# Advanced Metal Matrix Nanocomposites
## 3 Reinforcing Agents  
The aim of reinforcement of metallic materials with nano-particles or fibers is always aimed to improve specific properties [82], typical development goals for composite materials being:  
* Increase in yield and tensile strength at room temperature or elevated temperature while maintaining ductility;
* Increase of creep resistance at elevated temperatures;
* Increased fatigue strength;
* Improvement of thermal shock resistance;
* Improvement of wear resistance;
* Increase of the Young's modulus; or
* Reduction of coefficient of thermal expansion. Figure 13: 3D representation of tensile strength (**a**) and elongation (**b**) versus the volume fraction and the number of cycles [81]. Ceramic nanoparticles and carbon-based nanotubes have been available in the last few years at significantly lower prices than in the 1980s or 1990s and has culminated in impressive promotion of the research and development of nanoparticle-reinforced light metals. Figure 14 shows the number of publications on metal matrix nanocomposites over the last 25 years. The requirements for ceramic nanoparticles and their use in the metallic matrix to bring about the aforementioned improvements in properties, however, are rather customized by the combination of particles and matrix; low density is a requirement in order to curb the density of the composite. Furthermore, an obligatory mechanical compatibility exists, which is provided by a small difference in the coefficient of thermal expansion. Chemical compatibility is important at the outset to prevent the particles from dissolving in the melt but, at the same time, ensures good adhesion to the matrix. The melting point of the particles should be significantly higher than that of the matrix, which is typically the case with ceramic particles. A high modulus of elasticity and high strengths are vital and so is good wettability for metallurgical processing. Finally, the reinforcement component must also be inexpensive and, therefore, economical to procure. Carbides, borides, nitrides, and oxides are used to reinforce metallic materials in the nanoparticle form [83; 84], usually produced industrially at low cost and deployed in the grinding and polishing industry; especially, SiC, B\({}_{4}\)C, Al\({}_{2}\)O\({}_{3}\), AlN, BN, and TiB\({}_{2}\) are technically and economically relevant examples. Table 1 shows some relevant properties of the ceramics listed above and Table 2 lists common ceramic nanoparticles useable for metal matrix nanocomposite production. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Type** & **SiC** & **AIN** & **Al\({}_{2}\)O\({}_{3}\)** & **B\({}_{4}\)C** & **TiB\({}_{2}\)** & **TiC** \\ \hline Crystal structure & \(\alpha\): hdp & hdp & \(\alpha\): hdp & rhomb & hdp & cub \\ \hline Lattice parameters [nm] & a: 0.307 & a: 0.311 & a: 0.476 & a: 0.559 & a: 0.303 & a: 0.432 \\  & c: 1.008 & c: 0.498 & c: 1.299 & c: 1.205 & c: 0.322 & a: 0.432 \\ \hline Melting T [\({}^{\circ}\)C] & 2300 & 3000 & 2045 & 2450 & 2900 & 3140 \\ \hline Young’s modulus [GPa] & 480 & 350 & 410 & 450 & 370 & 320 \\ \hline Density [g/ cm\({}^{3}\)] & 3.22 & 3.26 & 3.98 & 2.53 & 4.49 & 4.92 \\ \hline Mobs hardness & 9.6 & - & 6.5 & 9.5 & - & - \\ \hline CTE [10\({}^{-6}\) K\({}^{-1}\)] & 4.9 & 6.0 & 8.3 & 5.4 & 7.4 & 7.4 \\ \hline supplier of nanoparticles & \multicolumn{4}{c}{loLiTe Nanomaterials, Skyspring Nanomaterials, Inc., Auer-Remy GmbH,} \\  & \multicolumn{4}{c}{ChemPur Feinchenikalien und Forschungsbedar GmbH, Alpha Aesar} \\ \end{tabular}
\end{table} 
Table 1: Properties of common particles used as nanoparticles for metals reinforcement [85; 86]. Figure 14: Number of publications on metal matrix nanocomposites in Web of Science. The size and shape of nanoparticles varies greatly. The particle sizes of a few nanometers in diameter are available while their shape varies between round, block-shaped, rectangular, flat, and filamentary. Both carbon nanotubes (CNTs) and nanodiamonds (NDs) are allotropes of carbon.