Effects of nanostructures on the fracture strength of the interfaces in nacre
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Effects of nanostructures on the fracture strength of the interfaces in nacre F. Song and Y.L. Bai State Key Laboratory of Nonlinear Mechanics (LNM), Institute of Mechanics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China (Received 29 December 2002; accepted 12 May 2003)
A strengthening mechanism arising from the mineral bridges in the organic matrix layers of nacre (mother of pearl) is presented by studying the structural and mechanical properties of the interfaces in nacre. This mechanism not only increases the average fracture strength of the organic matrix interfaces by about five times, but also effectively arrests the cracks in the organic matrix layers and causes the crack deflection in this biomaterial. The present investigation shows that the main mechanism governing the strength of the organic matrix layers of nacre relies on the mineral bridges rather than the organic matrix. This study provides a guide to the interfacial design of synthetic materials.
The nacre of mollusks, which is considered a natural layered composite containing about 95 vol% interlocking aragonite platelets staggered in successive layers and separated by a 5% organic matrix, has fracture strength and toughness properties that are orders of magnitude higher than those of monolithic aragonite.1,2 In particular, the microstructure and the performance of the interfaces in nacre, which play a crucial role in this biomaterial service, have become an attractive target for the interfacial design of synthetic materials.1,3–6 Previous studies deemed the organic matrix layers of nacre (i.e., the interfaces in nacre) to be the structure of biological organic macromolecules composed only of polysaccharide and protein fibers.1,2,7,8 However, Schaffer et al.9 clearly observed many nanopores in the interlamellar organic matrix sheets of nacre in terms of various microscopic observations, and then pointed out that there might be a number of nanoscale mineral bridges in the organic matrix layers of nacre and gave a statistical distribution. Song et al. confirmed the existence of the inorganic nanostructures in the organic matrix layers of nacre and proposed a distribution law of the nanostructures in the organic matrix layers.10 This is an indication that the mortar layers of nacre should be considered as a composite composed of organic matrix and mineral bridges rather than mere organic matrix, as shown in Fig. 1. These findings have led to further investigations on the mechanical behaviors of the interfaces in nacre. J. Mater. Res., Vol. 18, No. 8, Aug 2003
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In terms of the strength, Sarikaya et al. indicated that the fracture strength of nacre was related to several factors, including the size and structure of the aragonite platelets and the interfaces between the inorganic and organic components.1 Smith et al. pointed out that the key role to nacre’s fracture resistance resided in the polymer adhesive of the organic matrix layers.11 Evans et al.12 and Wang et al.13
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