**2.13 MXenes**

The conflict between people and energy has grown increasingly acute since the dawn of the twenty-first century, and humanity now faces a significant energy crisis as a result of high population growth and extensive fossil fuel exploitation. Every year, 20% of the world's energy is used to overcome friction, and it is possible to decrease this energy loss by using new system design to create innovative materials, efficient lubricants [124–126]. For the energy crisis to be slowed and energy loss to be minimised, an efficient lubricant is essential. MXenes are carbonitrides, nitrides, or carbides of transition metals [127]. Early transition metals are denoted by M (such as Ti, V, Zr, Ta, V, Mo, orNb), X (such as C and N), and Tx (such as surface functional groups like -O,-F, and-OH) [91, 128, 129] (**Figure 8a**). Different MXene

### **Figure 8.**

*(a) element compositions of MAX and MXene, (b) the preparation process of MXene, and (c) SEM image of multilayer Ti3C2TxMXene [130].*

#### *Thermal Characteristics and Tribological Performances of Solid Lubricants: A Mini Review DOI: http://dx.doi.org/10.5772/intechopen.109982*

surface terminations are produced by various synthetic techniques. MXenes are produced from MAX phase via HF etching [131]. As shown in **Figure 8b**, etching techniques [132] produce multilayer MXenes by selectively removing A-atom layers. A SEM image of Ti3C2TxMXene in accordion form is shown in **Figure 8c**. Single-layer MXenes may be created using intercalation and delamination [133]. The terminations of HF-etched MXenes are -F, -OH, and -O. Fluorine-free MXenes have been produced using Fenton methods, electrochemical, and hot alkali [134]. MXenes have applications in many different industries, such as biology, catalysis, sensors, electronics, electromagnetic, and tribology, and energy storage. MXenes can travel between layers when under pressure because they have weak interlayer connections [135–137]. Because MXenes have a large specific surface area, it is simpler to create lubricating or transfer films, which reduce wear and friction. Their voluminous surface groups facilitate control and modification while increasing polymer affinity.

MXenes are possible lubricant choices in a variety of tribological applications due to simplicity of modification, easily film manufacturing, having adjacent layers with low shear strength and improved interaction with polymeric matrixes. Element compositions of MAX and MXene is shown in **Figure 9a**. The growth rate of the tribology literature reports on MXenes has significantly outpaced that of MXenes in

#### **Figure 9.**

*(a) applications of MXenes, (b) statastics of MXenes publications from 2015 to 2021, and (c) the development of MXenes in tribology [130].*

2021, as shown in **Figure 9b**. These reports continue to rise every year. As lubricants, MXenes are underappreciated. The development of MXenes' tribology is summarised in **Figure 9c**. Ti3C2Tx, the first MXene, was discovered in 2011 [127], although it wasn't employed in liquid lubrication until 2014 [138].

In 2016 [39], Ti3C2Tx MXene was used for the first time as a polymer reinforcement in solid lubrication. The composites demonstrated exceptional tribological and mechanical properties. MXene was sprayed on copper discs as a solid lubricant in 2018 [40]. Recent years have seen a rise in MXene research. MXenes attained superlubricity in both solid and liquid forms in 2021 [41–43]. MXene is a fantastic lubricant, according to a recent research on friction. A thorough review article is required to comprehend the present state and issues with MXene-based lubricants since no focused study consistently summarises and analyses MXenes in the course of tribology research. From mechanical behaviour to simulation findings, this paper explains the lubricating potential of MXenes before summarising, analysing, and contrasting their tribological characteristics. Solid lubricants, lubricant additives, and reinforcing phases are the three tribological uses of MXenes (**Figure 10**). A summary of previous studies on MXenes and MXenes-based composites is provided, including information on preparation techniques, the characterisation of the materials, friction and wear tests, and lubrication mechanisms. Applications for MXenes in tribology encounter several challenges. We propose workable solutions and future research prospects for MXenes based on the research state and barriers. This paper provides a thorough summary of the state of the MXenes lubrication research, closes a gap in MXenes tribology, suggests research directions based on unsolved issues and real-world applications, and stimulates MXenes research.

Concluding the above discussed MXenes as solid lubricants, it can be demonstrated that although these have shown greater lubricity in solid lubrication, it needs a lengthy break-in time. Fast superlubricity is achieved in GO, and attempts

**Figure 10.** *A categorisation of MXenes' tribological applications [130].*

*Thermal Characteristics and Tribological Performances of Solid Lubricants: A Mini Review DOI: http://dx.doi.org/10.5772/intechopen.109982*

are required to be taken to accomplish the same in MXene. In addition, MXene's superlubricity technology is in its infancy, and its lubrication processes have not yet been confirmed, necessitating more research. MXene-based lubricants have scope in various industries, such as, spacecraft components, automotive industries, NEMS, machining industries, etc. These lubricants can decrease the environmental pollution and energy consumption significantly. These lubricants are expected to cope-up with the energy crisis and to achieve sustainable development.

#### **3. Solid lubrication mechanism**

In the absence of any liquid, gas, or lubricant, there is a significant amount of adhesion between two solid surfaces that are in perfect condition (ultra-high vacuum or new metallic surface). Chemical or physical forces can adhere [15]. In sliding and spinning equipment, adhesion leads to wear and friction. When exposed to extreme conditions such as high velocity, load and temperature, strong adhesion at tribostressing surfaces results in scuffing, cold welding, friction damage, or even disintegration. Adhesion is affected by surface characteristics such cleanliness, temperature, velocity, atmosphere, contact time, and loadas well as interface factors like mutual solubility, crystallographic orientation, chemical activity, and charge separation [15]. Components are exposed to severe wear and fatigue, high velocity, and high mechanical dynamic and thermal loads due to plastic deformation during hot metal forming [4]. During forming at high temperatures, the proper lubrication is required for decreasing the friction stresses, and avoiding seizures and galling. Tool wear occurs during high-speed dry machining due to abrasive, chemical, adhesive, and electrochemical wear [4, 5, 15]. Pantograph contact strips [4] deteriorate from arc discharge attack-induced adhesion and high-temperature mechanical impact. For casting thinstrip steel at high temperatures, refractory side dams withstand mechanical stress, corrosion, wear, and thermal shock. The lubrication mechanism differs with temperature variation under the application of friction force, which can be understood taking the solid-lubricated Ni3Al composite [4]. **Figure 11** shows different lubrication mechanism over wide range of temperature under sliding conditions.

The gas turbines must have abradable seals in compressor and turbine sections that can withstand abrasion, thermal corrosion, fatigue wear, and blistering. Extreme adhesive wear, also known as scuffing or smearing, and fatigue spalling during

#### **Figure 11.**

*The lubricating mechanism of solid lubricated Ni3Al matrix over wide temperature range [4].*

cyclic contact strain are the two main causes of rolling-contact bearing failure [139]. Significant anisotropy is seen in lamellar solids with poor interplanar cohesion. Cleavage occurs at low shear stresses and lowers sliding friction in materials with anisotropic mechanical properties. These lamellae self-lubricate as a result of crystallographic slip under light shear pressures [140]. Traditional solid lubricants (MoS2, graphite, etc.) have layered structures that are simple to shear and provide selflubricity; however, they lose their effectiveness at high temperatures (MoS2at 350°C and graphite at 450°C) due to structural degradations brought on by oxidation. When applied as a thin coating on rubbing surfaces, chemically stable fluorides and nonlamellar soft solids like indium, lead, tin, silver, gold can lessen wear and friction.

The second solid lubrication method entails the constant, ongoing formation of soft solid layers while sliding. Although the hard substrate controls the contact area, the thin solid sheet dictates the shear strength of contacting asperities. The contact area and asperity shear strength determine the frictional force in these circumstances. The contact surface between efficient soft solid lubricants develops a strongly adherent transfer layer after a limited run-in period. While thin lead transfer films offer sliding lead-based alloys self-lubrication, the composite coatings of plasma-sprayed Ni80Cr20-Cr2O3-Ag-CaF2/BaF2with fluoride and silver eutectic components have a low shear strength layer (PS3O4) materials for low-friction bearings and seals [1, 7, 15]. For sliding and rolling contact components, soft films and lamellar solids are frequently used. To create lubricious tribo-chemical coatings, intermetallics, metals, and ceramics react with water vapour or air. Metal or ceramic surfaces are kept out of direct touch by these tribo-chemically reacted coatings' low friction and shear strength. Vanadium or chromium as alloying elements produce tenacious and lubricious oxide layers in nitride or metal coatings that minimise friction at high temperatures [2].

Extremely high-temperature lubrication may be offered by microstructurally designed thermally stable oxides, particularly in oxidising conditions. Seven lubrication methods exist for lubricious oxides. (1) The ability to shear readily due to the crystal-chemical concept of cation screening by surrounding anions [9, 141]; (2) Oxides soften in the temperature range of 0.4–0.7 Tm (K), which is the ductile-tobrittle transition temperature. Upon meeting the operating temperature to a critical temperature for typical soft oxides, the lubricious behaviour is contributed for by material softening and plastic smearing [142]; and (3) The process of viscous flow originating from very thin liquid films, such as glass lubrication in hot metal formation, is analogous to the low friction characteristic of melting wear for the oxides by surpassing Tm [79, 103, 140].

Environmentally assisted oxidation results in the formation of ternary oxides with low melting points such as tantalates, tungstates, vanadates, niobates, and, silver molybdates, which get easily sheared. The nitride hard coatings follows a self-adaptation process through the lubricious oxides formation at high temperatures with the vanadium addition to it, due to which the wear resistance increases and these coatings can be application for metal cutting applications [73]. Lubricants made of polycrystalline oxide with nanometre-sized grains may become more ductile. The grain rotation and grain boundary sliding at low temperatures results in deforming the nano-sized oxides during tribo-stressing. On rubbing surfaces, adaptable and the introduced/created lubricious oxide nanofilms may show significant viscous flow and plastic deformation to increase the operating temperature range [143, 144].

From the previous research work, it can be concluded that for sliding friction the friction force has two contributing sources. First, an adhesive force is created at the actual contact area between the surfaces (the asperity junction). Second, a force of

#### *Thermal Characteristics and Tribological Performances of Solid Lubricants: A Mini Review DOI: http://dx.doi.org/10.5772/intechopen.109982*

deformation is required to plough or slice the asperities of the harder surface through the softer. The resultant friction force is equal to the total of the two contributing sources: adhesion friction and deformation and/or fracture friction. The adhesion is caused by the attractive forces between the contacting surfaces. This model provides a foundation for comprehending how thin surface layers can reduce friction and provide lubrication. However, it should be understood that one of the contributing sources frequently influences the other. In other words, the two sources cannot be considered entirely separate. Also, Solid lubrication is accomplished by introducing a solid or self-lubricating substance with low shear strength and strong wear resistance between the interacting surfaces in relative motion. To reduce the coefficient of friction, the shear strength of the interface, the surface energy, the real area of contact, and the ploughing or cutting contribution must be minimised. In general, reducing wear requires limiting these characteristics while enhancing the hardness, strength, and toughness of interacting materials. In order to lower the coefficient of friction and wear rate of materials, surface design and engineering of solid lubricants can be implemented.
