Reciprocating movement of pistons inside engine cylinders, rotation of bearings and crankshafts, walking or running on tracks/roads, engaging and disengaging of brakes and clutches etc., are affected by a phenomenon called friction. Friction is the force which tries to resist motion of one body sliding over another. The classic law of friction was discovered back in 15th century by Leonardo da Vinci, even though it was not published then. It was Guillaume Amontons who rediscovered the laws in 1699. The laws were called Amonton’s laws of dry friction, which summarily stated that the force of friction (F) is directly proportional to the applied normal load (N) or that the coefficient of friction (μ) is directly proportional to the friction force and inversely proportional to the normal load.

Ever since then, research has been on-going on friction and tribology. Tribology is the science of friction, wear and lubrication. Light metals such as Al, Ti and Mg are applied in aerospace, automotive, agricultural and electrical industries because of their low density and their high strength per unit weight, besides other excellent properties. However, they are easily eroded and worn away by friction and wear which undermine their utility and efficiency in service. However, this anomaly can be augmented by introducing reinforcement which improves their tribological properties. Polymers, on the other hand, are more resistant to chemicals, are cost effective and require no post-production treatment like metals do, but they are weaker to friction and wear. So, to eliminate this challenge and improve durability and efficiency in service, they require some reinforcement with nanomaterials. Ceramic materials have high hardness, good thermal stability and high wear resistance, but they are very brittle. So, to improve their fracture toughness, they are reinforced with high-strength and high-modulus nanoparticles or nanomaterials. Lubrication can be referred to the practice of applying oil, grease or nanomaterial on a surface in order to minimize friction and wear. Among all the known nanoparticles/nanomaterials used in lubrication and improving wear resistance, strength and fracture toughness of materials, carbon nanotubes (CNTs) stand out ^{[1][2][3][4][5]}[1,2,3,4,5].

It was in 1991 that Iijima discovered, for the first time, a particular carbon with a structural configuration that looked like a tubule [6]. It consisted of some tens of graphite layers (walls) that were later identified as multi-walled carbon nanotubes (MWCNTs) or simply (CNTs) (Figure 1). Successive walls of the tubule are 0.34 nm apart, with diameter of 1 nm and massive aspect ratio. When graphite exists in 2 dimensional forms, it is called graphene (Figure 1a). Two years on, Iijima and his co-workers developed single-walled carbon nanotubes (SWCNTs) (Figure 1b), which were formed when graphene was rolled into a seamless cylindrical tube [7]. Carbon nanotubes (CNTs) have been recognized as the stiffest material (bequeathed by its SP² hybridization) which buckles elastically under massive bending or compressive stress. It possesses huge tensile strength of about 100 GPa and exceptional elastic modulus of 1.27 TPa, its thermal conductivity ranges from 3000–6000 W/mK, and its current density is about 1015 A/m², with a conductance of 13 kᲲ^{−1 [6][7][8][9][10]}[6,7,8,9,10].

These exceptional characteristics of CNTs have provoked the interests of researchers on this material. The production techniques of CNTs are manifold, and they include: electric arc discharge, laser ablation, vaporization induced by a solar beam, and chemical vapor deposition (CVD) ^[11][12][13][11,12,13]. Consequent upon the essential properties of CNTs, they have been applied as a reinforcement for metal, polymer, ceramic and carbon matrices. Based on its mechanical-cum-tribological characteristics in metal matrices, Park, Keum [14] reported three types of strengthening mechanisms in CNTs-reinforced composites, which included Orowan looping, thermal mismatch and load transfer kinetics. Orowan looping occurs during plastic deformation and is as a result of dislocation of atoms in the crystals. Thermal mismatch occurs when there is a wide difference between the coefficients of thermal expansion (CTE) of the matrix and reinforcement, just like in CNTs (2.1 × 10⁻⁶ K⁻¹) and Al (2.36 × 10⁻⁵ K⁻¹) [15]. In this case, thermal mismatch raises the dislocation density through strain hardening which culminates in the strengthening of the composite [16]. More so, CNTs can efficiently absorb the load incident on its base material through the dynamic load transfer mechanism, thereby raising the load bearing capacity of the composite system. Among the three mechanisms, it was reported that load transfer kinetics contributed more than 50% of the improvements [14].

Nonetheless, there is a relationship between the mechanical strength and tribology of composite systems. It was in 1953 that Archard [17] propounded that the wear volume loss (V) of a material is inversely proportional to its hardness (H) and directly proportional to the sliding distance (S) and normal load (L). So, it can be inferred that materials with high strength (hardness) experience lower wear loss than those with low strength and vice versa. Meanwhile, the tribology of a composite system entails: the interaction of the surface with another surface when they are in a relative motion; the coefficient of friction (COF) of the system; the wear rate and wear loss of the system; and the lubrication properties of the system. Therefore, since it has been established that CNTs reinforcement improves the mechanical properties of composites, it is logical to assert that it improves the tribological properties of materials too. In addition, it has been shown that CNTs is a good solid lubricant [18]. Hence, incorporation of CNTs into a base material not only reduces the wear volume loss of the material but reduces the COF of the system which is promoted by the decrease in both the adhesive and deformation components of its coefficient [19]. Metal matrices reinforced with CNTs have exhibited excellent tribological properties, and this has caused the expansion of its engineering applications, industrial utility, automotive usage, aerospace, electrical and electronics applications ^[20][21][20,21]. In an experiment to investigate the tribological properties of Nickel-CNTs composite, it was discovered that 2 vol.% of CNTs gave the least COF of 0.22. When the volume fraction was increased to 5%, the COF increased, and other properties depreciated, because there was agglomeration of the reinforcement in the microstructure [22]. It was shown that the COF of Ni-2CNTs composite improved by 332% in comparison with monolithic Ni. The information herein is that optimizing the volume fraction of reinforcement is of paramount importance. This is because the high aspect ratio of CNTs provokes its agglomeration. So, once the optimal volume fraction is exceeded, homogenous dispersion becomes difficult. The COF improvements of different metals, ceramics and a number of polymers reinforced with CNTs are shown in Table 1. It can be observed in Table 1 that ceramics accommodated a higher volume fraction of CNTs than metals and polymers. A small volume fraction of CNTs (0.7 wt.%) improved the COF of polymers optimally. For metals, 4 wt.%–5 wt.% produced the optimal COF. This trend was equally observed by Watanabe et al. [23] who posited that ceramics are more hydrophilic than polymers due to their surface roughness. So, the degree of hydrophilicity of base materials controls the volume fraction of reinforcements that can be absorbed. Ceramics possess the highest hydrophilicity, followed by metals and then polymers.

Furthermore, when CNTs reinforcement is incorporated into polymers, strong interfacial bonding between the nano-carbon chains and polymer matrices are achieved, and this increases the modulus of polymer composites and expands its industrial, engineering, pharmaceutical and agricultural applications [41]. There was a report that there exists strong interfacial adsorption energy in polymer-CNTs reactivity, which helps to bond the polymer chains on the surface of CNTs stoutly, thereby enhancing the strength of the CNTs-polymer composite [41]. Campo et al. [42] studied the effect of CNTs addition into epoxy matrix composite. The results showed that addition of 0.5 wt.% CNTs into epoxy matrix reduced the mass loss by 900%, decreased the wear rate by 1367%, and decreased the COF by 275%. However, the improvements were more pronounced when the CNTs were functionalized, as functionalization decreased the Van der Wall forces and hydrogen bonding in CNTs molecules. Wang et al. [43] studied the tribological behavior of plasma-sprayed carbon nanotubes-reinforced TiO₂ coatings and discovered that the coating that had CNTs reinforcement exhibited about a 93.6% decrease in wear volume with a moderate decrease in the COF when compared with unreinforced TiO₂. By and large, CNTs reinforcement has proven to be a good tribological nanomaterial for composite development. Reports on the tribological properties of CNTs-reinforced composites are scant in the literature [44]. Singh and Beloka [45] reported Al- and Mg-based composites reinforced with CNTs; Chan et al. [46] studied the effect of nanofillers on polymer composites; Goswami et al. [47] concentrated on mechanical and tribological properties of CNTs/graphene-reinforced alumina, while Choudhary et al. [48] dwelt more on the effect of processing route, processing parameters and CNTs length on the properties of CNTs-reinforced composites.

Two factors that are responsible for high strength of materials reinforced with CNTs are as follows: (a) the interlocking carbon-to-carbon covalent bonds of CNTs providing strong metallurgical bond to the composite and increasing its dislocation density. (b) The carbon nanotube existing as one large molecule which does not possess grain boundaries at all, let alone weak ones that separate crystal grains of high strength materials such as steel or tungsten [49]. Hence, there is no danger of grain boundary weakness or failure. Figure 2 gives a schematic idealization of a CNTs-reinforced composite system. As shown in Figure 2, each particle of the matrix is actively held together or bonded by the CNTs molecules making it difficult to be broken without application of enormous force. So, the SP² covalent bond as well as single unit molecule of CNTs account for the exceptional strength of CNTs-based composite materials.

Researchers have studied the traditional fibers such as carbon-, glass- and Kevlar- based composites via computational and empirical techniques. Energy densities, ballistic impact capacity, wear and penetration resistances of these traditional fiber-based composites were investigated. Challenging issues such as delamination, compressive failure in the fiber/matrix interface, matrix cracking, and tensile failure of yarns were all identified as the principal failure mechanisms after characterization ^[50][51][52][50,51,52]. However, it was observed that failures in these conventional composites occur within the range of a micrometer. This implies that reinforcement based on nanomaterials such as CNTs can withstand such failures. Hence, they can improve the mechanical/tribological properties of nanocomposites under steady and repeated loading and prevent such failures that occur at the micrometer scale. So, conventional composites are now being replaced by nanocomposites and hybrid composites in most advanced applications, due to their excellent mechanical, tribological, and thermal properties [53]. Improvement in strength and energy absorption of CNTs-reinforced polymer/glass composites has been reported [54]. Also reported is the improvement in tribological, thermal, corrosion and mechanical properties of CNTs-reinforced metal/ceramics matrix composites ^[55][56][57][55,56,57]. It is because of the excellent mechanical and tribological properties, thermal and electrical conductivities that carbon-based nanofillers such as CNTs and graphene are employed in the development of nanocomposites ^[58][59][58,59]. It was reported that CNTs-reinforced polymers nanocomposites are presently being used in the place of steel in the development of bullet-proof shields, jackets, wear resistant surfaces, shock and impact absorbers, and ballistic impact-resistant materials ^[60][61][60,61], and they are much cheaper, less dense, more corrosion resistant and more robust than the conventional steel-based military armors and shields. CNTs-reinforced metal matrix composites, on the other hand, are used in the development of aerospace components, automotive piston and sleeves, nozzles, turbine blades, transmission conductors, electronic sensors and transmitters ^{[20][62][63][64]}[20,62,63,64], as they outperform monolithic Al, Ti, Mg or steel alloys. Fracture toughness is one of the principal properties imparted on ceramic matrices by CNTs. Hence, CNTs-reinforced ceramic composites are used in the development of high temperature nozzles and turbines, cutting and drilling tools, blades of lathes, pulverizers etc. ^[65][66][65,66], and they are cheaper, less dense and more robust than tungsten, diamond and steel alloys that were the traditional materials for these equipment.

The production techniques of CNTs-reinforced nanocomposites are manifold. They can be grouped based on the similarity of processes involved. These groups include (i) powder metallurgy, (ii) electrochemical techniques, (iii) casting method, (iv) thermal spraying and (v) other novel techniques. Table 2 shows sketches of the production techniques used in the development of nanocomposites.

Wear and friction characteristics of composites developed with CNTs reinforcement are presented in Table 3.