Biomimetic Processing of Ceramics and Ceramic-Metal Composites
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BIOMIMETIC PROCESSING OF CERAMICS AND CERAMIC-METAL COMPOSITES M. YASREBI, G. H. KIM, K. E. GUNNISON, D. L. MILIUS, M. SARIKAYA, and I. A. AKSAY Department of Materials Science and Engineering; and Advanced Materials Technology Center, Washington Technology Centers, University of Washington, Seattle, Washington 98195 USA
ABSTRACT Biomimetic design and processing of laminated B4 C-AI cermets, based on knowledge gained from the microstructure-property characterization of abalone shells, is described. In the nacre section of the shell, the microstructure is highly organized as CaCO 3 (aragonite) crystals, with a thickness of 0.25 itm, separated by a layer of organic matter 300-500 A thick. This organization forms a miniature "brick and mortar" microstructure. The resultant strength and fracture toughness of the nacre, i.e., 180 MPa and 7 MPa-mI/ 2 , are many orders of magnitude higher than those of monolithic CaCO 3. The processing of laminated B4C-AI cermets, based on the microstructure of the nacre, was performed by a combination of tape casting of the ceramic and infiltration of the metal. The resultant cermets displayed a 40% increase in both fracture toughness and strength over monolithic B4C-AI cermets.
INTRODUCTION The use of composite materials has become popular and frequently necessary to meet the requirements of technology. The employment of composites is well appreciated firom the fact that unique arrangements of constituent materials not only satisfy specific requirements but also exhibit properties superior to the sum of each component. Laminated composites which have planar geometry are one example and can be found both in nature and early civilizations. Examples are seashells, glued woods, and metallic armor. Even though exotic techniques and materials are introduced in the processing of laminated composites for modern applications, the flexibility of offering specific anisotropic properties, such as a specific strength, toughness, and stiffness, makes laminated composites attractive and provides an edge over single-phase materials. The discovery that modern laminates formed from composites with a nanoscale architecture have enhanced properties has drawn much attention. It has led to the reexamination of the structure-property relationship in composites with different scales of size. Eutectic composites, frequently known as in situ composites, exhibit a well-aligned fibrous or platelet-reinforcing phase by directional solidification or by phase transformation, the classical example being the pearlite structure. A wide range of materials belongs to this group such as metal-metal,' metal-ceramic,2 internietallic-intermetallic,1 metal-intermetallic,3 and ceramic-ceramic4"6 systems. In all cases, the size dependency of properties was observed and is known as the Il all-Petch relationship, a = (T, + kd-1/ 2, for ductile phases and the Orowan relationship, of = YKicd- 1 / 2 , for brittle phases. Although directional solidification is a thermodynamicallydriven process, precise control of the interfacial cha
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