# Advanced Metal Matrix Nanocomposites
## 8 Conclusions, Current Challenges, and Future Remarks  
Metal matrix nanocomposites (MMNCs) are lightweight, high-strength materials that have found widespread use in a wide variety of industrial and biomedical applications. A matrix usually comprising a lightweight metal, such as aluminum or magnesium, is reinforced by a variety of high-strength nano-particles, namely nano-ceramics, nano oxide/non-oxides, carbon-based nano-allotropes, elements, etc. This review aims to provide comprehensive information about the manufacturing methods, nano-reinforcing agents, dispersion mechanisms (such as cavitation de-aggomeration), underlying strengthening mechanisms, and the effects of different process parameters on mechanical, microstructural, and tribological properties.  
The selection of synthesis techniques and process parameters are critical for developing MMNCs with superior properties; stir casting, disintegrated melt deposition, accumulative role bonding, and powder metallurgy are some of these most prevalent processing techniques. Among the methods, special attention has been paid to the liquid-based manufacturing processes where a homogenous good dispersion of nanoparticles in the melt is still a very challenging proposition in view of poor wettability between nano-particles and the metal host. Although the dispersing techniques (e.g., melt ultrasonication) during casting/solidification is advantageous to avoid agglomeration and enhance wetting conditions, there is still a paucity of experimental or simulation studies available to suggest the best dispersing conditions. Direct in situ imaging of deagglomeration of nanoparticle clusters in molten metals will provide a clear insight about the processing, mechanisms, and parameters of the advanced metal matrix nanocomposites. Such experiments can provide precise understanding of the deagglomeration dynamics under certain treatment processing besides large-scale atomistic modeling, which may also offer similar understanding. The hurdle is that the chemical potentials of the interfacial atoms between liquid metal alloys and ceramic nanoparticles are usually not available owing to the charge re-distribution at the liquid metal/solid nanoparticle ceramics, which can only be determined by quantum mechanics. Thus, in the future, quantum-based chemical potentials for various MMNCs systems need to be developed so that large-scale atomistic modeling can be performed to study the deagglomeration dynamics. Moreover, from the industrial perspective, the upscaling of reliable processes for industrial application of MMNCs is presently one of the most challenging issues that needs to be investigated in future activities. Finally, MMNCs are usually processed under lab-scale conditions; a continuous industrial scale has not yet been developed, wherein the melt flows into a mold and nanoparticles are continuously homogenously dispersed to reinforce the metal matrix.  
Figure 25: Comparison of wear loss of Al-Al\({}_{2}\)O\({}_{3}\)/SiC composites produced by the casting route or powder metallurgy. Wear loss of PM samples is higher than those samples fabricated by casting regardless of the type of reinforcing agent (Al\({}_{2}\)O\({}_{3}\) or SiC) [191].