25. Development of Carbon Nanotube-Reinforced Ceramic Matrix Nanocomposites for Advanced Structural Applications

Ceramic matrix composites containing fiber reinforcements possess superior mechanical and tribological properties, as compared to their monolithic counterparts, that render them better suited for engineering applications demanding high strength, wear resistance, and resistance to thermal shock. Among the wide range of reinforcements used for toughening the otherwise intrinsically brittle bulk ceramic materials, carbon nanotubes (CNTs), owing to their excellent physical, mechanical, and thermal properties, are considered to be one of the most promising reinforcement types. The exceptional mechanical properties of CNTs offer excellent opportunities toward the development of considerably stronger and tougher ceramic nanocomposite systems for potential applications in aircraft and aerospace industries. However, there are many challenges with respect to the processing of CNT-reinforced bulk ceramic materials that limit their commercial applications to a considerable extent. Additionally, dispersion of the CNTs, optimization of the CNT volume fractions, development of suitable CNT/matrix interfaces, and distribution within the sintered polycrystalline ceramic microstructures are some of the aspects that need particular attention. Continuing research efforts have been directed toward addressing issues related to such aspects, in a bid to attain best possible combination of mechanical and tribological properties. With regard to microstructure development, achieving uniform distribution of well-dispersed CNTs within the sintered polycrystalline ceramic matrix (i.e., reinforcing the grain interiors and not just the grain boundaries with CNTs) has been found to be particularly difficult, until very recently. In these contexts, after discussing some of the basic aspects of carbon nanotubes and ceramic-CNT composites, the present chapter provides a comprehensive review of the overall status of research and development in CNT-reinforced ceramic matrix composites, with particular emphasis on a variety of processing techniques investigated to date in a bid to optimize the quality of CNT dispersion, character of the CNT-matrix interfaces, eventual densification of the composites, and also cost-effectiveness. The influences of CNT reinforcements on the properties of the some of the important ceramic systems for advanced structural applications are discussed, with an emphasis towards fracture behavior and the possible toughening mechanisms. This review also highlights the more recent research efforts that have been conducted to address the issues concerning inhomogeneous dispersion and distribution of CNTs within the ceramic matrix, thus aiming toward the realization of the full potential of CNTs as reinforcing fibers. Lastly, the various potential applications for ceramic-CNT composites as structural materials have been highlighted, with an outlook toward the scope for future developments and issues that need to be further addressed.