Mechanical properties of nacre constituents and their impact on mechanical performance
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The mechanical properties of nacre constituents from red abalone were investigated. Electron microscopy studies revealed that the tablets are composed of single-crystal aragonite with nanograin inclusions. Both nanoasperities and aragonite bridges are present within the interfaces between the tablets. By means of nanoindentation and axial compression tests, we identified single tablet elastic and inelastic properties. The elastic properties are very similar to those of single-crystal aragonite. However, their strength is higher than previously reported values for aragonite. A finite element model of the interface accounting for nanoasperities and the identified properties revealed that the nanoasperities are strong enough to withstand climbing and resist tablet sliding, at least over the initial stages of deformation. Furthermore, it was observed that the model over-predicts strength and under-predicts ductility. Therefore, we conclude that other interface features must be responsible for the enhanced performance of nacre over its constituents.
I. INTRODUCTION
Some structural materials found in nature exhibit remarkable mechanical performance. It is believed that these are the results of material design over several length scales, organized in a hierarchical fashion. Nacre is a typical example of such biomaterials. It exhibits remarkable properties not achievable in current man-made ceramic composites. For this reason, the investigation of nacre mechanics is becoming the focus of a growing number of research groups within the materials science community. Nacre is the iridescent material found in the inner layer of some seashells such as oyster or abalone. It is a composite material mainly composed of an aragonite phase (about 95 vol%) arranged in microscopic polygonal flat tablets, bonded together by a biopolymer thin film (Fig. 1). Aragonite is a very fragile ceramic. However the addition of a small amount of the organic polymer and a well-designed microstructure results in a material with strength and toughness 20–30 times that of monolithic aragonite.1 The toughening mechanisms involved in nacre have been the focus of significant research efforts in the past ten years. Several possible deformation mechanisms have been suggested, but definite identification of a)
On leave from Department of Structural Engineering, Politecnico di Milano, Milano, Italy. b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0239 J. Mater. Res., Vol. 21, No. 8, Aug 2006
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the microstructural features leading to this performance is yet to be established.2 The motivation behind these studies is to draw lessons from nature that would enable the development of design rules applicable to manmade composites such that orders of magnitude improvements in performance over their constituents can be achieved. Tablets of nacre have been investigated in the past, but their exact structure has not been fully elucidated. Transmission electron mi
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