Review of Graphene-Based Materials for Tribological Engineering Applications

Abstract

Graphene-based materials have great potential for tribological applications. Graphene’s unique properties such as low shear resistance, high stiffness, and thermal conductivity make it an attractive material for improving the properties of lubricants in a wide range of industrial applications, from vehicles to house refrigerators and industrial machinery such as gearboxes, large compressors, etc. The current review aims to give an engineering perspective, attributing more importance to commercially available graphene and fully formulated lubricants instead of laboratory-scaled produced graphene and base oils without additives. The use of lubricants with graphene-based additives has produced e.g., an increase in mechanical efficiency, consequently reducing energy consumption and CO₂ emissions by up to 20% for domestic refrigerators and up to 6% for ICE vehicles. Potential effects, other than purely friction reduction, contributing to such benefits are also briefly covered and discussed.

1. Introduction

Graphene and graphene-based materials are relatively novel 2D materials with great tribological potential. Graphene is inherently low-friction, very high stiffness, and its thermal conductivity may reduce friction and wear. Despite the stricter definition for Graphene being an “allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure” [1], the term “graphene” has been used also for up to 6 atom layers, Graphene Nanoplatelets (GNP), Nano Carbon and even micro graphite (graphene nanosheets) are also being generically and for marketing reasons being called as “Graphene” or “Graphene-based materials”. Despite the efforts to harmonize the nomenclature by ISO [1] and others [2,3,4], there is still a lack of consistency among different publications. According to ISO [1], the correct definitions are “FLG”, Few-Layer Graphene, a two-dimensional material consisting of 3 to 10 well-defined stacked graphene layers; “GNP”, graphene nanoplate or nanoplatelet: a 2D material consisting of graphene layers. Being a review, along with this paper, the original nomenclature used in the reference will be reproduced, with some comments when necessary.

This review paper will concentrate more on graphene engineering applications, especially its use as an additive for lubricants. As most of the current lubricants are oil-based, the use of graphene on aqueous fluids will be only sparsely covered. The review will also give more emphasis to macroscopic real-life studies. More fundamental, nanoscale, works will be covered only to help to discuss the macro-scale effects. For more fundamental discussions, the readers are encouraged to read one of the handful of reviews already published [4,5,6,7,8,9].

Literature about graphene and carbon-based 2D materials in lubricants is abundant and growing. Graphene offers several distinct advantages over other traditional lubricating materials, making it a promising candidate for improving the performance of lubricants in a variety of applications. Some of the main advantages [5,6,10,11,12,13,14,15] of graphene include the following:

• -. Excellent strength and mechanical stability;

• -. Low coefficient of friction;

• -. High thermal and electrical conductivity, allowing it to quickly dissipate heat generated by friction, helping to prevent overheating of mechanical components;

• -. Chemical stability over a wide range of environmental and chemical conditions, making it suitable for applications in harsh or reactive environments;

• -. Compatibility with different fluids, through functionalization, to improve its dispersibility and adhesion to different lubricating liquids;

• -. Self-lubricating properties, due to its crystalline structure, reduce the need for additional additives in some applications;

• -. Multifunctionality, since it can be chemically modified to incorporate different functional groups, allows the customization of its properties for specific applications.

These advantages make graphene an attractive material for improving the properties of lubricants in a wide range of industrial applications, from vehicles, house refrigerators, and industrial machinery such as gearboxes, large compressors, etc. The current work aims to give a more engineering perspective. Table 1, adapted from [16] summarizes the reduction of energy consumption on domestic refrigerators with the use of carbon nanoparticles. On Internal combustion engines (ICE), Refs. [17,18,19] reported significant fuel savings when using graphene additives on the lubricant oil that cannot be explained only by friction reduction. Ahead in this paper, the potential effects of lubricant-improved thermal conductivity and even combustion improvers will be briefly covered.

Despite the significant R&D efforts, there are still few real-life applications of graphene as an additive. Figure 1 reproduces a tentative timescale till full commercialization made in 2019 [26]. According to the Graphene Investment Guide [27] updated to June 2023, only three companies have posted positive share outcomes since June 2022. A potential driver for the use of graphene as lubricant additives is the crescent use of Battery Electric Vehicles (BEV). As the electrical motor and the gearbox use the same lubricant, improved thermal and electrical lubricant properties are required [28]. In BEVs, the lubricating fluid is in contact with both the electrical motor and the gearbox, demanding superior electric properties such as electrical conductivity dielectric constant, and dielectric strength along with good thermal management, and material adaptability [29].

Figure 2 shows the main methods for mass production of graphene sheets by exfoliation of bulk graphite. Figure 3 shows examples of shipment of few-layer graphene [30].

Considering tribology in more macroscopic terms, it is estimated that ~23% of the world’s total energy consumption originates from tribological contacts. Of that 20% is used to overcome friction and 3% is used to remanufacture worn parts and spare equipment due to wear and wear-related failures [31]. Therefore, the potential savings of introducing advanced tribology solutions can be as much as 40% in a time frame of less than 10 years. See Figure 4, Figure 5 and Figure 6.

Lubrication regimes can be divided into boundary, mixed, and hydrodynamic, depending on the ratio between oil film thickness and surface combined roughness. See Figure 7. Lubricant Friction modifiers acting on the moving surfaces will influence only the boundary and somehow the mixed regimes, while oil viscosity and viscous modifiers will mostly influence the hydrodynamic regime [32,33,34,35,36]. As an example, Figure 8 shows friction losses on a four-cylinder, 2.0 L Daimler M111 gasoline engine motored test; with oil main gallery temperature of 93 °C. Boundary friction losses dominated till 1000 rpm, and the regime became hydrodynamic at higher rpms. Notice the modest effect of the OFM additive and the higher hydrodynamic losses, but lower asperity ones of the higher viscosity oil.

It should be remarked that despite Friction Force and Coefficient of Friction being the more common indicators in the scientific literature for tribological performance, friction energy losses (load multiplied by speed) are what affect mechanical efficiency. For example, a small friction reduction in the hydrodynamic regime at higher speeds can have more effect on friction losses, hence mechanical efficiency, than a large reduction at the boundary regime. Figure 9 shows measurements in a Floating Liner Engine test with a DLC-coated cylinder. Notice that the high friction forces at reversal points have almost no relevance in terms of energy.

Another point to remark on when investigating micro and nanometric tribological phenomena is that surface roughness profiles are usually shown with a vertical scale much larger than the horizontal one. This representation may lead one to think that the nanoparticles would be unable to serve as contact cushions or tribofilms. Indeed, when the roughness profiles are brought to the same vertical-horizontal scales, most of the engineering surfaces are quite smooth. Figure 10 shows a measurement on a relatively rough surface from an engine cylinder.

2. Graphene-Based Materials

Graphene is part of a broad family of carbon nano allotropes and graphene itself has a family, consisting primarily of sp2 carbon atoms arranged in a hexagonal network [39,40,41], as represented in Figure 11. The National Physical Laboratory (NPL), in collaboration with international partners, through the ISO/TS 21356-1:2021 [1], has been working in terminology and definitions that provide a common and consistent understanding of the different types of graphene worldwide, Table 2. Nevertheless, despite their efforts, there is still no unambiguous nomenclature on different publications and, therefore, in this paper, the original nomenclature used in the reference will be cited, with some comments when necessary.

Additionally, other carbon variants are being investigated as lubricant additives. Wolk 2018 [42] used graphene quantum dots (GQD), covalently functionalized with dodecyl amine and obtained a reduction in friction coefficient from 0.17 to 0.11 on the macro scale in addition to significantly inhibiting corrosion. Graphene types can be transformed before or during usage. See Figure 12 from Georgakilas, 2015 [40]. As mentioned, one should be careful when reading that a kind of graphene was used in the experiment since the original graphene type could suffer exfoliation, degradation, and transformations during the tribological process. It can be, for instance, wrapped up to form 0D fullerenes, rolled up to establish the cylindrical structure of 1D carbon nanotubes, or stacked to form 3D graphite.

Any amount of graphene-based material for industrial applications will contain particles with variation in terms of size, number of layers, etc. Even if the sample contains mostly graphene particles with 1 to 6 layers, the relative share of such small particles in area and volume will be much lower than the number of particles. Figure 13, reproduced from [43], shows the relative share of the number of particles, area, and volume for two samples having mostly particles with few layers. Despite containing mostly particles of few layers, the volume share on both samples is dominated by particles of six or more layers. The reader should be aware that when along this review a given reference is said to use a given type of graphene, the reported effect may be caused by another graphene type contained in the study sample. The contribution of the different sub-populations on tribological applications still needs to be studied and probably is different for different properties, e.g., friction and wear being influenced more by the nanoparticles area and volume while electrical and thermal conductivity by the number of such nanoparticles.

3. Functionalization of Graphene-Based Materials for Tribological Applications

Graphene-based materials, as already mentioned in this paper, are very promising for tribological applications due to their self-lubricating lamellar structure and film-forming abilities, providing excellent anti-wear and friction-reducing performances. Nevertheless, despite those features, there are challenges to be overcome, such as the great propensity to aggregation, before they fully achieve the advertised properties [6], Penkov 2014 [44], Chouhan 2020 [45] and as e.g., to make it hydrophilic, Kinoshita 2014 [46]. A very studied way to solve that problem is by modifying the graphene surface through functionalization with different functional groups (oxygen-based, nitrogen-based, sulfur-based, halogen-based, etc.); ions; molecules; or particles (Hu 2018 [47], Wang 2021 [48], Guo 2021 [49], Rabchinskii 2020 [50]. As summarized in Figure 14, the physicochemical properties of graphene can be tailored via a covalent or non-covalent approach. Non-covalent functionalization preserves the conjugated π system of graphene, maintaining a low degree of defects in the sheets and they are usually easier to process than covalent functionalization; however, the bond between the coupling agent and graphene is relatively weaker, which can affect the stability of the obtained material. Covalent functionalization on the other hand, although it promotes changes in the conjugated π system, by the covalent bonds formed in the basal plane and edges, allows edge-selective reactions (Shi Q, 2020 [51]). That is relevant because the obtained material will add the beneficial properties of the coupling agent, but it may preserve most of the intrinsic properties of graphene [26]. Regardless of the approach, it is essential that the interactions/bonds formed be (1) properly selected, to render, for instance, dispersibility in polar and/or non-polar fluids; (2) stable; (3) selective; and (4) in enough extension to achieve the desired property.

The most common methods for graphene functionalization described in the literature are summarized in Figure 15. The chemical reactions make use of well-established synthesis routes from organic chemistry (radical reactions, cycloadditions, nucleophilic additions, and substitutions) to manipulate the graphene’s properties. Electrochemical reactions enable the functionalization and exfoliation to take place simultaneously, in addition to not using aggressive reagents, and are easily scalable. Regarding irradiation, among the available routes, microwave and ultraviolet-assisted reactions deserve mention. In terms of mechanochemistry, we can highlight processing via ball mill, sonication, and liquid-phase exfoliation (Georgakilas, 2012 [52], Dey, 2016 [53]). For example, Hendrix et al. [54] studied the functionalization of polyamide PA66 during continuous exfoliation of graphite using high shear, from a solvent-free condition. The results showed an increase in mechanical properties and improved stress transfer between graphene and its polymer matrix. Several other researchers have investigated graphene functionalization; more details can be found in Appendix A.

4. Graphene Functionalization for Use in Lubricants

Additives with enhanced dispersibility and stability enable formulators to increase their concentration in the lubricant without inducing aggregation and sedimentation, which is important for shelf-life and, therefore, for real-life applications ([6,55]). As a derivative, GO inherits many physical and chemical properties from graphene, making it an attractive base material for the synthesis of lubricant additives with enhanced dispersibility. The abundant oxygen-containing functional groups provide uniform water dispersion to its nanosheets. However, it is challenging to produce straight GO-based oil dispersions (Gao, 2022 [9]). That is why approaches such as the GO modification with a long organic chain have been studied, i.e., to promote homogeneous and long-term dispersion of graphene in oil-based lubricants. Nevertheless, the commonly used graphene modifiers, such as oleic and stearic acid, tend to decompose given the heating generated during the friction process. That lead Bao et al. [56] to use a reflux reaction to covalently modify the GO with polyisobutylene succinimide (PIBS) and evaluate the tribology properties of the resultant material. The GO-T154/oil dispersion showed stability over one year and provided a wear rate and coefficient of friction of less than 60% and 54%, respectively, compared to those of base oil. Moreover, GO-T154 exhibited higher heat stability. Such results were attributed to a synergistic interaction between the precursors, which led to the formation of uniform and continuous carbon film on the contacting surface. See Figure 16.

Nyholm and Espallargas [6] in their review discussed the steric stabilization and electrostatic stabilization mechanisms, see Figure 17, by which the surface functionalization can enhance dispersion stability and inhibit agglomeration of carbon nanostructures with potential application as lubricant additives. Steric stabilization prevents agglomeration mainly via the steric effect, a result of the repulsive forces induced by long-chain functional groups, typically polymers, attached to the surface of the nanostructure. Those groups can form a brush-like layer that shields the particles from attractive interactions. This concept was studied by Yu 2023 [57] from DFT calculations performed with different Organic Friction Modifiers (OFM), and amphiphilic surfactant molecules, combined with graphene (Spikes, 2015 [58], Ouyang, 2021 [59], Cyriac, 2021 [60]). The authors observed that the materials that presented lower friction coefficients in the tribological tests also presented high adsorption energy, meaning stronger adsorption on the substrate surface. Under load and shear force, the hydrocarbon tail from OFM-moiety can easily slip between opposite polar groups, resulting in a decrease in friction coefficient. Additionally, during the friction process, the adsorbed layer will inevitably be destroyed, but the molecules with higher adsorption energy can recompose themselves more quickly to recover the brush structure to keep the friction coefficient low throughout the whole friction process. Thus, from the experimental data, the authors inferred that lubricant molecules with those features can conduct tribochemical reactions during the friction process to form protective tribofilm leading to a low wear rate [57]. Electrostatic stabilization relates to the functionalization of the nanocarbon surface with electrostatically repulsing functional groups that increase the apparent surface charge and the double layer thickness of the structure, so that when the particles get closer, their electrostatic repulsion may overcome the attractive van der Waals forces. Electrostatic and steric stabilization mechanisms can also be combined in a so-called electrosteric approach, by selecting large sterically hindering groups with functional terminations that repel each other, such as a polyelectrolyte. In aqueous systems, both mechanisms prevail, while in non-aqueous systems, steric stabilization is dominant.

The authors also argued about a highly advanced strategy for surface modification. That involves a specific selection of functional groups to intentionally undergo tribochemical reactions under harsh operating conditions, acting as a precursor to the formation of a tribofilm with improved friction-reducing and anti-wear characteristics. That approach is illustrated in Figure 18 [6]. For example, Zhao 2018 [61], investigating the lubrication properties of graphene additives with different degrees of exfoliation, and Liu 2021 [62], from the study of graphene/N-butyl pyridinium tetrafluoroborate ionic liquid, have reported nano-structural evolution and the formation of tribochemically active functional groups, respectively, on tribofilms characterized after tribological testing performed with functionalized graphenes.

5. Tribological Mechanisms of Graphene-Based Materials

The effect of 2D materials, in particular graphene, on tribological performance may be due to different mechanisms. Graphene can act as a protective tribofilm, as a viscous modifier, etc. To ease this review organization, the main mechanisms were divided according to Figure 19 and will be discussed in separate sub-topics. The reader must keep in mind that more than one mechanism may have occurred in the case described in the reference as well as in the real-life applications.

Other graphene-tribological-related applications such as dry lubricant, coating, etc. are discussed in separate chapters. Table 3 and Table 4 summarize the main references of using graphene as an oil and grease additive and the proposed mechanism for the reported benefits. The use of graphene on aqueous solutions or as metalworking fluids are not covered in this review.

5.1. Protective Film Formation

The more commonly proposed tribological mechanism is graphene-based materials acting as typical protective tribofilms, avoiding direct contact between the moving surfaces. See Figure 20. The protective film will bear the loads with several benefits [6]: (a) original surface asperities are protected from micro welding and subsequent adhesive wear and (b) elastic and plastic deformation will occur in the tribofilm-reducing material’s surface fatigue.

Acting as a low shear resistance friction modifier, one layer sliding over the other, Graphene will act similarly to other FM additives such as MoS₂ and graphite. See Figure 21. Graphite has a planar and layered structure in which the carbon atoms within each layer are arranged in a hexagonal lattice with strong in-plane bonding, while weak van der Waals forces act between the layers. This contrast between intra-layer and inter-layer bonding strength allows individual graphitic layers to easily slide over one another. Some of the unique features of graphene would be its higher stiffness, and potential mild attack on the surface when compared with conventional FM additives containing sulphur.

5.2. Filling and Mending Effect

Another tribological mechanism proposed in the literature is that graphene nano particles and/or platelets or even graphene agglomerates can fill the surface roughness. See Figure 22. This effect is also referred to as the mending or self-healing effect because wear scars could be filled or mended by the nanoparticles (Gan 2020, [98]). Although it can be argued that the Graphene nanometric size can impair the effect on a macro scale, several of the already mentioned references observed the presence of larger and relatively thicker tribofilms. It can be expected that in an intermediate stage, the growing tribofilm at least partially filled the surface roughness.

5.3. Polishing Effect

Reducing roughness peaks is a common approach to reducing friction on lubricated regimes (Sedlaček 2009 [99]). It can be hypothesized that Graphene can act as a polishing particle due to its high mechanical properties. However, is not clear how the Graphene’s low shear strength will not avoid such mechanisms and/or if the particles will not be as roller bearings as discussed before. The polishing effect is generally only reported for nanodiamonds due to their high hardness [6].

5.4. Nano Roller Bearings

Some references [79,100,101] propose the ball and roller-bearing mechanisms. To promote rolling, nanoparticles should likely be of diameter comparable to the surface roughness and have high compressive strength and high resistance towards shear stresses. Graphene derivatives usually have low shear resistance; CNTs, carbon Onion-like, and others are not large enough to serve as rolling bearings on engineering surfaces. Graphene agglomerates will exfoliate, otherwise, they could probably scratch the surfaces. Anyway, Wang & Gong 2019 [79] decorated graphene-layered nanosheets with mono-dispersed silver (Ag) nanospheres. See Figure 23. The Ag nanospheres evenly grew on the layered graphene sheets, and this regularly laminated structure further guaranteed an enhanced lubricating effect. Tribological experiments demonstrate that 0.1 wt% addition of this composite could readily reduce the friction coefficient and wear spot diameter by 40% and 36%, respectively. Detailed lubricating mechanism experiments demonstrate that self-lubrication induced by such layered structure, the change from sliding friction to rolling friction, and the self-repair effect due to Ag nanospheres synergistically contribute to the excellent lubricating performance. See Figure 24 and Figure 25.

5.5. Hydrodynamic Effects

Graphene has also shown benefits on hydrodynamic lubrication regimes, which hardly can be explained only by surface effects such as tribofilm, polishing, etc. Graphene is known to change the lubricant rheology. Two recent works, Giudice 2017 [102] and Hamze, 2021 [103], review dozens of references about the rheology of graphene-based materials. Most of the works are on aqueous dispersions or polymers and few discuss the tribological effects. On hydrodynamic regimes, friction increases with viscosity, so if a given system is already working on a hydrodynamic regime, increase in viscosity will increase the friction losses unless a lower oil viscosity oil would be used before being added to graphene.

Zhang and Zhou 2011 [82] investigated graphene sheets on a four-ball tribometer. The lubricant with optimized graphene concentrations of 0.02–0.06 wt% showed enhanced friction and anti-wear performance, with friction coefficient and wear scar diameter reduced by 17% and 14%, respectively. The benefits were credited to the effects of the graphene sheets on the lubricant film. See Figure 26 and Figure 27.

Wang and Gao 2022 [95] used Few-layer graphene (FLG) as a nano-additive to lithium complex grease (LCG) and investigate the influence of FLG on the microstructure, viscoelasticity, friction, and wear properties of such additive greases. On reciprocating tests with 90 N (equivalent to 2 GPa Hertzian pressure) load, the baseline grease without and with FLG showed CoF~0.18 till 40 s, and then different contents of FLG began to behave differently. See Figure 28.

• -. The baseline presented relatively stable CoF till 220 s, the boundary film gradually failed, and the friction coefficient began to rise. CoF reached about 0.2 at 320 s;

• -. 0.5% FLG presented a larger CoF fluctuation from 40 s to 160 s. During this process, the disordered FLG in the grease gradually becomes ordered due to the tangential forces, and a protective tribofilm gradually forms. Continuous minor reduction on friction until the end of the friction experiment;

• -. 1.0 wt% did not fluctuate much from 40 s to the end of the test and only fluctuated significantly between 900 s and 1000 s. CoF was higher than with 0.5%;

• -. 2.0 wt% began to fluctuate greatly at 40 s, and until 330 s, CoF gradually increased to about 0.21. From 330 s to the end of the test, CoF showed irregular fluctuation. This is due to the FLG content being too large and it is easy to agglomerate; however, under the action of the tangential force, the FLG tended to change to an orderly state.

The CoF fluctuation was explained by the FLG changing continuously between two states of agglomeration and order. Under shearing stress caused by the test, the graphene sheets assumed different ordering and agglomeration. See Figure 29.

Ali 2018 [18] observed friction reduction on both boundary and hydrodynamic regimes on a reciprocating test when adding Graphene nanosheets plus oleic acid to a 5W-20 ICE oil. The oils were tested are relatively mild conditions (Hertzian pressures of 2 to 8 MPa). The average CoF was 0.05 for the 5W-20 oil, down to 0.03 with 0.4 wt% graphene. Friction presented the typical zigzag of reciprocating tests, with CoF peaks at reversion points and in the carried tests, as low as 0.02 CoF at mid-stroke. See Figure 30. Despite the author claiming that the regime was on boundary/mixed (based on calculated oil film calculation), the CoF low values suggest that at least during mid-stroke the regime was hydrodynamic. Ali also found up to 17% in fuel savings in a dynamometer ICE test. Such benefits could not be explained only by friction reduction. Potential effects on combustion are discussed ahead on item 7.

Several authors reported non-Newtonian effects when adding graphene to oils and greases. Mohamed 2020 [85] added MWCNT and GNS to calcium grease and observed that shear stress and viscosity increased with the nano additives’ concentration. Wang 2022 [95] concluded that the addition of FLG to a lithium grease makes the grease more compact, with greater structural strength, yield strength, apparent viscosity, storage modulus, and loss modulus. Gallegos 2022 [70] concluded that GNS addition to MO made the lubricants non-Newtonian shear-thinning and increased their viscosity at high shear rates, helping to reduce the wear on the tested metallic interfaces, and enhanced the thermal stability and oil anti-shudder property of the wet clutch. Opposite behavior was seen on greases on [27] when the addition of GNP to greases produced an increase in viscosity at low shear rates and shear thinning at higher ones. It was speculated that the graphene sheets can both act as barriers and “skis” for large molecules. See Figure 31.

5.6. Thermal Conductivity

While amorphous carbon and C₆₀ have a conductivity of the same order as the oils, graphene and even graphite have thermal conductivities of more than 500 times higher than lubricant oils. See Figure 32. Such a unique property makes graphene-based materials an excellent material candidate to remove heat from tribological pair and even work as coolant additive.

Ota 2015 [87] dispersed graphene on clay-based grease and obtained thermal conductivity 25–40% higher in comparison with the same concentration of graphite. Then the author tested the graphene in a gear oil using a block-on-ring tribological tester and observed a reduction of friction and temperature in the test. See Figure 33. Raman analysis of the surface after the test identified tribofilms of single or bilayer of graphene. See Figure 34.

Contreras 2019 [105] observed modest increases in thermal conductivity by adding GNP with volumetric concentrations of 0.01 to 1% on a mixture of ethylene glycol and water but a reduction of heat transfer on an automotive radiator. Mohamed 2020 [85] observed an almost linear increase in thermal conductivity with the addition of MWCNT and GNS to calcium grease. See Figure 35. The grease temperature dropping point also increased with the nano additives. Alqahtani, 2022 [64] obtained a 20% increase in thermal conductivity on an SAE 5W-30 with a GN concentration of 0.09 wt%. See Figure 36.

Nassef 2017 [86] tested ball roller bearings lubricated by lithium grease at different concentrations of graphite, MWCMT, and rGO. The author observed a significant reduction in the operation temperature, see Figure 37, as well as a reduction of bearing vibration. The greases with rGO also presented an increase in LCC load in the Timken test with LCC 25, 50, and 100% higher with, respectively, 2, 3.5, and 5 wt%.

Rasheed 2016 [19] tested two graphene variants in an air-cooled SI engine for 100 h and observed a significant reduction of engine temperature, especially after the functionalization of graphene. See Figure 38.

Sarafraz, 2019 [106] investigated GNP (0.025 to 0.100 wt%) in an aqueous solution and observed up to 89% increase in the thermal conductivity and attributed that to the thermophoresis effect, Brownian motion, and the enhancement in the thermal conductivity of the nanofluid due to the presence of the GNP nanoplatelets. See Figure 39.

As mentioned before, carbon materials form a variety of allotropes, and have a significant place in the field of thermal nanomaterials. As shown in Figure 32, the thermal conductivity of different allotropes of carbon has a large range value, from 0.01 for amorphous carbon to 2000 W·m⁻¹·K⁻¹ for Diamond and graphene. The heat conduction in graphene is attributed to phonons and electrons, with the main contribution being made by phonons, which are the quantized modes of the crystalline lattice vibrations (Balandin, 2011 [107]; Fu et al., 2020 [108]; Zhang et al. [109]).

According to Balandin (2011) [107], the thermal conductivity of graphene varies significantly in different directions due to their structural differences affecting its performance. An optothermal Raman study (Ghosh et al., 2010 [110] found that the thermal conductivity of low-layer graphene decreases with increasing number of atomic planes (n), i.e., the number of layers. In the xy plane, the atoms interact with each other through covalent bonds, so the thermal conductivity in the plane is very high, while in the z-direction, the Van Der Waals force (which is very weak) governs the interaction between the layers, therefore, thermal conductivity in this direction is normally very low, therefore, the transfer heat in their materials is anisotropic (Fu et al., 2020 [108]).

The thermal–physical properties in fluids whereby nanoparticles improve these properties can be mainly divided into some aspects: (I) Brownian Motion and Micro Convection Effect of Nanoparticles, (II) Agglomeration and Percolation Structure of Nanoparticles, and (III) Liquid layer formed by nanoparticle and base liquid. The Brownian motion theory shows that, in liquid suspensions, the nanoparticles keep moving randomly and are affected by the surrounding liquid molecules. The higher movement speed is caused by the smaller particles, therefore, the higher frequency of energy and change with the matrix occurs improving the thermal conductivity.

As with solids, phonons generally dominate heat conduction in nanoparticles such as graphene. Its structure can provide an efficient transfer of vibrations between atoms interacting by strongly connected covalent bonds. The increase in concentration in the liquid base can improve heat transfer properties through the formation of a continuous network, which provides a way to accelerate the rate of phonon transmission. See Figure 40. Therefore, the formation of a percolation network is essential for increasing the heat flow in composites. Otherwise, the high loading of graphene causes agglomerations that affect the properties and processability of the nanolubricants, new hybrid structures can be developed to overcome the limitations and thermal barriers observed, and functionalization of graphene to enhance these properties (Zhang et al., 2020 [109]).

6. Other Tribological Aspects

Although this review focuses on the use of graphene as a lubricant additive, some other tribological applications as dry lubricant, and coating will be briefly covered in the next topics.

6.1. As Dry Lubricant

Graphene was also investigated as a dry lubricant. Due to its nano size, water or other fluids are usually used as a vehicle to apply graphene on tribological surfaces. Different from graphene as an additive, here the expected action is the graphene serving and/or helping as a dry lubricant. Pape 2020 [90] carried out tests on a rolling bearing test rig under typical load conditions using GNP as a dry lubricant on the bearing contact surface. In addition, the bearings were lubricated with grease containing GNP and compared to tests with graphite nanoparticles as a dry lubricant and graphite-containing grease. See Figure 41.

6.2. Coatings

Due to its nanometric size, there are few works on Graphene as a tribological coating for real applications. Anyway, as early as 2004, Tambe [111] discussed the use of ultrathin graphene layers, even in multilayer configurations, which can be applied to nano- or microsystems such as microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) with oscillating, rotating, and sliding contacts to reduce friction and wear. Table 5 summarizes some of the recent works. As mentioned, coatings are usually of nanometric thickness, and their durability will be limited. A different approach was investigated by Brittain 2023 [112]. GNP was used as a sub-layer for a DLC coating and its addition into the DLC matrix reduced both friction and wear by creating a highly graphitic transfer film on the counter-body. See Figure 42 and Figure 43.

More commonly, Graphene has been used as a structural fortifier in polymer-bonded low-friction coatings. Common binders include epoxy, polyester, and polyurethane resins. Considering the environmental issues and the improvement in tribological properties, water-borne coatings are a promising option (Bandeira 2016 [122], Li 2018 [123], Liu 2019 [124]). Graphene sheets protruding from the polymer matrix impart to the coatings unique tribological, thermal, and electric properties that are not usually observed in conventional materials, Qi 2017 [125].

As discussed before, since graphene is a hydrophobic material, the primary challenge for the development of liquid formulations is the production of stable homogenous graphene dispersions. Surface modifications, involving chemical grafting of hydroxy, carboxy, amine, and other hydrophilic groups, have been performed on the surface of graphene through the edges or defect sites to improve graphene’s dispersibility Kuila 2012 [126], Berman 2014 [127]. Besides that, such chemically modified graphene sheets can cross-link with the binder, producing exceptionally strong nanoparticle-polymer composite coatings Presser 2004 [128].

Graphene has also been successfully tested as a filler in filaments for additive manufacturing Guo 2019 [129], Maldonado 2022 [130], and anticorrosive coatings Hamidon 2021 [131], Othman 2020 [132]. There are quite a few commercial products available on the market, such as the CorroNite^® corrosion protection system developed by Tribonex, graphene-fortified car waxes developed by TurtleWax, graphene-filled 3D printing filaments manufactured by Graphmatech, and many more.

7. Graphene as Combustion Improver

Ali 2018 [18] tested 5W-30 oil using GNP as a lubricant additive on an ICE dynamometer test and obtained 5% lower engine Brake Specific Fuel Consumption (BSFC) and 17% lower fuel consumption on a European emissions test cycle. Such reductions hardly could be explained only by friction reduction since friction losses on an ICE correspond to about 25% of fuel consumption at urban conditions, less at full load. Typical fuel savings with improved lubricant are around 2% [31,32,33,34,35,36,133], Tormos [133]. Other than pure tribological mechanisms should exist for graphene in the oil producing substantial fuel savings. A possible explanation for some fuel savings and/or higher engine maximum power reported by the literature when using graphene-based materials as oil additives for internal combustion engines could be that the graphene in the oil has somehow improved the combustion. Some small quantity of oil reaches and is burned in the combustion chamber, during engine operation. It can be speculated that graphene on the burned lubricant may have served to improve the combustion and the formation of graphene tribofilms in the cylinder wall has somehow contributed to improving the combustion. Tian and Koser [134], using regular lubricant oils, has reported sporadic but significant increases in Peak combustion pressure and engine power when small lubricant oil droplets reach the combustion chamber. See Figure 44. Potentially, graphene at a nano size but with a large aspect ratio can enlarge such an effect. Indeed, several authors [135,136,137,138,139,140,141,142,143,144,145,146,147] have investigated the use of graphene derivatives in fuels, especially for biofuel. See Table 6 and Figure 45.

Ooi [145] investigated the effects of graphite oxide (GO), and single-walled carbon nanotubes (SWCNTs) nanoparticles on the combustion, performance, and emission of a four-stroke single-cylinder light-duty diesel engine under various engine loads. Shortened ignition delay (ID) by up to 10%, advanced combustion phasing (up to 18%), shortened combustion duration (up to 15%), improved brake specific fuel consumption (BSFC) by up to 15%, reduced CO emission (up to 23%), and lowered UHCs emissions (up to 24%) were achieved with the 25 ppm of SWCNTs nanoparticle on fuel. See Figure 46.

Another potential effect of graphene in the oil reducing ICE emissions is that some graphene from the oil accumulates in the aftertreatment device and acts as a catalyzer to the emission gases. Jeevahan 2021 [148] investigated using graphene as a coating in the engine aftertreatment catalyst converter and observed a significant reduction in CO₂ and NO_x emissions. See Figure 47. Such benefits may open the opportunity to use graphene derivatives, instead of costly earth rare materials, for catalyzer coating on engine aftertreatment devices.

8. Summary and Future Outlook

Compared to other tribological materials, Graphene and its derivatives are new. Thus, it is important to emphasize that their success, especially as lubricant additives, still requires deeper studies. One of the current challenges involves the preparation of the lubricant, for example in terms of the selection of the graphene variant, graphene concentration, and how functionalization and/or structural modifications can positively and significantly enhance its compatibility and dispersibility to obtain appropriate formulations to meet desired lubricating performance. Along these lines, routes to produce the selected graphene and graphene-based materials must be scalable and economically viable.

In terms of tribological performance, significant advances were obtained recently regarding the mechanisms responsible for low friction values at the microscale. However, the understanding of how these mechanisms translate into the friction coefficient value at the macroscale is not as developed, despite some publications on the subject.

Another important aspect refers to ways to guarantee good performance over time, which include issues regarding product storage and longer-duration tests for real applications. These issues are imperative to evaluate stability as well as transformation and degradation under operation.

Author Contributions

Conceptualization, E.T., original draft preparation, all co-authors; writing—review and editing, all co-authors. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. The milestones of graphene commercialization. Adapted from Barkan 2019 [26].

Figure 2. Four typical methods for the mass production of small graphene sheets via the exfoliation of bulk graphite. From Ren 2014 [30]. Reproduced with permission.

Figure 3. Commercial mass production examples (a) shipment of few-layer graphene manufactured by Deyang Carbonene Technology and (b) graphene-coated aluminum current. From Ren 2014 [30]. Reproduced with permission.

Figure 4. Key technologies for reduction of CO2 emissions in order to limit global warming to 2 °C. From Holmberg [31]. Reproduced with permission.

Figure 5. Potential savings over current state of the art by introducing advanced tribology solutions. From Holmberg [31]. Reproduced with permission.

Figure 6. Global impact due to friction and wear on (a) Costs (b) CO2 emissions. adapted from [31].

Figure 7. Schematic Strobeck curve illustrating the different lubrication regimes and the relative influence of asperity contact and hydrodynamic effects on the total friction coefficient.

Figure 8. Friction losses on motored engine tests. (a) KV100 of current SAE 0W-12. (b) Same viscosity but with organic friction modifiers. (c) SAE 15W-40 with no FM. Adapted from Taylor 2019 [34].

Figure 9. Floating liner engine test, DLC-coated cylinder. 2500 rpm, IMEP 9.3 bar. (a) Friction forces. (b) Same data but with friction losses. Red circles indicate the TDCAF (Top Dead Center After Firing) angle, where friction force is maximum but friction losses negligible. Adapted from [37].

Figure 10. Surface roughness profile (a) typical magnification. (b) Same profile but with an equal scale on both axes. Adapted from [38].

Figure 11. Classification of carbon allotropes to their dimensionality and types of graphene. From Wick 2014 [39] and Georgakilas [40], respectively. Reproduced with permission.

Figure 12. Scheme showing interconversions among various carbon nanoallotropes. From Georgakilas [40]. Reproduced with permission.

Figure 13. Number of layers, area, volume. (a) Graphene of few layers, data from the MGgrafeno. (b) GO nanosheets, [43].

Figure 14. Main routes for the functionalization of nanomaterials.

Figure 15. Main routes of covalent routes for functionalization of nanomaterials.

Figure 16. Dispersion of GO variants dispersions on PAO 4: (a) after sonication; (b,c) after settling for 1 day; (1) GO-T154; (2) GOOS; and (3) GO(T154). Reproduced from Bao 2014 [56].

Figure 17. Functionalization scheme: (a) steric stabilization and (b) electrostatic stabilization.

Figure 18. Functional groups can act as precursors for active tribofilm formation. Adapted from Nyholm [6].

Figure 19. Graphene’s tribological mechanisms. (a) Typical FM tribofilm (b) Surface filling and mending (c) Polishing effect (d) nano roller bearings (e) Hydrodynamics at (1) low shear rate, (2) high shear rate. (f) Thermal effects (g) Superlubricity, incommensurable contact.

Figure 20. Protective surface films prevent wear by replacing metal-on-metal contacts with metal-on-additive or additive-on-additive contacts. Adapted from Wang and Zhang 2019 [80]. (a) Oil without EP Or FM additive. (b) oil with Graphene additive.

Figure 21. Scheme of the shear mechanism on tribofilms.

Figure 22. Filling and mending effect. From [6]. Reproduced with permission.

Figure 23. Schematic illustration for the formation of the L-Ag@rGO nanocomposite grown by simple laser irradiation strategy in solution. From Wang and Gong [79] Reproduced with permission.

Figure 24. Reduction of different additives compared to non-additive paranific lubricant. Adapted from Wang and Gong [79].

Figure 25. (a–c) SEM images of three typical wear surfaces lubricated by pure PL, oil sample with Ag@GO and L-Ag@rGO. (d–f) Corresponding EDS spectra, and (g–i) Raman spectra of the measured surfaces. From Wang and Gong [79]. Reproduced with permission.

Figure 26. Four-ball test results: (a) friction (b) wear (c) proposed mechanism. From Zhang and Zhou 2011. [82]. Reproduced with permission.

Figure 27. Surface after test: (a) base oil (b) with 0.06 wt% graphene and (c) with 5.0 wt% graphene. From Zhang and Zhou 2011 [82]. Reproduced with permission.

Figure 28. Reciprocating tests. Baseline grease and that with graphene have different concentrations. From Wang and Gao [95]. Reproduced with permission.

Figure 29. FLG states in the grease. (a) Unordered aggregation (b) ordered aggregation. Adapted from [95].

Figure 30. CoF on reciprocating tests. Adapted from Ali, 2018 [18].

Figure 31. Proposed mechanism for graphene increasing viscosity at (a) lower shear rates and (b) decreasing it at higher ones. From [27].

Figure 32. Thermal conductivity of carbon materials and of oil. Adapted from Angayarkanni, 2015. [104].

Figure 33. Block-on-ring test. Effect of 0.5% of graphene on (A) temperature, (B) friction, and (C) thermal conductivity. From Ota 2015 [87]. Reproduced with permission.

Figure 34. Block after a test (a) with graphite, (b) with graphene, and (c) Raman spectrum from the worn track with graphene. From Ota 2015 [87]. Reproduced with permission.

Figure 35. Thermal conductivity with different concentrations on calcium grease. From Mohamed 2020 [85]. Reproduced with permission.

Figure 36. Thermal conductivity with different GN concentrations, adapted from Alqahtani, 2022 [64].

Figure 37. Rolling bearing average temperature. Adapted from Nassef 2017 [86]. Reproduced with permission.

Figure 38. Effect on engine temperature with different graphenes. Adapted from Rasheed [19].

Figure 39. The proposed mechanisms involved in the enhancement of the heat transfer with GNP/water nanofluid inside the microchannel. From Sarafraz, 2019 [106]. Reproduced with permission.

Figure 40. Thermal conductivity enhancement (a) due to liquid layering at liquid/particle interface (b) due to increased effective volume of high conductivity clusters: (i) FCC closely packed particles arrangement, (ii) simple cubic arrangement, (iii) loosely packed particles in physical contact, and (iv) clusters of particles separated by liquid layers thin enough to allow for rapid heat flow among particles. From [104]. Reproduced with permission.

Figure 41. Average Frictional torque (a) graphene and graphite greases (b) dry graphene and graphite lubricated. From Patel [89]. Reproduced with permission.

Figure 42. Coefficient of friction with different concentrations of GNP, from Brittain [112]. Reproduced with permission.

Figure 43. Tribological mechanisms in a DLC-GNP composite (a) GNP low concentration (b) high concentration, with the removal of high-asperity GNP islands. from Brittain [112]. Reproduced with permission.

Figure 44. Increase in engine combustion pressure due to small lubricant oil droplets reaching the combustion chamber. From Tian and Koser, 2023 [134].

Figure 45. Number of publications with “graphene” and “diesel” in Google Scholar. Citations and patents are not included.

Figure 46. Brake-specific fuel consumption on a diesel engine with carbon nano-additives. Adapted from Ooi [145].

Figure 47. Reduction of engine emissions, adapted from Jeevahan 2021 [148].

Table 1

Use of graphene on compressor lubricants.

Graphene Type	Conc.	Surfactant	Oil	Energy Consumption	Ref.
Fullerene C60	1 to 3 g/L	Span-40 and tween-60	Mineral oil	4.5% lower	[20]
Amorphous carbon	0.2 g/L	None	POE/Mineral oil	15% lower	[21]
Graphite	0.05–0.5 wt%	none	naphthenic mineral oil	4.5% lower	[22]
Graphene nanosheets	10–30 mg/L	none	SUNISO 3 GS	20% lower	[23]
Graphene	0.2–0.6 g/L	none	Mineral oil	20% lower	[24]
MWCNT	0.05–0.1 vol	Triton X-100	POE (SW-22, Castrol)	17% lower	[25]

Table 2

Nano-carbo and graphene-based materials.

Terms *	Definitions *
GrapheneGraphene layerSingle-layer grapheneMonolayer graphene	A single layer of carbon atoms with each atom bound to three neighbors in a honeycomb structure. Can be abbreviated as 1LG to distinguish from bilayer graphene (2LG) and few-layer graphene (FLG). It has edges and can have defects and grain boundaries where the bonding is disrupted.
Bilayer graphene2LG	Two-dimensional material consisting of two well-defined stacked graphene layers.
Few-layer grapheneFLG	Two-dimensional material consisting of three to ten well-defined stacked graphene layers.
Graphene nanoplateGraphene nanoplateletGNP	Nanoplate consisting of graphene layers. Typically have thicknesses of between 1 nm and 3 nm and lateral dimensions ranging from approximately 100 nm to 100 µm.
Graphene oxideGO	Chemically modified graphene prepared by oxidation and exfoliation of graphite, causing extensive oxidative modification of the basal plane. Graphene oxide is a single-layer material with a high oxygen content, typically characterized by C/O atomic ratios of approximately 2, depending on the method of synthesis. The functional groups found include hydroxyl (OH), carboxyl (COOH), and epoxide (COC). **
Reduced graphene oxiderGO	Reduced oxygen content for graphene oxide. If graphene oxide was fully reduced, then graphene would be the product. However, in practice, some oxygen-containing functional groups remain and not all sp3 bonds will return back to the sp2 configuration. Different reducing agents will lead to different carbon-to-oxygen ratios and different chemical compositions in reduced graphene oxide. It can take the form of several morphological variations such as platelets and worm-like structures.
Carbon NanoTubes (CNTs)	Carbon with a diameter of nanometers and a length of micrometers (where the length-to-diameter ratio exceeds 1000)

* [1], ** [41].

Table 3

Carbon-based materials as an additive to oil.

Lubricant	Graphene Type	Graphene Concentration	Dispersion/Surfactant	Benefit	Prop. Mechanism	Ref.
PAO4	3 GnP: 300, 600 and 750 m2/g (<2 nm, 1–2 µm lateral size)	0.5 wt%	none	Increased wear resistance and thermal conductivity, especially in electric conditions		[63]
SAE 5W-30	Graphene nanosheets	0.03, 0.20, 0.40 and 0.6 wt%	Oleic acid	Lower friction and wear. Significant fuel saving	Tribofilm (self-healing)	[18]
SAE 5W-30	Powder (1.3 nm thick) and graphene nanoplates	0.03 to 0.15 wt%	N-dimethylformamide	15% lower wear, 35% lower friction. 77% higher TC, 30% viscosity	tribofilm	[64]
HDD CH-4 20W-50	Commercial N002-PDR	0.5 to 3.0%	Lipophilic polymer, WinSperse 6020	20% higher TC	thermal	[65]
SN-500 base oil	GO nanosheets	0.02, 0.04, 0.06, 0.8 wt%	none	Reduced friction and wear on four-ball	Protective film	[66]
SN-150 base oil	GO functionalized with DtBHBA	0.2 to 0.8 mg/mL	none	40% lower CoF, 17% lower wear under rolling conditions	tribofilm	[45]
PAO 06	GN and fluorinated graphene (FGN)	0.005 to 0.020 wt%	T161	Reduced friction	tribofilm	[67]
Engine oil	Graphene and GO	0.02 to 0.06 mg/mL	none	Reduced friction	Mending and tribofilm	[68]
20W-50	MWCNT, Graphene Nanosheets, C nanoballs, and Fullerene Nanoparticles (C60)	0.1 and 0.2%		10% higher TC with C nanoballs	thermal	[69]
Mineral oil	GNS (grade C-750, 900, 407 Sigma Aldrich (St. Louis, MO, USA), <2 µm size)	0.1, 0.5, 1.0 wt%	not informed	Reduced wear		[70]
GL-4EP90 (degraded)	Graphite, Graphene and [70]	Respectively 0.5, 0.5, 0.15 wt%	SiO2 for the GO, No dispersants	16% lower wear	tribofilm	[71]
500N base oil	Expanded graphite and Potassium Borate	0.03, 0.05, 0.10 wt%	-	30% lower CoF, 36% lower wear	TC and Tribofilm	[72]
Group lll base oil	Functionalized graphene (AGO-C(n))	0.005, 0.01, 0.02 wt%	-	22% lower CoF with 0.01 wt%	tribofilm	[73]
5W-40 synthetic oil	RGO	0.01 to 0.2 wt%	none	5% lower CoF, 3% lower wear	tribofilm	[74]
PAO and PAO + additives	GN (3–8 layers)	1 and 5%			tribofilm	[75]
HD-50 oil	Modified GNP	0.005–0.1 wt%	Oleic acid and sodium dodecyl presulfate	Up to 35% lower wear	tribofilm	[76]
20W50 SN/CF and SJ/CF	Graphene, thickness 8, 12, and 60 nm thick.	0.01%	none	70% higher Thermal conductivity, reduced wear	Thermal	[19]
Maritime engine oil	GO MXene-Nitrogen-doped	0.01%	none	7% oil thermo conductivity increase and reduced viscosity.	Thermal and Hydrodynamic	[32]
SN 150 oil	GO nanosheets	0.1 wt%	polyisobutenyl succinic acid-polyamine ester	Friction and wear reduction on boundary, mixed, and EHL lubricant regimes	tribofilm	[77]
oil	G, rGO, MoS2, hBN	0.4 wt% each and 0.2, 0.4	None, stability improved by mixing process	Up 80% lower wear, 42% lower CoF	Filling and mending	[78]
pure paraffin liquid (PL) oil and with commercial additives	Graphene layered nanosheets	0.1 wt%	mono-dispersed in silver (Ag) nanospheres	Reduction of Friction 40% and 36% on wear	Roller bearing and protective film	[79]
SAE 10W-30	Gr by liquid exfoliation	0.05 to 0.20 wt%		40% lower CoF and 36% lower wear	Protective tribo film	[80]
PAO 10	GO, rGO, and graphene-like covalent-organic frameworks (GCF) nano-sheets	0.002 to 0.08 wt%				[81]
Lubricant film	Graphene sheets	0.02–0.06 wt%		The friction coefficient and wear scar diameter were reduced by 17% and 14%, respectively		[82]

Table 4

Carbon-based materials as an additive to grease.

Grease	Graphene Type	Graphene Concentration	Dispersion/Surfactant	Benefit	Prop. Mechanism	Ref.
Bentone Grease	Multilayer Graphene	?	ethanol	Higher dropping point. Lower wear and friction	tribofilm	[83]
Grease	Graphene (C 94%, O 6%)	1 to 4%	none	CoF and wear reduction. 55% increase in Thermal conductivity		[84]
Calcium grease	MWCNT and G nanosheets	0.5, 1, 3 wt%		Up to 30% higher dropping point, 60% lower CoF, 74% lower wear		[85]
Cheavy-duty lithium grease	rGO, graphite, and MWCNT	0.5, 1, 2, 3.5, 5 wt%		Increase in Timken test LCC, lower wear, friction, and temperature on rolling bearing tests. Increase in vibration damping.		[86]
Base oil	Mixture of single and multilayer		poly isobutylene succinic imide (PIBSI) based	Reduced wear and friction	Tribofilm and thermal	[87]
Lithium grease	3D hierarchical porous Graphene	0.1, 0.3 and 0.5 wt%	none	52% lower wear, 20% lower friction	tribofilm	[88]
group II-III base oil	3 variants of rGO	0.01 wt%		Reduced wear and friction		[89]
Grease (and also as dry lubrication)	Graphene platelets, 2, 6–8, 11–15 nm	1 wt%		Reduced friction and wear	tribofilm	[90]
Ca and Li greases	CNT	7.5 wt%	MoS2 was also added to the grease	Reduced friction and wear	tribofilm	[91]
commercial lithium grease, mineral oil	rGO	0.2, 0.4, 0.6 wt%	toluene	CoF lower 30% for rolling, 20% for sliding-induced-rolling	tribofilm	[92]
Li Grease	GN and Graphite	0.2 to 2.0%			tribofilm	[93]
Li-based grease	GN	0.2 to 2.0%		Friction and wear reduction	Tribofilm and enhancement of FeO2 and LiO2 tribofilms	[94]
Lithium complex Grease	FLG	0.5, 1.0 and 2.0 wt%		52% lower wear and 20% lower CoF	Tribofilm and ordered state of the graphene sheets (Hydro?)	[95]
Polyurea Grease	FLG	0.5, 1.0 and 2.0 wt%		Reduced wear and friction. Improved rheology	Tribofilm	[96]
Lithium hydroxide monohydrate, mineral oil (KN4010)	GFL 0.5 to 1.5 nm thick	0.1, 0.5, 1.0 and 2.0 wt%	none	Reduced wear and friction. 1.6x higher welding point		[97]
Li grease	NanoCarbon and GNPs	0.2 wt%	Proprietary	Reduced friction and wear		[27]

Table 5

Graphene coatings.

Graphene Type	Tested Base Materials	Test	Benefit	Ref.
A few layers of graphene with the addition of graphene solution droplets	440C steel pair	Pin-on-disk	Decreased wear by almost four orders of magnitude and friction coefficients by a factor of six.	[113]
Solution-processed graphene coating	Steel vs. steel	Pin-on-disk on dry N2 ambient	Friction reduction from ~1.0 to 0.15	[114]
CVD graphene	GN-coated bronze vs. steel	Pin-on-disk	Wear resistance increased while the coated Graphene did not degrade to Amorphous carbon	[115]
Single and few-layer graphene	440C steel pair	Pin-on-disk at N2 and H2 ambiances	Increase in wear resistance	[116]
Graphene monolayer flakes dropped on a textured surface	Coated M2 Steel vs. AISI 52100 steel	Ball-on-disc	Reduced wear and friction (influenced also by the surface texturing)	[117]
CVD graphene	GN-coated copper vs. stainless steel	Flat-on-flat	Wear reduction, no benefit on CoF due to the high roughness	[118]
direct growth graphene (DG) and transferred graphene coating (TGC)	GN-coated steel vs. steel	Pin-on-disk	DG sample has better wear strength, while TGC samples are better at reducing CoF	[119]
CVD graphene and self-assembled graphene	GN-coated aluminum vs. steel	Pin-on-disk	Friction reduction during the little time that the graphene lasted	[120]
direct-grown graphene on bulk Cu transferred graphene, and self-assembled graphene from graphene flakes	GN coated copper vs. 100Cr6 steel ball	Ball-on-disk	Dependent on graphene type, wear reduction for the self-assembled graphene coatings. None for the CVD graphene	[121]

Table 6

Applications of graphene as a fuel additive.

Nanoparticle	Conc.	Fuel	Benefit	Ref.
G, Graphite Oxide	0.1 to 3.0 wt%	NATO F-76 diesel	Slight increase in PCP, leaner combustion	[136]
GNP, GO	20, 40, 60 ppm	Diesel, B20 biodiesel	Reduction of 29% smoke, 26% NOx	[137]
GO and GO-TiO2	50 mg/L	Commercial diesel	Higher PCP	[138]
CNT		Dieseohol with B2	Increase of 15% on Torque and power. SFC reduction of 12%	[139]
GQD	30 ppm	ethanol-biodiesel blends	Increase in power of 28%. SFC reduction of 14%	[140]
GO	50 ppm	Sapota seed biodiesel	39% lower NOx. Lower CO and HC emissions	[141]
CNT, GNP	25, 50 and 100 ppm	Biodiesel	Higher brake thermal efficiency, NOx decrease (but HC increase)	[142]
GO	20, 40, 60 ppm	dairy scum oil biodiesel	Higher brake thermal efficiency, emissions reduction	[143]
CNT, G, GO, GNP, MWCNTs	25 ppm	Jatropha biodiesel	dependent on biodiesel and nanoparticle content. See original work	[144]
GO, SW	25 ppm	Ultra-low sulfur Diesel	15% higher BSFC	[145]
MWCNT	30, 50 and 80 ppm	Diesel and Diesel plus H2	13% higher thermal efficiency	[146]
GO, GNP, MWCNT	50 mg/L	Blend of jatropha methyl ester and n-butanol	Significant reduction in SFC and emissions	[147]

Appendix A. Graphene Derivatives and Chemical Aspects

There are several publications summarizing the preparation methods of graphene and its derivatives (Spear, 2015 [149], Chouhan, 2020 [45], Sarno, 2020 [150], Zhao 2021 [151]). About combination of graphene nanosheets with other nanomaterials, reported approaches include hydrolysis or pyrolysis reaction (Meng 2019 [152]), hydrothermal method (Ismail 2021 [153]), solvothermal method (Sun 2022 [154]), in-situ reduction method (Shi [51]), in-situ chemical deposition (Meng 2015 [155]) and co-precipitation (Sammaiah 2018 [156]). Among them, hydrothermal and solvothermal methods are commonly used since are easy to handle and can provide high crystalline materials. The oxygen-containing functional groups of GO enable chemical or physical interaction with different organic molecules to generate the desired nanomaterials. Examples include the reaction of the epoxy group on the basal plane with n-octyl, through nucleophilic substitution (Yang 2018 [157]), and hydrogen-bonded GO/polyethylene glycol (GO/PEG) prepared by a one-step sonication approach, (Chen 2022 [158]). However, regardless of the synthesis approach, the challenge is always the control of the composition and microstructure of the desired material, which is directly related to the control of process parameters. Factorssuch as pH, concentration, oxidation degree, microstructure of graphene nanosheets, and doping heteroatoms are reported to affect the tribological performance of graphene-based materials (Gao 2022 [9]). Among the challenges to be overcome for graphene-based lubricant additives are: (1) some organic and inorganic graphene modifiers, such as SDS (Peng 2014 [159]) and MoS₂ (Ismail 2021 [153]), can cause the release of pollutants; (2) the degree of modification (the amount of modifier on graphene), the structure-property relationship, the synergetic effect between different components, the lubrication mechanism, as well as the influence of each functional group and component on the tribological properties and their interaction with the lubricants, need to be further explored; (3) the long-term dispersion stability in various liquid lubricants has yet a significant issue, considering the organic modifiers are prone to degrade due to the friction-induced heat, inducing re-aggregation of the graphene layers; (4) most of the studies were carried-out at room temperature and on a laboratory scale. Additionally, low-cost, large-scale preparation routes and tribological performance evaluation in real applications are crucial for the practical application of these additives (Gao 2022 [9]).

Appendix A.1. Chemical Aspects

The unsaturations are assigned as the origin of graphene’s reactivity and yet, graphene is chemically stable. That is because the pz atomic orbitals are strongly coupled and stabilized in a giant and delocalized p bonding system that usually prevents covalent addition reactions (Figure A1). Nevertheless, the p-ligand type feature renders versatile complexation reactions with organic compounds and transition metals through p–p, H–p, and metal–p interactions. In addition, the associated anti-bonding p* molecular orbitals can accommodate electrons, facilitating adsorption between graphene and electron-rich particles such as ions and alkali metals. Furthermore, graphene has defects in its structure that can work as active sites for chemical reactions. Among the defects are edges, vacancies, and folds or creases. The edges, unlike the internal atoms of the graphene sheet, can rotate more easily, without causing extra stress on the sheet, favoring covalent functionalization. Holes also have a high tendency to be functionalized due to the greater freedom of rotation of the carbon atoms. For the folds, there is a decrease in the overlap of the π orbitals, making them more reactive (Yan 2012 [160]).

Figure A1. Graphene reactivity. Adapted from Yan 2012 [160].

Furthermore, it is settled that to achieve the desired application is imperative to control the synthesis process of graphene and its derivatives. In that regard, there are two main approaches to synthesizing graphene: (1) bottom-up, from alternative carbon sources, such as methane (CH₄); and (2) top-down, from exfoliation of graphite (Coros 2018 [161]). Figure A2 shows a summary of the methods used to produce graphene, divided into top-down and bottom-up approaches. Considering large-scale production, currently, three representative techniques can be highlighted: (1) chemical vapor deposition (CVD); (2) wet chemical synthesis (such as hydrothermal and solvothermal), and (3) liquid exfoliation. However, those approaches still face challenges to be overcome for spread industrial commercialization, such as size and thickness inhomogeneity, wrinkle formation, cracking and contamination, low durability and instability, as well as the use of non-environmentally friendly chemicals (Choi 2021 [25]).

Figure A2. Representation of graphene synthesis methods. Adapted from [161].

Appendix A.2. Graphene Hydroxylation

A striking feature of graphene is its hydrophobicity, which limits its processability and applications. Many applications can be improved with the use of GO, however, in recent years, the development of graphene-based composites requires a carbon structure with minimal defects to obtain the maximum potential of its properties. The functionalization of graphene with the VIA group (chalcogens), which includes oxygen and sulfur, creates monovalent and divalent bonds with carbon. The development of graphene derivatives with defined stoichiometry allows for greater control of defects in the basal plane of the carbon structure while maintaining the intrinsic properties of graphene. The insertion of Hydroxyl (-OH) groups, that is, monovalent C-O bonds, known as Hydroxygraphene or Grafol, has become a very strategic route to confer properties to this material, such as hydrophilicity, making it compatible with water and polar and low-defect solvents (Eigler 2017 [162]).

Appendix B. In-Situ Graphene Formation, “Superlubricity”

The term superlubricity is selected to define a contact condition where a very low coefficient of friction is obtained. Some differences are often observed regarding the exact threshold below which the system is considered superlubric, but a value equal to 0.01 is found in many cases [163,164,165]. Carbon-based materials are potential candidates for superlubricity [166], but the necessary and sufficient conditions are complex and allow different approaches, which vary depending on the nature of the surfaces in contact and on the scale considered.

Several studies exist trying to explain the reasons for low coefficients of friction of layered (2D) materials at the nanoscale. These studies include atomistic simulations and experimental analysis, as reviewed by Zhang [167]. A key point of the analyses at the nanoscale is to define if contact sliding occurs (i) between neighboring layers-homojunctions, or (ii) between the layered material and other counter surfaces- heterojunctions.

Models related to homojunction conditions are more frequently found and, in this case, the orientation of the adjacent layers is significant in determining the intensity of friction loads. Figure A3 presents a model from Zhang [168] to study not only the effect of the misorientation angle θ between adjacent layers but also the effect of stiffness in interlayer friction. Those authors, similar to others [129], found that lower friction coefficients may be obtained for given values of angle different from zero, for example, 30°, a condition defined as incommensurate. Conversely, as indicated by Nyholm [6], commensurate contact provides restriction to lateral shearing, leading to higher friction coefficients as well as stick-slip motion.

Figure A3. Representation of a molecular dynamics analysis to study the misorientation [Forumla omitted. See PDF.] between adjacent graphene layers, as well as the stiffness supporting a graphene substrate [168].

Still at the nanoscale, superlubricity may also be obtained for layered heterojunctions, which indicate a condition where sliding occurs between graphene and another material. This condition was studied, for example, by Tian [169], for graphene in contact with WS₂, and by Song [170], for the contact of graphite and hexagonal boron nitride. In the first case, friction coefficient values as low as 0.003 were observed.

As indicated in Section 1, for the great majority of engineering applications involving contact, surfaces are rough, and localized interactions appear between surface asperities. Therefore, to obtain very low coefficients of friction at the macroscale level, superlubricity conditions must predominate throughout the contact points. In other words, if low friction conditions are found in only a small fraction of the contacts, the friction coefficient measured at the macroscale would not be in the superlubricity condition. Moreover, engineering applications often aim to have low friction values (at the macroscale) over time, and not for small periods, such that this predominantly superlubric contact must not be short-lived, for example as a result of structural imperfections generated by deformation [171]. In an attempt to obtain superlubricity at the macroscale, Li [172] conducted macroscale pin-on-disk tests with a steel ball sliding against highly oriented pyrolytic graphite (HOPG). The curve indicating the evolution of the friction coefficient as a function of time presented several peaks and valleys, with maximum values of 0.08 and minimum values on the order of 0.001, which indicate superlubricity conditions. According to the authors, contact conditions lead to localized and random formation of multilayer graphene nanoflakes (MGNF) at part of the contact points (Figure A4). Since macroscale friction results from statistical frictional forces at each contact, this random formation of MGNF leads to a random appearance of superlubricity with a short duration.

Figure A4. Schematic of the contact zone between a steel ball and highly oriented pyrolytic graphite (HOPG). Localized formation of multilayer graphene nanoflakes (MGNF) throughout the contact. (a) Several MGNFs attached on the asperities, preventing the direct contact between asperities and HOPG. (b) Top view of several MGNFs (dispersed randomly in the contact zone) sliding on the HOPG. Four different types of contact between MGNFs and HOPG substrate, sliding on the atomically smooth region: (c) sliding on the step region (d,e), and (f) pushing the step region. Reproduced with permission from [172].

In recent years, the search for superlubricity conditions at the macroscale was addressed in several publications [173,174,175,176,177]. Part of these examples involve the direct deposition of graphene on at least one of the contacting surfaces [173,174,175]. Conversely, the approach selected by Li relies on how to maintain adequate heterojunctions over time, which was obtained either by forming graphene/transition-metal dichalcogenide (TMDC) heterostructures during the running-in stage [176] or by pressure distribution and reduction on the rate of destruction of graphene/MoS₂ heterojunctions due to stress concentration [177].

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