Carbon Nanomaterial-Based Lubricants: Review of Recent Developments

Abstract

This review article summarizes the progress of research on carbon nanomaterial-based lubricants witnessed in recent years. Carbon nanomaterials, such as graphene, carbon nanotubes (CNTs), fullerenes and carbon nanostructures, are at the center of current tribological research on attaining superior lubrication performance. The development of nanomaterial-based solid lubricants, lubricant additives and bulk materials and the related issues in their processing, characterization and applications as well as their tribological performance (coefficient of friction and wear rate) are listed in a structured tabulated form. Firstly, regarding nanomaterial-based solid lubricants, this study reveals that carbon nanomaterials such as graphite, graphene, graphene-based coatings and diamond-like carbon (DLC)-based coatings increase different tribological properties of solid lubricants. Secondly, this study summarizes the influence of graphene, carbon nanotubes, fullerene, carbon nanodiamonds, carbon nano-onions, carbon nanohorns and carbon spheres when they are used as an additive in lubricants. Thirdly, a structured tabulated overview is presented for the use of carbon nanomaterial-reinforced bulk material as lubricants, where graphene, carbon nanotubes and carbon nanodiamonds are used as reinforcement. Additionally, the lubricity mechanism and superlubricity of carbon nanomaterial-based lubricants is also discussed. The impact of carbon nanotubes and graphene on superlubricity is reviewed in detail. It is reported in the literature that graphene is the most prominent and widely used carbon nanomaterial in terms of all four regimes (solid lubricants, lubricating additives, bulk material reinforcement and superlubricity) for superior tribological properties. Furthermore, prospective challenges associated with lubricants based on carbon nanomaterials are identified along with future research directions.

1. Introduction

Although friction is not always undesirable, as in machining, fabrication or grinding, it is thought to be the cause of 20% of global energy consumption [1]. Specific detrimental effects of friction include: (i) the hindered motion of the body arising from the frictional forces, (ii) energy loss from the work required to overcome the friction, (iii) the loss of materials and (iv) an increase in temperature as result of heat generation. Friction reduction needs to be mitigated or addressed in a wide range of fields, from aerospace to machinery, including in cars, machines, mechanical joints and others, because of the lower costs associated with consuming less energy and greater environmental advantages [2,3,4]. One of the best ways to prevent or lessen the impact of friction is to control the friction and wear through lubrication and the use of energy efficient lubricants since they not only significantly improve energy efficiency but also reduce CO₂ emissions [5]. Because friction wastes a significant amount of energy, lubricants have been utilized since 1400 B.C. [6]. Leonardo de Vinci (1452–1519) was the first to explain the basic concepts of friction and lubrication as well as the impact of lubrication on the coefficient of friction between two sliding bodies [7]. Lubricants are chemicals introduced between two tribosurfaces, i.e., two sliding solid surfaces in contact, to minimize or diminish the effect of friction. The ability to reduce friction is known as lubricity or slipperiness, and the action of doing so is known as lubrication [8]. Most lubricants are oils and greases, and in some cases fluid and solids are used for lubrication. Based on their physical states, lubricants can be solid, semisolid, liquid or gaseous, but their tribological performances greatly differ. Based on the United Nations Industrial Commodity Statistics (ICS) database, the worldwide production, import and export of lubricants amounts to a staggering quantity [9]. On the FUCHS Capital Markets Day 2022, it was revealed that the market demand for high-tech lubricant is increasing day by day despite world lubricant demand being flat or slightly increasing (0.14%) [10]. With the development of advanced technologies, the need for proper lubrication grows, as it can directly affect the global energy and material losses, i.e., the sustainability of mechanical performance and efficiency. Therefore, it is of paramount importance to expand our knowledge of lubrication and fill the gap.

Researchers around the world are continuously striving to improve the tribological performance of lubricants to reduce friction and wear. Several methods such as nanomaterial-based anti-friction and anti-wear additives, low viscosity oils and vapor phase lubrication, etc., have been reported to further improve the tribological performance of lubricants [11,12]. The incorporation of anti-wear and anti-scuffing additives is one of the most commonly used methods to improve the tribological performance of lubricants [2,3,13,14,15,16,17]. A wide range of metallic and nonmetallic additives, such as polymers, nanoparticles and carbon nanomaterials, etc., have been used to meet the growing demand for high-efficiency lubricants. Depending on the type and structure of the additives, different types of lubricity can be improved.

Carbon nanomaterials possesses outstanding mechanical, electrical, thermal, optical and chemical properties that make them ideal materials for tribological applications, with them exhibiting sustainable mechanical performance and efficiency [2,18,19,20,21,22,23,24,25,26]. Due to the success and prevalence of graphite as a solid lubricant in industry for centuries, carbon nanomaterials are one of the focal points of current tribological research. The significant number of the publications regarding carbon nanomaterial-related issues on tribology published during 2012–2022 shows the depth of interest in the tribological features of carbon nanomaterials (Figure 1). The data were obtained from the Elsevier Science Direct database using customary methods. “Carbon nanomaterial Tribology” is entered as a search term on the Science Direct website’s keywords search page. A total of 998 results were displayed, with the number of articles for each year indicated on the left-hand side. A graph showing the number of articles by year was then created on 15 August 2022. A wide range of carbon-based compounds and structures available as carbon can form strong covalent bonds in different hybridizations states. With the development of nanotechnology, carbon nanomaterials have become increasingly popular in tribological applications. The most commonly used carbon nanomaterials in terms of lubrication are shown in Figure 2.

Before the discovery of carbon nanomaterials, the most well-known structures used for lubrication were graphite and diamond. The coefficient of friction on the macroscopic scale was found to be 0.1 for graphite only and 0.05 for diamond/diamond contact without any lubrication [33,34]. Recently, Morstein, C. and M. J. W. Dienwiebel applied graphite coatings on iron with a thickness of 10 µm or more at higher mechanical loads and observed a steady increase in the coefficient of friction (COF) due to an increase in the contact area and resistance [35]. The minimum COF was found to be 0.12 for a 0.2 µm graphite coating using 402 mN of normal force [35]. Since the discovery of carbon nanomaterials such as fullerenes by Kroto, Smalley and Curl in 1985 [36], carbon nanotubes by Iijima in 1991 [22,37] and carbon onions by Ugarte in 1992 [38], researchers around the world have put effort into investigating the tribological properties of carbon nanomaterial-based lubricants. Carbon nanomaterials that are studied for tribological applications can be classified as zero-dimensional (0D), one-dimensional (1D) and two-dimensional (2D). Fullerenes, carbon nano-onions (CNOs), nanodiamonds (NDs), carbon quantum dots (CQDs) and graphene quantum dots (GQDs) are zero-dimensional (0D), as all three dimensions of these materials are at the nanoscale [21,39,40,41]. One-dimensional carbon nanomaterials include carbon nanotubes (CNTs), carbon nanofibers or nanowires and carbon nanohorns (CNHs), as one dimension of these materials are outside of the nanoscale [42]. Two-dimensional (2D) carbon nanomaterials include graphene, graphene nanoribbons and few-layer graphene with sheet-like structures with nanoscale thicknesses [9,18,43,44,45,46,47].

This review presents the development of carbon nanomaterial-based solid lubricants, lubricant additives and bulk materials in a structured tabulated form for easy readability and understanding. The development of carbon nanomaterial-based lubricants, such as graphene, carbon nanotubes (CNTs), fullerenes and carbon nanostructures as well as their processing and experimental methods, along with their operating conditions, applications and tribological performances, are discussed in this review of contemporary research lubricants. The lubricity mechanism of the lubricants is also discussed. Graphene and CNTs have proven to be excellent candidate nanomaterials for superlubricity regimes [48]. Their impact on superlubricity is outlined in a tabulated form along with their experimental method, operating conditions and applications. Reducing friction between tribosurfaces remains challenging due to the decreased lubrication efficacy of traditional lubricants. Carbon nanomaterials have unique characteristics that make them the ideal alternative for tribology and have attracted the keen interest of researchers [1]. As a result, this analysis provides an overview of the current state of carbon nanomaterial-based lubricants, including solid lubricants, lubricant additives and bulk materials. Furthermore, prospective challenges concerning lubricants based on carbon nanomaterials are also identified. Finally, a comparative analysis between graphene and graphite is presented, as the tribological performance and anti-wear capabilities of lubricants depends on the operative environment as well as the formation of a continuous film.

2. Lubricity Mechanism of Carbon Nanomaterials

The lubricity mechanism depends on the tribological characteristics of a material and the operating conditions and it can be (i) a fluid film, a thick film or hydrodynamic lubrication, (ii) a thin film or boundary lubrication or (iii) extreme pressure lubrication. Carbon-based nanomaterials lubricants are operated in the context of boundary or hydrodynamic lubrication or in a mixed regime [20].

In thick film lubrication, tribosurfaces are separated by a thick and continuous film of the lubricant where the load is low and the speed is high. As the surfaces are not in contact, viscosity plays crucial role in reducing friction. Hydrocarbon oils are widely used for fluid film lubrication. In thin film lubrication, tribosurfaces are not completely separated and exist under a high load and low speed. Here, the surfaces are in contact and the performance of the lubricant depends on its ability to stick to the surface, forming a continuous film. Vegetable oils, animal oils, graphite, MoS₂ and graphene, etc., are used in boundary lubrication.

3. Carbon Nanomaterials as a Solid Lubricant

Solid lubricants have low friction behavior and are employed in scenarios where liquid substitutes fail because of high temperatures, vacuum conditions or heavy mechanical loads. At present, the tribology of carbon nanomaterials has greatly improved, making them a promising lubricant for their versatile structures and mechanical, electrical, chemical and thermal properties. Carbon nanomaterials are employed in tribology as solid lubricants, additives and super lubricants. Solid lubricants are used in places where liquid and semi-liquid lubricants fail to give satisfactory results. A solid lubricant has significant properties at high temperatures and pressure. Solid carbon nanomaterials such as graphite and graphene and their coating on a surface increases different surface properties.

Table 1. Tribological properties of carbon nanomaterial-based solid lubricants.

Solid Lubricant	Experimental Method, Conditions, Materials and Application	Tribological Properties		Ref
		Friction Coefficient (µ)	Wear Rate
Graphite	Steel–steel pair test; 1 N, dry N2 humid air; ethanol solution; solid lubricant coating on steel.	Dry N2 µ = 0.8 Humid air µ = 0.17	Dry N2: 5.07 × 10−7 mm3/Nm Humid air: 4.94 × 10−8 mm3/Nm	[18]
Cu-coated graphite/Al2O3/Cu	HT-1000 ball-on-disk tribo-meter; 5 N, 360 rpm; electrolytic copper, ethanol; deposited on the 304 stainless steel substrate	10 wt% Cu-coated graphite/10 wt% Al2O3 µ = 0.29	2.2 × 10−4 mm3/Nm	[53]
Ni-grown graphene	Home-built micro tribometer; 5–70 mN, 50 μm/s; poly (methyl methacrylate) (PMMA); deposited onto the SiO2 substrate.	µ = 0.12	-	[54]
500 mg/L graphene/Ni	Tested by a reciprocating ball-on disk method; 1 N, 150 mm/s; mixture of H2SO4, NaNO3 and KMnO4; composite coatings sliding against an M50 steel balls.	µ = 0.10	8.6 × 10−4 mm3/Nm	[55]
Graphene oxide	Reciprocating ball-on-plate tester (MFT-R4000); 400 mN, 25 mm/s; N-type polished single-crystal silicon, ultra-pure water; film as solid lubricant fabricated onto silicon wafer.	µ = 0.05	-	[56]
MoWS4/graphene heterogeneous composite	Steel ball as counterpart ball and with the loads of 1 N under 3 Hz reciprocating motion mode in dry argon; N-methyl-2-pyrrolidone, ethanol; composite coatings on steel–steel interface.	µ = 0.008	1.6 × 10−6 ± 1.1 × 10−6 mm3/Nm	[57]
Graphene coating/Au vs. TiN	High vacuum tribometer with a ball on disk contact geometry method in dry nitrogen (900 mbar) at room temperature; 1 µm thick gold film; graphene coatings with Au vs. TiN substrates.	µ = 0.15	-	[58]
Graphene coating	AFM (MFP-3D) tester; spring constant 2.8 N/m, resonance frequency of 75 kHz; pyrolytic graphite, 5 nm thick Cr and 95 nm thick Au thin film on Si wafer; graphene coatings at electrical contact interface.	µ = 0.028	-	[59]
3D graphene/hexagonal boron nitride composite coating	Ball-disc tribological test with friction pair GCr15 bearing steel and DLC films at load 5 N, 5 hz; 0.1 wt% h-BN ethanol solution; composite coating at steel-DLC contact.	µ = 0.01	3.0 × 10−7 mm3/Nm	[60]
Diamond-like carbon (DLC) film with hydrogen content	Tribological test in ultra-high vacuum with a reciprocating pin-on-flat configuration; sliding speed of 1 mm/s and 1 Gpa; thin amorphous silicon layer (<5 nm); deposited on polished 440 C steel or silicon substrates.	µ = 0.02	-	[61]

summarizes the tribological performance of recently developed carbon-based nanomaterials used as solid lubricants. A study by Klemenz et al. [49] revealed that graphene coatings improved load bearing capacity, protected the surface from wear and increased the contact stiffness and elastic capacity. It has been demonstrated that graphene coatings on aluminum can enhance the load-carrying area and alter the deformation behavior, which has a considerable impact on the indentation depth and the associated load even at higher system temperatures [50]. Covering copper with a graphene coating improves its mechanical properties [51,52].

3.1. Graphite

As a typical 3D carbon material, graphite has been widely used in the tribology field as a lubricant. Graphite consists of a stack of carbon atom sheets, with each sheet containing a hexagonal arrangement of atoms, and best suited for lubrication in air. Graphite exhibits better lubricating properties in humid air than in dry air [62]. For the lubrication of graphite, water vapor plays a crucial role, as adsorbed water lowers the bonding energy between graphite’s hexagonal planes. Friction is decreased when water molecules move into the area between graphite layers. Berman et al. [18] compared the tribological properties of graphite with their prior studies of graphene [63,64]. The tribological test results showed that graphite powder lubricant exhibited comparatively low friction in humid air and high friction and high wear losses in a dry nitrogen atmosphere, but the wear of graphene was significantly reduced in both humid air and dry nitrogen environments (Figure 3). Graphite can also serve as a promising reinforcement in bulk composites [65,66,67]. Ravindran et al. [68] prepared composites with 5 wt% graphite as reinforcement and observed significantly decreased friction and wear due to the self-lubricating effect of graphite.

Graphite acts as a lamellar solid to provide lubricity [62]. It resembles a series of cards placed one above another, with these behaving as loosely bound graphite layers. When the two sliding surfaces come into contact with each other, graphite imparts lubricity by rolling up the series of loosely arranged layers and in doing so behaves like roller bearings [69]. Due to this roller mechanism, the contact surfaces slide with a comparatively lower coefficient of friction. The roller mechanism alternative coefficient of friction is influenced by the sliding zone temperature and the sliding environment, i.e., vacuum or dry air, water vapor, etc. [70]. Bulk graphite is very effective at reducing friction under humid condition due to the intercalation of water molecules in between the graphite layers [62].

Concerning graphite-based lubricants, the concentration of graphite and the size of the graphite particles largely influences the coefficient of friction [71]. Fasihi et al. confirmed that graphite-based solid lubricants with a lower carbon content reduce the coefficient of traction more remarkably [72]. Su et al. concluded that with an increase in the volume fraction and a decrease in the size of graphite nanoparticles, the coefficient of friction reduces in the context of graphite nanoparticles being added to vegetable-based oil as an additive [73]. The underlying lubricating mechanism in this case involved the graphite nanoparticles forming a physical deposition film between the contact surfaces due to their small size and high surface energy. Again, contact surfaces are not ideally smooth and they may be comprised of many convex and concave zones. The convex zones initially come in contact while sliding and cause an increased coefficient of friction. But when graphite nanoparticles are added to oil as an additive, this reduces the friction in two ways. Firstly, they can penetrate the concave zone between the contact surfaces to repair the damage. And secondly, graphite nanoparticles can be laid on the rubbing surface to produce a thin separation in terms of the contact zones [73].

The dry superlubricity of graphite is induced by weak van dar Waals forces due to the formation of a convenient share plane (smooth contact interfaces) at their atomic level [74,75]. In the case of bulk graphite, dry superlubricity occurs due to the formation of a transfer layer as a result of the transfer of triboinduced material from the graphitic substrate to the contract surface [75,76,77]. The transfer layer primarily features nanosized flakes, which are assumed to configure in a misfitting way, imparting interfacial crystalline incommensurability and resulting in a low coefficient of friction, i.e., 10⁻³ [35,78].

3.2. Graphene

Graphene is a 2D carbon nanomaterial that has outstanding friction and wear properties. It is a single-atom sheet of sp² hybridized carbon atoms. Graphene has received the attention of researchers around the world for its outstanding mechanical, thermal and electrical properties and is considered one of the greatest discoveries of the twenty-first century [28,79,80,81,82,83]. The key advantages of graphene are its high chemical inertness, low surface energy, extreme strength and easy shear capability on its densely packed and atomically smooth surface along with its outstanding tribological behavior [54]. One of the major advantages of graphene is that the tribological properties of graphene is independent of the environment, as it can effectively reduce the wearing of tribosurfaces in addition to drastically reducing friction in humid and dry environments [18]. However, this special property cannot be found in graphite. In humid conditions, the coefficient of friction of graphene and graphite lie in the range of 0.15–0.17 [18]. But graphene can greatly reduce the wearing of tribosurfaces compared to graphite, with this being attributed to the formation of a thin film with better surface coverage as well as the easy shear of the thin film at the contact interface of the tribosurfaces. Graphene can be applied to nanoscale or micro-scale systems such as microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) with oscillating, rotating and sliding contacts to reduce friction and wear because even with multilayers it is ultrathin. Jibin et al. demonstrated that a graphene–fullerene C60 hybrid film that was created on the surface of silicon (Si) exhibited a good friction reduction, load carrying capacity and anti-wear ability [32]. The formation of (3-aminopropyl) triethoxysilane(APTES)-a-graphene-fulleren C60 film, which can achieve a very low nanofriction coefficient, as low as 0.1, is shown in Figure 4 [32]. The cobblestone effect of regularly oriented C60, which results in the creation of curved tops, was responsible for the improved tribological performance. The creation of a film embedded with fullerene C60 led to a reduction in the frictional area between the sliding microsphere and the APTES-a-graphene-C60 film. The presence of graphene played a crucial role in reducing the nanofriction coefficient of the hybrid fil, with graphene having potential in the context of MEMS applications.

3.3. Graphene-Based Coatings

Graphene is one of the thinnest and strongest nanomaterials available. A flat monolayer of tightly packed carbon atoms in a two-dimensional honeycomb lattice makes up a graphene structure [28]. As solid lubricants, graphene-based coatings are extensively used in sliding steel surfaces, electrical contact interfaces, iron contact surfaces and space applications, etc., thanks to their superlubricity, electrical, ultralow wear and low friction properties [1,32,41,50,57,58,59,60,84,85,86,87,88,89,90,91,92,93]. The friction and wear reduction of coatings facilitated by monolayers, few-layers, multi-layers and reduced graphene oxide emphasizes graphene’s inherent advantage in lubricating coatings [2]. According to tribological and mechanical studies of graphene at the nanoscale, the strong electron–phonon coupling in single-layer epitaxial graphene [94] and the puckering effect [95] cause a rise in friction as the number of atomic layers decreases. The sp³ functionalization of the graphene surface may also have an impact on the friction of graphene at the nanoscale. This is due to the possibility of the creation of larger band gaps and fewer van der Waals interactions, which lower adhesion forces and the number of free electrons [96,97]. The outstanding protective stability and wear resistance of graphene as a covering has been shown to have their origins at the level of the monolayer [98]. In experiments, monolayer graphene exhibited a twofold increase in friction compared to bilayer graphene [99]. As shown by sliding contact studies [100,101,102,103], the effect of the testing conditions on the friction performances of multi-layer graphene become more notable at the macroscale. A low friction coefficient of 0.08 was obtained when multi-layer graphene coatings slid across the hydrogenated-diamond-like carbon coating in comparison to coatings against steel (0.15) and TiCN (0.26) under the same conditions [100]. In an experiment, Lang et al. discovered that graphene exhibited ultra-low friction (the coefficient of friction was roughly 0.028), ultra-high current stability (the coefficient of variation was as low as 4.74%) and ultra-high conductivity effective wear-resistance properties at electrical contact interfaces [59]. Another experiment, by Jiang et al. [57], revealed the superlubricity in steel–steel interface obtained by using MoWS₄ and graphene-based heterogeneous composite as a solid lubricant. They found the friction coefficient to be as low as 0.008 and the wear rate to be 1.6 × 10⁻⁶ ± 1.1 × 10⁻⁶ mm³ N⁻¹ m⁻¹ (Figure 5). In this instance, the MoWS₄ and graphene-based heterogeneous composite was primarily concentrated on the top surface of the tribofilms, generating a tribofilm–nanoscroll–tribofilms friction pair that produced stable macroscale superlubricity. Steel now performs better against wear as a result.

3.4. Diamond-like Carbon (DLC)-Based Coatings

Diamond-like carbon (DLC), a prominent 3D allotrope of carbon compounds, is characterized by sp3 bonding between carbon atoms. DLC has drawn a significant amount of interest from researchers due to its wide bandgap, excellent hardness and exceptional chemical stability [104,105]. The exceptionally smooth surfaces of carbon nanodiamonds are crucial for the friction and wear characteristics of the self-mated tribology system [39,106,107,108,109,110,111,112]. Due to their high hardness, excellent lubrication performance, extremely low friction and chemical inertness, diamond-like carbon (DLC) coatings are widely utilized in metal contact and engine systems [86,105,113,114,115,116,117]. In an experiment by Donnet et al. [61], it was demonstrated that the friction coefficient of a DLC film in an ultra-high vacuum with a hydrogen content of 42 at.% was as low as 0.02. However, in humid conditions, the ultralow coefficient of friction of DLC film with hydrogen content increases as the water vapor pressure rises [118,119,120,121,122].

4. Carbon Nanomaterials as Additives in Lubricants

Lubricant additives play a critical role in developing new properties or compensating for drawbacks of lubricants. The performance of a typical lubricant depends on the viscosity of the base oil and the additive. Without an additive, both boundary and hydynamic lubrication can adversely affect tribosurfaces [123]. Carbon nanomaterial additives are very promising in the development of high performance lubricant with reduced friction and anti-wear capabilities, and they are also chemically stable and environmentally friendly in comparison to traditional organic lubricant additives [124,125]. Nanomaterials such as graphene, carbon nanotubes, fullerenes, carbon nanodiamonds and other carbon nanostructures have greater surface areas, which allows them to cover larger areas in terms of the tribosurface, resulting in reduced friction [45].

4.1. Graphene as Lubricating Additives

As a 2D nanomaterial, graphene possesses superior tribological properties with a high chemical stability, high strength and easy shear capability. Graphene is ultrathin and can be applied to microelectromechanical systems (MEMS) and nanoelectromechnical systems (NEMs). Berman, D. et al. used graphene nanoplates as oil additives for ceramics/steel sliding components and found that graphene entered the rubbing interface and prevented ceramic/steel pairs from mechanical contact by forming a protective film, contributing to reduced friction [64]. Zin, V. et al. improved the tribological and thermal properties of lubricants by using graphene-based nano-additives and found the tribological properties of graphene-based lubricants to be promising [126].

Table 2. Carbon nanomaterials as additives in lubricants: previous papers.

Lubricant	Additive	Experimental Method, Conditions and Application	Tribological Properties		Ref
			Friction Coefficient (µ)	Wear
Polyalphaolefin 6 oil (PAO-6)	Modified graphene (0.5 wt%)/dispersant (1 wt%)	Reciprocating tribotester (UMT-5, Brucker) with a ball-on-disk bearing steel mode. Load 2 N, sliding speed 5 mm/s. Modified graphene/dispersant in PAO6 oil.	µ = 0.10	Wear track depth 21 nm	[127]
Pure water	Graphene oxide (1 wt%)	Diamond-like carbon vs. stainless steel contact by a pin-on-disk tribometer. Load 10 N. Graphene oxide in pure water.	µ = 0.06	-	[128]
Graphene enhanced	Graphene dispersion (110 µg/mL)	Reciprocating tribotester (UMT-3) with a ball-plate bearing steel mode. Load 2 N. Graphene enhanced lubricant	µ = 0.105	Wear volume 0.8 × 10−5 mm3	[129]
Pure water	Graphene oxide (0.1 wt%) and nano diamond (0.5 wt%) concentration	The tribotester ball on plate tribopairs employed were Si3N4 balls and Si wafers. Load 5 mN. Graphene oxide and nano diamond in water.	µ = 0.03	Wear track depth 5 nm	[130]
Deionized water	Carbon dots (0.1 wt%) concentration	Reciprocating tribotester (UMT-3) with a ball on disk. Load 15 N, 5 Hz, 25 °C. Carbon dots in water-based lubricants for amorphous carbon (a-C) coatings.	µ = 0.03	Wear volume 0.9 × 10−5 mm3	[30]
SAE 10 W-30 lubricating oil	Liquid phase exfoliated graphene (0.05 wt%) concentration	The pin-on-disk tribometer test. Pressure 10 Mpa, Speed 0.3 m/s. Liquid phase exfoliated graphene in SAE 10 W-30 lubricating oil.	µ = 0.033	Wear rate 2.91 × 10−7 mm3/Nm	[131]
Esterified bio-oil	Graphene/MoS2 (0.5 wt%) mass ratio 3:2	MQ-800 four-ball tribometer test. Load 300 N, 1000 rpm. Graphene and MoS2 dispersed in esterified bio-oil for steel–steel contact.	µ = 0.017	Wear scar diameter 0.43 mm	[132]
Refined oil	Edge-oleylaminated graphene (0.1 wt%) concentration	Ball milling method. Load 396 N, 75 °C. Edge oleylaminated graphene as ultra-stable lubricant additive for friction and wear reduction.	µ = 0.05	Wear scar diameter 0.35 mm	[133]
Base liquid	Polyethylene glycol 200 modified carbon dots (CDs-PEG200) (0.2 wt%) concentration	The universal friction and wear tester (MMW-1) with steel balls. Load 40 N, 600 rpm. Synthesis of polyethylene glycol modified carbon dots as a kind of water-based lubricant additive.	µ = 0.045	Wear volume 0.4 × 10−6 mm3	[134]
10 W40 engine oil	(Zinc oxide) ZnO/MWCNTs (multiwalled carbon nanotubes) hybrid nanomaterial (0.25 wt%), mix 3:2	Bronze alloy-steel contacts using linear reciprocating ball-on-disk tribotester. Load 35 N. Zinc oxide/MWCNTs hybrid nanomaterials in engine oil.	µ = 0.044	Wear volume 0.09 mm3	[135]

and

Table 3. Carbon nanomaterials as additives in lubricants: most recent papers.

Lubricant	Additive	Experimental Method, Conditions and Application	Tribological Properties		Ref
			Friction Coefficient (µ)	Wear
PEG200 oil	Reduced graphene oxide (0.2 mg mL−1) concentration	Ball-on-disc nanotribometer. Load 500 mN. Reduced graphene oxide (rGO) in PEG200 oil.	µ = 0.06	-	[19]
Base oil	Few-layer graphene larger interlayer spacing (FLG-Ls) (0.5 wt%) concentration	Reciprocating sliding tester (UMT-3 CETR, USA). Load 2 N. Graphene oxide in pure water.	µ = 0.08	-	[136]
Hydraulic oil	Graphene oxide sheets include regular edges (RG) (1 wt%)	Reciprocating tribotester (UMT-3) with a ball on disk mode. Load 2 N, 0.5 Hz. Reduced graphene oxide sheets on lubrication properties as a lubrication additive.	µ = 0.0614	Wear scar depth 0.151 µm	[137]
water	1-hydroxyethyl-3-methyl imidazolium tetrafluoroborate functionalized graphene oxide (ILCAs-GO) 0.8 mg/mL concentration	CETR UMT-3 multi-function sliding test. Load 5 N. Hydroxyl-terminated ionic liquids functionalized graphene oxide in water.	µ = 0.172	Wear volume 0.6×105 mm3	[138]
Hydraulic oil	Graphene (TRGO)	UMT-3 (CETR, USA) tribometer in a ball-on-disk reciprocating friction and wear mode. Load 3 N. The graphene (TRGO) as additive in hydraulic oil.	µ = 0.081	Wear scar depth 0.68 µm	[139]
4010 aviation lubricant (4010 AL)	Graphene (0.075 wt%) concentration	Four-ball tester. Si3N4 ceramics/GCr15 steel tribo-pairs. Load 392 N and 1450 r/min. Graphene nanoplates as oil additives for ceramics/steel sliding components	µ = 0.068	Wear scar depth 0.516 µm	[140]
Polyalphaolefin oil	Graphene platelets (5 wt%) concentration	The ball-on-disk tribometer test. Load 35 N. Graphene-containing polyalphaolefin oil on DLC-coated surfaces.	µ = 0.025	-	[141]
SAE 5W-30 oil	Graphene nanoplate (GN) (0.12 wt%) concentration	Four ball tribo tester method Load 800 N, 10,050 rpm. Engine oil with graphene nano-additives.	µ = 0.0425	Wear scar depth 0.75 mm	[142]
Polyalphaolefin 4 oil (PAO-4)	Carbon spheres (CS-700) + SMO surfactant	High-frequency reciprocating rig tribotester. The test relative humidity was 35–40%., the oil temperature 100 C, the contact pressure 1 GPa and the sliding speed 0.06 m/s. Micro-carbon sphere additives for enhanced lubrication in PAO-4 oil.	µ = 0.10	Wear coefficient 49 × 10−18 m3/Nm	[143]
Polyalphaolefin 6 oil (PAO-6)	Highly exfoliated reduced graphite oxide (heRGO-4) (0.5 wt%) concentration	Reciprocating tribotester (UMT-3 CETR, USA) with a ball-on-disk mode. Load 2 N. Highly exfoliated reduced graphite oxide powders as efficient lubricant oil additives in PAO-6 oil.	µ = 0.084 ± 0.005	Wear scar depth 31.6 ± 12.6 nm	[144]
canola oil	Graphene sheets (0.7 wt%) concentration	Pin-on-disk tribometer. Load 10 N, contact pressure 700 MPa, sliding speed, 20 mm/s. Graphene in canola oil.	µ = 0.064	Wear rate 1.5 × 10−5 mm3/Nm	[145]
5W30 engine oil	Carbon spheres (CS) (0.5 wt%) concentration	Ball-on-disk tribometer. Relative humidity, 35–40%; oil temperature, 100 C; load 10 N. Submicrometer carbon spheres in engine oil.	µ = 0.04	Wear volume 0.9 mm3	[146]
paraffin oil (PO)	Alkylated fullerene which bears three eicosyl chains (3,4,5-C20C60,1) (1.0 wt%) concentration	Steel–steel contacts using an Optimol SRV-IV oscillating reciprocating friction and wear tester. Load 200 N, 25 Hz. Alkylated fullerene in paraffin oil.	µ = 0.16	-	[147]
Mineral oil	Fullerene nanoparticle (0.5 wt%) concentration	Disk-on-disk type tester. Load 200 N. Fullerene nanoparticle in mineral oil.	µ = 0.02	Surface roughness 0.048 µm	[148]
pure calcium grease	Two-dimensional (2D) graphene nanosheets (3 wt%) concentration	Four-ball tester. Load 200 N, 1200 rpm, 60 min. Graphene nanosheets in grease.	µ = 0.01	Wear scar depth 0.40 µm	[149]
Water	Aminoborate-functionalized reduced graphene oxide (rGO-AmB) (0.2 w/v %)	The ball-on-disk tribometer test. Load 0.5 N, 50 rpm. rGO-AmB as additive in water.	µ = 0.12	Wear scar width 98 µm	[150]

summarize recent progress regarding the use of using graphene-based nanomaterials.

4.2. Carbon Nanotubes (CNTs) as Lubricating Additives

CNTs, formed of rolled-up graphene sheets with a cylindrical hollow structure, are used as additives in dry and liquid lubricants to improve their tribological properties [31,151]. Due to the unique 1D structures of CNTs, such as their high strength and superior thermal conductivity and electrical and optical properties, they have been widely investigated for triblogical applications. Compared to conventional materials, CNTs possess a higher surface energy and tension that cause them to wind and form bundles easily [42,152]. A strong propensity to form bundles is responsible for reduced friction capabilities as the tribofilm becomes discontinuous. Thus, a stable and homogeneous dispersion of CNTs is required for enhanced lubrication performance. Therefore, CNT surfaces are modified to minimize the effect of van der Waals attraction force through covalent functionalization or by wrapping a CNT surface using various function molecules. Xie, H. et al. (2021) functionalized MWCNTs (multi-walled carbon nanotubes) and SWCNTs (singl-walled carbon nanotubes) in carboxylic acid for proper dispersion, and then oil-based and water-based lubricants were prepared [153]. The inclusion of MWCNTs in the oil-based lubricant resulted in a better performance and reduced the coefficient of friction, which was as low as 0.063 at a lower concentration (0.01 wt%). Water-based lubricants show improved tribological performances at higher SWCNT concentrations [153]. Fan et al. modified CNTs using ionic liquids that improved the dispersion of CNTs in oil and greatly enhanced the lubrication properties [154]. CNTs have been used with other materials to study their synergistic effect on lubrication. Zhang, L. et al. (2015) integrated GO and MWCNTs with ionic liquids that effectively reduced friction [155]. MWCNTs easily roll between the friction pair, resulting in very low friction coefficients at a low applied load, in contrast to GO layers, which are easily absorbed by composite coatings and the counterpart steel ball by sliding in one direction. Under a high applied load, GO layers can form nanobearings between friction pairs, leading to the direct formation of nanosheets for a reduced friction coefficient and wear rate [155].

4.3. Fullerene as an Additive in Lubricant

The fullerenes C60 and C70 were first discovered by Kroto and Kratschmer [36,156]. As a promising carbon-based nanomaterial, the fullerene C60 has a great impact on lubrication as a lubricant additive due to its perfect spherical shape, high load-bearing capacity, strong intra-molecular forces, weak inter-molecular forces and low surface energy [157,158,159]. However, due to its poor solubility in organic solvents [160], certain suitable chemical modifications on the intrinsically hydrophobic and lipophobic C60 sphere are needed to dissolve it in organic oil solvents. An experiment by Lui et al. [147] used an alkylated fullerene which bears three eicosyl chains (3,4,5-C20C60, 1) and was synthesized via cycloaddition at 25 °C as a lubricant additive in paraffin oil. They found that the average friction coefficient was 0.16 with a 1% concentration of alkylated fullerene added in paraffin oil under a constant load of 200 N and a frequency of 25 Hz using an Optimol SRV-IV oscillating reciprocating ball and disk friction and wear tester in steel–steel contacts. Another experiment by Gupta et al. [161], performed on a steel disc in a ball-on-disc tribotester, revealed that the friction coefficient was decreased by about 20% from that of raw mineral oil and the reduction in the width of the wear scar was from 300–380 μm to 120–130 μm when a higher additive concentration (5 wt%) of C60-rich (85% C60 and 15% C70) nanoparticles were added in mineral oil. Furthermore, Lee et al. [148] showed that by controlling the volume concentration of fullerene nanoparticle additives in mineral oil, friction and wear can be reduced. The friction coefficient was 0.02 and surface roughness was reduced to 0.048 μm from 0.106 μm with fullerene nanoparticles (0.5 vol% concentration) when performed on a disk-on-disk type tester at a load of 200 N. The investigation pointed out that the volume fraction of fullerene nanoparticles has a significant impact on controlling the friction coefficient and the magnitude of wear. Another experiment by Lee et al. [162] revealed that when using 0.1 vol% fullerene nanoparticles as lubricants in compressors, the friction coefficient of the oil containing the fullerene nanoparticles was 90% lower than that of raw oil. Moreover, fullerene is effective as an additive when added to lower viscosity lubricant oil [163].

4.4. Carbon Nanodiamonds as an Additive in Lubricant

As a promising carbon nanomaterial, nanodiamond (ND) particles have become a major focus of interest for researchers all over the world within the last decade due to their unique properties such as their sharp particle size distribution, high and tunable surface energy, biocompatibility, high hardness, chemical inertness, high thermal conductivity as well as their optical properties. Several experiments reported that the addition of ND particles as additives into liquid lubricant [164,165,166] reduced the friction and wear of sliding surfaces in contact. Due to the nontoxicity and chemical stability [2] of ND particles, they have an advantage as an additive over other traditional environmentally hazardous additives. They are very effective as an additive in boundary and extreme pressure sliding lubrication. NDs have been used as additives in many different liquid lubricants such as commercial oils [164,167,168,169,170], paraffin oil [165,166], poly-alphaolefin, polyester and poly-ethylene glycol. They reduce surface roughness significantly when compared to microcrystalline diamonds, which is important for the friction and wear behaviors of self-mated tribosystems. Small micron-size NDs, which are less than 200 nm in size, with this small size enhancing their anti-friction and wear-reduction properties, are believed to decrease friction and wear effectively [171]. Novak et al. [111] observed that, when using a ball-on-disc test and a UMT-2 tribometer with 3–10 nm sized (1 wt%) DNDs (detonation nanodiamonds) in P100N paraffinic oil at a 1.2–1.7 GPa load and a 100–500 mm/s sliding speed, the friction was reduced but the wear increased. However, an experiment by Zhai et al. [172] revealed that the addition of 0.2 wt% NDs in base oil reduces the friction torque from 0.12 Nm in terms of pure oil to 0.08 Nm, and the wear volume also decreases significantly on steel/copper test pairs at a 106 N load. Moreover, when used as an additive in mineral oil, ND particles can reduce wear and friction in the presence of sliding contact. Mechanical tribological performance increases proportionally with the increase in concentration of nanodiamonds. The tiny diamond particles fill in asperities and provided a protective ball bearing-like cover over the wear surface. Thus, Marko et al. [110] conducted an experiment using a four ball tribotester at a 391 N load and 1200 rpm. The results showed that adding a 0.01% weight concentration of NDs in pure mineral oil reduced the friction coefficient from 0.02 in terms of pure mineral oil to less than 0.005, and increasing the weight concentration of the NDs reduced the friction coefficient of the lubricant. Furthermore, an investigation by Elomaa et al. [173] revealed that increasing the ND concentration from 0 to 1.1 wt% resulted in a continuous reduction in the friction coefficient, from 0.16 to 0.11. Minimum wear for both the disc and counter ball occurred when the ND concentration was 2.2 wt% or 0.55 wt% at 100 N load.

4.5. Carbon Nano-Onions as an Additive in Lubricant

Carbon nano-onions (COs) are synthesized by the annealing of nano-diamond powder at a high temperature (typically between 1400 and 1900 °C). Depending on the annealing conditions, nanoparticles exhibit a nested, circular shape with or without a remaining diamond core. After annealing at 1600 °C, round shape carbon onions are formed but they contain a residual diamond core. After annealing at 2000 °C, the diamond core disappears but the carbon onions become facetted [174]. An unfinished diamond graphitization process is an easy explanation for the occurrence of a remnant diamond core. Carbon onions either have a residual diamond core inside the graphitic shells or do not, depending on the synthesis technique used. Carbon onions without the diamond core present have better anti-wear properties. However, an investigation by Pottuz et al. [175] revealed using carbon onions without diamond cores (CO2), carbon onions with diamond cores (CO1) as additives in PAO oil. The two carbon onion samples’ friction coefficient values were determined to be extremely low (between 0.07 and 0.12), indicating excellent friction-reducing properties for these two carbon onions samples. For both samples, a common and intriguing pattern could be seen: the higher the contact pressure, the lower the friction coefficient (0.07 at 1.42 GPa for CO2). Matsumoto er al. [176] conducted an experiment on a high frequency reciprocating rig sliding tester using cylindrical test samples against highly polished flats. The lowest friction coefficient, 0.07, occurred at 0.94 Gpa for PAO with carbon onions. Therefore, it was found that the contact pressure has a significant impact on the friction-reducing ability of nanocarbon materials

4.6. Carbon Nanohorns and Carbon Spheres as an Additive in Lubricant

Carbon nanohorns (CNHs) are conica-shaped carbon nanostructures with exceptional properties compared to other carbon nanostructures. Harris et al. [177] were the first to investigate carbon nanohorns made from the waste product of the shoot created during the arc discharge method of producing fullerene. After heating to 2500–3500 K with a positive-hearth electron gun for B4 h, the waste product was converted into two types of nanohorn, single walled (SWCNHs) and multi walled (MWCNHs). Zin et al. [126] studied the tribological properties and behavior of the dahlia-like shaped SWCNH nanoparticles in an SAE 40 grade Mobil Pegasus 1005 synthetic base oil using a ball-on-disc Cetr UMT-2 tribotester at 25 °C. The best results were obtained using a 0.01 vol% dispersion in the base oil. This reduced the overall average friction coefficient from 0.085 (for the base oil) to 0.074 or 9% and the wear rate from 109 ± 7 to 74 ± 5 (μm³/mm/min), i.e., by 30%.

K.mistry et al. [143] conducted an experiment whereby low-density polyethylene plastic bags were used to create spherical carbon particles in a specially made, sealed autoclave reactor while being heated by argon. Friction and wear experiments were conducted with a high-frequency reciprocating rig tribotester, a relative humidity of 35–40% and an oil temperature of 100 C. The friction coefficient was 0.10 and the wear coefficient was 49 × 10⁻¹⁸ m³/Nm, with these being measured when 1 wt% of carbon spheres (CS-700) + SMO surfactant were added to PAO-4 oil. Another experiment by Lv et al. [146] used carbon spheres as a green lubricant additive in 5W30 engine oil to reduce friction and wear. Carbon spheres with a diameter of ∼400 nm were synthesized from glucose by using a hydrothermal process. A ball-on-disk tribometer measured the friction coefficient, 0.04, and wear volume, 0.9 mm³, when carbon spheres (CS) (0.5 wt% concentration) were used as an additive in 5W30 engine oil at a relative humidity of 35–40%; an oil temperature of 100 C and a load of 10 N. The above experiments demonstrate that when lubricating with base oil, carbon spheres are efficient in lowering both friction and wear when used as an additive.

5. Carbon Nanomaterial-Reinforced Bulk Material as Lubricants

In recent years, bulk carbon materials have been used extensively in the field of tribology for various application to reduce friction and wear. Bulk material such as ceramics, metals and polymers reinforced by carbon nanomaterials have a significant impact in terms of reducing friction and wear. Carbon nanomaterials (graphene, carbon nanotubes and carbon nanodiamonds) have been used for their unique tribological properties in metals, polymers and ceramics in various research works for improving tribological properties [29,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194]. Due to its low cost and superior self-lubricating qualities, graphite, an allotrope of carbon, is widely utilized as an efficient solid lubricant. Its lubricating effect and mechanism are well researched and understood. Solid lubricants, which are all forms of carbon, have various lubrication effects and mechanisms, and their levels of lubrication are not exactly the same either. The friction and wear properties of composites can be effectively improved by comprehending their individual lubrication effects and mechanisms and optimizing them as solid lubricants. This results in a wide range of application possibilities and significant engineering value, prolonging the lifespan and performance of mechanical systems. Graphene is considered to have potential as a solid lubricant to significantly improve the friction and wear properties of composites. In their investigation into the impact of graphene nanoplates on the tribological performance of Ni₃Al matrix composites, Zhai et al. [195] discovered that even very small amounts of graphene nanoplates had a noticeable impact on strengthening the tribological capabilities of composites. These outcomes were caused, in part, by graphene nanoplates, which generated a small protective layer on the contact surfaces to minimize friction while also enhancing the action of composites to produce better wear resistance. The tribological characteristics of copper matrix composites containing 5, 10, 15 and 20 vol% carbon nanotubes were studied by Lin et al. [196]. The lowest value in terms of wear rate occurs when the carbon nanotube content is 10–15 vol%, according to tribological studies, and the friction coefficient increases as the concentration of carbon nanotubes declines.

Table 4. Tribological properties of carbon-reinforced bulk material used as lubricants.

Bulk Material Lubricant	Experimental Method, Conditions and Application	Tribological Properties		Ref
		Friction Coefficient (µ)	Wear Rate mm3/Nm
10 vol% graphene/Cu	Ultrasonic dispersing and hot-press (HP) sintering. HT-1000 ball-on-disk high temperature tribometer. Load 2 N, 1 m/s. Graphene/Cu graphene-reinforced bulk material as lubricant.	µ = 0.17	1.8 × 10−4	[197]
4.0 wt% graphene/ polytetrafluoroethylene	Compression at room temperature and sintering. Inidirectional two-pin-on plate tribometer; 20 N, 0.1 m/s. Graphene/polytetrafluoroethylene graphene-reinforced bulk material as lubricant.	µ = 0.18	7.5 × 10−6	[198]
1.0 wt% graphene/polyethylene	Hot compression, 2D transducer of the triboindenter. 100 µN, 0.333 µm/s. Graphene/polyethylene graphene-reinforced bulk material as lubricant.	µ = 0.24	-	[199]
15 vol% multi-walled CNTs/copper	Pin-on-disc test. 24 N, 2.77 m/s. Electrolytic copper powder. Multi-walled CNTs/copper metal and ceramic matrix composite material as lubricant.	µ = 0.10	1.2 × 10−4	[200]
10 wt% multi-walled CNTs/Al2O3	Ball-on-reciprocating flat geometry test. 14 N, 10 mm/s. Multi-walled CNTs/Al2O3 metal and ceramic matrix composite material as lubricant.	µ = 0.11	-	[201]
1.0 wt% multi-walled CNTs/Al	Pin-on-disc test. 1 N, 0.5 m/s. Multi-walled CNTs/Al metal and ceramic matrix composite material as lubricant.	µ = 0.25	2.00 mm3/Kg m	[202]
1.0 wt% multi-walled CNTs/polyphenylene	Reciprocating-type ball-on-disc tribometer (HSR-2M). 20 N, 0.20 m/s. Multi-walled CNTs/polyphenylene polymer composite material as lubricant.	µ = 0.09	3.15 × 10−6	[203]
0.5 wt% multi-walled CNTs/epoxy	Pin-on-disc tribometer test. 10 N, 0.09 m/s. Multi-walled CNTs/epoxy polymer composite material as lubricant.	µ = 0.06	3.0 × 10−5	[204]

summarizes the tribological properties of carbon-reinforced bulk material used as lubricants.

5.1. Graphene Reinforced Bulk Material

As a reinforced carbon nanomaterial, graphene significantly increases the lubricity of bulk materials. Because of its excellent mechanical qualities, bulk material wear and friction are reduced. A well-dispersed form of graphene in the polymeric matrix has been used in numerous investigations to fabricate graphene/polymer composites. According to research by Kesavulu et al. [189], the combined loading of 2 wt% Al₂O₃ and 0.5 wt% graphene nano-platelet-reinforced epoxy nanocomposites resulted in an increase in micro hardness of 47.64% and a decrease in terms of average coefficient of friction and specific wear rate of 18.3% and 76.92%, respectively. Another study by Jin et al. [188] showed that considerable reductions in the coefficient of friction (COF) of grease were possible with trace levels of Mn₃O₄/graphene (as little as 0.02 wt%). The COF and wear depth were 43.5% and 86.1%, respectively, at a 0.1 wt% Mn₃O₄/graphene concentration, with these being lower than those of pure graphene. Xu et al. [197] investigated the tribological properties of NiAl matrix composites (NAMC) containing (MLG) multilayer graphene. Tribological tests were carried out in the air (relative humidity 40–50%) on a HT-1000 ball-on-disk high temperature tribometer. The investigation showed MLG possessed the lowest friction coefficient and wear properties. Another study by Bhargava et al. [198] involved an experiment on a unidirectional two-pin-on plate tribometer against a rotating polished SS 304 disk with a total normal load of 200 N. The friction coefficient was 0.185 when composites of polytetrafluoroethylene (PTFE) filled with graphene platelets were used.

5.2. Carbon Nanotube-Reinforced (CNTs) Bulk Material

The mechanical, electrical and tribological properties of polymer, metal and ceramic matrices have been effectively improved using CNTs. In polymer matrix composites, the appropriate portion of carbon nanotubes used as additives can enhance mechanical and tribological properties. Bastwros et al. [205] fabricated MWCNTs-Al composites via high energy ball milling, followed by the cold compaction of the mix and hot extrusion. The CNT content varied from 0–5 wt%, and the impact of CNTs was studied. With an increase in CNT content, both the hardness and anti-wear properties dramatically improved. The friction and wear of the composite containing 5 wt% CNTs decreased by 55.6% and 78.8%, respectively, among the various CNT concentrations. The self-lubricating qualities of carbon nanotubes (CNTs) are a result of their structure, which is comparable to those of graphite and fullerenes. CNTs may therefore be appropriate for abrasion resistance and anti-friction materials. Dong et al. [206] investigated the effects of various multiwalled carbon nanotube (MWCNT) concentrations (0–4 wt%) on the tribological characteristics of epoxy MWCNT nanocomposites in dry sliding against ordinary carbon steel. They discovered that the MWCNTs greatly boosted wear resistance and decreased the coefficient of friction of the nanocomposites, and they came to the conclusion that the nanocomposite with 1.5 wt% MWCNTs displayed the best wear behavior. At various sliding speeds and with various applied loads (40–120 N), Cui et al. [207] examined the friction and wear behavior of epoxy–MWCNT nanocomposites. However, they looked at lower concentrations (between 0 and 0.5 weight percent) and the impact of functionalized carbon nanotubes with amino and carboxyl groups. They demonstrated that the wear rate reduced with increasing MWCNT loading and that the nanocomposites with MWCNTs had a lower friction coefficient and wear rate than clean epoxy. However, Campo et al. [204] studied the wear behavior of epoxy matrix composites with different types and percentages of multiwalled carbon nanotubes (MWCNTs). The tribological properties of epoxy-MWCNTs nanocomposites were investigated using a “pin-on disc” wear testing machine under different conditions, and MWCNTs were dispersed into an epoxy resin via a calendering process. The results showed that the epoxy composites with a 0.5 wt% of amino MWCNT had the best tribological properties, with a coefficient of friction of 0.06 and 3.0 × 10⁻⁵ mm³/Nm wear rate. The composites with MWCNTs had a lower mass loss, friction coefficient and wear rate compared to neat epoxy, and these metrics dropped when the MWCNT fraction was increased.

5.3. Carbon Nanodiamonds as Bulk Material

Nanodiamonds have a significant impact in composites in terms of improving their tribological properties. Nanodiamonds/epoxy composites were made by Neitzel et al. [190], and they investigated the tribological behaviors of the materials at various length scales. Light microscopy, optical profilometer scans of wear tracks and counterpart results from pin-on-disk tests all showed that the epoxy containing nanodiamonds had dramatically increased macroscale wear resistance. The inclusion of 7.5 vol% nanodiamonds resulted in a four-fold decrease in the friction coefficient. In comparison to composites made with non-aminated nanodiamonds, those made with aminated nanodiamonds had superior tribological characteristics, with a friction coefficient of 0.08 ± 0.02, which might be attributable to the covalent integration of ND-NH₂ particles.

6. Superlubricity of Carbon Nanomaterial(CNM)-Based Lubricants

The degree of scientific interest in superlubricity has significantly expanded globally over the last ten years. Two touching solid surfaces can slide with minimal resistance due to superlubricity. Graphene and CNTs have been proven to be excellent candidate materials for the superlubricity regime [146,147,148,149,150,151,152,153,154,155,156,157]. At present, with the amount of nanomaterials and nano-devices increasing, nano-mechanical behavior is becoming an essential subject when considering increases in efficiency. Macro/microscopic superlubricity can cure core issues such as friction and wear. As device sizes decrease, it is crucial to pay close attention to minimizing friction and wear at the micro-scale [208]. However, superlubricity at the micro-scale is difficult to achieve and microscopic superlubricity is more difficult to apply in practical working conditions, as the superlubricity is acquired by reducing of Van der Waals force, surface force, adhesion and friction and a high vaccum, high temperature and specific materials are required [209,210,211,212,213]. The self-retracting motion (SRM) of graphite microscale superlubricity [208,214] is when, after being sheared from micron-sized mesas made of highly oriented pyrolytic graphite (HOPG), microscopic graphite flakes retract back onto the mesas. This effect is driven by a reduction in surface free energy. SRM speeds in nanotube systems can surpass 100 m/s, according to theoretical studies of gigahertz oscillators [215]. An experiment by Yang et al. [213] obtained SRM speeds using microscopic graphite mesas and an optical knife-edge technique over a wide range of temperatures. At various sample temperatures, repeatable SRM speeds ranging from 10⁻⁴ m/s to 25 m/s were observed. Hence, the self-retracting motion of sheared microscale graphite mesas exhibit remarkably high speeds, with speeds of up to 25 m/s having been measured, in an almost frictionless retracting motion. Moreover, Yang et al. developed an optical technique used for quantitative measurements of superlubric systems with applications in MEMS and NEMS. Carbon quantum dots (CQDs) are a promising carbon nanomaterial for use as a superlubricant thanks to their unique characteristics, such as their quasi-spherical shape, high surface area and size, which is below 10 nm [216]. Ma et al. [217] fabricated carbon quantum dots (CQDs) with ionic liquid (IL) nanoparticles, which, at a content percentage of 3.6%, showed an excellent superlubricity and obtained a significantly low friction coefficient, roughly 0.006, and a wear rate of roughly 0.7 × 10⁻¹⁴ m³/Nm. Superlubricity of CNM based lubricants that can be achieved with specific condition is shown in Table 5.

6.1. Carbon Nanotubes (CNTs) in Superlubricity

Concentric nanotubes (CNTs) have been shown to facilitate superlubricity due to the structural incommensurability between them, which allows them to move easily relative to one another along their shared axis [218,219]. However, CNTs have flaws, deformations and contaminations that can cause energy loss and limit superlubricity in engineering applications. Zhang et al. [220] evaluated the inter-shell friction of double-walled CNTs using a scanning electron microscope with nanomanipulators. A four-nanoprobe system and a silicon nanorode (35 mm in length, 200 nm in diameter) was used as the force cantilever (force constant K ≈ 1–10 nN µm⁻¹). According to the findings, a centimeter-long inner shell could be pulled out with a friction of less than 5 nN. In comparison to the theoretical result of 0.18 J/m², the dissipation energy for the pull-out process was approximately 0.218 J/m² [221]. Theoretically, the fluctuation of the total van der Waals interaction only depends on the crucial edge of the outer shell and ultimately depends on the outer diameter of the double-walled carbon nanotubes (DWCNTs). In addition, in centimeter-long DWCNTs, the existence of macroscale superlubricity is largely due to the as-grown CNTs’ flawless structures. The intershell interaction of multi-walled carbon nanotubes, MWCNTs, dramatically increases in the presence of defects. The ultralow friction observed here is rooted in the perfect structure of ultralong CNTs [214,222] as well as in the relative straightness of the DWCNTs. Thus, superlubricity is possible under ambient conditions in centimeter-long DWCNTs with perfect structures.

6.2. Graphene in Superlubricity

The primary elements of two-dimensional materials with a layered structure are monolayers or multilayers of graphene, both of which have an atomic thickness and an exceedingly low shear strength. Due to their enormous specific surface area, in-plane strength, weak layer-layer interaction and surface chemical stability, they have very low friction and wear-resisting properties [223,224,225,226,227]. Consequently, 2D materials have received a lot of attention. As a 2D material, graphene is one of best carbon nanomaterials for superlubricity. However, because of the incommensurate susceptibility of superlubricity’s to flaws and deformations, graphene’s superlubricity has predominantly only been realized in a small number of tests. Using graphene in conjunction with nanodiamonds and diamond-like carbon, Berman et al. [228] achieved stable superlubricity at the engineering scale for the first time. Using a solution method, the nanodiamonds were added to the SiO₂ substrate together with three or four layers of graphene flakes. The friction test performed by a CSM tribometer confirmed that the superlubricity state occurred over wide ranges of test conditions: the load varied from 0.5 to 3 N and the velocity changed from 0.006 to 0.25 m/s. Additionally, while sliding in a dry environment, the nanodiamonds aided in the development of nanoscrolls, resulting in the superlubric condition (a friction of 0.005 to 0.004).

Koren et al. [229] examined superlubricity in quasicrystalline twisted bilayer graphene. Structural superlubricity is a low friction phenomenon in incommensurate sliding surface contacts and can explained by a virtually zero energy barrier due the lack of crystal symmetry [230,231]. The sliding force is analyzed to study the friction characteristics of such quasicrystalline structures. The relationship with friction comes from the fact that whenever the actuator’s stiffness is less than the negative value of the displacement force gradient along the slide direction, energy dissipation spontaneously jumps from one marginally stable position to the following stable equilibrium position. Therefore, the friction force scales with the magnitude of the sliding force fluctuations. When compared to a Bernal stacked bilayer system of the same area, the force oscillations are extremely tiny and are on the order of a single sliding atom, i.e., ∼10 pN [232].

Table 5. Superlubricity of carbon nanomaterials.

CNM Lubricant	Experimental Method, Conditions	Friction Coefficient (µ)	Ref
C60 Fullerene	The friction coefficients obtained by linear fitting for all the cases are presented in C60 bearing system µ C60 (ϴ) and graphite system µ G (ϴ); friction coefficients of the C60 bearing and graphite systems for the scan directions ϴ = 0, 15 and 30; 10.27 nN.	µ = <0.001	[233]
Carbon quantum dots	Rotating ball-on-disc tester. Steel and Al2O3 balls (u = 3 nm); 200 r min−1, 20 mN.	µ = 0.0066	[217]
Nanodiamonds	CETR UMT-3 ball on disk test; 0.15 m s−1, 3 N; glycerol colloidal solution 30 wt%.	µ = 0.006	[234]
Graphene	0.5–3 N	µ = 0.004	[2]
Bilayer graphene	The sliding force exhibits a fractal structure with distinct area correlations. Zero scaling of the sliding force is demonstrated for a geometric sequence of dodecagonal elements.	Sliding force: 10 pN	[229]
Graphene flake	Graphene flake sliding on a graphite substrate, using molecular dynamics test. 100–600 m s−1.	Sliding force: 0–4 nN	[235]
CNT	1–10 nN/µm	Sliding force: 1.37–1.64 nN	[2]

Table 5. Superlubricity of carbon nanomaterials.

CNM Lubricant	Experimental Method, Conditions	Friction Coefficient (µ)	Ref
C60 Fullerene	The friction coefficients obtained by linear fitting for all the cases are presented in C60 bearing system µ C60 (ϴ) and graphite system µ G (ϴ); friction coefficients of the C60 bearing and graphite systems for the scan directions ϴ = 0, 15 and 30; 10.27 nN.	µ = <0.001	[233]
Carbon quantum dots	Rotating ball-on-disc tester. Steel and Al2O3 balls (u = 3 nm); 200 r min−1, 20 mN.	µ = 0.0066	[217]
Nanodiamonds	CETR UMT-3 ball on disk test; 0.15 m s−1, 3 N; glycerol colloidal solution 30 wt%.	µ = 0.006	[234]
Graphene	0.5–3 N	µ = 0.004	[2]
Bilayer graphene	The sliding force exhibits a fractal structure with distinct area correlations. Zero scaling of the sliding force is demonstrated for a geometric sequence of dodecagonal elements.	Sliding force: 10 pN	[229]
Graphene flake	Graphene flake sliding on a graphite substrate, using molecular dynamics test. 100–600 m s−1.	Sliding force: 0–4 nN	[235]
CNT	1–10 nN/µm	Sliding force: 1.37–1.64 nN	[2]

7. Conclusions and Outlook

This article provides a comprehensive review of current advancements in the field of lubricants based on carbon nanomaterials, including stability, methods for improving their tribological performance, tribological characteristics, and lubrication mechanisms. According to studies that have been reported in the literature, reducing friction between tribosurfaces is still a difficult task because traditional lubricants have a lower lubrication efficiency. Outstanding mechanical, electrical, thermal, optical, and chemical characteristics of carbon nanostructures make them the perfect material for tribological applications with long-lasting mechanical effectiveness. This review provides a more organized tabulated summary of the procedures, experimental method, circumstances, applications, and tribological characteristics of solid lubricants, lubricant additives, and bulk materials based on carbon nanomaterials.

The study has revealed the influence of carbon nanomaterials such as graphite, graphene, graphene-based coatings and diamond-like carbon (DLC)-based coatings on the superior tribological properties of solid lubricants. Among these, graphene-based solid lubricants provide the best topological properties. In comparison to the other solid lubricants investigated in the literature, MoWS₄/graphene heterogeneous composites offer the lowest frictional coefficient and wear rate.

Lubricant additives play a critical role in developing new properties or compensating for the drawbacks of a lubricant. Graphene, carbon nanotubes, fullerene, carbon nanodiamonds, carbon nano-onions, carbon nanohorns and carbon sphere-based lubricating additives were reviewed in detail. Nanomaterials such as graphene, carbon nanotubes, fullerenes, carbon nanodiamonds and other carbon nanostructures have greater surface areas, which allows them to cover larger areas of the tribosurface, resulting in reduced friction. Here, graphene is the most prominent and widely used nano lubricating additive. However, more recently, carbon nanodiamonds have become a major subject of research for their sharp particle size distribution, nontoxicity, chemical stability and reduced friction and wear rate.

In recent years, carbon nanomaterial-reinforced bulk material lubricants and have been used extensively in the field of tribology for various application to reduce friction and wear. Graphene, carbon nanotubes and carbon nanodiamond-reinforced bulk material lubricants were reviewed in this work in a comprehensive fashion. As a reinforced carbon nanomaterial, graphene significantly increases the lubricity and decreases friction and wear rate of bulk materials.

Graphene and CNTs have been proven to be excellent candidate nanomaterials for the superlubricity regime. Their impact on superlubricity has been outlined in a tabulated form along with their experimental methods, conditions and applications. The table indicates that graphene is a prominent contender to impart the superlubricity of a lubricant. Due to the success and prevalence of graphite as a solid lubricant in industry for centuries, carbon nanomaterials are at the center of current tribological research. Among them graphene is the most prominent and widely used carbon nanomaterial in all four regimes (solid lubricants, lubricating additives, bulk material reinforcement and superlubricity) discussed in this study. It is one of the thinnest and strongest 2D carbon nanomaterials and has outstanding friction and wear properties. The graphene concentration in lubricants may significantly change the coefficient of friction and wear rate. It was found that, for the same experimental method and condition, the coefficient of friction and wear rate vary according to the concentration of graphene oxide in water-based lubricant. The optimum value in terms of graphene oxide offers the minimum coefficient of friction and wear rate, and the optimum value is between the maximum limit and minimum limit in terms of graphene oxide concentration.

Future studies could focus on analyzing the tribological characteristics of biolubricants with added carbon nanoparticles. Additionally, it is possible to research how carbon nanolubricants affect real engines. According to literature, the majority of studies base their assessments of the lubricant’s tribological performance on a single type of nano-material that has been added (as additives or reinforcement) to the basic lubricants. But very little research has been done on how their combined impact affects tribologi-cal performance. The tribological characteristics of the resultant lubricants may be sig-nificantly influenced by the kinds, concentration, and processing technique of nano-particles with basic lubricants. It might be a useful direction for future research.

The international efforts to reduce carbon dioxide emissions continue to put pressures on different stakeholders including manufacturers across all transport modes with strong commitment to decarbonize this sector as a whole. Accordingly, McKinnon et. al (2015) summarized the three aspects through which vehicles can be improved and enhanced; increasing vehicle carrying capacity, improving energy efficiency and reducing externalities [236]. Energy efficiency improvements can come either from redesigning the vehicle to become more streamlined thus reducing air friction, or revamping the engine to consume less energy while at the sometime remain effective. On the hand, reducing vehicle externalities include carbon dioxide emissions abetment and noise problem amelioration. While it may require redesigning the vehicles to make them streamlined and therefore reduce the friction, it is not the case with the engine and other hydraulic parts, since the use of energy efficient lubricants not only significantly improves energy efficiency but also reduce CO₂ emissions [5].