## 3 Advantages of Composites over Traditional Materials
Under the combined influence of temperature and pressure, the material is easily deformed, which contributes to the achievement of high density. Al-MWCNT composites were prepared by hot pressing  
Figure 6: List of different processing routes for MM-CNT composites [5]. powder mixtures at 520 \({}^{\circ}\)C and 25 MPa [118] with a holding time of more than 30 min. The authors of [123; 124] obtained Al -MWCNT composites by ball-milling raw materials for different lengths of time. The Al particle size was \(\sim\)13 \(\upmu\)m, that of MWCNTs was l-5 \(\upmu\)m, and d \(\sim\) 10-20 nm. Subsequently, the mixture was subjected to hot pressing at 560 \({}^{\circ}\)C. It is shown that with an increase in the milling time, the degree of damage to MWCNTs increased, which was noted by an increase in the intensity of the Al4C3 phase. Tensile strength and yield strength for the composite Al-0.5 wt. % CNTs (6 h grinding), compared with pure Al, amounted to 42.3% and 18.4%, respectively. Hot pressing has the advantage of producing high-density (>95%) metal-MWCNT composites. The hot-pressing time to achieve the optimal density in most studies was \(\sim\)1 h. Long-term temperature exposure can lead to metal carbide formation, depending on the quality of the MWCNTs used. Some of these studies have found a reaction between MWCNTs and metal [124; 125], while others have followed the stability of MWCNTs. At present, an effective method of consolidation is spark plasma sintering (SPS), the basic principle of which is to heat the powder material by passing a pulsed current with simultaneous application of pressure [126; 127; 128]. Another paper [129] provides a general view of the strengthening mechanisms of Al-CNT nanocomposites fabricated using 1 vol. % CNT using the powder metallurgy method using the ultrasonic treatment and its combination with a ball mill (Figure 7). It is shown that this approach results in the formation of a homogeneous dispersion of CNTs with minimal damage. In addition, Al4C3 was found, which formed during the preparation of composites; Al4C3 plays an important role in transferring the load from the matrix to the reinforcement. As a result, the authors showed that the presence of Al4C3, CNTs, and a high density of dislocations near the latter is the key to improving the mechanical properties of nanocomposites. Carbon nanofibers and nanotubes are attracting a lot of attention not only because of their structural features and unique physical and mechanical properties but also because of their potential technological applications. In general, the use of CNFs and CNTs can be divided into two groups. The first group includes materials using ensembles of carbon nanostructures, such as composite materials (where CNTs and CNFs are used as reinforcing elements), materials for chemical power sources, adsorbents, catalysts, etc. Note that  
Figure 7: HRTEM image and inverse FFT showing the Al4C3 and CNT embedded in the Al matrix [129]. composites containing CNT/CNF have the best strength-to-weight properties among other materials [84,130]. The second group includes the use of individual CNTs due to their high mechanical and electrical properties [84,130]. Table 1 shows that the properties of MWCNTs are slightly lower than those of MWCNTs since destruction will begin with the outer, defective layer. There are much fewer studies aimed at reinforcing the ceramic matrix of CNTs, and metal-CNT composites have been little studied. In addition, the works [131, 132, 133, 134, 135] showed the successful reinforcement of the aluminum matrix with carbon nanostructures to improve the tribological characteristics of the composite material. According to [133], the greatest effect in reducing the friction coefficient and wear rate is observed in aluminum alloys reinforced with C\({}_{60}\) fullerenes and graphite. In [134], a study was conducted on the effect of the concentration of carbon nanotubes in the composition of the aluminum matrix on the wear resistance of the material (Figure 8).