Progress of Multidimensional Nano-Additives under Dry/Liquid Wear: A Review

Abstract

An investigation of the interaction between multidimensional nano-additives and tribofilms is crucial for enhancing mechanical efficiency, extending equipment lifespan, and reducing environmental impacts. Improved tribofilm performance is obtained via several mechanisms: filling surface defects with 0D nano-additives, directional lubrication for 1D nano-additives, interlayer slippage for 2D nano-additives, and improved film durability for 3D nano-additives. Under dry lubrication, the formation of tribofilms via mechanical mixing is influenced by material hardness, surface roughness, and frictional conditions, with their thicknesses increasing by 20–30% under high loading. Conversely, liquid-lubricated films result from the physical adsorption and chemical reactions of the lubricants, with extremely high pressure additives reducing the friction coefficient by 30–50% at high pressure. A greater understanding of these mechanisms is beneficial for optimizing industrial technologies and developing efficient, eco-friendly lubrication systems.

1. Introduction

Amidst the ongoing developments in engineering technologies, and with the continuous extension of their application fields, the demand for materials with properties of wear-resistance is increasing [1,2,3,4,5]. The performances of wear-resistant materials not only directly affect the service life and reliability of mechanical equipment but also relates to engineering costs and resource utilization efficiency [6,7]. To satisfy the needs of different fields, a variety of new wear-resistant materials and design strategies continue to emerge. In the design of wear-resistant materials, nanomaterials, as important functional materials, have shown great potential in increasing the wear resistance of materials due to their unique structures and properties [8,9,10].

Research into the performance of nanomaterials under dry/liquid wear aims to enhance mechanical system efficiencies and reduce friction, thereby decreasing energy consumption and boosting equipment performance, particularly in energy-intensive industries such as automotive manufacturing and aerospace [11,12]. Furthermore, an understanding of how nano-additives affect the structure and properties of tribofilms under dry/liquid conditions contributes to the optimization of lubricant formulations, thus enhancing energy transfer efficiency and yielding significant economic and environmental benefits [13,14]. According to the dimensional characteristics of nano-additives, they can be placed into the following categories: zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) nano-additives [15,16,17]. As an important type of nanomaterial, 0D nano-additives have the characteristics of being nanoscale in size and they often exist in the form of particles; they have a high specific surface area, excellent mechanical properties and surface activity, and form a stable friction film on the friction surfaces of materials, effectively developing wear resistance for these materials [18,19]. One-dimensional nano-additives, usually in the form of nanofibers or nanotubes, have a high aspect ratio, which can enhance the chemical stability of friction films, extend the service life of the material, and reduce maintenance costs [20,21]. Additionally, 2D nano-additives are principally made up of various nanosheets and their corresponding multilayer structures; examples of these structures are graphene, two-dimensional transition metal sulfide, and two-dimensional oxides, whose thicknesses range from a few nanometers to tens of nanometers, with pre-eminent mechanical performances and chemical stability [22,23,24,25,26]. Unlike the above-mentioned nano-additives, 3D nano-additives are nanohybrids, comprising one or more zero-dimensional, one-dimensional, or two-dimensional structural units, and usually including porous or highly porous structures [27,28]. Figure 1 presents a molecular structure diagram of multidimensional nano-additives (including nanoparticles, nanotubes, nanosheets, and nanodiamonds), along with the representative morphologies of the corresponding materials (such as A1₂O₃ nanoparticles, MoS₂ nanotubes, ZnO nanosheets, and BP/TiO₂ nanohybrids) that are widely used in various applications under dry/liquid wear conditions. Among them, 0D nano-additives enhance industrial lubricants, 1D nano-additives improve electronics and energy, 2D nano-additives enable transistor fabrication, and 3D nano-additives optimize battery electrodes for an extended service life. In particular, they can be added to the friction interface as a lubricant, thus leading to the formation of a stable friction film and a reduction in the coefficients of friction and wear.

The broad applications of nano-additives in dry wear and liquid wear scenarios provides significant options and possibilities for designing effective friction interface optimization strategies [34,35]. Through the rational selection and design of nano-additives, the performance of the friction interface can be effectively improved, and the goals of friction reduction, wear reduction, and interface lubrication can be achieved. In different dimensions, the contribution of nano-additives to the friction film is different. Zero-dimensional nano-additives, in the form of nanoparticles, can usually fill surface micro-pits, leading to an improvement of the friction film in terms of mechanical strength and smoothness [8,36]. The unparalleled “nano-rolling effect” in 1D nano-additives enables them to be mutative in the friction state, from sliding to rolling, and can form a stable friction film structure and improve the toughness and stability of the friction film [37,38,39,40,41]. Two-dimensional nano-additives, usually in the form of nanosheets, can form a continuous friction film on the friction surface, reducing the friction coefficient and wear rate [42,43,44]. Three-dimensional nano-additives have a larger specific surface area and surface activity, and they can form a porous friction film on the friction surface, improving the adsorption and lubrication properties of the friction film [5,45,46]. Therefore, in-depth research on the contribution of nano-additives of different dimensions under dry/liquid wear conditions is of great significance in guiding the design and preparation of new wear-resistant materials.

This review will provide a systematic overview of three important areas of nano-additives in tribology research (nano-additives’ effects and wear mechanisms under dry/liquid conditions). Firstly, the effects of various nano-additives in dry/liquid wear processes are examined. Due to their unique sizes, shapes, and chemical properties, dense and wear-resistant tribofilms are formed on the frictional interface, thus effectively reducing friction and wear while enhancing the thermal and chemical stability of tribofilms. Secondly, the mechanisms and factors by which nano-additives enhance wear-resistant materials are analyzed. The existence of nano-additives improves the friction interface’s load-bearing capacity and significantly extend the material lifespan under high loading; additionally, they have a role in the optimization of surface morphologies and in the regulation of environmental conditions, further enhancing integral material properties. Finally, the research results and future trends of multidimensional nano-additives in tribology are explored. Future studies should focus on their performance in complex environments and explore their compatibilities with new materials.

2. Zero-Dimensional Nano-Additives in Tribology

To explore efficient strategies in tribological optimization, 0D nano-additives like Al₂O₃, Fe₃O₄, and CuO nanoparticles have been widely applied in diverse dry/liquid friction applications for the purposes of controlling friction and wear [47,48]. The unique near-spherical structures of 0D nano-additives confer excellent tribological potential upon material systems, which manifests as a contribution towards the formation of lubricating films [49,50,51]. Lubricating films formed under 0D nanomaterials can consist of all of the known forms of nanoparticles and have easy shear, providing the tribological properties of friction reduction and anti-wear protection. In the subsequent sections, the tribological behavior relating to the wear-resistant and friction-reduction properties of 0D nano-additives under dry and liquid friction is investigated; the wear mechanisms of 0D-enhanced systems are analyzed.

Table 1. Tribological performance of 0D nano-additives under dry/liquid friction.

Frictional State	Matrix	Reinforcement	Test Conditions	Tribological Results		Ref.
				COF	Wear
Dry friction	TC4 alloy	6 wt% Al2O3 nanoparticles	Si3N4 ball (ball-on-disk; 10 N; 400 °C)	0.185	Wear rate: 3.6 × 10−5 mm3 N−1 m−1	[30]
	Co-based Satellite 6 alloy	0.6 wt% Nano-Y2O3	Alumina disk (pin-on-disk, 10 N, 0.1 m/s, 25 °C)	~0.650	Specific wear rate: 1.35 × 10−5 mm3 N−1 m−1	[50]
	Ni-P coating	1 g L−1 Nano-SiC	AISI52100 steel pin and AZ31 alloy disk (pin-on-disk; 300 °C)	0.380	Wear rate: 4.2 × 10 −5 mm3 N−1 m−1	[52]
	Ni coating	8 vol% Nano-SiC	Silicon nitride ball and A356 aluminum alloy disk (ball-on-disk; 100 N; 20 Hz)	0.095	Wear rate: 1.13 × 10−6 mm3/N m	[53]
Liquid friction	PEG oil	1.0 wt% CDs-PF6	GCr15 steel balls (Four-ball tester, 392 N, 1200 rpm, 75 °C)	0.030	Wear loss: 0.199 × 10−3 mm3	[54]
	PAO8 oil	0.35 wt% TiO2-OA	100 Cr6 steel ball and 100 Cr6 steel disc (ball-on-disc, 120 °C, 213 rpm)	0.098	Wear scar diameter: 2.89 × 10−4 mm	[51]
	PAO4 oil	1.0 wt% Cu nanoparticles	AISI5120 steel balls (Four-ball tester, 392 N, 1200 rpm, 75 °C)	0.065	The smallest wear scar diameter	[55]
	Base oil	0.06 wt% N@MCNs nanospheres	AISI52100 steel balls and AISI52100 steel disks (ball-on-disk, 150 N, 50 °C, 25 Hz)	0.110	Wear volume: 6.05  ×  104 μm3	[56]

COF: friction coefficient; Ref.: references.

shows the detail values of the tribological properties for 0D nano-additives under dry/liquid friction, respectively.

2.1. Contributions of 0D Nano-Additives on Lubrication Films under Dry Wear

Many nanomaterials can serve as functional additives in optimizing the properties of solid matrices (metal matrices, ceramic matrices, polymer matrices); this fact has attracted tremendous research interest [57,58,59]. This close research attention on nanomaterials has led to the acceleration of incentives for the application of 0D nano-additives in solid materials. Zero-dimensional nano-additives, including various nanoparticles and nanospheres measuring 1–100 nm, have been proved to be widely used in tribology.

Metal matrices have excellent ductility, which is advantageous for the compression of nanoparticles, enabling them to fill tiny defects on the surfaces that are a result of wear, thus producing a continuous friction layer. The good dispersion of 0D nano-additives plays a crucial role in determining the structure and properties of a composite. Additionally, the dispersion indirectly affects the formatting of a given tribofilm [60,61]. An essential prerequisite for the uniform dispersion of nano-additives is that the concentration must reach a certain range. The prevention of particle agglomeration is necessary if one is to avoid the creation of conditions that favor the formation of friction films through abrasive wear. The scattering of particles on a given surface is not uniform, and in some areas, particles are aggregated [52]. The specific parameters of such tribological behaviors have been presented in Table 1. It was noteworthy that, even when using nano-reinforced systems with appropriate concentrations, one cannot completely guarantee the formation of a high-performance tribofilm. The formation of a tribofilm is closely related to the contents of the elements on a given substrate, the tribological reaction on the surface in question, and the operating environment used [62,63]. For example, reducing the sodium content of used lubricants would limit the formation of a tribofilm; this would significantly reduce the friction and anti-wear properties of the lubricants. In contrast, the Fe₃O₄ surface was most favorable for tribofilm formation because it produces the most species and molecules [64,65].

The current density has a noticeable effect on coatings with a high particle content [53]. As shown in Figure 2a, the SiC content in the coating increases depending on the increase in the current density. Figure 2b shows the microhardness curves for different SiC contents. With the increase in the SiC content in the coating, the hardness of the coating also increased; this was due to the dispersion and strengthening effect of the ceramic particles in the coating. The specific parameters in such tribological behaviors are presented in Table 1. Under dry conditions, polytetrafluoroethylene (PTFE)-combined wear chips produced a fluoro-based tribological film under the sliding surface; this changed the wear process into a sliding process between the smooth tribofilms. In turn, the component was crushed into worn fragments, which improved the quality of the formation of the tribofilm, while compacting the transfer film so that it reached a thin and uniform consistency. In addition, the tribo–chemical reaction of SiC nanoparticles promoted the formation of a strong tribofilm with a complex structure; this had protective and shielding effects [66,67].

The production of a PTFE nanocomposite filled with Al₂O₃ nanoparticles was carried out in dry conditions using a freshly created dual-pins-on-disk tribometer [68]. The results revealed that a concurrent effect between the two polymer components appeared, thereby improving the tribology performance of the hybrid system. The formation of a tribofilm and its different characteristics were elucidated via a surface analysis of the different run-in stages. For rough surfaces, the loosely adhered transfer film was formed at the beginning of the sliding process in order to form a durable transfer film. Inversely, for smooth anti-surfaces, durable transfer films were formed from the outset. A tribofilm with a thickness measuring microns was observed on the surface of the PTFE composite after the run-in; the film had an excellent low-friction performance [69,70]. Particularly, the tribological effect also plays an important function in the tribological behaviors of nanocomposites. Because of the insufficient strength of polymers, it was easy to conduct the removal and transferal of materials for polymer nanomaterials under friction, resulting in the formation of transfer-film-embracing additives. The mechanical performances and the bonding strengths of these films exerted a crucial influence on the tribological behaviors of the polymers.

After being exposed to the friction surface, the wrapped Al₂O₃ nanoparticles not only play a role in enhancing the load-carrying capacities of lubricating films, they also act as “micro-bearings” between the friction pairs [30]. Obviously, as shown in Figure 2c,d, in comparison to TC4 in all tested temperatures, TC4-Sn-Ag-Cu (TC4-SAC) achieved smaller coefficients for the friction and wear rates; these may be attributed to the outstanding lubricating characteristic of Sn-Ag-Cu. The detailed parameters of such tribological behaviors are displayed in Table 1. Further, Figure 2e shows the self-lubricating mechanism reaction. Under the action of low mechanical stress, a small amount of coated Al₂O₃ nanoparticles microporous channels were precipitated; only a local lubrication film was formed on the worn surface. When the load continued to increase, the coated Al₂O₃ nanoparticles exposed to the frictional surface could not only enhance the bearing capacity of the lubrication films but could also play a role of “micro-bearing” for the inner parts of the friction pairs. In addition, the wear surface was covered with a relatively smooth and uniform lubrication film during the frictional process, which can mainly be attributed to the addition of Al₂O₃ nanoparticles [71,72]. The decrease in the wear coefficient was mainly controlled by the formation of a tribofilm on the friction surface, which improved the friction and wear behaviors of the composites [73]. Structurally, these tribofilms were quite complex and may consist of all known forms of nanoparticles. For tribology, they were easy to shear and provided ultralow friction and anti-wear protection [74,75].

By introducing antifriction materials into ceramic matrices, the friction force which ceramics are subjected to can be reduced; synchronously, the tribological properties of ceramics can be improved [76]. A prediction model that could reflect the friction performance of antifriction materials on ceramics was established to predict the wear rate of friction films under dry friction conditions; this model provided theoretical guidance for ceramic designers [77]. Based on a process of calculating the wear rate through a mathematical model formula, with increase in normal force, the friction coefficients of a ceramic matrix began to decrease and then enter a state of stable fluctuation. Correspondingly, during the friction process, the extrusion of antifriction nanomaterials from the substrate can be carried out, forming a large area of lubrication-film-containing nano-additives, thus optimizing the friction coefficient. The presence of low-friction-coefficient films impede the direct contact of the friction pair and thus reduce friction.

The improvement in the tribology properties of lightweight alloys that is achieved through an increase in the presence of additive-surface chemical interactions is sufficiently noteworthy. In addition to the remarkable contribution of experimental results to our understanding of the formation mechanism of tribofilm, chemical interactions at the atomic level can be effectively explained by computer simulation, which also provides valuable viewpoint for its formation on metal substrates. Based on density functional theory simulations, the interaction of zinc dialkyldithiophosphates (ZDDP) with lightweight metallic substrates has been investigated [78]. The results of the study demonstrated that, compared to aluminum (Al) and magnesium (Mg), the molecular fragments from ZDDP were adsorbed more strongly on the intermetallic Mg₁₇Al₁₂ phase, owing to the higher surface energy of the mixed substrate. The tribofilm formed by ZDDP was insufficient in preventing the wear of the aluminum alloy. In this situation, the tribological activity of the nano-additives on the inner worn interface led to the formation of a tribofilm, to a certain extent. In summary, near-spherical 0D nanomaterials have a roller bearing function that is preponderant to the bearing capacity of tribofilms; this is easy to oxidize. Thus, it is suitable for the photoelectric properties of the material. However, nanoparticles are easy to agglomerate, which affects catalytic performances. The tribological effect of 0D nano-additives on a worn surface significantly influences the establishment of a tribofilm.

2.2. Enhancements of 0D Nano-Additives on Lubrication Films under Liquid Wear

The use of solid reinforcing agents remains the preferred choice in complex and harsh space environments; however, liquid lubricants are increasingly emerging as a potent means for a reducing in friction and wear within mechanical contact areas [79]. Under liquid wear, the wear mechanisms involve the formation of a lubricating film on the tribology surface, thereby preventing direct contact among the frictional pairs [80].

The synergistic effect between multicomponent nanoparticles performs an important role in the formation of high-performance tribofilms [81,82]. The introduction of nano-hexagonal boron nitride (nano-hBN) (0.75 wt%) and nano-Al₂O₃ (0.25 wt%) into calcium sulfonate complex grease (CSCG) results in optimal tribology performances: a 11.3% reduction in the mean friction coefficient, a 6.1% decrease in the wear scar diameter, and a significant improvement in worn surface morphology were observed in comparison with CSCG [83]. This can be explained through the synergistic effect: on the one hand, nano-hBN could promote the formation of a protective layer that is mainly composed of calcium carbonate; on the other hand, nano-Al₂O₃ could inhibit the agglomeration of hBN and strengthen the antifriction effect of calcium carbonate.

The modification of lubricants using nanoparticle additives to enhance the tribological performance of lubricants has become a promising approach in this technology [84]. The excellent lubrication ability of such materials has been found to lead to increased working efficiency, application accuracy, and service life for numerous moving contact parts. This ability is closely related to the mechanical characteristics of 0D nano-additives, like hardness and shear modulus. Nanoparticles have many advantages, such as high hardness, good thermal stability, and low cost. Nanoparticles can roll or slide between surfaces, repair surface defects, and form tribofilms via physical embedding or chemisorption.

On the basis of the intensity of friction chemical reactions, the process of the formation of tribofilms can be divided into two stages [85]. In the initial stage, particles gather in front of the contact area, and particle accumulation begins to move back and forth with the surface of the plate. Van der Waals forces combine with the electrostatic interaction of the nano-additives on the friction surface, making it easy for the films to adsorb onto the substrate surface; thus, they form physical protective films in an adsorptive manner, preventing any chemical reaction from occurring. The presence of adsorption films improves the lubrication conditions of the two contact surfaces, making it easier for the lubricating oil to enter the contact area, thus increasing the thickness of the oil film [86]. At this point, the lubrication film primarily plays a significant function in preventing direct contact with the frictional surface and reducing wear. Meanwhile, as the friction time continues, the deposited physical adsorption films begin to disintegrate. Because of the high load, friction heat, and mechanical energy produced by friction, tribo–chemical reactions occur on the sliding surface; this signifies that a formation of tribofilms have entered the subsequent stage. In the process of tribological chemical reactions, new tribofilms are formed; these mingle and become progressively displaced by the physical film, which significantly enhances the tribological properties of the friction system.

The tribological properties of magnesium silicate hydroxide (MSH) nanoparticles were explored using a four-ball test [87]. The results indicated that MSH could effectively increase the thickness of oil films, thus achieving a reduction in wear; this was ascribed to the formation of a tribofilm containing Mg, Si, and O on the worn surface. Figure 3a presents the change curve for the friction coefficients; additionally, one can see the corresponding scanning electron microscope (SEM)-captured surface morphologies in different periods [88]. During the stable period, the area that was left exposed after stripping following the stabilization period was rich in Mg and Al elements. In addition, the newly formed surface had higher tensile strength and yield strength, which could reduce cracks caused by fatigue wear. These potential tribological behaviors have led to extensive research being conducted. Under temperature conditions of 250 °C, the wear rate of the sample disc was the highest when using a 0.25 wt% nano-Al₂O₃ lubricant, as shown in Figure 3b.

CDs-PF₆ was obtained via a method seeking to modify carbon nanodot–ionic liquid (CDs-IL) hybrids with hexafluorophosphate (PF₆). CDs-PF₆ nanoparticles, as polyethylene glycol-based (PEG) additives, possess outstanding antifriction and wear-resistant properties [54]. Figure 3c shows the curves of the friction coefficients for CDs-PF₆ in PEG with various concentrations under loads of 392 N. CDs-PF₆ could markedly enhance the anti-wear and friction-reduction characteristics of PEG at low concentrations (0.25–1 wt.%). CDs-PF₆ achieved a remarkable concurrent improvement in anti-wear (96.5%) and friction-reduction (72.7%) characteristics; this is a result of the formation of a protective carbon-based layer at the contact regions. Table 1 shows the definite values of the friction coefficients and wear rates. However, the CDs-PF₆/PEG were weakened at a concentration of 2 wt.%. This might be attributed to the competitive adsorption of CDs-PF₆ and PEG at the friction interface, in addition to the fact that CDs-PF₆ are more prone to agglomeration at high concentrations. The excellent tribological properties of PEG were mainly attributed to the formation of physical adsorption films, where nanoparticles can interact with the frictional surface. As shown in Figure 3d, the combination of the negatively charged anions of the CDs-IL and the positively charged sites of the grooves promoted the aggregation of CDs-IL on the rubbing surface during the friction process. With the continuation of the friction condition, the deposited physical adsorption films began to decompose and tribo–chemical reactions occurred because of the high pressure, the frictional heat, and the mechanical energy; this formed a protective transfer film on the metal surfaces and exhibited a “self-mending effect” by filling the deep scratches. This process grants it an outstanding anti-wear and friction-reduction property.

To enhance the tribological characteristics of the base oil, the use of nanoparticles as lubricant additives has attracted more and more attention. As shown in Figure 3e, the tribological properties of the PAO₄ base oil were affected by the presence of Cu nanoparticles at certain concentrations [55]. This phenomenon was attributed to the dissemination and infiltration of nano-Cu on the friction interface when the nanoparticles were mixed with the base lubricants, forming a protective layer on the friction interface, and thus achieving the best tribological performances under optimal concentration conditions. The details of the values of the PAO₄ base oil, containing 1.0 wt % nano-Cu, and how they influenced the tribological performance, are listed in Table 1. Furthermore, the friction coefficients increased due to the fracture of the synovial film or the surface layer, which was caused by increasing concentration; this provides a practicable solution for the separation of friction pairs and the related tribological optimization. The effect of the addition of eichhornia crassipes nanotube (EC-CNT) particles on the tribological property of the lubricants was examined [89]. When compared to the base oil, the tribological effects of the EC-CNT additive at different concentrations (0.5 wt%, 1 wt%, and 1.5 wt%) were studied at different temperatures and with a steady load of 100 N, as shown in Figure 3f. When the temperature increased from 50 °C to 100 °C, the average friction coefficients decreased more strongly, relative to the base lubricants. However, the reduction in the friction coefficient and in the unidirectional mode was small compared to those of the reciprocating orientation. A 49% reduction in friction was achieved using a 1 wt% EC-CNT nanoparticle under the unidirectional testing mode; meanwhile, a 58.1% reduction was obtained in the reciprocating mode compared to the base lubricant [90].

Nanoparticles can be used as additives to enhance the lubrication properties of films. However, due to their tendency to aggregate and settle, introducing 0D nano-additives into lubrication oil is a complicated issue. The homogeneous and stable diffusion of nano-additives has a significant sway in promoting the creation of high-performance tribofilms. The wear resistance of carbon-based nanoparticles was due to the protective layer that formed on the contact surface, minimizing friction and wear [91]. Overall, 0D nano-additives markedly enhance the wear resistance of friction systems. In mechanical systems, the addition of nanoparticles to base lubricants could improve the efficiency of systems by reducing the friction coefficient. With an increase in the concentration, the mean values of the friction coefficient decreased, reaching a low value when the nanoparticle concentration was 1.0 wt %, and then rose. A protective layer formed on the surfaces of the mechanical mating components when the nanoparticles were mixed with the base lubricants, resulting in the highest tribological performance, which reached the optimal concentration of lubrication. Furthermore, there was an increase in the friction coefficient because of the rupture in the lubricating films or the formed surface layers. Compared with pure PAO₄ oil, the mean friction coefficient of the PAO₄ oil containing the 1.0 wt% Cu nanoparticle was decreased by 51.85. This can be explained by the fact that the effective dispersion led to the formation of a protective tribofilm between the frictional pairs. The tribofilm prevented direct contact between the friction interfaces, thus improving the wear resistance.

In addition to oil-based lubricating materials, water-based lubricating materials can perform an important role in tribology. Adding an ionic liquid additive (IL-16) to water-based lubricants achieves low-friction and -wear characteristics [92]. Water-based lubricants with IL-16 (1.6 wt.%) exhibit excellent antifriction, anti-wear, and anti-corrosion properties at a load current of 1.5 A; this was mainly ascribed to the formation of stable physical adsorption films on the surfaces of the friction pairs, thus reducing friction and wear. It can be seen that the addition of an ionic liquid additive enables the creation of high-performance tribofilms. Furthermore, the leading product of frictional chemical reaction between nano-additives and ILs was found to be of great significance for the antifriction and anti-wear properties of tribofilms.

In conclusion, the formation of tribofilm plays an important role in the tribological behaviors of liquid lubrication systems. During the period of physical adsorption, 0D nanoparticles within a lubrication film also provide a function similar to that of ball bearings; the exceedingly small size of the particles facilitates the repair of defects on worn surfaces. These characteristics directly ensure that a lubricant has a low friction coefficient and outstanding anti-wear capacities. However, the high surface area of nanoparticles can result in their self-aggregation, which can lead to high performance among the lubrication films with a short service life. Consequently, improving the dispersion stability of nanomaterial contributes to increasing their applicability and facilitates their development in new fields. It is noteworthy that surface modification is essential for the effective dispersion of nanoparticles in lubricating oil. In order to effectively suspend nanoparticles, the compatibility between nanoparticles and biological lubricants must be considered. Meanwhile, surfactants as well as dispersants can be utilized to reduce agglomeration. The excellent dispersion capability also provides sufficient conditions for the formation of tribofilms.

2.3. Wear Mechanisms of 0D Nano-Additives

The wear mechanical and tribological properties of nanocomposites can be effectively improved due to the size effect of nanoparticles and the matrix’s bonding strength. Due to their unique structure, near-spherical 0D materials are beneficial in reducing friction resistance and providing a workable approach for enhancing the bearing capacity and lubrication property of the tribofilm. Moreover, in the context of fluid lubrication, 0D materials, i.e., particles with relatively small particle sizes, are applicable in repairing surface flaws of friction pairs, producing a “self-healing” effect in lubrication systems; this leads directly to exceptional lubrication performances. Meanwhile, due to their high specific surface area and high energy, 0D nano-additives typically have a strong tendency to self-aggregate.

Solid lubricating films have great advantages in extreme operating conditions. It is noteworthy that combining nanomaterials with a plastic deformation matrix contributes to the formation of high-performance solid lubricating films. In liquid lubrication, the friction interface undergoes two stages: physical adsorption film and tribo–chemical film. Among them, in the formation mechanism of tribofilms in solid–liquid composite systems, material phenomena such as deformation, damage, and reaction play crucial roles [93,94]. Furthermore, the physical characteristics of a lubricant, its molecular composition, and its potential reactions with various other wear byproducts are all pivotal factors in the mechanisms involved in generating tribofilms [95].

In the physical adsorption stage, there is no “stress assist” or “thermal effect” in the film-formation process; a tribofilm can be generated by surface electrostatic potential attraction. The adsorption film begins to decompose and a tribo–chemical reaction occurs under conditions of high pressure, friction heat, and mechanical energy. Subsequently, the formation of tribofilms enters into the next stage. It should be emphasized that, in this stage, fatigue wear caused by alternating stresses in the reciprocating sliding process induces three-body wear and accelerates the wear of the amorphous carbon film (a-C film) between friction interfaces, as shown in the nanofriction mechanism [96]. When an a-C film is scratched, it becomes difficult to form a transfer film under low shear stress. Owing to the elastic deformation and passivation of the a-C suspended bond, a-C films require higher stress and longer sliding distance to form transfer films. During the sliding process of an a-C film, the energy stored in the attached phase dissipates by increasing the contact area until the stable stage is reached.

The formation of tribo–chemical film goes through the initial phase of fast growth at a high reaction temperature; then, it progressively decelerates and reaches its final thickness. Nevertheless, high shear stress results in the elimination of the protective layer, thus leading to the surface being clean during wear. On the contrary, under light lubrication (low or moderate load), the interfacial shear stress is inadequate in effectively eliminating tribofilms. Friction stress is accountable for the good bond that forms between a tribo–chemical film and a worn surface, thereby resulting in a clean and smooth surface [97].

As shown in Figure 4a, Wang et al. obtained novel porous carbon nanospheres doped with nitrogen (N@MCNs) via melamine-assisted carbonization [56]. A ball-on-disc test was conducted on a 500 SN base oil containing 0.06 wt% N@MCNs; the results showed that the friction coefficients and wear volume were reduced to 0.11 and 6.05 × 10⁴ μm³, respectively. The related values of the tribological performances are presented in detail in Table 1. Meanwhile, the friction and wear mechanisms of N@MCNs have been investigated through SEM and X-ray photoelectron spectroscopy (XPS) analyses of worn surfaces, as shown in Figure 4b,c. The images demonstrate that, during the friction process, protective tribofilms, including nitrides and metal oxides, form on contact interfaces as a result of tribo–chemical reactions between N@MCNs nanoparticles and an iron atom/iron oxide. The formation of tribofilms contributes to the prevention of friction during direct contact; thus, lower friction is achieved, improving the anti-wear and friction-reduction properties of the base oil. It is worth noting that an enlarged pore size on porous clusters leads to a decreased surface damage, which could be ascribed to a broadened contact area and a reduced penetration depth.

In a study by Chen et al., dendritic mesoporous silica (D-mSiO₂) nanospheres featuring tunable structures were prepared, having particle sizes of 30–140 nm, pore diameters of 3–9 nm, surface areas of approximately 780 m²/g, and pore volumes of ~1.5 cm³/g. Figure 4d shows schematic diagrams for the interfacial contact behavior that was observed between D-mSiO₂ and the substrate surface [98]. Low-modulus D-mSiO₂ nanospheres might deform elastically when a load is applied, thus resulting in a reduction in the contact pressure at the contact interface and a decrease in the indentation depth of the particle on the surface. Excellent uniformity and dispersity of D-mSiO₂ nanospheres could prevent the surface scratches that are caused by particle agglomerations. In addition, with increased reaction temperatures in the wear process, spherical topographies and uniform nanospheres are still well maintained, and the specific particle size enlarges from 45 nm to 90 nm, as shown in Figure 4e(I–IV).

Related research has evaluated the tribological features of the nanocomposites reinforced with aluminum nano-oxides (Al₂O₃) and titanium nano-oxides (TiO₂) [99]. To study their wear mechanisms, SEM images of the worn surfaces of poly methyl methacrylate (PMMA)/Al₂O₃/TiO₂ composites are presented in Figure 4f(I–IV). The morphologies of the rubbed surfaces changed in correspondence with the Al₂O₃/TiO₂ filler amount used. Pure PMMA presented with numerous delamination layers, many voids, and lots of wear debris because of the ploughing of its rubbed surface, which in turn led to a reduction in weight. Moreover, the eliminated weak layers could enhance the shear resistance. Furthermore, the brittle failure of the rubbed surfaces resulted in an increase in the surface roughness, which in turn led to a reduction in toughness. The delamination wear mechanism, which typically elevates the friction coefficients and wear rate, was the predominant phenomenon observed in the case of pure PMMA. The surface morphology of other PMMA/Al₂O₃/TiO₂ nanocomposites, instead, appeared relatively smooth, which could be attributed to the observed enhancement in the nanocomposite strength and hardness, resulting from the increase in the amount of Al₂O₃/TiO₂ filler used.

The sample with a filler content of 2.0 wt% was demonstrated to have the most substantial enhancement in hardness, reaching a 9.75% increase compared to pure PMMA, as illustrated in Figure 4g. The improvement in PMMA in the mechanical properties indicated that the hardness of the TiO₂ and Al₂O₃ nanoparticles was significantly greater than that of pure PMMA, thereby enhancing the load-bearing capacity. Furthermore, the incorporation of the filler into the microstructure of the matrix embedded the dislocation movement during loading, thereby enhancing the strength and modulus of the composites. As shown in Figure 4h, nanocomposites with 0.8 wt% filler content presented a remarkable performance among various filler amounts. To sum up, the 0D nano-additive not only transforms the friction condition from sliding to rolling, but also forms a protective tribofilm between the surfaces through physical adsorption or chemical reaction under the boundary lubrication condition, thus improving the friction performance. Furthermore, the elucidation of the relationship between macroscopic tribological behavior of material and nano/micro-scale properties of tribofilm is of great importance for understanding the mechanisms of friction and wear. Nevertheless, the characterization of the mechanical properties of tribofilms is a challenging task due to its low thickness and the inhomogeneous distribution it has on metallic surfaces.

3. One-Dimensional Nano-Additives in Tribology

Similar to 0D nano-additives, well-dispersed 1D nano-additives at higher contact pressures and shear run-in processes also contribute to the formation of high-performance lubrication films [100,101]. Many studies have been carried out to enhance the outstanding tribological characteristics of composites strengthened by 1D nano-additives [102,103]. This section primarily addresses the tribological behaviors and wear mechanisms of 1D nano-additives under dry and liquid friction conditions.

Table 2. Tribological performance of 1D nano-additives under dry/liquid friction.

Frictional State	Matrix	Reinforcement	Test Conditions	Tribological Results		Ref.
				COF	Wear
Dry friction	Ni-B coating	0.6 g/L Ni-coated MWCNT	ZrO2 balls (rotary tribo-tester, 350 r/min, 45 °C, current density, 2.5 A/dm2)	0.486	Wear capacity: 1.546 × 10−2 mm3	[101]
	PI	1.0 wt% CNTs	GCr15 steel ball (ball-on-disc, 10 N, 20 mm/s, 0.12 GPa)	0.050	Wear rate: 1.0 × 10−6 mm3/Nm	[104]
	PI	CuO nanowires	Steel ball and 45# steel disk (ball-on-disk, 3 N, 200 r/min)	~0.250	Wear scar width: 0.365 mm	[105]
	PEEK	10 CF/5 ATP	GCr15 bearing steel disc (pin-on-disc, 1 m/s, 3 h, 3 Mpa)	~0.150	Specific wear rates: 0.6 × 10−6 mm3/Nm	[106]
Liquid friction	PAO	5 wt% MWNTs	AISI52100 steel balls and DIN 100Cr6 steel disks (ball-on-disc, 10 N, 0.005 m/s)	~0.068	Wear loss: ~3 × 10−5 mm3	[29]
	NBR	CNTs	ZCuSn10Zn2 copper alloy ring (10 N; 3.3 m/s constant-temperature)	~0.048	Wear volume: 0.32 mm3	[107]
	Base Oil	0.6 wt% M-HNTs	Cr12MoV cold-working die steel pin and 45# steel disk (pin-on-disk, 80 N, 500 rpm, 0.6 m/s, RT)	0.129	Wear volume: 0.29 mm3	[108]
	Water-based lubricant	0.2 wt% MWCNTs-g-PAM	GCr15 bearing steel balls (four ball friction and wear tester; 392 N; 1450 rpm; RT)	~0.044	Wear scar diameter: ~0.254 mm	[109]

summarizes the improvements made in the frictional performances of 1D nano-additive materials under dry/liquid friction.

3.1. Contributions of 1D Nano-Additives to Lubrication Films under Dry Wear

One-dimensional nanomaterials are increasingly being recognized as appealing solid lubricants for a broad spectrum of tribological applications [110]. Their superb tribological potential presents a reasonable and effective way to protect the friction and wear of the surfaces of frictional pairs. One-dimensional nanomaterials, used as additives, exhibit ultralow interfacial shear strength and excellent tribological properties; these are similar to those of 0D nano-additives and contribute to the generation of high-performance tribofilms [111].

It is very important to repair crack growth and failure modes in ceramic materials. The load strengths of these materials are considered, in addition to the effects of mechanical fatigue owing to the periodic load applications, as they have the potential to produce defects and affect structural behaviors [112]. In this case, one-dimensional nano-additives could increasingly play a toughening role [113]. The ameliorative roughness brought by SiC nanowire deposition contributes to the enhanced bonding strength between matrices and composites [114]. As shown in Figure 5a(I–VI), compared with the Si layers (Si30, Si60, and Si90), the SiCnws-Si layers (SiCnws-Si30, SiCnws-Si60, and SiCnws-Si90) possessed compact structures, which increased the bonding strength between the matrix and the SiC nanowire layer. Meanwhile, the addition of carbon nanotubes effectively improves the microhardness of the composites, which is due to the grain refinement and enhancement of the carbon nanotubes in the matrix [115]. As shown in Figure 5b, the hardness values of the Mg/steel joints increases significantly when Cu and carbon nanotubes (CNTs) are added at the same time, compared to copper powder alone.

The generation of a transfer film plays a vital role in the tribological behaviors of polymers. Consequently, researchers are increasingly interested in developing productive strategies to enhance the tribological properties of transfer films. Unlike ceramic matrices, polymer matrices in friction systems are prone to material transfer, owing to their relatively low strength. Yin et al. [116] explored the tribological properties of PMMA nanocomposites reinforced with diamond nanothreads (DNTs) using atomistic simulations and density functional theory calculations. The results indicated that the friction coefficients of PMMA nanocomposites decreased by 25.98% after DNT addition, which can be attributed to the excellent interfacial interactions such as Van Der Waals interaction and mechanical interlocking, as shown in Figure 5c,d. Furthermore, due to the rougher topography of molecular electronic potential and the presence of a higher binding energy, a better antifriction performance can be accomplished using nitrogen-doped DNTs.

By comparing the materials before and after modification, the variation trend of the friction coefficient for different composites with time was obtained [117]. Figure 5e shows the friction mechanism of Ti₃C₂T_x@SiCnws/TP-FC, including tannic acid–polyethyleneimine (TP) and fabric liner composites (FCs). It is worth noting that the modified FC maintained a minimum, stable friction coefficient. Meanwhile, the wear resistance of the composite was improved due to the synergistic enhancement between Ti₃C₂T_x@SiCnws, leading to enhanced interface adhesion and the dispersion of the filler. Meanwhile, after friction, a large number of microcracks and fiber fractures appear on the surface of the FC, transfering to the corresponding surface, forming a solid tribofilm. The formation of a tribofilm here led to it playing the role of a buffer layer in the process of friction.

During the run-in process, the high contact pressure and shear can lead to the generation of a composite tribofilm that has low shear strength, thus resulting in a state of super-lubrication. A higher concentration of copper nanowires could accelerate the run-in time and improve the formation of a tribofilm. However, a thicker film is more prone to flaking, which hinders its ability to meet the required friction and wear properties [118].

CNTs, as one-dimensional nano-additives, are widely used in diverse solid substrates such as metals, ceramics, and polymers, mainly on account of their remarkable tribological optimization mechanisms [119,120]. Studies on CNTs as additives into polymer materials to improve composites’ mechanical properties have been conducted. The dispersion and adhesion properties of carbon-nanotube-reinforced epoxy composites have been investigated using various means of analysis. Experimental outcomes have indicated that CNTs with bigger particle sizes (50–100 nm) had a more uniform dispersion, thus obtaining better adhesion properties and forming nanostructured tribofilms [121]. For such films, the orientation of the CNTs is important for the tribological characteristics. More accurately, when CNTs are aligned perpendicularly to the friction direction, this tends to lead to a slide/roll motion occurring between the paired friction surfaces, thereby leading to decreased friction and wear for the composites [122]. It is worth noting that the rolling effect of 1D nano-additives is an effective approach in improving the lubrication properties and load-bearing capacities of tribofilms.

Tailoring interface nanostructures to reduce friction and wear provides a feasible tactic for creating high-performing polymer-on-metal sliding pairs. Hu et al. [104] obtained polyimide (PI) composites through the in-situ addition of multiwalled CNTs. A ball-on-disc test was conducted on PI composites reinforced with CNTs; the results demonstrated that the friction coefficients and wear rates for PI composites with 1.0 wt% CNTs decreased by 90.7% and 82.0% in comparison to those of pure PI, respectively. The specific parameters of this study are listed in Table 2. The proposed explanation for this phenomenon is that the continuous introduction of CNTs into sliding interfaces could turn into carbon nanospheres and graphene nanosheets, which promotes the formation of robust and lubricated tribofilms, thus achieving low friction. Uniform CuO nanowires on carbon fiber cloths (CFCs) might be able to improve interfacial bonding with PI matrices [105]. Meanwhile, CuO nanowires could introduce a lubricating phase into PI/CuO@CFC composites to boost their mechanical and tribological performances. PI/CuO@CFC composites have been shown to have excellent tribological properties. The specific relevant parameters of this study are listed in Table 2. The enhancement mechanism was put forward through an analysis for the composite interface and worn surface. In general, 1D nano-additives with a high aspect ratio have a high specific surface area, a high length–diameter ratio, high mechanical strength, high thermal conductivity, and other characteristics. It also provides an outstanding stress-transfer mechanism, as well as exhibiting a nano-rolling effect, which is essential in minimizing frictional resistance. Nevertheless, in applications of photocatalytic devices, one-dimensional nanorods have a low photocatalytic performance owing to their difficult morphologies. Customizing the interface nanostructures to reduce the friction and wear offers a feasible strategy for developing high-performance sliding pairs.

3.2. Enhancements of 1D Nano-Additives on Lubrication Films under Liquid Wear

Similar to 0D-nano-additive-enhanced lubricants, the uniform dispersion of 1D nano-additives in a mixture is vital in ensuring that their full tribological potential is reached. The dispersion stability of nano-additives in the lubricant medium is one of the most important factors affecting tribological properties; dispersion also plays an important role in the formation of lubrication films [123]. The enhanced tribological performances of lubricants incorporating 1D nano-additives is becoming increasingly acknowledged. Nevertheless, this beneficial lubrication is a short-term achievement due to the self-induced reaggregation of the additives in the mixture. Consequently, the first problem to be solved is the dispersion stability of additives in the base lubricating oil. The homogeneous and stably dispersed one-dimensional nano-additives can stick to the friction surface and progressively gather and densify under the applied load to form a lubrication film with a thicker boundary, which is very important in ensuring wear resistance.

The synergistic antagonism between MoS₂ nanotubes and the representative oil additives also performs a significant role in the production and tribological properties of lubrication films [124]. The use of MoS₂ nanotubes as an additive into oil exhibited excellent properties, including load-bearing capacities, resistance to wear, and friction reduction between moving mechanical parts. Optical micrographs for wear scars showed that wear was evident in tests using only a base oil, with unique plastic deformation occurring. Furthermore, the wear of the nanotube-lubricated mixture was greatly reduced, which showed evidence of tribofilms’ presence after the test, with the blend of pure base oil (PAO) containing MoS₂ nanotubes (PAO+NTs). In addition, the friction coefficient was closely related to the sliding distance. The typical variation of friction coefficient and sliding distances between PAO and oil containing 5 wt% MoS₂ nanotubes was investigated. It could be seen that the friction coefficient was stable and the run-in conditions were smoother and faster. The difference was even greater as the duration of the test increased. The specific tribological behaviors and test conditions for this are recorded in Table 2.

The addition of CNTs provides a feasible strategy for improve the wear resistance of nitrile butadiene rubber (NBR) [107]. As shown in Figure 6a, CNTs can be intertwined with the NBR molecular chain to prevent the rubber surface from being torn, thus improving the wear resistance of the rubber. However, when the physical adsorption caused by the Van der Waals force was difficult to resist, and the external force on the matrix materials increased, internal cracks appear in the parts where the interaction between the nanoparticles and the matrix material was weak; thus, carbon nanotubes are extruded from the nitrile rubber matrix during the wear process, which provides a possible route for the formation of excellent tribofilms.

The potential tribological performance of MoS₂ multi-wall nanotubes (MWNTs) as additives in lubricating oil is dependent on the formation of the tribofilms [29]. A ball-on-disc test was conducted under 1 GPa contact pressure and 0.005 m/s sliding velocity; the results indicated that the addition of MWNTs to a base PAO oil significantly reduced the friction coefficient in comparison to that of a pure base PAO oil, as shown in Figure 6b. The specific values of these friction coefficients are shown in Table 2. These results are ascribed to the wear-resistant and low-shear films that formed on the surface; these formed through the attachment of MoS₂ nanosheets to the surface.

Halloysite nanotubes (HNTs) have gradually become the focus of research owing to their excellent biocompatibility and potential film-forming properties with respect to tribology; the tribological properties of Span 80-modified HNTs into PAO₁₀ oil were investigated using a pin-on-disk tribometer by the group of Qin et al. [108]. The results revealed that HNTs at low concentrations (0.1–0.6 wt%) exhibited excellent anti-wear performances, which were ascribed to the formation of a composite tribofilm on the surface, consisting of an exterior layer of soft amorphous carbon film and an interior hard layer containing Al₂O₃, SiO_x, and iron oxide. Specifically, the friction coefficients and the wear scar size were determined and calculated, as shown in Figure 6c,e. As a result, designing the special double-layer film structure with a soft inside and a hard outside provided a feasible strategy for attaining excellent tribological properties. It is worth noting that the crucial factor in the generation of the composite tribofilms was the crushing of HNTs that occurred when loading was applied. Specific nanotube structures may act as rolling bearings to reduce the friction coefficient; this is possible because the nanotubes can only act as rolling bearings in specific directions. Spherical and quasi-spherical nanoparticles can roll when in contact with a surface, and they can change the sliding friction type into a mixture of sliding and rolling friction.

As shown in Figure 6g, sodium carboxymethylcellulose (CMC-Na)-functionalized CNTs (CNTs@CMC-Na) were obtained via a grafting technique using a microwave-assisted ball-milling apparatus [125]. A four-ball tribometer test was conducted following the addition of CNTs@CMC-Na (0.2 wt%) to water; the results indicated that tribological functions of the nano-additives in reducing friction (50.13%) and wear (58.3%) were superior to those of the pure water–CNTs-COOH condition, as illustrated in Figure 6h. These excellent tribological properties were attributed to the formation of high-quality lubricating films in the friction interface; they acted as a separation layer to prevent the concavity of the contact surface’s friction interface, enabling repairs and protection. Furthermore, it provided lubrication and protection during the friction process; this significantly decreased the generation of oxidation and metal–oxide particles at the frictional interface.

In a study by Jiang et al. [126], the tribological and lubricating properties of poly alpha olefin oil reinforced with MoS₂/h-BN hybrid nanoparticles were investigated in conditions both with and without oleic acid (OA) modification. Exceptional tribological characteristics were identified in the hybrid lubricants; these were attributed to the generation of a tribofilm on the surface of the tribo-pair. It is noteworthy that, in the lubrication process with friction pairs under the same working conditions, the friction reduction and wear resistance performances of mixed nano-lubricants were more prominent than those of single nano-lubricants, because two or more nano-additives can compensate for the performance defects of individual nano-additives; there is often a synergistic effect between two nanomaterials. In conclusion, unlike 0D materials, the 1D nano-additives with an aspect ratio cannot provide a repair function; the particular structure resulted in one-way rolling only. Therefore, the orientation of one-dimensional materials in the adsorption film determines lubrication performance. Nevertheless, as the friction time extends, the deposited physical adsorption film starts to disintegrate, and the high reactivity of 1D materials is beneficial in increasing the occurrence of tribological chemical reactions. A one-dimensional friction chemical reaction film can be strongly attached to a worn surface and this contributes to the long-term effectiveness of the lubricant.

3.3. Wear Mechanisms of 1D Nano-Additives

The success of liquid lubrication is closely related to the generation of protective tribofilms and their characteristics [127,128,129]. It has been proven that adding multidimensional nano-additives can effectively improve the bearing capacity, adhesion strength, and lubrication performances of tribofilms [130,131,132]. However, nanomaterials with different structural parameters have different effects on the formation and tribological properties of tribofilms. Similar to 0D-nano-additive-enhanced lubricants, the dispersiveness and stability of 1D nano-additives also play important roles in the formation of lubrication films.

Surface modification technology is used to lower the surface tension and alter the surface polarity of nano-additive materials, thus improving the stability and lubrication performances of lubrication systems. Polyacrylamide (PAM)-grafted multiwalled carbon nanotubes (MWCNT-g-PAM) serving as additives for water lubrication were examined [109]. The results showed a 32% reduction in the friction coefficient and a 26% reduction in the wear rate when compared to pure PAM, as shown in Table 2. In a comparison between two-body wear and three-body wear conditions, CNTs can act as tiny bearings in the friction process, thus turning sliding friction into rolling friction. In addition, the changes of the hollow tubular structure for CNTs also plays an important role in the wear process [133]. As the load was increased, the axial structure of the CNTs undergoes a gradual destruction, resulting in the transformation of the cylindrical hollow structure into a lamellar structure. This process ultimately leads to the formation of a lamellar structure that is similar to that observed for graphene. The lamellar structures can be seen in sliding or rolling contact with the friction pairs. CNTs can be subjected to high contact pressure, resulting in their flattening or crushing. This process generates abrasive wear on the friction subsurface, which serves two functions: firstly, it forms effective protective films on the frictional surface; secondly, it contributes to a reduction in the friction coefficient for lubricating oils. The hollow tubular structure of CNTs allows them to roll in the wear process. This can be attributed to the good dispersal of functionalized MWCNTs in the base lubricant, with an increase in the presence of a grafted PAM. The reduction in the frictional coefficient implies that the MWCNTs-g-PAM composites can serve as spacers, preventing rough contact from occurring between counterparts, and thereby considerably improving the tribological properties of the base lubricant.

The content and dispersal state of 1D additives in lubricants performs a crucial role in improving the antifriction and anti-wear performances of lubricants. MoS₂ nanowires were prepared by Zhang et al. via the electrospinning method as additives for perfluoropolyether (PFPE) base oil [134]. After the four-ball test, it was found that PFPE oil containing 1.0 wt% MoS₂ exhibited excellent friction reduction and anti-wear properties; more specifically, these properties achieves a minimal value for the friction coefficient (0.11) and the wear scar diameter (452 μm), as shown in Figure 7a,b. This significant improvement for lubricant oil was mainly attributed to the distinctive nanowire morphology and the favorable dispersion of MoS₂ nanowires in PFPE. It is worth noting that, when a concentration is less than optimal, the complete lubricating protective film cannot form on a friction surface, and antifriction and anti-wear properties become limited; in contrast, large particles that gather on a frictional surface occur as a result of the addition of excessive additives, thereby adversely affecting the whole friction system.

Wang et al. [106] obtained hybrid nanocomposites made from mixed exfoliated attapulgite (ATP) nanofibers with polyetheretherketone (PEEK); these were reinforced with short carbon fiber (CF). The authors explored the function of attapulgite in the tribological properties of polyether-based materials through a pin-on-disc test. The findings revealed that, in comparison to pure PEEK, the friction coefficients of the PEEK-based materials reinforced with 10 CF/5 ATP at 5 Mpa were reduced by up to 67%, as shown in Figure 7c. The presence of the ultra-high tribological property was ascribed to the growth of nanostructured tribofilms with high resilience on the surfaces. More importantly, the physiochemical products of the attapulgite nanofibers played an important role in the growth of a resilient tribofilm. As shown in Figure 7d, during the running-in process, more and more attapulgite nanofibers were released from the hybrid nanocomposite to the sliding interface, thus feeding the growth of a high-resilience tribofilm. It is worth noting that, by virtue of the high flash temperature, the friction products of attapulgite, residual PEEK, and abraded iron oxide were finally “tribo-sintered” into a solid tribofilm, thus providing prominent solid lubrication.

Han et al. [135] investigated the effects of CNTs in a reinforcing phase on the micro-structure and wear properties of an interstitial CoCrFeNi-CNTs coating. The results showed that, compared with that of a CoCrFeNi coating (10.54 × 10⁻⁵ mm³ N⁻¹ m⁻¹), the wear rate of the CoCrFeNi-CNTs coating (2.18 × 10⁻⁵ mm³ N⁻¹ m⁻¹) was significantly reduced. These improvements in wear performance stem from the strengthening of the solution of carbon on the grains and the precipitation strengthening of (Fe, Cr)₇C₃ carbides on the grain boundaries. In addition, as shown in Figure 7e,f, the lower friction coefficients and the average friction coefficients further demonstrate that the wear resistance of the CoCrFeNi-CNTs coating could be markedly enhanced by the addition of CNTs. Correspondingly, the microstructure evolution of the CoCrFeNi-CNTs coating on a worn surface is shown in Figure 7g. More specifically, in Figure 7g(I), the CoCrFeNi-CNTs coating exhibited pronounced groove and delamination characteristics on the surface undergoing initial wear. Thus, the wear mechanisms exhibited characteristics of both abrasive wear and fatigue wear during this period. In Figure 7g(II), carbides showed exfoliation in large quantities but were redeposited at a rapid rate. At this stage, the predominant wear mechanism was abrasive wear. In Figure 7g(III), carbide hardening layer was fully formed on the worn surface. Concurrently, the degree of carbide exfoliation was also significantly reduced. The wear mechanism underwent a transformation from an initial combination of strong abrasive wear and fatigue wear to a predominantly weak abrasive wear [136]. In general, during the wear process, the lamellar (Fe, Cr)₇C₃/α eutectics were deposited on the worn surface and ultimately formed a carbide-hardening layer structure, thus preventing the grievous wear of the CoCrFeNi-CNTs coating.

In conclusion, 1D nano-additives with a high aspect ratio fail to provide a repair function; instead, their use results one-way rolling only due to their unique structure. The high aspect ratio and outstanding mechanical properties endow one-dimensional materials with a stress-transfer effect, which provides an effective approach for improving the bearing capacity and lubrication performance of tribological chemical films under dry friction. Furthermore, in liquid lubrication systems, the surface defects of 1D nano-additives are readily reactive with water molecules or oxygen in the liquid, dispersing uniformly and stably on the friction surface. Under the influence of an external load, these defects gradually gather and compact, forming a thick boundary lubrication film that exhibits excellent anti-wear characteristics. Lubrication films that are rich in one-dimensional materials can effectively prevent the direct contact of friction pairs and reduce the adhesion of worn surfaces, which provides an effective strategy of restraining the friction stress concentration. For one-dimensional nanomaterials, the key parameters controlling the friction and wear behavior are the uniformity of the dispersion and the stability of nanomaterials. Evenly dispersed nano-additives can form high-performance tribofilms with low shear strengths under a high load, which provides a feasible strategy for preventing the direct collision of the friction pairs.

4. Two-Dimensional Nano-Additives in Tribology

Two-dimensional nano-additives have the following advantages: ultra-thin structure; high specific surface area; good thermal stability; low surface energy; smooth surface atoms [137,138]. Attributing to their unique layered structure and their excellent mechanical and self-lubricating properties, 2D nano-additives are effective in reducing friction coefficients via the interlayer slip, and they can form friction adsorption films that repair and protect sliding contact surfaces [139,140]. This section mainly discusses recent progress in research on 2D nano-additives (such as 2D-graphene and its derivatives, 2D-MoS₂, and 2D-MXenes) under dry/liquid friction conditions. Details of such studies are presented in

Table 3. Tribological performance of 2D nano-additives under dry/liquid friction.

Frictional State	Matrix	Reinforcement	Test Conditions	Tribological Results		Ref.
				COF	Wear
Dry friction	Aluminum alloy	APS-GO-POCl3 composite film	GCr15 steel ball (2 N, 1 Hz, 2 mm)	~0.150	Wear volume: 1.002 × 106 μm3	[141]
	Ni coating	0.4 g/L GO	Bearing steel ball (ball-on-disk, 1 N, 40 rpm, RT)	~0.320	Wear track: 0.320 × 103 μm3	[142]
	Epoxy	5 vol% g-C3N4 nanosheet	GCr15 steel ring (plate-on-ring, 100 N, 0.05 m/s)	0.570	Wear rate: 15.64 × 10−6 mm3/Nm	[143]
	Phenolic coating	1 wt% g-C3N4 nanosheet	SISI-1045 block (ring-on-block, 320 N, 2.5 m/s)	~0.125	Wear rate: 2.2 × 10−17 m3/Nm	[144]
Liquid friction	GTL8 oil-based lubricant	0.075 mg/mL ODA-RGO	SAE52100 steel balls and 316 L disk (ball-on-disk, 10 N, 2 Hz; 30 min)	~0.100	Wear rate: ~7.5 × 10−6 mm N−1 m−1	[145]
	Water-based lubricant	0.2 wt.% SGO	40CrNiMoA pin and M50NiL disk (pin-on-disk; 1 N; 650 MPa; 160 rpm; RT)	0.370	Wear scar diameter: 463 µm	[146]
	Pure water	0.05 wt% PEI-RGO	GCr15 bearing steel ball and 201 stainless steel disk (ball-on-disk, 4 N; 3 Hz)	~0.268	Wear track width: 164.2 μm	[147]
	Liquid crystal	0.010 wt% h-BN nanosheets	GCr15 steel ball and GCr15 steel disk (ball-on-disk, 2 N, 200 rpm)	0.049	Wear scar diameter: 129 μm	[148]

. Note that the focus here will be on significant achievements in the field, and studies on the ulteriorly correlative wear mechanisms of 2D nano-additives are included.

4.1. Contributions of 2D Nano-Additives on Lubrication Films under Dry Wear

The ultra-thin structure and high specific surface area of 2D nanomaterials provides a viable solution for improving both wear resistance and antifriction properties in tribofilms [149,150]. Two-dimensional nano-additives have attracted much attention due to their outstanding tribological potential [151,152,153]. The exceptional qualities of 2D nano-additives, including their high mechanical strength, superior thermal stability, and low surface energy, as well as their atomically smooth surface, have obtained significant interest due to their potential uses in tribology applications.

Research into macroscale and nanoscale applications of 2D nano-additives performs an important role in exploring effective strategies for reducing wear and friction. Recently, Wang et al. [154]. comparatively investigated the macroscopic tribological properties of a graphene oxide (GO)-based coating using ball-on-disk tests in different environments (including air, nitrogen gas, and vacuum). The results demonstrated that a GO-based coating showed superior tribological properties in a dry nitrogen environment and in a vacuum environment, due to the formation of special carbon nanoparticles, in comparison to a humid air environment. In addition to the macroscale applications, the frictional behaviors of graphene at the nanoscale also play a vital role in the advancement of micro- and nanomechanical systems. The frictional behavior of graphene on stainless steel was investigated via molecular dynamics simulations [155]. The simulation results evidenced the effect of the multilayer structure and strong adsorption effect of graphene on stainless-steel, revealing the primary source of the super-low friction of graphene.

When a solid lubricant film is present, the shear behavior of the contact interface is governed by the shear strength of solid lubricants [156]. A graphene–diamond interfacial structure was built to improve the mechanical properties of the diamond substrate [157]. Three types of graphene–diamond interface structures were constructed to investigate the properties and mechanisms of the suppression of the frictional wear of the diamond substrates by graphene, as shown in Figure 8a. With the increase in the number of layers, the increase in the rigidity outside the layer reduced the elastic deformation at the contact point, thus reducing the “wrinkling” effect. The wrinkling effect of graphene was analyzed to investigate the suppression mechanism of the surface friction and the subsurface wear of the diamond by graphene, as shown in Figure 8b. The potential energy of the graphene sliding with the indenter was formed. It is worth noting that the surface defects and the occurrence of other terminating groups may influence the frictional behavior of graphene.

MoS₂, an advanced 2D nanomaterial, has been increasingly used in improving tribological properties owing to its lamellar shear and high mechanical strength. Mohammad’s group [158] demonstrated that the addition of 2.5 wt% MoS₂ (optimal concentration) into a copper matrix caused a decrease in the wear loss and frictional coefficient, synchronously improving the wear resistance of the tribo systems; this was mainly attributed to the generation of tribofilms. As for Ni-based coatings, a new interface structure with a 19.63% micropore density facilitated a reduction of 20% in the friction coefficient and a decrease of 65% in the wear rate [159]. The prominent tribological property was ascribed to the cooperative interaction between the MoS₂ lubrication layer and the hard Ni-based coating.

In comparison to other 2D nanomaterials, MXene has outstanding characteristics, such as a larger outer surface area, better thermal conductivity, high porosity, and environmental friendliness [160]. As a result, MXene has commonly been used in a number of remedial settings for the treatment of groundwater pollution, surface water, industrial and municipal effluents, and seawater desalination [161,162]. An effect of fewer monolayers on tribological pairs was observed for MXene; they have thicker monolayers in comparison with other 2D materials. These results were helpful in building an understanding of the mechanisms of friction that occur during MXene use, revealing the potentiality of MXene as a tunable lubricant at the nanoscale. To prove frictional behaviors between the surfaces of SiO₂ and Ti₃C₂T_x MXene, the layer-number-dependent friction coefficient of the MXene sheet was measured by an atomic force microscope [163]. Figure 8c(I–IV) shows that the MXene/SiO₂ heterostructure has a strong friction anisotropy with a six-fold symmetry. The increase in the layer number was found to significantly reduce the friction anisotropy, and a number of layers higher than 10 was no longer related to the thickness of the film. In addition, the multidimensional site friction system, consisting of MXenes and other additives, reflected the lubrication mechanism for tribofilms through complex synergies [164,165]. In addition, the synergistic effect between conductive polyaniline and MXene was utilized to combine them together to improve the electrical conductivity for the composite, thus enhancing the wave absorption performance of the composite [166].

Through the application of an assembly technology, GO-based composites show excellent tribological properties; this provides valuable guidance for the design of metal-based composite films. A GO-based composite grafted with POCl₃ was fabricated onto a metal substrate [141], as shown in Figure 8d. Tests were conducted to determine the tribological properties of the GO-POCl₃ nanocomposite; the results demonstrated that the friction coefficient and wear volume were reduced for the nanocomposite in comparison with bare-metal substrates. The main results for the tribological properties for GO-POCl₃ nanocomposites are listed in Table 3. The excellent friction-reduction performance was ascribed to the tribo–chemical reaction, forming tribofilms and a transfer layer. A formed tribofilm consisting of GO nanoflakes and metallic phosphate was discovered as a friction process in which the layered GO nanosheets aligned along the direction of the friction, which was dominated by the tribological properties. Similarly, Zhang et al. [142] successfully fabricated a nickel/GO nanocomposite coating via ultrasonic-assisted electrodeposition. They gradually improved the tribological behavior of the nickel/GO coating; this was recorded through the increasing ultrasonic power. The main reason for this was grain refinement and the improved surface quality of the composite coatings. The specific details of the tribological performances of the nickel/GO composite coatings are listed in Table 3.

Figure 8. (a) Simulation model of diamond-coating graphene; (b) graphene wrinkle effect during sliding; reproduced with permission from the authors of [157]. (c) Investigation on frictional behavior between SiO₂ surface with Ti₃C₂T_x Mxene: including (I) the experimental AFM diagram, (II) structural monolayer for the Ti₃C₂T_x Mxene, (III) height profiles image of tested Ti3C2Tx sheet, and (IV) TEM image for thin MXene monolayer. Reproduced with permission from the authors of [163]. (d) Fabrication process for SS-APS-GO-POCl₃. Reproduced with permission from the authors of [141].

Figure 8. (a) Simulation model of diamond-coating graphene; (b) graphene wrinkle effect during sliding; reproduced with permission from the authors of [157]. (c) Investigation on frictional behavior between SiO₂ surface with Ti₃C₂T_x Mxene: including (I) the experimental AFM diagram, (II) structural monolayer for the Ti₃C₂T_x Mxene, (III) height profiles image of tested Ti3C2Tx sheet, and (IV) TEM image for thin MXene monolayer. Reproduced with permission from the authors of [163]. (d) Fabrication process for SS-APS-GO-POCl₃. Reproduced with permission from the authors of [141].

The tribological behavior of a WC/Co/graphene platelet (GPL) composite was investigated using a ball-on-disc tribometer [167]. The results showed that the frictional coefficients and the wear rates of the WC/Co/GPL nanocomposites were reduced to 0.343 and 1.38 × 10⁻⁶ mm³ (mN)⁻¹, respectively. The tribological properties of the WC/Co/GPL nanocomposites were attained through the formation of tribofilms. Enhancing the tribological properties of the tribofilms required that the adhesion of GO to the substrate was strengthened [168]. The composite film Si-APS-GO-HOAF, formed through a self-assembly technique driven by chemical bonds consisting of GO and hexafluoropropylene-oxide oligomer acid fluorides (HOAF), displayed outstanding tribological properties; strong interfacial interaction of chemical bonds was observed, alongside the synergistic effect between GO and HOAF. However, the formation of carbon oxide fragments and silicon oxides resulted in damage to the assembled layers, thereby causing lubrication failure.

The tribological behavior of the epoxy (EP) nanocomposite reinforced with a g-C₃N₄ nanosheet was investigated by Yu et al. [143] via plate-on-ring tests. Compared with pure EP, a 27% decrease in the frictional coefficient and an obvious 83% reduction in wear rate were obtained for the 1% g-C₃N₄-reinforced nanocomposite, as shown in Table 3. Excellent tribological nanocomposite properties were ascribed to the formation of a robust tribofilm consisting of g-C₃N₄, EP molecules, and graphitic carbonaceous materials. In a similar report [144], 1 wt% g-C₃N₄ nanosheets/phenolic coatings were proved to exhibit optimal tribological properties at a loading of 320 N and a sliding speed of 2.5 m/s, as shown in Table 3. The improvement of the friction-reduction and anti-wear properties of the phenolic composite coatings was attributed to two crucial factors: the formation of uniform tribofilms; the strong interfacial adhesion. In conclusion, in contrast with 1D materials, the presence of a larger surface area and a unique layered structure ensure that 2D nano-additives can easily cover worn surfaces, thereby promoting the formation of high-performance adsorption membranes. Lubricating films consisting of physical deposition films as well as tribological chemical films have excellent tribological properties. Nevertheless, under high frictional stress and heat, the decomposition of the functional groups for the additive surfaces leads to the tribofilms having decreased lubrication capacity; 2D nano-additives with a high friction coefficient should be studied further.

4.2. Enhancements of 2D Nano-Additives on Lubrication Films under Liquid Wear

Extensive studies have shown that the structures and shapes of nano-additives play vital roles in reducing friction and wear [169,170]. Due to the double-layer structure and the excellent self-lubricating properties of 2D nano-additives, they have received growing attention in the tribological field; this has extended to applications in improving the efficiency, durability, and environmental compatibility of mechanical systems.

Adding an amount of 2D nano-additives to a lubricating oil can effectively decrease frictional coefficients through an interlayer slip or the formation of adsorption films. Protective tribofilms enable nano-additives to repair and protect sliding contact surfaces, thus significantly improving the antifriction and wear resistance of lubricants [171]. The test results showed that the addition of some elements in the friction area resulted in the formation of protective films through tribo–chemical reactions, which prevented the direct contact of tribological pairs, decreasing friction and wear.

The addition of GO significantly enhances the tribological properties of water-based lubricants. However, the poor dispersion stability of graphene-based nanosheets in aqueous solutions poses a key challenge for their further application in the industry. Thus, it is important to explore GO derivatives that can be stably dispersed in water for long periods of time. The replacement of groups with 2D nano-additives provided feasible strategies for their effective and stable dispersion in water [145]. Moreover, reduced graphene oxide (RGO) was grafted with octadecylamine (ODA) as modifier to obtain ODA-RGO, which made up for the disadvantage posed by the poor dispersion of graphene-based materials. The lubrication mechanisms of ODA-RGO are presented in Figure 9a. Thin plates such as ODA-RGO could avoid direct contact between fitted sliding parts. A long-term lubrication effect was observed in the friction test. During friction, ODA-RGO was broken into small pieces, as shown in Figure 9b. Fractured ODA-RGO may be redeposited into a sheet structure that can exhibit better friction reduction and wear resistance. The results showed that RGO has good lubrication and antifriction effects, and its excellent tribological properties are a result of the formation of RGO protective films, which ensure what hardly any direct contact occurs between tribological pairs. Recent works on the tribological performances of RGO-reinforced composite coatings are listed in Table 3.

The composite tribofilm provides an effective way of significantly improving the tribological properties of greases. Figure 9c,d show the process of preparation and the lubrication mechanisms of porous oil-impregnated GO/polyamide 6 (PA6) composites [172]. After the introduction of a solid–liquid dual-lubrication structure, GO and lubricant oil produce synergistic friction reduction and anti-wear effects. The lubricating oil in the pore structure of the porous PA6 composite was precipitated to the wear surface under the joint actions of friction and load. This could avoid direct contact occurring between the steel ball and the composite under the combined influence of stress and frictional heat. The extrusive lubricating oil was combined with a well-dispersed GO to form a solid–liquid double-lubrication structure, thereby endowing the GO/PA6 composite with outstanding tribological properties.

Compared with GO, SGO shows excellent lubrication properties; this may be due to the stronger adsorption capacity of sulfonic acid groups in comparison with hydroxyl and carboxyl groups. Wu et al. [146] studied the tribological properties of sulfonated graphene oxide (SGO) as water-based lubricant additives. The significant improvement of the tribological properties was related to the micro-layered structure of the SGO nanosheets. This structure and charge effect ensured that the SGO nanosheets had easy access to the contact interface, enabling them to adsorb on the worn surface, thereby achieving protective tribofilms and averting the direct contact of the tribological pairs and preventing any material transfer. In addition, adding SGO could enhance the thickness of water films. As shown in Figure 9e,f, the friction and wear resistance of the friction pairs were increased by 74% and 15.7%, respectively, and the surface roughness of the worn track was significantly reduced after SGO lubrication. More works on the tribological properties of SGO nanosheets are listed in Table 3. This was attributed to the adhesion of SGO onto a friction surface after sliding and the generation of protective films during the aging phase of the friction process. GO nanosheets were more likely to aggregate to produce large clusters, which resulted in fewer nanosheets entering the friction gap. SGO showed better lubricity than GO, which could be due to the stronger adsorption capacity of sulfonic acid groups than hydroxyl and carboxyl groups. Due to the uniform distribution, the small size, and the ultra-thin structure of SGO nanoplates, they easily floated on the friction interface during the sliding process, thus avoiding direct surface contact. Under a normal load, the adhesion degree of SGO nanoplates on the frictional surface increased with the increase in the sliding time, which contributed to the formation of protective films. However, GO had lower lubricity than SGO, which was attributed to the fact that GO nanoplates were more likely to polymerize and produce large clusters, resulting in fewer nanoplates entering the friction gap.

Atomic oxygen (AO) irradiation has a significant influence on the tribological behavior of WS₂ films [173]. As shown in Figure 9g, compared with a WS₂/P201 nanohybrid (0.06), the average friction coefficient of an AO-irradiated (0.03) WS₂ film was slightly lower. This may be ascribed to the generation of thickness for the dense layer. As shown in Figure 9h, in frictional layers and transfer films, WO₃ effectively hinders hydrocarbons and oxygen; increasing the dense layer’s thickness enhanced the durability of the AO-irradiated WS₂ films. In a WS₂/P201 hybrid system, WS₂ migrated from the worn surface to the counter ball, forming a smooth transfer film. WS₂ originated from fractured loose columnar crystals and remained suspended in oil, where they functioned as an additive bound by adsorption. Mixed-oil films could isolate gases and ensure the sustained lubrication of friction tests.

Multilayer 2D nanomaterials bond well under weak Van der Waals forces, and adjacent layers are facile to shear, which gives them unique interlayer slip behavior; they are increasingly considered to be excellent lubricant additives [174]. Due to the weak Van der Waals forces in the interlayer, Ti₃C₂T_x was prone to slip on the surface and formed uniform and continuous tribofilms during the friction process, thus preventing direct contact between friction pairs. Meanwhile, increasing the temperature would change the wear mechanism. At lower temperatures, the synergistic effect reduced the wear of friction pairs. At a temperature of 100 °C, the synergistic effect disappeared and the anti-wear effect of the Ti₃C₂T_x lubricating film was present. When the temperature increased further, the lubrication film failed, and the friction changed from abrasive wear with the lubricating oil as the main lubrication phase to adhesive wear.

The smoothness of tribofilms is very important in ensuring excellent friction- and wear-reduction performances of lubricants under fluid lubrication. It is worth noting that the generation of tribofilms is largely dependent on the dispersion and stability of nano-additives. The two-dimensional material black phosphorus (BP), as an oil-based nano-additive in OA, showed a good tribological performance in hard contact under high pressure, resulting in macroscale superlubricity [175]. Similar to the finding so the report by Liu et al. [176], researchers have presented a new lubrication system for achieving superlubricity with a BP coating at the macroscale; the system involves the formation of a BP coating with deposited BP nanoflakes, followed by water lubrication. This is mainly attributed to the shear-induced interfacial structural conversion of BP to phosphorus oxide, leading to the formation of tribofilms on tribological pairs with extremely low shear strength.

Alongside nanomaterials of different sizes, two-dimensional additives of appropriate concentrations prove to be equally crucial in lubrication systems. Wang et al. [177] investigated the effects of graphene/Fe₃O₄ in tribological properties under water lubrication conditions. This reputably frictional property is closely related to the generation of tribofilms on steel surfaces. Based on previous reports, it can be difficult for graphene to attach to steel surfaces. This is a result of the fact that graphene is a hydrophobic material that agglomerates in water. It is difficult for graphene to enter the gap between the two friction surfaces. But Fe₃O₄ films can change the surface morphology and properties of steel, allowing large amounts of graphene to enter the gap; this enables graphene to be deposited on its surface more easily. The more intense surface oxidation is, the more graphene deposition can be achieved. Meanwhile, the thickness of graphene and oxide films increase with the increase in soaking time.

The eminent dispersibility and stability of water-based fluids play vital roles in the formation of tribofilms, enabling them to have a low-friction performance. Polyethylenimine-reduced graphene oxide (PEI-RGO) nanosheets were fabricated via a modified Hummer’s method with the water bath method [147]. In contrast to pure deionized water, water with 0.05 wt% PEI-RGO showed a prominent reduction in the friction coefficient (54.6%) and the wear rate (45.0%). The values of the tribological performances of RGO-reinforced composites are listed in detail in Table 3. Significant anti-wear effects on the wear surface can be attributed to the form of water-based lubricant films. Furthermore, the form of a low-shear-resistance protective layer on wear surfaces is also an important factor that cannot be ignored [148]. With the addition of 0.01 wt% h-BNn, compared to that of a pure 4-n-pentyl-4′-cyanobiphyl liquid crystal lubricant, the friction coefficient and the wear scar diameter on the ball can be reduced by 15.5% and 33.8%, respectively. The values for the influence of nano-h-BNn on tribological properties are summarized in Table 3. In conclusion, multilayered 2D nanomaterials can be successfully combined under the action of weak Van der Waals forces; the adjacent layers are easy to shear, which is increasingly recognized as an excellent feature for lubricant nano-additives. The proper concentration of 2D nano-additives can effectively decrease the frictional coefficient via the production of interlayer slip or adsorption films; these perform important roles in lubrication systems. However, the poor dispersion stability of 2D nanomaterials in lubricants is a major challenge that faces their further industrial applications. By adjusting the chemical composition and structure of 2D nanomaterials, the lubrication performance can be effectively controlled, and the stability and dispersion of two-dimensional nanomaterials in lubricants can be further improved. Therefore, further research and development of multilayer two-dimensional nanomaterials that can be stably dispersed in lubricants for a long time have important scientific significance, with great value in engineering applications.

4.3. Wear Mechanisms of 2D Nano-Additives

The adsorption and tribo–chemical reactions of 2D nanosheets at contact surfaces can create films to enhance tribological behaviors. Two-dimensional nanostructures bond well under weak Van der Waals forces and have a relatively low shear strength. Their tiny, layered structures makes it easy for them to enter into contact interfaces and adhere to the frictional surfaces, thereby forming a tribofilm that prevents friction, which might otherwise occur from direct contact and material transfer. Meanwhile, 2D nanostructures with strong chemical bonds in their layers mean that multilayer structures have excellent mechanical properties; which is conducive to the rapid establishment of stable contact zones and short run-in times, enabling the generation of high-performance tribofilms in an inductive manner. In addition, their larger surface area allows 2D nano-additives to cover a larger worn surface than the 1D nanomaterials can; 2D nanomaterials in lubricants can easily enter into contact areas to fill concave holes, thus repairing defects on the contact surface and enhancing the wear resistance of the contact surface and promoting the production of a high-performance adsorbent film. When used in lubricants, 2D materials also act as a catalyst for the formation of a tribofilm.

The tribological properties of the CZrP/UHMWPE composites (α-zirconium phosphate/ultra-high molecular weight polyethylene) were investigated under dry friction conditions [178]. The results demonstrated that, in comparison to the pure UHMWPE, a frictional coefficient and the wear loss of 5 wt%CZrP/UHMWPE composites were, respectively, reduced by about 30% and 58%, as shown in Figure 10a,b. This phenomenon could be explained by the fact that, during the dry friction process, the dense protective layer formed by the accumulation of CZrP on the friction surface helps to reduce the deformation and tearing of the composite materials; this simultaneously enables certain lubricating effects. In addition, the unique interlayer sliding effect that is endowed to composites with higher carrying capacity assists in achieving a lower friction coefficient.

Nanocomposites made from tungsten disulfide/graphene oxide (WS₂/GO) were fabricated using the hydrothermal method and assembled on amorphous silicon coatings [179]. Meanwhile, the tribological properties and lubrication mechanisms of WS₂/GO nanocomposites in ambient air were systematically investigated. The results showed that a robust and stabilized WS₂/GO membrane formed at the frictional interface, thereby realizing a low friction coefficient (0.12). This lubrication mechanism was ascribed to the fact that nano-WS₂ had a positive role in extending the layer spacing of GO and promoting the interlayer slip of GO, as shown in Figure 10c. In addition, compared to the lubricating film formed by a single GO, WS₂/GO tribofilms in the contact interface exhibited more robust reliability, which was of significance for improving tribological performances for Si-based coatings. Under a higher load, the remarkable tribological properties attributed to the friction continued; thus, the films were exfoliated and transferred to the surface of the ball, generating easy shearing and protective films, and thus achieving a certain resistance to friction and wear. Liu et al. [180] fabricated a lubricating composite film with GO and binary ionic liquids (ILs) as an interlayer for a Si surface via self-assembly and spraying; then, they comprehensively evaluated the tribological performance of the film using the ball-on-disk tribometer method under a dry friction condition. The results indicated that, in comparison to the Si substrate, the nanoscale adhesion and friction forces were reduced by about 65% and 60%, respectively.

Adding 2D nano-additives into the composite lubrication system can achieve a nano-roller bearing effect, thereby transforming sliding friction into rolling friction and significantly reducing friction and wear. Yang et al. [181] designed a composite lubrication coating comprising a diamond-like carbon (DLC) film used as solid lubricant, PEG 200 as a liquid lubricant, and MoS₂ as solid additive. The corresponding evolutionary mechanism for the heterogeneous lubrication film, a formation of the C/MoS₂ (C element was distributed on MoS₂) heterostructure tribofilm, is shown in Figure 10d,e. Under the combined influence of the contact stress and the frictional heat, MoS₂ decomposed into molybdenum, which then recrystallized with DLC particles to form heterostructure transfer films with varied orientations, enhancing the system’s overall shearing ability. As a result, the tribofilm not only prevented physical contact between counterpart pairs, but also performed a significant role in load bearing, which further reduced friction and wear. However, in the context of performance deterioration occurring as a result of the air oxidation effect and a short service life caused by weak mechanical strength, a rational multicomponent nanostructure design was required to optimize the tribological properties. A related work reported that tailor-designed nanostructured Mo-S-Cu-B films with a superb self-lubricant ability possessed high-performance features of ultralow friction and wear in various working environments, such as inert gas and terrestrial air [182]. Figure 10f showed the function of the Cu and B on the microstructure of the nanostructured Mo-S-Cu-B film and the mechanism for the consequent ultralow friction and wear. Under the influence of transverse shear force, the randomly distributed MoS₂ nanosheets were reoriented so they were parallel to the sliding direction; this made the contact between the frictional pairs transfer from being between the Mo-S-Cu-B film and the corresponding ball to being between the MoS₂ nanosheets and the MoS₂ nanosheets. Furthermore, the parallel arrangement between the passivated MoS₂ layer and the hydrophobic substance Cu₂O was helpful in resisting the invasion of oxygen and water vapor; its tribological property in humid air was improved. More specifically, the mean friction coefficients and the wear rate of Mo-S-Cu-B films were reduced to 0.078 and 7.9 ×10⁻⁷ mm³/N·m, respectively, as shown in Figure 10g,h. Overall, the concept and establishment of tailor-made nanostructured films provide a straightforward design strategy for developing nanocomposite materials with excellent tribological behaviors, thus extending the application scenarios for MoS₂-based self-lubricating films. Meanwhile, the advanced self-lubricating coating film displayed strong operational characteristics that are capable of adapting to the harsh working environments that are commonly found in various practical applications.

In summary, lubricating films consisting of physical deposition films and tribological chemical films have exceptional tribological behaviors. Well-dispersed 2D nano-additives improve the tribological properties of a matrix; this effect can be attributed to their self-lubricating behaviors, which make it easy to for the film to slip between cambium and provides an effective strategy for further reducing friction. The weak Van der Waals force endows the shear strength for the lower layers of the multilayer structure, and the structural characteristic of easy sliding directly produces a splendid lubrication performance. However, the interlaminar structure failure caused by friction leads to a deteriorative self-lubricating behavior in two-dimensional nanocomposites. Research should be conducted on 2D nano-additives with higher frictional durability and on inferior inter-slip forces. In addition, research should address the potential effect of thermoelectric properties on performance values, which is a challenge that arises in the use of 2D nanomaterials.

Figure 10. (a) Friction coefficient, (b) wear volume, and wear rate of CZrP/UHMWPE composites. Reproduced with permission from the authors of [178]. (c) Schematic diagram of lubricating mechanism for WS₂/GO film on coating. Reproduced with permission from the authors of [179]. (d) Schematic diagram for formation of C/MoS₂ tribofilm. (e) Worn condition of solid–liquid lubricating system. Reproduced with permission from the authors of [181]. (f) Schematic diagram of nanostructure design and ultralow friction mechanism. Mean frictional coefficient (g) and wear rate (h) in humid air. Reproduced with permission from the authors of [182].

Figure 10. (a) Friction coefficient, (b) wear volume, and wear rate of CZrP/UHMWPE composites. Reproduced with permission from the authors of [178]. (c) Schematic diagram of lubricating mechanism for WS₂/GO film on coating. Reproduced with permission from the authors of [179]. (d) Schematic diagram for formation of C/MoS₂ tribofilm. (e) Worn condition of solid–liquid lubricating system. Reproduced with permission from the authors of [181]. (f) Schematic diagram of nanostructure design and ultralow friction mechanism. Mean frictional coefficient (g) and wear rate (h) in humid air. Reproduced with permission from the authors of [182].

5. Three-Dimensional Nano-Additives in Tribology

Advanced 3D nanohybrids that are able to exhibit attractive mechanical and chemical behaviors have been explored; they have been implemented in dry/liquid friction conditions, and have received attention due to their potential in friction-reduction and anti-wear applications [183,184]. In this final analysis, the advantages of 3D nanomaterials are found in their integration, which can form composite films that combine the various properties of nanomaterials, and can even show a synergistic effect which is superior to that of traditional materials [185]. Analogously to other nanomaterials, plenty of research works have focused on the unique properties of 3D nanomaterials during wear applications [186]. This section primarily examines the tribological behaviors and wear mechanisms of the 3D nano-additives under dry/liquid friction conditions.

Table 4. Tribological performance of 3D nano-additives under dry/liquid friction.

Frictional State	Matrix	Reinforcement	Test Conditions	Tribological Results		Ref.
				COF	Wear
Dry friction	Rubber	CBp/MWCNTs hybrid	Tribometer (5 N, 70 r/min, 1 Hz, 150 °C)	~1.400	Wear width: 1.272 × 10−3 mm	[184]
	Ni coating	0.04 g/L CNTs/GO	Si3N4 ball and AZ91D magnesium alloys disk (ball-on-disk, 3 N, 10 mm, 10 min)	~0.060	Wear loss: 0.44 × 10−6 kg/m	[187]
	Epoxy coating	RGO/ZnS hybrid	GCr15 steel ball (ball-on-disk, 15 N, 0.033 m/s, 30 min)	~0.055	Wear rate: ~2.08 ×10−6 mm3/Nm	[188]
	EP coating	HBN-TiO2 hybrid	GCr15 steel ball (10 N, 2 Hz, 5 mm, 30 min)	~0.520	Wear rate: 4.6 × 10−4 mm−3 N−1 m−1	[189]
Liquid friction	PAO oil	0.50 wt% MXene@Cu	AISI52100 steel ball and AISI52100 steel disk (ball-on-disk, 25 Hz, 50 °C)	0.110	Wear volume: ~1.190 μm3	[190]
	Castor oil	0.10 wt% MnZnO/FMWCNTs	Steel ball and bronze disk (ball-on-disk, 40 N, 10 Hz, 30 min)	~0.025	Wear volume: ~0.023 mm3	[191]
	Water-based lubrication	GO/Chitosan-g-PNIPAM nanohybrid	Steel ball and 316 L stainless steel disk (ball-on-disk, 100 N, 25 Hz)	0.130	Wear rate: 5.1 × 10−2 mm3 N−1 m−1	[192]
	Lithium grease	0.3 wt% 3D HPGS	GCr15 steel ball (four-ball tribo-tester, 700 N,1200 rpm, RT, 30 min)	0.065	Wear volume: 0.194 mm3	[193]

provides a summary of the significant improvements that have been observed in the friction and wear performances of materials related to 3D nano-additives under dry/liquid friction conditions.

5.1. Contributions of 3D Nano-Additives on Lubrication Films under Dry Wear

Compared with other reported nanoscale materials, 3D nanomaterials possess rich nanostructures and their potential under dry friction has attracted significant attention from researchers [194,195]. The advantage of using 3D nanomaterials comes from their easy integration, resulting from their being composite films that integrate the diverse properties and functions of nanomaterials and even show synergistic effects that are superior to those of traditional materials [196,197].

The design of 3D nano-additives is increasingly recognized as a dominant factor in achieving excellent tribological behaviors [198]. The multidimensional synergistic effects of the nanometer complex compound is exhibited by 3D nano-additives, similarly to how the combination of 0D nano-additives and 2D nano-additives can create a collaborative effect, carrying the mechanisms of two kinds of additives [199]. This effectively provides solution strategies to the problems posed by poor dispersion that might be attained through the use of single nano-additives and the deficiency in the matrix’s mechanical strength [200]; such a solution is helpful in ensuring the formation of high-performance tribofilms. As shown in Figure 11a, the architectural features and frictional processes of Ni-GO-CNTs composite coatings are discussed in [187]. Table 4 presents the performance values of Ni-GO-CNTs composite coatings for tribological applications. When carbon nanotubes are added, they can only form a partial complex with GO. Both GO and carbon nanotubes have self-lubricating properties, even though they did not yet form a spatial network structure. After the coating was achieved, the ground and shed GOs and CNTs enter tribological pairs and adhere to the frictional surface, preventing direct the contact between tribological pairs and reducing the friction coefficient. Due to the high toughness and tubular structures of CNTs, the friction behavior changed from sliding friction to rolling friction, which played a role in the protection and lubrication provided by the coatings. Furthermore, CNTs with a dispersion strengthening effect were scattered in the matrix, which improved the hardness of the composite coatings. CNTs embedded in a Ni matrix resist removal during friction due to their large diameter. The microstructure strengthened the composite’s adhesion phase, inhibiting crack propagation and surface spalling, and enhanced the coating’s plastic flow and plow resistance, thereby improving wear resistance. As can be seen from the results of the friction coefficient in Figure 11b, the addition of CNTs has a significant impact on the friction coefficient. It can be seen from the curve that, when the concentration of CNTs reaches 0.04 g/L, the frictional coefficient of Ni-GO-CNTs coatings is at a minimum. The initial friction coefficient for coatings increased as the CNT content reached higher; this can be attributed to the greater deposition of the Ni matrix on the sample surface. Additionally, CNTs and GOs formed a stable suspension; during friction, the high strength and toughness of the GO caused both the GO and the CNTs to be gradually released from the coating surface.

The tribological properties of epoxy coatings which can be enhanced by graphene oxide/ZnS (RGO/ZnS) compounds were investigated in [188]. When comparing the various tribological property parameters of ZnS, RGO, and RGO/ZnS, the results clearly showed that the RGO/ZnS hybrid had a lower frictional coefficient and a higher wear resistance than single ZnS or RGO, as shown in Table 4. The splendid tribological properties of the RGO/ZnS nanocomposite were attributed to the synergy between ZnS nanoparticles and RGO nanosheets, and to the formation of transfer films on the steel balls. Moreover, the structure of the in situ-transferred hetero-film, shown in Figure 11c, was beneficial in achieving wear resistance and long durability [201]. Due to the rough surface for the pristine GS film, as well as the low-shear strength of the PTFE ball, the exfoliated PTFE chips were quickly transferred to the grooves of the GS film, achieving the downward transfer in the pre-friction process against the former PTFE counterpart. The results of the friction coefficient were indicated in Table 4.

Excellent interfacial physical and chemical effects could effectively promote the stress transfer that occurs during the friction and wear processes, thus preventing the pulling-out function of 3D hybrid materials. Due to the cross-linked network in the EP formed during curing, it becomes brittle and has a weak bond to the substrate for the EP coating. As a result, the EP coating has poor wear resistance and a shortened service life, resulting in poor friction resistance [202]. Figure 11d,e show the tribological behaviors of the EP coating and its composite coatings which are reinforced with filler-modified h-BN [203]. With comparison to pure EP, the reduced friction and anti-wear performances of the h-BN/ZnO/EP hybrid possess a small and steady friction coefficient. The reinforcing effect of the h-BN/ZnO hybrid is a result of its synergistic lubricating effect, including the self-lubricating characteristic of the h-BN layered structure and the decoration of the ZnO nanowires. Furthermore, the mixture of graphite and h-BN particles has excellent friction and wear resistance, and the tribofilms produced here show excellent protection against the friction produced by sliding metal surfaces [204]. As a result, the addition of nanohybrid power is considered to be an effectively enhanced strategy for improving load-carrying capacity through pressure transfer.

Figure 11. (a) Effect of concentration of CNTs on frictional coefficient of Ni-GO-CNTs coatings. (b) Schematic illustration of frictional process for Ni-GO-CNTs coating. Reproduced with permission from the authors of [187]. (c) The process from preparing of GS-PTFE hetero-film to testing. Reproduced with permission from the authors of [201]. (d) Friction coefficient, wear rate, and (e) variation of friction coefficient of EP and its composite coatings. Reproduced with permission from the authors of [203].

Figure 11. (a) Effect of concentration of CNTs on frictional coefficient of Ni-GO-CNTs coatings. (b) Schematic illustration of frictional process for Ni-GO-CNTs coating. Reproduced with permission from the authors of [187]. (c) The process from preparing of GS-PTFE hetero-film to testing. Reproduced with permission from the authors of [201]. (d) Friction coefficient, wear rate, and (e) variation of friction coefficient of EP and its composite coatings. Reproduced with permission from the authors of [203].

Insights into the synergistic effect of nano-complexes (g-C₃N₄-CuO) have shown that g-C₃N₄ is less prone to agglomeration than traditional powders (including nanoparticles, nanowires, nanorods, nanosheets, and nanotubes); this is conducive to the formation of transfer films, thus greatly improving film performance [205]. CuO played an important role in enhancing the strength of films. The interaction of g-C₃N₄-CuO leads to the uniform distribution of CuO nanoparticles and improves the compatibility of the films. Thus, this compound is an attractive anti-wear additive for polymer-based materials.

Compared with pure Ti, the average friction coefficients of Ti₃C₂-Go nanocomposite films were reduced by 76%; the wear volume of Ti₃C₂-Go nanocomposite films is 94% lower than that of GO films; the wear volume of Ti₃C₂ nanocomposite films was 86% lower than that of Ti₃C₂ films [206]. These excellent tribological properties were due to the in-situ lubrication caused by the films and the synergistic action of Ti₃C₂ and GO. The lubricating films composed of Ti₃C₂ and GO nanosheets avoided direct contact between frictional pairs, and the synergistic action of Ti₃C₂ and GO further improved the friction performance.

Because both of the interlayers slip, and due to h-BN and TiO₂’s high hardness, the abrasion-induced transfer films may have an unexpected tribological property [189]. The mechanical properties of the composite coatings, especially their hardness and tensile strength values, as well as their wear resistance, significantly improved. The addition of nano-filler might effectively improve the load-bearing capacity of the films through stress transfer. For instance, the use of two-dimensional materials (h-BN) could reduce friction through an interlayer slip. In addition, the h-BN-TiO₂ composite exhibited excellent wear and friction resistance, which was attributed to a key role in the material structure of TiO₂, including the high-load-carrying capacity and the reduced self-lubricating effect. The synergistic effects of HBN and TiO₂ can also be implemented to improve the tribological properties of films.

In addition to the polymer transfer film mentioned above, the key role of 3D nano-additives in ceramics is receiving increasing research attention. This 3D nano-network structure is characterized by low density, high porosity, and high specific surface area. The mechanical strength, wear resistance, and fracture toughness of the substrate could be enhanced by mixing it with hard ceramics. The tribological properties of the Al matrix nanomaterial composite, reinforced with graphene (AGNC), were studied in [207]. The results showed that the mechanical properties of Al were significantly improved with the addition of graphene. So far, the maximum hardness of AGNC has been increased by 242%, the elastic modulus has been increased by 77%, the yield strength has been increased by 182%, the tensile strength has been increased by 215%, the fracture strain has been increased by 134.7%, and the compressive strength has been increased by 586%. The synergistic effect of nanohybridization was of great significance in improving the wear resistance and self-lubrication properties of composites. Peculiarly, graphene-rich tribofilms are thought to play a crucial role. Graphene, for instance, effectively reduces the friction coefficient by sliding between layers. The combination of low weight, improved mechanical properties, and good thermal and electrical properties in the AGNC make it a potential candidate for a variety of applications in various industries, including automotive, aerospace, energy storage, and sports.

Well-designed structures provide a feasible method for the dispersion and stability of nano-additives, which is of great significance in the formation of protective lubrication films. In a study conducted by Ma et al. [208], where they constructed hierarchical interfaces, boron nitride (BN) was used as a rigid unit, and polyethyleneimine (PEI) and polydopamine were used as flexible units; this approach contributed to the formation of tribofilms and created an efficient thermal conduction network. They also significantly reduced wear damage, which is greatly important in the ameliorative tribological properties of resin-based composites. With continuous friction, the effective repair agents BN, PEI, and abrasive chips can fill in the pits and holes on the surfaces of friction materials, further promoting the formation of a smooth synovium. In conclusion, the designed 3D nano-additives are characterized by low density, high porosity, and a great specific surface area; these characteristics provide a feasible way of improving the dispersion and stability of nano-additives. Meanwhile, 3D nano-additives have been confirmed to inherit the advantages of 0D–2D additives, with their adjustable structural characteristics showing attractive tribological potential. Unluckily, the complexity of the required technology and its high costs are presently a hinderance to its wide-spread application in tribology. Future research should be devoted to the simplification of this process and the reduction in the cost of it. Furthermore, the sustainability and environmental friendliness of 3D nano-additives should be considered in efforts to avoid negative impacts on the environment.

5.2. Enhancements of 3D Nano-Additives on Lubrication Films under Liquid Wear

The extensive applications of 3D nano-additives in tribology effectively provide improved approaches to improving the friction and wear properties of lubricants. A large number of studies on 3D nano-lubricating materials show that interlaminar shear stress plays a dominant role in reducing friction and wear. In addition, the synergistic effect between nanoparticles, the rolling or sliding effect of fiber bars, and the tribofilms formed by the friction chemical reaction between the friction interface also contribute to excellent lubrication performance [33,209]. Three-dimensional nano-additives are widely used in tribology, owing to their unique composite properties, which provide an effective way of improving the tribological properties of lubricants.

The loading for the metal particles into two-dimensional layered materials can effectively enhance interlayer spacing and interlaminar sliding, while also achieving excellent dispersibility, which offers the viable strategy for designing lubricating films with the exceptional tribological properties. Cui et al. [190] obtained the MXene@Cu nano-composites via the simple solution-phase reduction method, then testing the tribological properties of them. The addistion of 0.5 wt% MXene@Cu to PAO oil significantly reduced friction coefficient from 0.434 to 0.110, decreased wear volume by 88.1% in comparison to the pure PAO, and increased load-carrying capacity to 650 N, as illustrated in Figure 12a,b. Excellent tribological performance was ascribed to the stable protective films formed on the worn surface, thereby achieving reduced fiction and wear. Figure 12c summarized the reduced friction and wear mechanisms of MXene@Cu. During the friction process, the MXene nanosheets readily adsorbed onto the surface, forming a protective film that prevents direct contact between the tribological pairs. With a generation for heat, the tribo–chemical reaction occurred between fragmented MXene nanosheets and Cu nanoparticles (Cu NPs), thereby persistently and stably forming a tribo–chemical reaction film. Furthermore, the presence of Cu NPs could not only repair frictional interface, but also change frictional form for interface of contact from the sliding friction to the rolling friction. It was seen that protective adsorption film, tribo–chemical reaction layer, and repair effect of Cu nanoparticles collectively endowed the MXene@Cu nanocomposites with the excellent wear resistance and a high load-bearing capacity.

The addition of 3D nanomaterials as an additive to oil significantly improves the lubricating properties of the base oil. Recent works investigated the tribological properties of MnZnO/FMWCNTs hybrid nanomaterials that were obtained by anchoring the functionalized multiwalled carbon nanotubes (FMWCNTs) to the surfaces manganese-doped zinc oxide (MnZnO) nanosheets [191]. The tribo-tester results demonstrated that, in the case of 0.10 wt% MnZnO/FMWCNTs hybrid nanomaterials, castor oil with this optimum concentration showed superior tribological properties, including a 26.21% reduction in the frictional coefficients and a 89.09% decrease in wear volume, compared with the castor oil alone. The outstanding lubrication behaviors of the MnZnO/FMWCNTs hybrid nanomaterials correspond to the following two points: the synergistic mechanisms between Mn-doped ZnO nanosheets and FMWCNTs; the formation of a boundary film on a frictional surface. In addition, owing to their unique microstructure and high thermal conductivity, MnZnO/FMWCNTs can readily penetrate the friction area, fill up the grooves and valleys, and consequently form a lubricating film on the worn surface; this prevents direct contact between the metal surfaces and enhances heat dissipation during the friction process.

To advance the development of green nano-additives for water-based lubrication, the rational integration of 2D materials with brush-like copolymers is used as a novel approach, contributing to the formation of a high-performance lubrication film. Wei et al. [192]. fabricated GO/Chitosan-g-PNIPAM nanohybrids through an in situ non-covalent assembly strategy. The nanohybrids’ tribological performance was investigated under a high-contact-load condition; a tribological evaluation of the prepared compound showed that significant antifriction and anti-wear properties were achieved as compared with the pure water or single GO options. Superior friction-reduction and anti-wear properties may be attributable to the dispersion stability of modified GO and the synergistic effect of the brush-like copolymer in combination with two-dimensional GO additives. Recent research has provided evidence for the tribological properties and lubrication mechanisms of water-based lubricants with GO/SiO₂ hybrid nanostructures, as shown in Figure 12d,e. Lubricating films composed of deposited C and SiO₂ were dynamically formed on a frictional interface [210]. The presence of C in the film reduces the shear resistance, thus achieving a lower friction coefficient. SiO₂ nanoparticles in the film acted as polishing media to remove roughness, resulting in a finer worn surface. The improvement of the abrasive performance was attributed to the co-lubricating behavior of the GO/SiO₂ nanostructures that formed in the slurry; these reduced the friction and surface roughness of the abrasive–matrix interface.

Xie et al. [211] explored the tribological behaviors of CNT/SiO₂ combinations as lubricant additives in deionized water by using a linear-reciprocating ball-on-plate test. The final test data demonstrated that the CNT:SiO₂ (0.2:0.3) composite nanofluid showed superior tribological properties; the load-bearing capacity and lubrication film stability improved in comparison to individual nanofluids. This effect was mainly ascribed to the sliding tribofilm of CNT as well as the ball-bearing effect produced by the nano-SiO₂. An FESEM-EDS analysis revealed that the deposited tribofilm effectively repaired the grooves on a worn surface, resulting in a relatively smooth surface, as shown in Figure 12f. On the other hand, the tribofilm resisted direct contact between friction pairs and protected the matted surface against friction and wear. Furthermore, CNT/SiO₂(0.2:0.3) combination nanofluids combined the advantages of nano-CNT and nano-SiO₂, thus achieving excellent friction-reduction and anti-wear properties. The lubrication mechanism of CNT/SiO₂ composite nanofluids is shown in Figure 12g. The differing morphological characteristics of CNT and nano-SiO₂ led to distinct antifriction and anti-wear mechanisms under identical test conditions; this affected their tribological performances. However, the effectiveness of the CNT/SiO₂ composite nanofluids was highly dependent on their mass ratios. The combinational CNT/SiO₂ (0.2:0.3) nanofluid, which leveraged the strengths of both CNT and nano-SiO₂, demonstrated exceptional friction-reduction and anti-wear properties.

The participation of graphene significantly improves the antifriction and anti-wear abilities of greases, making the worn surface smoother [193]. In particular, the wear volume loss and friction coefficient of greases containing 3.52 wt.% graphene were decreased by 0.20% and 3.3%, respectively, in comparison to the original greases. The excellent anti-wear and antifriction properties were attributed to the fact that graphene could be coated on the friction interface and form protective tribofilms, thus preventing tribological pairs from being in directly contact and reducing friction and wear. Graphene could be used as a protective deposition film for the substrate, promoting the formation of a composite tribofilm.

Proper film thickness could provide the best tribological properties. In contrast, film that is too thin or too thick may lead to an increase in the friction coefficient and wear [212]. Chen et al. [213] proposed an approximate method of estimating film thickness. Based on the theory of elastic hydrodynamic lubrication (EHL), the deformation of a ball during wear is consistent with the Hertz contact theory. The radius of the contact area with point contact can be expressed by Formula (1).

$$a={\left({\displaystyle \frac{3RF}{2E^{\prime }}}\right)}^{{\displaystyle \frac{1}{3}}}$$

(1)

where $a$ is the radius of the Hertz contact area, $R$ is the radius of the ball, $F$ is the normal load, and $E^{\prime }$ is the effective elastic modulus. The contact radius increases with increases in the applied loading and decreases with increases in the elastic modulus or the decreased radius of the ball. Ulteriorly, the wear scar can be thought of as the Hertzian elastic deformation of an imaginary ball larger than the real ball under the original load. The radius of the imaginary sphere can be expressed by Formula (2).

$$R^{\ast }=R{\left({\displaystyle \frac{a^{\ast }}{a}}\right)}^3$$

(2)

where $R^{\ast }$ is the radius of the imaginary ball and $a^{\ast }$ is the radius of the wear scar on the real ball. This radius, as key data in calculating the minimum film thickness, determines the size and shape of the contact area and directly affects the pressure distribution in the contact area. In practical engineering, this calculation can be used to predict the performance of mechanical parts during contact and wear. Finally, the minimum film thickness can be obtained using Formulas (3)–(7):

$$U=\eta V/E^{\prime }{R}^{\ast }$$

(3)

$$G=\alpha E^{\prime }$$

(4)

$$W=F/E^{\prime }{R}^{\ast 2}$$

(5)

$$H_{min}=3.63{\displaystyle \frac{U^{0.68}{G}^{0.49}}{W^{0.073}}}\left(1-e^{-0.68k}\right)$$

(6)

$$h_{\min }=H_{\min }\ast R^{\ast }$$

(7)

where $U$ is the speed parameter, $\eta$ is the bulk viscosity of the lubricating solution, $V$ is the averaged linear velocity of the ball and the disk, $G$ is the material parameter, $\alpha$ is the viscosity–pressure coefficient, $W$ is the load parameter, $H_{min}$ is the minimum oil film thickness of the imaginary ball, $k$ is the ellipticity parameter, and $h_{\min }$ is a minimum film thickness. It can be concluded that the calculated value minimum film thickness is not only influenced by the inlet rheology but also by the viscosity response of the lubricants under high pressure. Based on this, the corresponding tribological properties can be successfully assessed [214]. Firstly, selecting an appropriate viscosity grade is necessary in maintaining a stable film thickness across varying temperatures and pressures. Additionally, reducing the contact surface roughness not only decreases lubricants’ shear forces but also enhances oil film thickness, thereby minimizing friction and wear. Meanwhile, an adequate supply of lubricants is essential in maintaining continuous lubrication of the contact surfaces. By optimizing these parameters, film thickness can be effectively managed, thus ensuring the reliable performance of mechanical systems under diverse operating conditions.

To summarize, in cases of lubrication systems containing a significant mass of 3D nano-additives, the designed 3D materials exhibit stable dispersibility, exceptional mechanical performances, and prominent reactivity, thereby facilitating the formation of high-performance tribofilms. However, in terms of applications in the industrial field, the complex technology and demanding preparation conditions of mass production are still obstacles. Thus, further emphasis should be placed on conducting basic research to optimize the process and reduce the challenges associated with the preparation of these materials, thereby ensuring the attainment of the desired tribological properties. Additionally, in order to effectively match the performance and working conditions of nanomaterials, it is imperative to conduct thorough research on the stability and durability of 3D materials under varying environmental factors to ensure their long-term reliability. It is worth noting that determining an appropriate film thickness through experiments and tests is crucial in ensuring optimal tribological performance. A lubrication film that is too thin may fracture or peel off under high pressure, resulting in a loss of lubrication effectiveness. Conversely, a lubrication film that is too thick can create air pockets between contacting surfaces, leading to excessive localized deformation and wear.

5.3. Wear Mechanisms of 3D Nano-Additives

Awareness of the friction and wear behaviors of composite materials under high contact stress point contact is very important for their applications in practice [215]. In-depth analysis and research on the wear mechanisms of these kinds of materials can help us to understand their performances under harsh conditions, so as to optimize the available design and application strategies for the material. In this respect, composite films containing 10 wt% Cr₃C₂ have shown remarkable advantages. The corresponding composite film displays an extremely low friction coefficient (~0.24) under high contact stress. This result indicates that Cr₃C₂ nanoparticles may form an effective lubricating film on frictional interfaces, thus realizing a reduction in the direct contact between the surfaces [203,216].

The utilization of functionalized two-dimensional nanomaterials for the preparation of lightweight, bifunctional (wear and corrosion resistance), and high-performance composite coatings represents a highly promising strategy. Wu et al. [217] obtained MXene@PTA-Zn(II) hybrids by introducing zinc ions (Zn(II)) into MXene@PTA nanosheets that were functionalized with poly(tannic acid); the researchers were concurrently investigating the tribological properties and wear mechanisms of WEP (water-borne epoxy)-based nanocomposites containing these hybrids. The ball-on-disk test results showed that a WEP-based coating with 0.4 wt% MXene@PTA-Zn(II) exhibited the lowest average friction coefficients and wear rates, as shown in Figure 13a,b. Superior wear resistance was attributed to the uniform dispersion of MXene in the WEP matrix. More importantly, the enhancing effect of poly (tannic acid) (PTA) and Zn(II) modification on the compatibility between the nanohybrid material and WEP performed a dominant role in achieving the effective dispersion. In addition, the wear mechanisms of the coated samples were investigated further by using wear tracks, as shown in Figure 13c. It is clear that, in comparison to blank WEP, a WEP-based coating with 0.4 wt% MXene@PTA-Zn(II) showed relatively low plastic deformation (Figure 13d), which was due to the presence of MXene sheets in the coating, which could slide relatively easily when subjected to shearing force, thereby reducing the influence of shear force on the coating. Similarly, in Figure 13e, the wear mechanisms of the coatings suggested that appropriate and evenly dispersed fillers could effectively elaborate the lubricating role of MXene, thus improving the wear resistance of the composite coatings.

In addition to the use of additives in improving the tribological properties of the coatings, nanomaterials have great advantages as solid lubricating films under extreme working conditions. Based on Figure 13f, it can be summarized that the friction coefficients of a ZrMoNeAg composite film with an Ag content of 11.88 at.% decreased with an increase in the temperature [218]. As the temperature rose, the soft Ag phase in the ZrMoN-Ag film diffused and accumulated on the surface, forming a lubricating Ag layer. The surface Ag content increased significantly after the temperature reached 200 °C, in comparison to the as-deposited film, with the formation of AgO and Ag₃O₄ oxidation phases leading to a reduction in the friction coefficient. This reduction in the friction coefficient was attributed to the diffusion of Ag and the formation of oxidized phases. At 600 °C, the friction-induced oxidation led to the formation of Ag₂MoO₄ and laminar MoO₃, which are key contributors to lubrication.

In liquid lubrication systems, composite and mixed nanomaterial additives can fully exploit their individual characteristics and the synergistic effects; they can do so far more than single nano-additives can, thereby facilitating the development of high-quality lubricating oils which have excellent anti-wear performances, meeting the growing demands for clean energy solutions. In a study conducted by Ouyang et al. [219], the tribological property of castor oil containing 0.5 wt.% 3D hierarchica porous graphene (3D HPG) and 0.7 wt.% multiwalled carbon nanotube (MWCNT) nanoparticles was investigated using a linear reciprocating wear test. The characterization results indicated that 3D HPG/MWCNT (0.5:0.7) accomplished reductions in the friction coefficient and the wear volume of up to 26% and 48% under low-speed and heavy-load conditions, respectively. This outstanding tribological performance can be ascribed to the special rolled-bearing-like synergistic lubrication effect that was achieved between the 3D HPG and MWCNT nanoparticles, as shown in Figure 13g. More specifically, the smooth surface was constructed by filling the grooves and pits in the surfaces of the tribo-pairs using MWCNT nanoparticles, which contributed to the suspended 3D HPG nanoparticles that adsorbed onto the friction surface and formed a dense and uniform protective film. In addition, under a sustained shear force, the sandwich composite graphene nanosheets/MWCNT/graphene nanosheets lubrication structure was formed between the friction interfaces, exerting a synergistic effect between the adsorption of the outer nanosheets and the rolling of the inner nanoparticles, further improving the friction-reduction and anti-wear properties. Analogously, the combination of carbon nanotubes (CNTs) and hexagonal boron nitride(h-BN) exhibited an excellent anti-wear stability [220]. More specifically, the addition of 6 wt.% carbon nanofiber (CNF) and 6 wt.% h-BN into polyalphaolefin was demonstrated to have the lowest average friction coefficient at the stable friction stage. This was ascribed to the formation of uniform protective tribofilms on the surface. In addition, the mechanism of the synergistic effect of h-BN and CNF on the antifriction and anti-wear performance was investigated, as shown in Figure 13h. On the one hand, h-BN was responsible for the rolling and deposition functions; meanwhile, under repeated stress, h-BN and the self-lubricating CNF formed a uniform protective film on the surface. On the other hand, h-BN performed a role as a gasket, which reduced the entanglement of CNF and formed a better network structure, thus providing more heat-conduction channels. As a result, an environmentally friendly, mixed nano-lubricating oil can be synthesized through the addition of multicomponent nano-additives to traditional lubricating oil; this formulation can meet the complicated and tricky working conditions of real-world applications, as well as meeting the demands for green solutions.

Besides the influence of nano-additives, the formation of tribofilms depends, to a significant extent, on the nanostructure effect of the matrix, the composition of the lubricating matrix, the surface roughness and morphology for sample, and the operating conditions used [221,222]. In general, the matrix of composite materials can be divided into metals, ceramics, and polymers. Choosing suitable nano-additives for different material systems is key to realizing high-performance frictional films. The ductility of metal matrices is conducive to the formation of stable and effective lubrication films which are helpful in friction conditions. The choice of hard 0D nanoparticles in such systems can enhance the bearing capacity of friction films; meanwhile, 2D nanomaterials easily form lubricating films due to their layered structure, thus improving the wear resistance and frictional reduction performance of the system [223]. In contrast, the formation of tribofilms for ceramic-based materials depends more on the reactivity of the additives in question. Soft nanoparticles can form a stable frictional film through chemical reactions, while 1D nanomaterials can improve the overall performance by improving the toughness of the film. In polymer transfer films, adjustable 3D nano-additives can enhance the adhesion and mechanical properties of tribofilms, and can further optimize the lubrication effect. An understanding of the interaction mechanism between different material matrices and nano-additives, combining theory with experiment, can provide effective solutions for tribological challenges in various industrial applications.

In conclusion, 3D materials can inherit the components and characteristics of other nano-additives (including 0D, 1D, and 2D nano-additives), which enables them to show excellent stability, dispersibility, and remarkable synergistic effects in different lubricating environments. It is necessary to emphasize the maintenance of an appropriate concentration, because concentration directly affects the dispersibility and lubricating effect of nano-additives in different substrates. Meanwhile, the formation of high-performance tribofilms enables a reduction in the friction coefficient and wear rate, which significantly extends the service life interval and energy efficiency of systems. More specifically, during the process of repeated friction, three-dimensional nanostructures are torn into two-dimensional complex combinations under high contact stress; combined with damaged materials, they compress or react to form smooth surfaces or even tribofilms, preventing direct contact between friction pairs and improving the wear resistance against damage caused by friction and wear in systems. Additionally, the diversity in the structure and large composition spaces of films provide possibly unlimited research routes for optimizing friction and wear systems. When combined with surface texture, nano-additives can effectively reduce frictional coefficients and improve lubrication performances; in return, the surface texture can further optimize lubrication conditions through changes in the contact surface shape and increases in the thickness of the oil film. This synergy performs an important role in the following aspects: reducing the wear of mechanical parts; prolonging the service life of mechanical parts; improving the mechanical efficiency of mechanical parts. In the future, it is of great scientific and practical value to study how textures and nano-additives affect the formation processes of frictional films and their respective roles in this process, contributing to the development of lubrication systems with better performances, and leading to satisfaction with the diverse needs of industrial applications.

6. Conclusions and Prospect

Friction and wear are inevitable phenomena that arise from the contact or motion between two parts; these phenomena are inherent in both everyday life and industrial production. They can be mitigated through the utilization of advanced technologies such as nano-additives. The incorporation of nano-additives facilitates reduced friction and wear on inner moving surfaces, thereby enhancing the efficiency, service life, and application accuracy of mechanical systems, while ensuring the production of high-quality parts and surface treatments. Due to their exceptional resistance to friction and wear, nano-additives have garnered significant attention in various tribological applications within micro- and macrosystems. An increasing number of experts and scholars have conducted comprehensive and profound research on nano-additives. Their perspectives are summarized here:

• Various nanoparticles have demonstrated their effectiveness as additives for solid lubrication. The unique structural morphologies, superior molecular structures, and excellent chemical inertia of 0D nano-additives contribute to their exceptional tribological properties in friction pairs. Furthermore, the incorporation of 0D nano-additives can optimize coatings and enhance the wear-resistance, friction-reduction, and load-bearing capacities of lubricants. However, nanoparticles often agglomerate in matrix materials or lubricants, forming large aggregates that can create interface defects and stress concentrations, leading to the acceleration of wear and material failure. In addition, functionalized nanoparticles may lose their surface modifications under high temperatures or in extreme conditions, which limits their use in harsh environments. Despite these challenges, with advances in optimization techniques and safety assessments, 0D nano-additives continue to hold significant promise in the field of tribology.
• Representative studies have confirmed that the one-dimensional nanomaterial systems exhibit exceptional resistance to high-pressure-induced wear. This is because the introduction of the nanomaterial alters the primary wear mechanisms that occur during friction: a reputable “nano-rolling effect” transforms the surface contact type from being sliding contact to rolling contact; this is reflective of outstanding tribological properties under both dry and lubricated conditions. Additionally, the special toughening mechanisms can enhance the effective fracture toughness of composites. Thus, one-dimensional nanomaterial systems are deemed to be an ideal additive for anti-wear purposes. Unfortunately, 1D nanomaterials with high aspect ratios often become entangled in lubricants or matrix materials, thus impacting their performances at frictional interfaces. Additionally, their long-term stability under extreme conditions, such as high temperature, pressure, or chemical corrosion, remains a concern. These harsh environments may compromise the surface-modification layer, thereby limiting their applicability.
• The relatively low shear strength that occurs between the layers of two-dimensional nano-additives endows them with exceptional shear properties, facilitating active loading transfer even under extreme pressure, effectively reducing the run-in time. This in turn promotes the formation of high-performance tribofilms. Moreover, their mechanical properties, including the hardness and toughness of the coating structure, can be significantly enhanced by refining through two-dimensional nano-additives. At the same time, tribofilms at the contact interface can be formed to directly prevent contact between friction pairs. However, the practical applications face several limitations: sheet overlap easily occurs in the matrix. With regard to this problem, it is urgent that researchers develop effective dispersants and optimize the dispersion processes; these increase process complexity and may affect material properties. Additionally, synthesizing high-quality 2D nanomaterials requires precise condition control, often resulting in higher processing costs.
• Well-crafted three-dimensional nanomaterials not only inherit the primary properties of their components but also play a direct role in optimizing the mechanical and tribological behaviors for the substrate. Moreover, multidimensional interactions endow three-dimensional nanocomposites with unique advantages that can address issues such as the poor dispersion of a single nano-additives, the insufficient mechanical strength of metal matrices, and the induction of tribofilm formation. For instance, the incorporation of two-dimensional nano-additives plays a pivotal role in impeding the agglomeration of zero-dimensional nanomaterials; meanwhile, in order to enhance the surface properties of nanomaterials with other dimensions, zero-dimensional nanoparticles should be employed for modification. Currently, the achievements surrounding low friction and high wear resistance as well as strong adhesion in tribofilms are contingent upon the utilization of nano-mixtures with optimal dispersions. Nevertheless, applications of 3D nano-additives as nano-fillers are confronted with limitations and potential threats, which may bring health and environmental risks; this situation requires comprehensive evaluation and management.
• Many studies on the formation mechanisms of friction films show that well-designed functional nano-additives contribute to the regulation of nanostructures and the self-lubricating properties of tribofilms. Meanwhile, the design of new nano-additives and lubricating oils provides broad application prospects for development in various industries. However, the effects of nano-additives on tribological behaviors are complex because they are not immune to the defects and contamination threats that might reduce tribological performance and reproducibility. Thus, the complexities of the tribological properties of nanocomposites should be comprehensively evaluated to obviate the potential risks that might face their tribological applications. In our exploration of the potential of nano-additives, understanding the specific mechanisms and limitations of the formation of tribofilms is especially crucial. Tribofilms form through physical adsorption, chemical reactions, and mechanical mixing, and each of these mechanisms is faced with its own challenges: physical adsorption films can easily fail; chemical reaction films are difficult to control; and mechanical mixed films are often uneven. A deep comprehension of the pros and cons of these mechanisms is essential for developing more efficient nano-additives and lubrication systems; such an understanding will contribute to our ability to overcome performance instability, thereby expanding the potential applications of the composites across various industries.