Search

Ceramic composites found in nature, such as bone, nacre, and sponge spicule, often provide an effective resolution to a well-known conflict between materials' strength and toughness. This arises, on the one hand, from their high ceramic content that ensures high strength of the material. On the other hand, various pathways are provided for stress dissipation, and thus toughness, due to their intricate hierarchical architectures. Such pathways include crack bridging, crack deflection, and delamination in the case of layered structures. On the basis of these inspiring ideas, we attempted here to create simultaneously strong and tough laminated alumina composite with high ceramic content. Composites were prepared from high-grade commercial alumina with spin-coated interlayers of ductile polymers ( PMMA and PVA). The specimens' ultimate properties (strength, fracture toughness, and work of fracture) were measured by a four-point bending method. In some cases, fracture toughness of the composites was increased by up to an order of magnitude, reminiscent of the natural layered composites. It is proposed that this increase may be attributed to an interlocking mechanism, often encountered in biological composites. The significance of sample architecture and the role of the interfacial and bulk properties of the interlayer material are discussed. [ABSTRACT FROM AUTHOR]

'Smaller is stronger' is now a tenet generally consistent with the predominance of evidence. An equally accepted tenet is that fracture toughness almost always decreases with increasing yield strength. Can 'smaller is tougher' then be consistent with these two tenets? It is taught in undergraduate engineering courses that one design parameter that allows for both increased strength and fracture toughness is reduced grain size. The present study on the very brittle semiconductor silicon proves this exception to the rule and demonstrates that smaller can be both stronger and tougher. Three nanostructures are considered theoretically and experimentally: thin films, nanospheres, and nanopillars. Using a simple work per unit fracture area approach, it is shown at small scale that toughness is inversely proportional to the square root of size. This is supported by experimental evidence from in situ electron microscopy nanoindentation at length scales of less than a micron. It is further suggested that dislocation shielding can explain both strength and toughness increases at the small scales. [ABSTRACT FROM AUTHOR]