The Quest for Stronger, Tougher Materials: A Letter and a Response

Dr. Ritchie writes: "The perspective “Structural Nanocomposites” (Y. Dzenis, 25 January, p. 419) describes a quest for improved structural materials and indicates that composites with nanoscale reinforcements would have “exceptional mechanical properties.” Is this true? Why would reinforcements that are small in size or volume offer any particular benefit over larger-scale reinforcements? As the Perspective correctly asserts, if the composite material is to be used for a small-volume structure, clearly the reinforcements must also be small. In addition, small-volume reinforcements are stronger, as has been known since the early days of research on whiskers (1). In this regard, reinforcement by carbon nanotubes, for example, which are thought of as one of the strongest materials in existence (2), would seem ideal. The problem with this notion is that new materials are not limited by strength, but by resistance to fracture (also known as fracture toughness). It is not by accident that most critical structures, such as bridges, ships, and nuclear pressure vessels, are manufactured from materials that are low in strength but high in toughness. Indeed, the majority of toughening mechanisms mentioned by Dzenis—i.e., crack deflection, plastic deformation, and crack bridging—are promoted by increasing, not decreasing, reinforcement dimensions [e.g., (3)]. Is it any surprise that “results obtained so far are disappointing”?... Dr. Dzenis's reply is included.