Improved Fracture Toughness in Advanced Nanocrystalline Ceramic Composites

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Improved Fracture Toughness in Advanced Nanocrystalline Ceramic Composites Joshua D. Kuntz, Guo-Dong Zhan, and Amiya K. Mukherjee Department of Chemical Engineering and Materials Science University of California - Davis Davis, CA 95616 ABSTRACT Nanocrystalline materials have demonstrated very interesting changes in physical, chemical and mechanical properties at severely diminished length scales. This article focuses on the topic of nanocrystalline ceramic composites specifically designed for applications requiring improved fracture toughness. The difficulty in producing fully consolidated ceramic composites that retain a nanocrystalline structure is the main hurdle for thorough investigations in this area. This obstacle has been overcome in the current investigation through the use of a fast, comparably lower temperature, sintering technique e.g., Spark Plasma Sintering. Alumina based nanocomposites incorporating carbon nanotubes and additionally incorporating nanocrystalline niobium have yielded fracture toughness values that have exceeded that for pure nanocrystalline alumina by more that 300%. This introduces the question of whether this improvement is merely additive or evidence of a synergistic toughening mechanism involving ductile phase and fiber toughening.

INTRODUCTION Advanced ceramic materials are attractive because of their low density, chemical inertness, high strength, high hardness and high temperature capability. Nanocrystalline ceramics are commonly defined as having a grain size of 100nm or less. While this definition is rather arbitrary, nanocrystalline ceramics are known to possess unique physical and mechanical properties, including 1) enhanced superplasticity, 2) superior strength, and 3) optical transparency in usually opaque ceramics. Pilot-scale facilities for nanopowder synthesis and the commercialization of sizable quantities of certain types of nanosize powders have been achieved. The fabrication of nanopowders into fully dense components that retain a nanocrystalline grain size has lagged behind powder synthesis and characterization. In part, the gap between powder synthesis and fabrication is related to an incomplete theoretical understanding of the mechanisms and consequences of densification and sintering when grain interfacial regions dominate. Equally relevant is the incomplete understanding of the particular experimental conditions that yield high-density compacts without microstructural coarsening. The available experimental work in this area clearly demonstrates that the conditions for densification and sinterability of nanocrystalline ceramics is system-specific and is not readily deduced from theory alone at the present time. Nanocrystalline ceramics do not appear to possess higher fracture toughness than conventional microcrystalline ceramics. Therefore, it is very likely that, in spite of the significant advantage in other properties, toughness is likely to be the bottleneck that controls the application of nanocrystalline ceramics in both structural and functional