**4. Nanofluids application fields**

Diverse studies on nanofluids have been carried out by many researchers. This section deals with literature reviews on nanofluids; nanofluids preparation and characterization; thermophysical, electrical, and tribological properties; as well as nanofluids applications, which lays foundation and basis for further investigations. Some of the main fields of application for these systems are thermal management and tribological, which are described in the following sections.

### **4.1. Thermal performance of nanofluids**

Heat transfer is classified into various mechanisms, such as thermal convection, thermal radiation, and thermal conduction. In diverse fields, thermal transport is a critical parameter to obtain efficient performance of components and devices. Heat convection occurs when bulk flow of a fluid (gas or liquid) carries heat along with the flow of matter in the fluid, this process could be "natural," by density differences in the fluid occurring due to temperature gradients, or "forced," where fluid motion is generated by an external source such as a pump, fan, or other mechanical means. Radiation heat transfer is the transfer of energy by means of photons in electromagnetic waves in much the same way as electromagnetic light waves transfer light. On the other hand, heat conduction is the direct microscopic exchange of kinetic energy of particles through the boundary between two systems. When an object is at a different tem‐ perature from another body or its surroundings, heat flows so that the body and the sur‐ roundings reach the same temperature, at which point they are in thermal equilibrium. The thermal conductivity (*k*) of liquids can be successfully measured if the time taken to measure *k* is very small so that the convection current does not develop [158]. Effective thermal conductivity (*k*eff) is described as the nanofluid thermal conductivity, compared to conven‐ tional fluid thermal conductivity. Diverse techniques have been proposed to measure nano‐ fluids thermal conductivity over the past years. The most common techniques to measure the effective thermal conductivity of nanofluids are the transient hot-wire method [15, 47, 99, 126, 158–161], steady-state method [35, 88, 105, 115, 162], cylindrical cell method [163], temperature oscillation method [62, 164–166], and 3-ω method [40, 167–169] to name a few.

graphene/DiW nanofluid, the enhancement is about 13.6% and 94.3% at 25 and 50°C, respec‐ tively. These high increments in thermal conductivity exhibited by the graphene-based nanofluids can be ascribed to the high aspect ratio of defect-free graphene sheets. Walvekar et al. [150] and Ding et al. [151] performed diverse studies on CNTs-water nanofluids, showing

Interfaces are ideal templates for assembling nanoparticles into 2D structures by the nature of the interfaces. At the interfaces, the nanoparticles are mobile and defects of the structures can be eliminated [152]. This ordered structure could have higher thermal conductivity than that of the conventional, therefore an enhancement of the effective thermal conductivity. However, some issues could be addressed when a surfactant or dispersant is used [66]. Interfacial layering refers to a phenomenon at the liquid–particle interface where liquid molecules are more ordered than those in the conventional liquid; therefore the interface effect could enhance the thermal conductivity by the layering of the liquid at the solid interface (giving that crystalline solids possess much better thermal transport that liquids) [45, 153], by which the atomic structure of the liquid layer is significantly more ordered than that of the conventional liquid. Various researchers have suggested that there is a liquid layering on the nanoparticles, which helps enhance the heat transfer properties of the nanofluid [151, 154–156]. Yu et al. [156] proved the formation of layers by the liquid molecules close to a solid surface, even though the thickness and thermal conductivity of the nanolayers are not well known yet. Ren et al. [157] found, through a theoretical model, that adding liquid layering on the nanoparticles an increase in layer thickness leads to higher thermal conductivity increment; as larger the size of the suspended particles, the weaker appear the effects of the nanolayer and the thermal

Diverse studies on nanofluids have been carried out by many researchers. This section deals with literature reviews on nanofluids; nanofluids preparation and characterization; thermophysical, electrical, and tribological properties; as well as nanofluids applications, which lays foundation and basis for further investigations. Some of the main fields of application for these systems are thermal management and tribological, which are described in the following

Heat transfer is classified into various mechanisms, such as thermal convection, thermal radiation, and thermal conduction. In diverse fields, thermal transport is a critical parameter to obtain efficient performance of components and devices. Heat convection occurs when bulk flow of a fluid (gas or liquid) carries heat along with the flow of matter in the fluid, this process could be "natural," by density differences in the fluid occurring due to temperature gradients, or "forced," where fluid motion is generated by an external source such as a pump, fan, or

that thermal conductivity is highly dependent on temperature as well.

*3.1.10. Interfacial layering on the liquid-nanostructure interface*

166 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

motion.

sections.

**4. Nanofluids application fields**

**4.1. Thermal performance of nanofluids**

Heat transfer fluids have been explored in diverse systems. Argonne National Lab research group with Eastman et al. reported a 40% enhancement with only 0.40 vol.% of copper oxide (CuO) particles of 10 nm in diameter [83], while Choi et al. reported a remarkable 160% increase in thermal conductivity of MWCNTs/engine oil nanofluid at 1.0 vol.% filler fraction of nanotubes [170]. Marquis et al. [19] reported a thermal conductivity enhancement of 45% at 1.0 vol.% concentration of highly pure SWCNTs in 15W-40 oil. In this same investigation, a remarkable enhancement of 175% with 1.0 vol.% of MWCNTs in poly-a-olefin (BP Amoco Ds-166) oil is obtained, similar to Choi found at 1.0 vol.% of nanotubes in oil [170]. Similarly,, other researchers have investigated the effect of CNTs in diverse fluids with aid of dispersants. For instance, Liu et al. [14] measured the thermal conductivities of nanofluids containing MWCNTs dispersed in EG and synthetic engine oil. The increase of thermal conductivity for MWCNT/EG at 1.0 vol% was ∼12.5%, meanwhile, for MWCNT/synthetic engine oil, an improvement of ∼9 and 30% for 1.0 and 2.0 vol.% filler fraction was observed. Wen et al. [66] investigated the effect of temperature on the thermal conductivity of MWCNTs/DiW (20–60 nm in diameter and micrometer size in length) nanofluids. In order to properly stabilize the MWCNTs within DiW, 20 wt.% of sodium dodecyl benzene sulfonate (SDBS) was added to all samples. At 0.84 vol.% filler fraction, thermal conductivity enhancements of ∼24 and ∼31% were achieved at 20 and 45°C, respectively. In general, an improvement on thermal conduc‐ tivity enhancement was observed as filler fraction and temperature increased. Assael et al. [78] studied the MWCNTs/DiW nanofluids with addition of 0.10 wt.% sodium dodecyl sulfate (SDS) as a dispersant. According to Assael et al., SDS would interact with MWCNTs, affecting their outer surface, enhancing interactions with DiW. It was found that at 0.60 vol.% MWCNTs, the enhancement in thermal conductivity was ∼38%. Hwang et al. [41] investigated the thermal conductivity of DiW- and EG-based nanofluids. MWCNTs (10–30 nm in diameter and 10–50 μm in length), CuO, and SiO2 (33 and 12 nm in diameter, respectively) were used. It was observed that thermal conductivity of nanofluids was improved almost linearly as filler fraction increased. For DiW-based systems, the addition of SiO2, CuO, and MWCNTs at 1.0 vol.% filler fraction showed an increase of 3, 5, and ∼12%, respectively. Also, CuO/EG nanofluid at 1.0 vol.% showed an increase of ∼9%.

Ding et al. [66] also investigated the effects of MWCNTs dispersed in DiW, with addition of 0.25 wt.% gum Arabic (GA) dispersant. For MWCNTs at 0.50 and 1.0 wt.%, an increase in thermal conductivity was achieved up to ∼30 and ∼38% at 25°C, and ∼35 and ∼80% at 30°C, respectively. It was found that these improvements were slightly higher than that results reported by Liu et al. [14], Wen et al.[151], Assael et al. [78], and Xie et al. [171], but lower than that showed by Choi et al. [170]. There are diverse factors that cause these discrepancies among the different groups; as mentioned by Wang et al. [138], these discrepancies should rely on the dependency of thermal conductivity is on diverse factors such as the structure and properties of the CNTs, aspect ratio, clustering, addition of dispersants, temperature and the experimental errors as well. Hong et al. [58] successfully developed stable and homogeneous nanolubricants and nanogreases based on CNTs in polyolefin oils. Thermal conductivity experiments showed an increment of 20% at 0.10 wt.% filler fraction; similarly, at 3.0 and 10 wt.%, thermal conduc‐ tivity increments were 50 and 80%, respectively. More recently, Walvekar et al. [150] analyzed the effect of CNTs on diverse temperatures ranging from 25 to 60°C. CNTs/DiW nanofluids were stabilized with the addition of GA as dispersant. Superb results showed improvements at diverse filler fractions, varying from 0.01 to 0.10 wt.%, and diverse temperatures, ranging from 25 to 60°C. A maximum thermal conductivity enhancement of ∼288% was shown for 1.0 wt.% CNTs/DiW nanofluids at 60°C.

Research on oxide nanoparticles have been conducted as well. Das et al. [62], for instance, showed strong temperature dependence of nanofluids with Al2O<sup>3</sup> and CuO particles as used by Lee et al. [144], which significantly improved the scope of nanofluids as an alternate for existing coolants. In 2005, Chon et al. [64] and Li et al. [172] confirmed this, but no temperature effect on thermal conductivity enhancement of nanofluids was observed in CNTs [173]. Li et al. [174] synthesized kerosene-based nanofluids with dispersed Cu nanoparticles (∼40–60 nm in diameter). Temperature dependence on thermal conductivity for Cu/kerosene-based nanofluids showed that as nanofluid temperature increases, thermal conductivities increased as well. For measurements at 25, 40, and 50°C, the effective thermal conductivity increased by ∼10, ∼13, and 15%, respectively, with 1.0 wt.% Cu nanoparticles. In other investigations, nanodiamonds (<10 nm) dispersed in EG (with addition of poly (glycidol) polymer) and MO (with addition of OA) were studied by Branson et al. [175]. It was observed that addition of 0.88 vol.% of nanodiamonds enhanced the thermal conductivity by ∼12%. In MO, for instance, with enhancements of ∼6 and ∼11%, filler fractions of 1.0 and 1.9 vol.% are achieved, respec‐ tively. According to Branson et al., the differences on enhancement efficiencies are attributable to divergence in thermal boundary resistance at nanoparticle/surfactant interfaces [175].

Several research studies have developed graphene-based nanofluids with high nanoparticle stability and significant enhancements [146, 176–183]. Shaikh et al. studied the effect of exfoliated graphite (2D sheets in micrometer size range) dispersed within poly-α-olefin (PAO) oil at various filler fractions, ranging from 0.10 to 1.0 vol.%. It was observed that addition of 2D structures improved the thermal conductivity from 18 to ∼130%, respectively [176]. Moreover, Yu et al. [180] investigated EG/graphene sheets (0.20–2.0 μm range, and 0.43 nm of interplanar distance), obtaining up to ∼86% increase in thermal conductivity with 5.0 vol.% concentration at 50°C. Hadadian et al. [184] prepared highly stable graphene oxide (GO)-based nanosheets. Thermal transport of EG increased by 30% with 0.07 GO mass fraction. Other EGbased nanofluids synthesized by Yu et al. [121, 180] have shown better enhancements of 61 and 86% with graphene oxide [121] and graphene nanosheets [180], respectively, at 5.0 vol.% loading. Similarly, a different study by Yu et al. [146] with graphene oxide nanosheets found enhancements of up to 30.2, 62.3, and 76.8% for distilled water, propyl glycol, and liquid paraffin (LP), respectively. Kole et al. [159] obtained a 15% in thermal conductivity with 0.395 vol.% exfoliated GnS dispersed in distilled water. Moreover, Aravind et al. [178] synthesized graphene and graphene-MWCNT composite nanoparticles and dispersed them in polar base fluids. Enhancements in thermal conductivity of de-ionized water of 9.2 and 10.5% were found for graphene and graphene-MWCNT, respectively. According to this study, a synergistic effect was found for graphene-MWNT additives; furthermore, MWNTs prevented restacking of graphene sheets.

Ding et al. [66] also investigated the effects of MWCNTs dispersed in DiW, with addition of 0.25 wt.% gum Arabic (GA) dispersant. For MWCNTs at 0.50 and 1.0 wt.%, an increase in thermal conductivity was achieved up to ∼30 and ∼38% at 25°C, and ∼35 and ∼80% at 30°C, respectively. It was found that these improvements were slightly higher than that results reported by Liu et al. [14], Wen et al.[151], Assael et al. [78], and Xie et al. [171], but lower than that showed by Choi et al. [170]. There are diverse factors that cause these discrepancies among the different groups; as mentioned by Wang et al. [138], these discrepancies should rely on the dependency of thermal conductivity is on diverse factors such as the structure and properties of the CNTs, aspect ratio, clustering, addition of dispersants, temperature and the experimental errors as well. Hong et al. [58] successfully developed stable and homogeneous nanolubricants and nanogreases based on CNTs in polyolefin oils. Thermal conductivity experiments showed an increment of 20% at 0.10 wt.% filler fraction; similarly, at 3.0 and 10 wt.%, thermal conduc‐ tivity increments were 50 and 80%, respectively. More recently, Walvekar et al. [150] analyzed the effect of CNTs on diverse temperatures ranging from 25 to 60°C. CNTs/DiW nanofluids were stabilized with the addition of GA as dispersant. Superb results showed improvements at diverse filler fractions, varying from 0.01 to 0.10 wt.%, and diverse temperatures, ranging from 25 to 60°C. A maximum thermal conductivity enhancement of ∼288% was shown for 1.0

168 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

Research on oxide nanoparticles have been conducted as well. Das et al. [62], for instance, showed strong temperature dependence of nanofluids with Al2O<sup>3</sup> and CuO particles as used by Lee et al. [144], which significantly improved the scope of nanofluids as an alternate for existing coolants. In 2005, Chon et al. [64] and Li et al. [172] confirmed this, but no temperature effect on thermal conductivity enhancement of nanofluids was observed in CNTs [173]. Li et al. [174] synthesized kerosene-based nanofluids with dispersed Cu nanoparticles (∼40–60 nm in diameter). Temperature dependence on thermal conductivity for Cu/kerosene-based nanofluids showed that as nanofluid temperature increases, thermal conductivities increased as well. For measurements at 25, 40, and 50°C, the effective thermal conductivity increased by ∼10, ∼13, and 15%, respectively, with 1.0 wt.% Cu nanoparticles. In other investigations, nanodiamonds (<10 nm) dispersed in EG (with addition of poly (glycidol) polymer) and MO (with addition of OA) were studied by Branson et al. [175]. It was observed that addition of 0.88 vol.% of nanodiamonds enhanced the thermal conductivity by ∼12%. In MO, for instance, with enhancements of ∼6 and ∼11%, filler fractions of 1.0 and 1.9 vol.% are achieved, respec‐ tively. According to Branson et al., the differences on enhancement efficiencies are attributable to divergence in thermal boundary resistance at nanoparticle/surfactant interfaces [175].

Several research studies have developed graphene-based nanofluids with high nanoparticle stability and significant enhancements [146, 176–183]. Shaikh et al. studied the effect of exfoliated graphite (2D sheets in micrometer size range) dispersed within poly-α-olefin (PAO) oil at various filler fractions, ranging from 0.10 to 1.0 vol.%. It was observed that addition of 2D structures improved the thermal conductivity from 18 to ∼130%, respectively [176]. Moreover, Yu et al. [180] investigated EG/graphene sheets (0.20–2.0 μm range, and 0.43 nm of interplanar distance), obtaining up to ∼86% increase in thermal conductivity with 5.0 vol.% concentration at 50°C. Hadadian et al. [184] prepared highly stable graphene oxide (GO)-based

wt.% CNTs/DiW nanofluids at 60°C.

Diverse theories explain the mechanisms that could affect the behavior of nanofluids; the most accepted being Brownian motion [40, 137–140], percolation theory [137, 138, 154, 173, 185], micro-convection cell model [137–140, 154, 185], and liquid layering theory [45, 137, 138, 153, 154, 185]. **Table 2** shows the influence of oil-based nanofluids on thermal conductivity. Similarly, **Table 3** shows the results from diverse investigations on other water-based nanofluids, and various materials and sizes used as reinforced nanoparticles. **Table 4** shows the influence of various nanofluids in thermal management properties, as well.



Notes: If not specified, measurements were conducted at room temperature. 1 With addition of dispersant (CH-5).

**Table 2.** Influence of oil-based nanofluids in thermal management.


**Filler Type of oil Nanoparticles**

**Graphite** Heat transfer oil<sup>1</sup>

**Exfoliated graphite**

With addition of dispersant (CH-5).

1

**morphology**

∼8–10 atomic layer thick

(LD320) at 30°C Spherical ∼10–30 nm diam. 0.34 vol.%

∼5 atomic layer thick

∼5 atomic layer thick

Diameter: 10–50 nm

Diameter: 10–30 nm

Diameter: 25 nm

Diameter: 10–50 nm

Diameter: 20–300 nm

Diameter: ∼25 nm

Diameter: ∼15 nm

Poly-α-olefin (PAO) 2D sheets ∼ μm range 0.10 vol.%

**Diamond** Mineral oil Spherical ∼<10 nm diam. 1.0 vol.%

**Graphene** Mineral oil (50°C) 2D sheets ∼500 by 500 nm

170 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

**h-BN** Mineral oil (50°C) 2D sheets ∼500 by 500 nm

**h-BN** Synthetic fluid 2D sheets ∼500 by 500 nm

**MWCNT** Engine oil (15W-40) Rods ∼ Length: 0.3–10 μm

**MWCNT** Mineral oil Rods ∼ Length: 10–50 μm

**MWCNT** Synthetic PAO oil Rods ∼ Length: 50 μm

**MWCNT** Synthetic engine oil Rods ∼ Length: μm range

**MWCNT** Poly-α-olefin (PAO) Rods ∼ Length: 1–100 μm

**MWCNT** Poly-α-olefin (PAO6) Rods ∼ Length: μm range

**CNTs** Poly-α-olefin (PAO) Rods ∼ Length: μm range

Notes: If not specified, measurements were conducted at room temperature.

**Table 2.** Influence of oil-based nanofluids in thermal management.

**Filler fraction**

1.9 vol.%

0.01 wt.% 0.10 wt.%

0.68 vol.% 1.36 vol %

0.01 wt.% 0.05 wt.% 0.10 wt.%

0.25 vol.% 0.5 vol.% 1.0 vol.%

0.10 wt.% 8%

0.5 vol.% ∼8.5%

1.0 vol.% 160%

1.0 vol.% ∼175%

1.0 vol.% 2.0 vol.%

0.04 vol.% 0.25 vol.% 0.34 vol.%

0.10 vol.% 0.60 vol.% 1.0 vol.%

0.60 vol.% 1.00 vol.% **TC**

∼5%

∼10%

∼5% ∼12% ∼36%

∼9% ∼10% ∼80%

∼10% ∼17% ∼45%

∼9%

∼9% ∼100% ∼200%

∼35% ∼96% ∼161%

∼18% ∼56% ∼130%

∼30% [14]

**enhancement**

∼11% [188]

∼80% [15]

**Ref.**

[126]

[15]

[49]

[19]

[76]

[170]

[19]

[189]

[176]

[176]


Notes: If not specified, measurements were conducted at room temperature. 1 With addition of cetrimonium bromide (CTAB).

**Table 3.** Influence of water-based nanofluids in thermal management.



Notes: If not specified, measurements were conducted at room temperature. 1 With addition of <1 vol.% of thioglycolic acid.

**Table 4.** Influence of diverse nanofluids in thermal management.

#### **4.2. Tribological performance**

**Filler Conventional fluid**

**CNTs** Water (RT ∼25°C)

Water (60°C) Water (60°C)

With addition of cetrimonium bromide (CTAB).

**MWCNTs** Water Rods ∼ Length: 30 μm

Notes: If not specified, measurements were conducted at room temperature.

**Al2O3** R141b refrigerant (20°C) Spherical ∼13 nm diam. 0.50 vol.%

**Al2O3** Ethylene glycol Spherical ∼28 nm diam. 5.0 vol.%

**AlN** Ethylene glycol Spherical ∼50 nm diam. 5.0 vol.%

**Cu** Toluene Spherical ∼ 40–60 nm diam. 1.0 wt.%

**Table 3.** Influence of water-based nanofluids in thermal management.

**Graphene & MWCNTs**

**Graphene nanoplatelets**

1

**Nanoparticles morphology**

**CuO** Water Spherical ∼ 25 nm diam. 0.10 vol.%

**TiO2** Water Spherical ∼15 nm diam. 1.0 vol.%

Water G sheets, 1 μm lateral;

172 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

Water (50°C) 4.0 vol.% ∼36%

**Graphene** Water Sheets, 1 μm lateral 0.40 vol.% ∼9% [178]

Water 2D sheets, ∼2 nm in thickness 0.01 wt.% ∼28%

**SiO2** Water Spherical ∼12 nm diam. 1.0 vol.% ∼3 % [76]

**Filler Conventional fluid Nanoparticles morphology Filler fraction TC enhancement Ref.**

**Al2O3** Ethylene glycol Spherical ∼10 nm diam. 5.0 vol.% ∼18 % [83] **Al2O3** Ethylene glycol Spherical ∼60 nm diam. 5.0 vol.% ∼30% [190]

**Au** Toluene Spherical ∼ 10–20 nm diam. 0.011 vol.% ∼9% [11] **Cu** Ethylene glycol<sup>1</sup> Spherical ∼ 10 nm diam. ∼0.30 vol.% ∼40% [83]

**Cu** Kerosene (@25°C) Spherical ∼ 40–60 nm diam. 1.0 wt.% 10% [174]

MWCNTs ∼19 nm diam.

Rods ∼ Length: 35 μm

Diam.: 20 nm

Diam.: 15 nm

**Filler fraction**

0.30 vol.%

5.0 vol.%

0.01 wt.% 0.10 wt.% 0.10 wt.%

2.0 vol.%

8.0 vol.%

10.0 vol.%

1.5 wt.%

0.40 vol.% ∼11%

**TC**

∼7%

∼18%

∼38% ∼126% ∼288%

1.0 vol.% ∼7% [171]

∼26%

∼25%

∼20%

∼12%

∼69% [17]

∼40% [115]

∼40% [195]

∼14% [174]

**Enhancement**

∼12% [194]

∼30% [104]

**Ref.**

[178]

[179]

[150]

Tribology is a science and technology that describes the interaction between surfaces and their relative movement, practices and materials associated, including friction, lubrication, and wear. Friction and wear are two major causes of energy and material losses in mechanical processes. Friction plays a crucial role in diverse processes such as drilling, cutting, working pair components and mechanisms, among others; a measurement for this property, which is becoming more relevant in today's life, is the COF (μ). Wear is a critical issue as components are in constant friction; a key measurement of the anti-wear properties of the lubricants and metal-cutting fluids is the WSD. Lubricants can be used to minimize contact friction between components, resulting in considerable energy and tooling savings [198]. The use of nanopar‐ ticles on lubricant nanofluids applications have the advantage of not to be temperature sensible and that tribo-chemical reactions are limited, compared to conventional additives [71, 199– 201]. A great advantage is that in cooling applications, there could be higher energy savings and less pollution emissions. Moreover, it is very important to mention that some of these used nanoparticles are environmental friendly.

**Figure 7.** Schematic diagram of the tribological mechanism of nanosheets as lubricant additives.

The addition of nanoparticles (or nanoadditives) has been shown excellent enhancements in tribological properties in numerous fluids. Moreover, diverse mechanisms by which dispersed nanoparticles in lubricants result in lower friction and wear have been shown in the literature. These mechanisms include (i) reacting with the surfaces creating a transferred solid lubricant film from nanoparticles under the contact pressure [202, 203], (ii) rolling of nanoparticles in the contact zone, where the nanoparticles serve as a third body, which decrease the contact between the asperities of the two mating surfaces [204], (iii) reducing asperity contact by filling the valleys of contacting surfaces [71, 205, 206], (iv) shearing of trapped nanoparticles at the interface without the formation of an adhered film [207], and (v) tribosinterization of nano‐ particles can occur on the wear surfaces forming a film which also prevent the direct contact of rubbing surfaces and reduce greatly the frictional force between the contacting surfaces [71, 208, 209]. As observed by Zhang et al. [204], a particular effect occurs when excessive concen‐ tration of nanostructures is added to nanofluids. A threshold is reached and even though with higher filler fraction there is an improvement in tribological properties, there is an optimal filler fraction where wear is minimized, as it is explained by the tribological mechanism depicted in **Figure 7**. During tribological evaluation (four-ball tribotesting), components are in sliding contact, nanosheets can form a protective layer on the surface of each steel ball at lower concentrations, which introduces the enhanced anti-wear performance. However, as the nanosheets loading exceed a critical value, the fluid film will become discontinuous, degrading the anti-wear properties, finally leading to a dry friction.

becoming more relevant in today's life, is the COF (μ). Wear is a critical issue as components are in constant friction; a key measurement of the anti-wear properties of the lubricants and metal-cutting fluids is the WSD. Lubricants can be used to minimize contact friction between components, resulting in considerable energy and tooling savings [198]. The use of nanopar‐ ticles on lubricant nanofluids applications have the advantage of not to be temperature sensible and that tribo-chemical reactions are limited, compared to conventional additives [71, 199– 201]. A great advantage is that in cooling applications, there could be higher energy savings and less pollution emissions. Moreover, it is very important to mention that some of these used

**Figure 7.** Schematic diagram of the tribological mechanism of nanosheets as lubricant additives.

The addition of nanoparticles (or nanoadditives) has been shown excellent enhancements in tribological properties in numerous fluids. Moreover, diverse mechanisms by which dispersed nanoparticles in lubricants result in lower friction and wear have been shown in the literature. These mechanisms include (i) reacting with the surfaces creating a transferred solid lubricant film from nanoparticles under the contact pressure [202, 203], (ii) rolling of nanoparticles in the contact zone, where the nanoparticles serve as a third body, which decrease the contact between the asperities of the two mating surfaces [204], (iii) reducing asperity contact by filling the valleys of contacting surfaces [71, 205, 206], (iv) shearing of trapped nanoparticles at the interface without the formation of an adhered film [207], and (v) tribosinterization of nano‐ particles can occur on the wear surfaces forming a film which also prevent the direct contact of rubbing surfaces and reduce greatly the frictional force between the contacting surfaces [71, 208, 209]. As observed by Zhang et al. [204], a particular effect occurs when excessive concen‐

nanoparticles are environmental friendly.

174 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

Eswaraiah et al. [210] synthesized ultrafine graphene thru solar radiation exfoliation techni‐ ques (sheets ∼300 nm by <2 nm thick). Nanofluids from these material and motor oil showed a decrease in COF of 80%, compared to base oil. This increase was attributed to the graphene tribological mechanism, which acted as nano-bearing within the oil, as well as for its excellent mechanical properties. It was explained by Hernández-Battez et al. [71] that nanoparticles could react with the surfaces, forming antifriction compounds and deposit on the wear surfaces by tribosinterization [71]. Moreover, Yu et al. [211] reported improved lubricating properties by adding 0.2 wt.% Cu nanoparticles to lubricant oil; in their study, Cu formed a soft film by friction-shearing and high pressure, reducing the COF up to 20%. As described by Peng et al. [212] during the friction process, a lubricating film of the nanoparticles is formed between the rubbed faces. The nanoparticles in the film not only bear the load but also separate the rubbing faces, dominating the reduction in the wear and friction.

Recently, Hu et al. [213] investigated the effects of MoS2 nanosheets (30–70 nm in thickness) dispersed in LP. Average COF for 0.5 wt.% filler fraction of MoS2/LP was reduced ∼60%, as well as WSD, which was reduced ∼8%, compared to pure LP. The anti-wear properties of the base fluid with MoS2 nanosheets were improved remarkably by increasing the MoS2 concen‐ tration up to 0.5 wt.%. According to Hu et al., due to the dimension and surface effect, it is ascribed that MoS2 could enter into the gap of the friction pair, functioning as lubricator. Wu et al. [214] studied the effects of 2D nanosheets of MoS2 with addition of 1.0 wt.% of span-80 (sorbitant monooleate) as a surfactant in LP. Results were also compared with MoS2 micro‐ particles (3–5 μm in diameter). It was shown that COF was reduced by ∼18% at 1.5 wt.% MoS2/LP; furthermore, the COF of nanosheets were lower and more stable than that of microparticles due to the surface area effect [215]. As explained by Wu et al., the lubrication mechanism of layered 2D-nanosheets of MoS2 was associated with the shearing of the weak Van der Waals bonds between molecular layers. When MoS2 is used as an additive in base oils, besides molecules of base oils, MoS2 powder is also adsorbed on the surface of substrates. Then the adsorbed MoS2 is burnished and forms stable films, which can endure high loads and improve tribological performances of the base oil. Therefore, with addition of MoS2 particles, the COF of base oil is reduced significantly. Similarly, Kao et al. [216] used TiO2 nanoparticles as additives in paraffin oil to reduce the friction between cast iron components. Tribological studies revealed an enhancement of ∼24% in COF at 60°C; it was concluded that spherical nanoparticles provide good rolling to reduce friction between two parallel specimens, as nanoparticles could fill rough cracks in a metal wall surface to reduce the COF.

For the protection of mechanical components, from friction and wear in aerospace, automotive, military, and various industrial applications, an efficient lubricant is demanded; graphene is a widely known material for this purpose. More recently, BN has attracted attention, since it has similar properties as graphene. Among diverse applications, BN could improve lubricity properties of composites under friction or wear applications as well. Zhang et al. [217] and Saito et al. [218], for instance, have observed a decrease in COF with increasing temperature in composites containing BN and have attributed this to the lubricating nature of BN. Silver (Ag) is also used due to its relatively larger coefficient of diffusion and its nature to form low shearing stress junctions at sliding interfaces, resulting in good lubrication. However, h-BN high thermal stability, good chemical inertness, and high thermal conductivity, makes it suitable candidate to be a "clean" lubricant [217]. There are studies available in the literature on the coatings prepared by addition of h-BN. Leon et al. reported that Ni–P–hBN autocatalytic composite coating with 35 vol.% hBN sliding against steel ball at room temperature had a COF of ∼0.2 [219], while steel on steel COF is ∼0.8. Avril et al. reported that laser melting hBN/α-Fe(Cr) coating showed lower COF and better wear resistance than untreated steels under dry sliding within a temperature range of 25–500°C [220]. Spikes [221] stated that the most promising 2D sheet structures are carbon-based graphitic materials and the inorganic fullerenes. These showed low friction in boundary lubrication conditions in laboratory tests. Hence, for metal-mechanic and oil industry which deals with drilling, cutting, or other friction characteristics with working tools, this research will be suitable. **Table 5** shows the influence of 2D nanoparticles on tribological applications with COF and wear performance.



Notes: If not specified, measurements were conducted at room temperature. 1 Oleic acid was added ∼5 vol.%. 2 On steel/440C systems. 3 On titanium/440C pairs.

**Table 5.** Influence of nanofluids (oil-based) in tribological applications.
