Structure-Mechanical Property Relationships In A Biological Ceramic-Polymer Composite: Nacre
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STRUCTURE-MECHANICAL PROPERTY RELATIONSHIPS IN A BIOLOGICAL CERAMIC-POLYMER COMPOSITE: NACRE
KATIE E. GUNNISON, MEHMET SARIKAYA, JUN LIU, and ILHAN A. AKSAY Department of Materials Science and Engineering, and Advanced Materials Technology Center, Washington Technology Center, University of Washington, Seattle, WA 98195
ABSTRACT The structure-mechanicalproperty relationships were studied in nacre, a laminated ceramicpolymer biocomposite found in seashell. Four-point bending strength and three-point bend fracture toughness tests were performed, and the results averaged 180 t 30 MPa and 9 t 3 MPa.m1 2, respectively, indicatingthat the composite is many orders of magnitude strongerand tougher than monolithic CaCO3,, which is the primary component of nacre. Fractographic studies conducted with a scanning electron microscope identified two significant toughening mechanisms in the well-known "brick and mortar" microstructure of nacre: (i) sliding of the aragoniteplatelets and (ii) ligamentformation in the organicmatrix. These toughening mechanisms allow for high energy absorptionand damage tolerance and thereby prevent catastrophic failure of the composite. The structure of the organic matrix and the interfacial structure between the organicand inorganiccomponents were studied with transmission electron microscopy by using both ion milled and ultramicrotomedsections with and without the intact aragonite platelets. We found that the organic matrix is indeed a multilayered composite at the nanometer scale but is thinner (about100 A) than reportedin the literature. The morphology of the interfacial region between the organic and the inorganiclayers suggests the presence of a structural "transitory"region that interlocks the two dissimilarphases.
1. INTRODUCTION In designing and processing materials for technological applications, valuable lessons can be learned from biological soft and hard tissues.1- 3 Biologically formed materials almost exclusively have composite structures 4 and the individual phases of these structures are often arranged in complex and highly ordered units to form hierarchically organized architectures. 2,4,5 The design of biological structures developed over time in order to meet specific materials requirements for the organism. Since biological systems have a limited supply of raw materials at their disposal, they must use the available raw materials in the most efficient manner possible to achieve the structures and the properties that are required for survival in their habitat. 6 The formation of biological materials systems, and the relationships between the structures and their resulting properties, can provide important information for the development of synthetic 7 materials.
Mat. Res. Soc. Symp. Proc. Vol. 255. (D1992 Materials Research Society
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We chose to study the property-structure relationship in the nacre of the red abalone, Halitois rufescens, because it has been shown to display a higher strength and toughness than other mollusk shell structures. 8-11 Nacre in mollusk shells
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