and then affects the tribological behaviors. The viscosity of ILs varies substantially despite a relatively narrow range of density (1.0-1.6 g/mm 3 ) because the rheological behavior is significantly affected by the cationic and anionic structures. Accordingly, due to the diversity of | https://mc03.manuscriptcentral.com/friction cationic/anionic and the complexity of their molecular structures, a definite law of influence on the viscosity has not been established (the existing influence factors include length, structure, and symmetry of alkyl chain, H-bonding between the ions, and so on) [117]. Viscosity has different effects in different lubrication regions, and suitable ILs can be designed according to particular working conditions.
Various oil-soluble ILs have been synthesized and optimized in the past ten years, and the development of oil-soluble ILs has solved a lot of problems, including corrosion, thermal instability, toxicity, high cost, and so on [127,128]. On the other hand, it is highly effective to enhance the anti-wear behavior of base oil, which is confirmed by lots of experimental results. Hence, the unique characteristics of ILs as lubricating oil additives indicate the potential for future industrial applications [117].

1D nanomaterials
1D nanomaterials have two dimensions at nanoscale with large length-to-diameter aspect ratios. Section 3.2 summarizes the research progress on 1D nanomaterials as lubricating oil additives. CNTs, oxides, sulfides, halloysite clay nanotubes (HNTs), and cellulose nanocrystals are primarily included in Section 3.2. An overview of 1D nanomaterials used as lubricant additives is listed in Table 2.

CNTs
As one form of 1D nanomaterials, CNTs or multiwalled CNTs (MWCNTs) have been widely explored for their excellent friction behaviors. With distinctive structure characteristics of coaxial cylindrical graphene with various layers, it can relatively move along the concentric axis of cylindrical graphene [129]. Theoretical modeling has predicted an ultralow interfacial shear force between layers of double-walled CNTs. Nevertheless, for non-concentric layers, the presence of defects in structure would remarkably increase the shear force, and it monotonically increases with the length of CNTs. On the other hand, the chemical inertness of bare CNTs causes little adsorption on the considered surfaces.
Significant progress in CNTs was achieved in recent years. To overcome the chemical inertness of CNTs, extensive surface functionalization was conducted by stearic acid [130], Co [131], carboxyl [132], and so on [133], and the density of modifier on surface has an impact on their friction behaviors [130,134]. Noteworthily, MWCNTs modified with polymeric aryl phosphates (PAPs) could dramatically improve the friction reduction (~60%) and anti-wear performances (~95%) with 0.08 wt% contents [135]. The detailed data of tribological properties are illustrated in Fig. 8.
The mechanism and rheological behavior of CNTs were also studied [134][135][136]. Meaningfully, CNT additives were investigated in the engine [137]. Conclusions showed that CNT additives in engine oil resulted in a reduction (7%) in the motoring torque; moreover, during normal operation, the dispersion of CNTs in engine was effectively improved by oil shear, and the agglomerates of CNTs were quickly removed by oil filtration system.

Oxides/sulfides
Progress was made in the preparation of metal dichalcogenide nanotubes, in analogy to CNTs, they can also exhibit good tribological performances attributing to layered cylindrical shape. Mo 6 S 4.5 I 4.5 [138] and Mo 6 S 3 I 6 [139] nanowires presented outstanding friction reduction properties, and COF both reached a value of 0.04. MoS 2 was formed in contact area during the friction process. However, these two results were almost identical, so the different chemical formulas were suspicious.
By sulfurization, MoS 2 nanotubes could be synthesized from Mo 6 S 2 I 8 nanowires, and it was found that MoS 2 nanotubes remarkably decreased the COF and wear loss [140]. Similarly, in-situ tribochemical sulfurization was occurred by using MoO 3 nanotubes as additives in base oil in the existence of sulfur-containing lubricant additive, and followed a MoS 2 -rich tribo-film formation [141,142]. The load-carrying capacity of this tribo-film was much higher than that of the common MoS 2 nanotubes [141]. Owing to the continuous sulfurization of MoO 3 nanotubes during sliding contact, common MoS 2 nanotubes progressively degraded and lost lubricity because of oxidation. In-situ sulfurization of MoO 3 nanotubes broke the temperature sensitivity limit of MoS 2 nanotubes and caused superb tribological performance up to 200 °C [142]. On the other hand, it 660 Friction 11(5): 647-684 (2023) | https://mc03.manuscriptcentral.com/friction can be concluded that the oxidation stability of MoS 2 is poor. WS 2 [144] and CuS nanorods [145] were also prepared and investigated, and their anti-oxidation behaviors were not good either [17]. Probably, the method of in-situ tribo-chemical sulfurization is helpful to overcome this problem, but the sulfur element cannot be avoided.

Others
As naturally occurring materials, HNTs with adequate hydroxyl groups [146,147] are considered effective additives in lubricating oil. It was proved that HNT additives could significantly improve the load-carrying capacity and raise the seizure load [148]. Meaningfully, a comparison of carbon nanoparticles (CNPs), TiO 2 , MoS 2 , MoO 3 , and HNTs was performed, indicating that the friction reduction properties of using HNTs under the same concentration in oil were equal to MoS 2 [149]. These results are displayed in Fig. 9.
The improvement of load-carrying capacity and tribological performances of HNTs may be attributed to the C 60 -like hollow nanostructure with a smaller Young's Modulus, which leads to the easier exfoliation of outer layers to form a protective tribo-film on the surface. Moreover, a rolling bearing mechanism may be also present under different working conditions [148]. On the other hand, as clay mineral nanomaterials, HNTs are not hazardous to the environment. The affluence in natural resources and the ability to minimize the quantity of sulfur and phosphorus in oil facilitate HNTs to be the promising, effective, and green additives in oil.
Besides, single crystalline magnesium borate (Mg 2 B 2 O 5 ) nanowires [150] and WSe 2 nanorods [151] as additives in lubricating oil both presented high load-carrying capacity that maybe benefit from the structure of column. BaB 2 O 4 nanorods [152] and cellulose nanocrystals [153] were also studied.

2D nanomaterials
With ultrathin longitudinal dimensions, 2D nanomaterials could decrease the shear resistance between the contact sliding surfaces, and thus optimize the applied conditions [23]. Moreover, strong covalent bonds and weaker layer-layer interactions in 2D nanomaterials ensure the mechanical stability of nanostructure [11]. In Section 3.3, recent progress in research on 2D nanomaterial additives in oil is summarized, typically including G, GO, MoS 2 , layered double hydroxide (LDH), black phosphorus (BP), and covalent organic frameworks (CFs). An overview of 2D nanomaterials used as lubricating oil additives is listed in Table 3.

Carbon
In the past few years, a great deal of graphene-based layered nanomaterials including G, GO [154], fluoro graphene (FG) [155], and reduced graphene oxide (rGO) have been added to base oil to improve the anti-wear and friction reduction behaviors for their high specific surface area, small-size nanosheets with only nanometers thickness [156], and the ability to facilely enter the contact surface. However, due to a large number of demands for acid aqueous solution and water, the synthesis process of graphene-based nanomaterials is not economical and environmentfriendly [15,21].
Pristine graphene-based layered nanomaterials are not dispersible in most oil because of chemical inertness that could be avoided by physical or chemical modification. They were functionalized by oleic acid [157], ILs [158], dodecylamine [159], phosphoniumorganophosphate [160] and so on. There are also several kinds of methods to improve dispersity. Highly-deoxygenated and less defective G was prepared by exfoliation of graphite oxide based on the focused solar electromagnetic radiation. It was also observed that COF reduction, anti-wear, and extreme pressure properties were all enhanced [161]. 3D porous G and high molecular weight polyisobutylene succinimide (T161) as hybrid additives were proposed [162]. The hybrid additives could effectively reduce the COF and wear rate of 316 ASS by 73.1% and 97.8%. The results are displayed in Fig. 10.
Moreover, the methods of ball milling, electrochemically exfoliation [163], liquid phase exfoliation [164], and arc-discharge [165] were also used to prepare high-performance additives. Interestingly, tribological performances of graphite nanosheets [166,167] and highly exfoliated reduced graphite oxide [168] gained by ball milling were both wonderful, and the functional groups could also be grafted on the surface of G by ball milling [169]. The preparation methods of functionalization are both in accord with Fig. 4(d). Obviously, ball milling is a promising way to produce high-performance carbon-based layered nanomaterials as lubricating oil additives.
G is a kind of flexible 2D nanomaterial. Because of the flexibility, their tribological performances are greatly affected by working conditions. Especially, under the mixed lubrication, exfoliated G sheets tended to be trapped in the wear area and induced the "puckering" effect. Then, their high mechanical strength resulted in a higher COF than base oil. Moreover, along with frictional time, the uniformity and dispersion were worsened, and their aggregation intensified. Accordingly, the adsorption film formation was difficult to occur, which was detrimental to their tribological performances [163].

Oxides/sulfides
MoS 2 with a sandwiched S-Mo-S nanostructure has attracted considerable attention. As mentioned above, MoS 2 can be synthesized into different nano-shapes, such as nanoparticles, C 60 -like, nanotubes, and nanosheets. Despite different geometries, MoS 2 nanosheets possess better tribological properties as a result of the ability to easily form linear tribo-film and supernatant performance [171]. Except for Fe 3 O 4 nanoflakes, there are few oxides with layer nanostructure as lubricating oil additives were studied. MoS 2 nanosheets synthesized by ball-milling the mixture of MoO 3 and S could strongly adsorb the surface of substrates to form stable tribo-films [170]. Ultrathin MoS 2 nanosheets prepared by the typical synthesis route [172], solid-state reaction [173], or electrochemically exfoliation [163] were both confirmed to enhanced anti-friction and anti-wear properties as lubricating oil additives. MoS 2 nanosheets with high concentration exhibited significant superiority over ZDDP in reduction of friction and wear in high load [175]. Coral-like MoS 2 prepared by a hydrothermal method could reduce the COF but not wear scar diameter [174].

Natural minerals
As a kind of natural mineral, LDHs can also be fabricated by chemical synthesis in lab, which are composed of metal ions located at the hexagonal crystal center of laminates and hydroxide ions occupying the apexes [177]. The metal ions in LDHs include Mg 2+ , Al 3+ , Ni 2+ , Co 2+ , Fe 3+ , Cu 2+ , Zn 2+ , etc. Due to the unique crystal nanostructures, diversity of chemical composition and size, as well as shape-memory, LDHs have been widely investigated in various fields. It is also interesting to explore the potential tribological properties as additives in lubricating oil. Co-Al-CO 3 -LDHs [178], Mg/Al-LDHs, Zn/Al-LDHs, and Zn/Mg/ Al-LDHs [179] were studied and compared, and Mg/Al-LDHs possessed the best lubrication. Moreover, LDHs with different metal ions and geometry features (spherical, plate-like, and flower-like) were discussed [180]. It was found that instead of different chemical compositions, morphology had a greater influence on the tribological behaviors, and flower-like LDHs with the high specific surface area were demonstrated to exhibit the best performance. Calcined [181] and modified LDHs [182] were also studied.
In addition, layered palygorskite and anthracite sheets as natural inorganic minerals were also studied [183,184].

Others
Zirconium phosphate (ZrP) with plate-like nanostructures was found to be effective as additive [185]. CFs are covalent crystalline porous polymers that achieve the assembly of organic units to create designed skeletons [186]. Triazine-based CFs as additives in oil exhibited high thermal stability, excellent dispersity, and tribological performances [187]. Several families of CFs have been constructed, but there are still challenges in exploring their application of additives in lubricating oil. Besides, a few studies have also shown the application of BP nanosheets in tribology. BP nanosheets prepared by ball milling could improve the load-carrying capacity, friction reduction, and anti-wear properties [188,189].

Nanocomposites
Due to the synergetic effect, nanocomposites usually possess superior tribological performance compared to the individual. According to the nanocomposites composited with different dimensions, they are divided into five major sections: composition of 0D nanomaterials composited with 1D nanomaterials (0D/1D), and others are marked as 0D/2D, 1D/2D, 0D/0D, and 2D/2D. Because few reports related 1D/1D, there is no detailed introduction in this review. A summary and comparison of nanocomposites as lubricating oil additives is shown in Table 4.
ZnO/CNTs nanocomposites with varied mass ratios and concentrations revealed that nanocomposites exhibited enhanced friction reduction and higher anti-wear capability than pure oil, individual ZnO, and CNTs [191]. Interestingly, magnesium silicate hydroxide nanotubes composited with Ni nanoparticles could form a smoother tribo-film through tribo-chemical reactions, which were devoted to friction reduction (15%) and enhanced anti-wear ability (78%) [192]. Besides, the composition of CNTs composited with Ni [193] or Cu [194] nanoparticles was also studied, and the results are listed in Table 4.
Montmorillonite is a resourceful laminar mineral with low cost that is of great potential as lubricating oil additives. MoS 2 /montmorillonite composites achieved the improvement of COF and wear loss [195], suggesting the application prospects as lubricant additives. Significantly, a sandwichlike nanostructure of Mn 3 O 4 nanoparticles and G sheets (Mn 3 O 4 @G) was prepared [12]. Even at a low concentration of 0.075 wt% and a high temperature of 125 °C, the COF and wear depth were reduced by 75% and 97%, respectively. Moreover, TiO 2 /F-rGO nanocomposites with a close link between particles and sheets displayed a low COF and excellent anti-wear capacity attributed to the synergistic lubricating effect, as shown in Fig. 11. It is worth noting that BP dotted with Ag nanoparticles prepared via a facile approach significantly enhanced the tribological performances, and the COF and wear rate were reduced by 73% and 92%, respectively.

1D/2D
With the innovative synthetic strategy, hybrid MoS 2 @CNT nanocomposites were obtained, and the compatibility in nonpolar oils was achieved by coating with oleylamine [209]. In this kind of nanostructure, the outer surface of CNT was sheathed with 1 to 3 MoS 2 nanosheets that achieved unique synergy lubrication.

2D/2D
The layer morphology allows deposition into the sliding area, protecting sliding surfaces from direct contact. Further, due to the different properties of 2D nanomaterials, composite nanosheets may overcome the limitations of individuals [215][216][217]221].
MoS 2 /G composite nanosheets as additives were discussed [218,219], and their outstanding tribological performances were attributed to stable surface boundary films formed during the sliding. Besides, boehmite/GO [220] and BN/G [221] nanocomposites were prepared and explored as lubricating oil additives, both exhibiting better friction reduction and anti-wear properties than the homogeneous individual nanosheet.
The poor oxidation resistance of MoS 2 is a problem for its application. Combination with other nanomaterials with stronger oxidation resistance is an effective method. For all nanomaterials with poor oxidation resistance, there are four methods: (1) As mentioned above, select one material with stronger oxidation resistance to prepare composites, and combine their advantages. The one with poor oxidation resistance could be protected by the cover of another material. Accordingly, tribological characteristics may be changed with the various material addition. (2) Add traditional antioxidants into oil to enhance the oxidation resistance. It is convenient and effective.
(3) Antioxidants are used as surface modification groups to coat nanomaterials that are easy to be oxidized, by which the good dispersion of nanomaterials can also be achieved. (4) In-situ tribo-chemical sulfurization of oxides. Select oxides as additives in base oil containing sulfur element, and in-situ tribo-chemical sulfurization occurs during sliding. This method was proved to be effective, and the tribological properties of tribo-film are better, but the sulfur element cannot be avoided.

Discussion
Due to a large number of articles and complexity of retrieval, it is impossible to cover entire published articles in this review, but most of the research results and statistics have reference value for nanomaterial design. According to many experimental results, the tribological characteristics of most nanomaterials are summarized below. However, due to the complexity of the friction process and the diversity of influencing factors, the conclusions are not absolute, which may be different from special nanomaterial and working conditions.

Relationship between morphology and tribological performance
Tribological performance is found to be affected by many factors. Morphology of nanomaterials plays an important role, which directly determines the pressure to which the nanoparticles are subjected to during loading.
As for 0D nanoparticles, the size of spherical structure is critical to tribological performance of nanoparticles. The smaller nanoparticles could more significantly improve the tribological performances than the larger ones, but the mixed nanoparticles with different sizes display better tribological performance than those with uniform size. Moreover, it has been confirmed by a large number of studies that load-carrying capacity can be significantly improved by 0D and 1D nanoparticles that maybe benefit from the structure of sphere and column.
2D nanomaterials with weak interlayer interactions could offer low interfacial shear force, which is beneficial to friction reduction. But for non-concentric layers, the presence of defects in structure would remarkably increase the shear force. By comparison, sheet-like structures generally outperform other morphologies [222]. As discussed in Section 3, nanosheets possess the ability to easily form linear tribo-film and supernatant performance [171]. The best example is that poorly crystalline onion-like MoS 2 nanoparticles own better lubricating properties than those of perfectly crystallized spherical particles because of their stronger exfoliation ability to produce sheets rapidly and assembled layer upon layer to form a tribo-film [223]. The morphology is also found as a critical role in dispersion stability of nanomaterials in oil. Ordinarily, the smaller-size nanomaterials are more easily dispersed in oil than the larger ones, but it will be changed when functionalization on nanomaterials' surface is conducted. Typically, the porous or flower-like nanomaterials with the relatively high specific surface area have a good dispersion in lubricating oil, which has been widely reported.
It is worthy to note that the influence of morphology is not absolute, and the aforementioned is just a summary of the most of existing experimental results. For example, the size of poorly crystalline onion-like MoS 2 nanoparticles do not impact their tribological performance, because they can be uniformly exfoliated into sheets to form tribo-film [9]; larger LDH nanosheets with a higher degree of crystallinity show the best and stable tribological performance than the smaller ones, because a tribo-film with superior mechanical property is formed during sliding as a result of high crystallinity [177]; aggregates are responsible for some tribo-films formation in point contact but not the nanoparticles with good dispersion in oil [190].
Briefly, although the morphology plays an important role, the tribological properties are affected by many factors, which should not be determined by the structure separately, considering that influence factors at the same time is necessary.

Statistical analysis of tribological data
Statistical analysis of friction reduction and anti-wear properties of four types of nanomaterials with different nanostructured features (0D, 1D, 2D, and nanocomposites) are directly shown in Fig. 12, including the data in Tables 1-4. The following comparison and discussion can be drawn.
1) It is relatively easy for nanomaterial additives to enhance the anti-wear property of base oil, but a significant reduction of COF may require more efforts. On the one hand, these results may be attribute to the reason that nanomaterials as lubricant additives can play as "a protective coating" or "bearing ball" between friction pairs, separating the sliding surfaces to avoid the direct contact to significantly enhance the anti-wear performance. On the other hand, the problem of COF reduction may be imputed to the physicochemical properties of lubricating oil and surface chemistry. The value of COF is closely connected to the pressure-viscosity coefficient and contact pressure [222], which can be easily explained by the Stribeck curve. The low COF value can be obtained under lubricants www.Springer.com/journal/40544 | Friction with a low pressure-viscosity coefficient, and the reduction of apparent viscosity can lead to the ordering of molecules. Generally, the pressure-viscosity coefficient of lubricating oil is relatively large, so it is challenging for lubricating oil to realize low COF value under relatively high contact pressure. Luo's group [224] that has been devoted to superlubricity believe that the ultralow COF is absolutely not obtained by using traditional steel/steel pairs under the same conditions, which is attribute to the surface chemistry. However, statistics show that the proportion of steel/steel pairs used in studies on nanomaterials as lubricating oil additives is 85%.
2) The nanomaterials that possess the ability to synchronously improve friction reduction and anti-wear properties of lubricating oil are mostly carbon-based nanomaterials, such as G and CNTs. The lubrication state of lubricating oil is mostly boundary lubrication, where the oil molecules are strongly adsorbed on the surfaces with only two or three molecular layers to form lubricant film. Benefitting from the developments of exfoliated technology, ultrathin or few-layered G and single-wall CNTs can be achieved by overcoming the strong interatomic-layer bonding. Ultrathin carbon-based nanomaterials could enter the thin oil film without damage to provide a lower shear stress sliding interface and play important roles in oil intermolecular interactions that are helpful to reduce friction. Besides, G is also considered to be an effective nanoadditive to improve the superlubricity system.
3) Compared with individual nanomaterials under the same experimental conditions, most nanocomposites own obvious superiority in tribological performances. However, according to the data of different reported articles that differ from preparations and friction conditions, generally, better tribological performances are not exhibited when nanocomposites are used as lubricating oil additives. It is shown that the different working conditions have great influences on tribological performances of nanomaterials [225], moreover, future experimental verifications and mechanism research are needed.

Lubrication regime analysis
It is well known that the Stribeck curve is widely used to determine the lubrication regime (Fig. 13), including boundary lubrication, mixed lubrication, and elasto-hydrodynamic lubrication, which can also be used to identify the role that nanomaterials play during friction [22]. Most nanomaterial additives could substantially improve the lubrication performance in all the lubrication regimes [226,227], and generally, the impacts on boundary lubrication and mixed lubrication are relatively greater [36,228].
In boundary lubrication, there is a little rolling effect, and the tribological performances are determined by a protective film, whose formation can be promoted by nanomaterial additives [39,54]. Therefore, the mechanical properties of nanomaterials play a crucial role in the mechanical properties of protective film. Specifically, under low frequency or high load, the strength of film is more necessary; but under highfrequency condition, the ductility becomes more important [54]. It also explains why spherical nanoparticles can significantly improve the load-carrying capacity of oil. Besides, tribological performances of nanomaterials are affected by the viscosity of base oil in boundary lubrication. The more effective performances by adding nanoparticles were observed in base oil with lower viscosity. Because COF decreases as the oil viscosity increases, the lubricant with higher viscosity is more difficult to be squeezed out of the contact area. A longer squeezing time achieves a shorter time for the asperities to contact and results in a smaller COF [229].
By adding some nanomaterial additives, in boundary lubrication, there are not only physical effects that  arise, but also tribo-chemical reactions. During the sliding, tribo-chemical reactions between nanomaterials and the iron atoms and/or iron oxide species occur, and their products could adhere to the steel surfaces and form a tribo-film with a low shearing strength [65,227].
In elasto-hydrodynamic lubrication, the dominant factor in determining the tribological performances is the flow feature of nanofluids, which can also be influenced by nanomaterial additives. A theory, based on some experimental results, was proposed: The nanoparticles induced a plug flow in the narrow area between sliding surfaces, leading to only a few layers of oil molecules sliding on each other, and then friction reduced [230,231]. The aggregation phenomenon among nanoparticles was also found in the Hertz contact area [54]. However, results in some research cannot be explained by this theory, e.g., ZrP additives with higher concentration led to a lower COF [185]; under some working conditions, the parts of COF reduction by GO additives are almost the same in all the lubrication regimes [232]. It can be seen that the process of friction is quite complex, and the influence of nanomaterials on the properties of nanofluids needs to be further studied.

Interaction between nanomaterials and sliding surface
The accepted lubrication mechanisms between nanomaterials and sliding surface are also discussed in many reviews that can be briefly summarized as follows: (1) Nanomaterials promote the formation of tribo-films, protection films, or adsorption films, which can separate the friction surfaces and change the surface property. (2) Physical function. The added nanomaterials in lubricating oil may result in promising tribological characteristics owing to the sliding and rolling effect within sliding surfaces depending on the normal and shear stress. Further, small-scale nanomaterials can fill the gaps on surfaces or polish the surfaces to reduce surface roughness, and it is called the polishing or smoothing effect. (3) Chemical function. Nanomaterials reformed because of certain reasons and deposited on the wear tracks during the sliding process. This function is called the repair effect. The main friction mechanisms are shown in Fig. 14.
In addition to the above mentioned, it is confirmed that nanomaterials are firmly associated with the interfacial interaction between nanomaterials and substrates. For instance, when the lubrication with the single layer is considered, the weak interlayer interactions are not the main factor in determining its lubrication performance, but the interaction between the layered nanomaterial and substrate surface is. Then the substrate properties have an increasing influence on the mechanical and dissipative properties of the layer nanomaterial [23]. It was reported that when exfoliated G was applied on surfaces such as mica, good contact with the substrate and strong adhesion were obtained. When applied on SiO 2 with atomically rough surfaces, the adhesive force between G and substrate was reduced, generating the "puckering" effect. Because the contact region was decreased after folding, G maintained the rough configuration, and the tribological performances were not improved [233]. However, GO with functional groups could be covalently grafted to the substrate surface to overcome abscission. In addition, with high out-of-plane flexibility, 3D nanostructural G is dominated by the contact substrate geometry, and tribological characteristics of G particularly depend on the roughness of sliding surface and interfacial adhesion by van der Waals forces. Hence, before, after, or in the process of sliding, the adhesion to substrate surface and morphology of nanomaterial are closely correlative to their tribological properties [23]. Similar to the aforementioned nanoparticles, the existence of 1D nanomaterial CNTs could also form the aggregations in the contact area, but during the sliding process, surface roughness of substrate led to a decrease in the number of aggregations, passing through the contact area. And an increasing film thickness, friction reduction, and wear drift were observed, originating from the transient propagation of CNT aggregation through the contact area affected by the interfacial interaction [135].

Interaction between nanomaterials and lubricating oil
The organization and distribution of nanomaterials in oil depend on their character of interaction forces such as van der Waals force, electrostatic force, Brownian motion, and steric interactions. The rheological performances and flow properties of colloidal fluids are governed by these forces together with dynamic interactions in oil owing to shear rate. For example, differing from graphite bulk [234], rGO is composed of loose layers with a lot of narrow cracks. This specific nanostructure reduces the van der Waals force between rGO interlayers, resulting in the decrease of resistance under shear force. The carbon layers with low cavities and defects can enhance the tribological behaviors [168]. The diagram of mechanisms is shown in Figs. 15(a 1 ) and 15(a 2 ). The polar nature plays an important role in the interaction between nanomaterials and lubricating oil [235]. Polar compounds can be adsorbed onto metallic surfaces, and this attraction results in a thin oil film. However, metallic nanoparticles could also be adsorbed by polar molecules, preventing their ball-bearing function as lubricating oil additives or disrupting the oil film formation [38]. Accordingly, for the polar nature of different lubricating oil, surface functionalization on nanomaterials can effectively transform the interaction between nanomaterials and lubricating oil, such as enhancement of dispersion stability and homogeneous distribution; however, not all treatments associated with surface functionalization are positive [27,53,205]. The diagram of mechanisms is shown in Fig. 15(b). The conventional view is that tribological performances of nanomaterials as lubricating oil additives are in proportion to their dispersion stability in oil. However, when nanoparticles were added into oil as additives in point contact, tribo-film formation originated from the aggregates of nanoparticles but not the well-dispersed [190]. This new point may promote the advancement of novel lubricating oil additives design in different working conditions.
The relationship between nanomaterials and oil film is also one of the focus points discussed in the tribology fields. One of the major affecting factors to consider is the size of nanomaterials compared to the thickness of oil film. When the size of nanomaterials is larger than oil film thickness, nanomaterial behaviors are dominated by contact kinematics conditions. At low sliding velocity, nanomaterials easily enter into the contact surface where an adherent boundary oil film is from. However, this oil film disappears with | https://mc03.manuscriptcentral.com/friction the sustained increase of velocity, where the state of lubrication is changed from mixed lubrication to elasto-hydrodynamic lubrication. At higher sliding velocity, most materials in nanofluids are rejected to directly contact the surface, inducing a decrease in oil film thickness [135]. Therefore, substrate surface trapping mechanisms should be highlighted.
As already mentioned in Section 4, the viscosity of base oil also has an impact on the tribological performances of lubricant, especially in boundary lubrication. The thermal conductivity and viscosity of oil are both important properties, which can be directly affected by temperature and nanomaterial concentration. Generally, most oil samples mixed with nanomaterials have Newtonian behavior. The viscosity of nanofluids rises with the increasing concentrations of nanomaterials and decreasing temperatures [91], moreover, it is more sensitive to the concentrations [131]. At high temperatures and heat fluxes, heat transfer in lubricant assumes an important role. The presence of nanomaterials with high thermal conductivity could enhance the heat transfer from the contact surface, and lead to higher viscosity and load-carrying capacity even at a low concentration [91,92].

Interaction between nanomaterials and other additives
With the development of oil additives, the coexistence of nanomaterials with conventional lubricating oil additives is unavoidable [236]. From the tribological data analyses of studies on lubricating oil mixed with nanomaterials and other additives, it is shown that in general, the tribological performances of base oil both added with nanomaterials and other additives are better than those of the bare nanomaterials, indicating the synergy between nanomaterials and other oil additives. Dispersants are the most commonly used with nanomaterials in lubricating oil, because the dispersed stability of nanomaterials can be improved by adding them. This method could easily solve the agglomeration problem of nanomaterials in lubricating oil that is more simple, economical, and easier to achieve than surface functionalization on nanomaterials.
The interactions between MoS 2 nanotubes and several oil additives were investigated, which depended on the contact conditions and lubrication surfaces [30,237]. The synergetic interaction was obtained between MoS 2 nanotubes with anti-wear additives under mixed rolling and sliding conditions. Besides, all the selected oil additives displayed a synergistic phenomenon with MoS 2 nanotubes under extreme pressure, indicating the importance of well-dispersed nanomaterial in boundary lubrication. However, higher antagonism was observed between the MoS 2 nanotubes and dispersants under reciprocating sliding. Those phenomena suggest that the effect of other oil additives on nanomaterials lubrication is also affected by working conditions and lubrication regime. As shown in Fig. 10, there was an excellent synergistic effect between 3D G and T161. The edge and surface of 3D G were modified by long-chain T161 to form a steric layer, which enhanced the dispersion stability of 3D G and ensured the sustained supply of 3D G to promote the homogenization of oil film. Under lower contact pressure, 3D G could fill the tracks on the wear surface, and 3D porous nanostructures were mostly retained, resulting in the improvement of tribological performances.

Interaction between nanomaterials in nanocomposites
It must be accepted that there are different interactions between nanomaterials in nanocomposites, such as synergy and antagonism effects. Different types and nanostructures of nanomaterials differ in their characters and advantages. For instance, metallic nanoparticles own unique properties of noncorrosiveness and low melting point; the small size (< 10 nm) of CDs; large length-to-diameter aspect ratios of CNTs; and mechanical stability of carbon-based nanomaterials. In general, small-sized nanoparticles can improve the friction reduction and anti-wear properties, and large-sized nanoparticles exhibit better load-carrying capacity [111]. The coexistence of nanomaterials' advantages in achievable conditions is always expected by researchers. Strong van der Waals forces and interactions are revealed due to the large surface area of a few graphene-based layers, and the tribological performance of oil mixed with graphene-based nanomaterials is www.Springer.com/journal/40544 | Friction limited by the aggregation of graphene sheets. Thus, there is no beneficial uniform coverage on the tribo-film. However, nanoparticles or nanotubes can effectively prevent graphene-based layers from stacking; moreover, they can roll between layers to reduce friction. The composites of 0D/2D and 1D/2D effectively combine the advantages of different nanostructures. The diagram of synergy effects with different dimensions are shown in Fig. 16.
Other parameters of nanomaterials in nanocomposites also need to consider, such as the concentration, the ability to deform, the affinity between different nanomaterials, and the mass ratio of different nanomaterials that can control the tribological performance of nanomaterials as lubricating oil additives.

Outlook
To date, a large number of applications in nanomaterials as lubricating oil additives have been carried out, indicating the prosperous development of this field. A preliminary suggestion is provided by this review for selecting appropriate nanomaterials to achieve superior tribological performances. However, analysis of experimental process and data indicates that there are still some challenges in applications. The following may be important future research suggestions in the development of nanomaterials as lubricating oil additives.
1) Trans-dimensional research. Nonconforming experimental data are obtained by using the same nanomaterials in different researches, which may be attributed to the different experimental processes, contact surfaces, and nanostructure of nanomaterials or others. Trans-dimensional research on tribology and molecular mechanism in depth is needed to explain the problem. Therefore, maybe several fields need to be paid attention to (i) the application of molecular dynamics simulation [238,239] of tribological behaviors for nanomaterials and the combination of molecular simulation and tribological tests; (ii) the systematic research on the transformation regularities of oil film, contact surface, nanomaterials, and energy throughout the process of friction and the change regularities of tribological performances under different working conditions; (iii) the molecular interaction system between oil, contact surface, nanomaterials, and other conventional additives.
2) Design of nanoparticles. In general, scholars prefer the study of nanomaterials that have already been discovered. Therefore, changing the traditional pattern of research is important to reveal and achieve new nanomaterials with targeted tribological behaviors. For example, the geometry, nanosize, and functionalization of MOFs and CFs can be precisely controlled and designed, and it is interesting in studying their application in lubricating oil. However, the study of MOFs as lubricating oil additives began in 2011, and CFs began in 2017; there is few research carried out. And the same is true for ILs. Nanomaterial design is difficult but may be most effective to obtain better tribological performances.
3) Preparation with the superior method. Many severe preparation methods of nanomaterial have been carried out to pursue better tribological performances,  | https://mc03.manuscriptcentral.com/friction but some drawbacks are displayed, such as high cost, small-scale production, poor repeatability, long time-consuming, and so on. It is hardly accessible in the current industry. One-step and low-cost methods that can achieve the preparation of nanomaterials with superior tribological performances are promising. On the other hand, due to the abundant resources and low cost, natural minerals are worth exploring in tribology. However, more efforts are required to optimize their tribological performance. 4) Research on synergistic mechanisms. Benefitting from the synergistic lubricating effect, the combination of two or more nanomaterials has better tribological behavior than individual nanomaterials under the same working conditions. On the other hand, the coexistence of nanomaterials with other conventional additives is positive and unavoidable. However, the synergistic lubricating effect and collaborative mechanism are not certainly confirmed at a molecular level, which is necessary for lubrication system design. 5) Environmental protection. It is important to minimize or eliminate the use of sulfur and phosphorus contained in lubricating oil additives. There is no doubt that eco-friendly nanomaterial is of great help to reduce energy consumption and carbon footprint. Eco-friendly nanomaterial additives are useful for environmental sustainability and meet the requirements of green tribology [240], which is a new area for tribologists.

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