## 3 Advantages of Composites over Traditional Materials
The interaction between an Al atom and an SWCNT is due to the formation of a covalent bond with the carboxyl functional group and directly with the SWCNT surface [112]. In practice, the surface of nanotubes is not ideal; therefore, depending on the degree of purity of CNTs and the number of defects and grafted functional groups on their surface, the formation of metal carbide is also possible, which occurs due to the substitution of carbon atoms. Variation of the Gibbs free energy of formation of various carbides (per mole of graphite) depending on temperature. According to the experimental data [113; 114; 115], aluminum is practically insoluble in carbon, and the solubility of carbon in aluminum does not exceed 0.02-0.04 wt. %. The main reaction that determines the interaction between carbon structures and Al is the formation of Al\({}_{4}\)C\({}_{3}\) carbide. The interactions of amorphous (glassy carbon), quasi-crystalline (carbon fibers), and crystalline (graphite, nanotubes) carbon structure with Al [116; 117] showed that aluminum carbide is formed at 970-1370 K at the time of isothermal contact of carbon materials with liquid aluminum for three or more minutes. When obtaining composites with dispersed or fiber hardeners, the main tasks are to achieve a uniform distribution of the hardener in the matrix and the formation of strong contact at the interface between the phases of the components. At present, there are many methods for obtaining composites based on a metal matrix with an arming component in the form of CNTs (MM-CNT): melt processing (casting, melt infiltration), gasothermal spraying (plasma spraying, cold spraying), electrochemical, (electro- and chemical deposition) new methods (powders mixing at the molecular level, nanoscale dispersion, sandwich processing, laser deposition), etc. (Figure 6). However, the most accessible and widely used methods for obtaining metal matrix composites reinforced with MWCNTs are solid-phase methods of powder metallurgy (PM) [88; 104; 118; 119]. A typical technological process of the PM method consists of the mixing of components and their consolidation. Mixing is usually carried out using mechanical alloying in mills of various types or ultrasonic processing of components in liquid media [61]. Consolidation methods based on the sintering of metal powders can be divided as follows: (i)-conventional sintering, (ii)-hot pressing. The use of this technological scheme, as a rule, leads to a chaotic arrangement of MWCNTs. In [120; 121], MWCNTs were grown on Al powder (Ni catalyst). The resulting powder was pressed at 600 MPa, sintered at 640 \({}^{\circ}\)C for 2 h, and additional pressing at 2 GPa was carried out to achieve the required density and properties. Microhardness increased from 0.15 GPa to 0.65 GPa and tensile strength from 140 MPa to 398 MPa. It is also reported that the high increase in strength is associated with a strong interfacial bond in the form of a thin layer of Al\({}_{4}\)C\({}_{3}\) between the components (Figure 7). Ag-based composites reinforced with MWCNTs were prepared by mixing powders in a mortar and compaction at 320 MPa, followed by sintering at 700 \({}^{\circ}\)C for 1 h [122]. In order to increase the adhesive bond with the matrix, the MWCNTs were treated with acid to roughen the surface. Due to insufficient density after sintering, the composites were pressed again at 400 MPa for further compaction. The authors stated that the uniform dispersion of MWCNTs without the formation of any clusters was achieved up to 8 vol. % MWCNTs in the composite after sintering at 700 \({}^{\circ}\)C [122]. Most studies on sintered metal MWCNT composites indicate insufficient density, which can be increased by reducing the particle size of the matrix powder, improving the uniformity of the distribution of nanotubes, or using additional deformation treatments [89]. In hot pressing, powder mixtures or cold pressed compacts are processed at high temperatures in a mold in a protective environment.