**5. Two-dimensional nanostructures**

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

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

of 2D nanoparticles on tribological applications with COF and wear performance.

**CuO** Poly-α-olefin oil Spherical ∼30–50 nm diam. 2.0 wt.% ∼54% – [71]

**CuO** API-SF engine oil Spherical ∼5 nm diam. ∼0.10 wt.% ∼20 % ∼17% [222] **CuO** SAE30 oil Spherical ∼5 nm diam. ∼0.10 wt.% ∼6% ∼79% [222] **Diamond** API-SF engine oil Spherical ∼10 nm diam. <0.10 wt.% ∼4% ∼43% [222] **Graphene** Engine oil Sheets ∼300 nm, <2 nm thick 0.025 wt.% ∼80% ∼33% [210]

**MoS2** Graphene solid lubricant Flakes (10–20 nm) 7–10 wt.% ∼23% ∼56% [223] **Graphene** Ionic liquid (IL) Sheets (monolayer 3.819 nm) 23 wt.% ∼56% ∼94% [224] **MoS2** PAO Nano sheets (5–10 nm thick) 5.0 wt.% ∼30–40 % ∼75% [225] **h-BN** CIMFLO 20 oil<sup>2</sup> Spherical ∼70 nm diam. 1.0 wt.% – ∼55% [205] **MoS2** CIMFLO 20 oil<sup>2</sup> Spherical ∼70–100 nm diam. 4.0 wt.% ∼10%<sup>2</sup> ∼65% [205]

Spherical ∼20 nm diam.

**Filler fraction**

0.20 wt. %

0.06 wt.% 1.00 wt.% **COF decrease**

∼5% ∼10% ∼20%

∼17% ∼13% ∼12% **Wear decrease**

∼13% ∼21% ∼23%

∼9% ∼14% ∼12% **Ref.**

[211]

[204]

**morphology**

**Graphene** Poly-α-olefin oil<sup>1</sup> Sheets ∼ μm size 0.02 wt.%

**Filler Type of Oil Nanoparticles**

**CuO** Poly-α-olefin (25°C)

Poly-α-olefin (80°C) Poly-α-olefin (140°C) Initial studies on 1-D nanostructures got immediate attention soon after the landmark paper by Iijima [4] on CNTs in 1991 and various types of organic-inorganic 1-D nanostructures were realized thereafter [226]. More recent advances in layered materials enable large-scale synthe‐ sis of various two-dimensional (2D) materials [5, 15, 108, 210, 227, 228], where atoms are arranged in flat layers, which can be stacked on top of each other. One of the most common naturally layered materials is graphene, which has been widely studied for its superb prop‐ erties and applications in diverse fields. 2D materials can be good choices as nanofillers in heat transfer fluids, as they have high surface area available for heat transfer.

A common production route of these layered nanostructures is exfoliation, where material individual layers are separate out from each other, either chemically or mechanically (i.e., abrasion) [15, 49, 108, 229–231]. It is important to mention that even though exfoliation can be achieved mechanically on a small scale [229, 230], liquid phase methods are required for diverse applications such as nanoelectronics, micro-electromechanical systems (MEMS)/ nanoelectromechanical systems (NEMS), chemical and pressure sensors, etc. [231]. Another possible route to obtain these 2D structures is by direct chemical growth of individual layers (i.e., graphene sheets) through chemical vapor deposition (CVD) technique on the surface of a metals catalyst (i.e., copper, silica) by heating at high temperatures (∼600–1200°C) and passing a carbon-containing gas such as methane over the catalyst [227]. A breakthrough research by Coleman et al. [108] showed that they were able to synthesize diverse 2D materials (MoS2 WS2, BN, Bi2Te3, MoSe2, MoTe2, NiTe2, etc.) by wet exfoliation technique. Exfoliation of 2D insulators such as Bi2Te3, Bi2Se3, and h-BN would reduce its residual bulk conductance, highlighting surface effects. Another important aspect is that changes in electronic properties, as the number of layers is reduced as expected [108, 232]. This class of materials represents a diverse and largely unexploited source of 2D systems with interesting physic-mechanical and electrical properties, with high specific surface areas that is important for sensing, catalysis, and energy storage applications [108]. Hence, like graphene [5, 233], layered materials must be exfoliated to achieve their full potential.
