NANO LETTERS
Toughening through Nature-Adapted Nanoscale Design
2009 Vol. 9, No. 12 4103-4108
Zaklina Burghard,*,† Lorenzo Zini,† Vesna Srot,‡ Paul Bellina,‡ Peter A. van Aken,‡ and Joachim Bill† Institute for Material Science, UniVersity of Stuttgart, Heisenbergstrasse 3, D-70569 Stuttgart, Germany, and Stuttgart Center for Electron Microscopy, Max-Planck-Institute for Metals Research, Heisenbergstrasse 3, D-70569 Stuttgart, Germany Received July 20, 2009; Revised Manuscript Received October 19, 2009
ABSTRACT The extraordinary combination of strength and toughness attained by nature’s highly sophisticated structural design in nacre has inspired the synthesis of novel nanocomposites. In this context, the organic-inorganic hierarchical design of nacre has been mimicked. However, two key features of nacre, namely the scaling of the structural components and the low content of the organic phase, have not been replicated yet. Here, we present thin nanocomposite films with properly adjusted thicknesses of the organic and inorganic layers, as well as a microstructure that closely resembles that of nacre. These films, which are obtained by the combination of low-temperature chemical bath deposition of titania with layer-by-layer assembly of polyelectrolytes, exhibit enhancement in a fracture toughness by a factor of 4, combined with notable increase in hardness, while the Young’s modulus is largely preserved in comparison to the single titania layer. Our findings highlight the significance of the 10:1 inorganic/organic layer thickness ratio evolved by nature, and provide novel perspectives for the future development of efficient bioinspired thin films.
Many structural biomaterials exhibit a unique combination of hardness, stiffness and fracture toughness that remains unattained by man-made monolithic ceramics and composites.1-4 One intriguing example is nacre, an iridescent layer coating the inner part of mollusc shells. It constitutes a layered nanocomposite composed of at least 95 wt % fraction of calcium carbonate (CaCO3) in the form of hexagonal aragonite platelets separated by thin, intermediate biopolymer layers consisting of proteins and/or polysaccharides, as exemplified by Red Abalone nacre in Figure 1a. The basic mechanical properties of nacre are contrasted to those of its constituents in Figure 1b. It is well established that the highly regular “brick-mortar” arrangement effectively combines the strength of the mineral platelets with the high elasticity of the protein layers between them, thus imparting a 2-fold increase in strength compared to the pure inorganic component.2 In addition, this architecture results in enhanced crack deflection along the interface, which promotes the effective redistribution of an applied fracture load, yielding an 8-fold increase of fracture toughness over monolithic CaCO3.3 In contrast, as a consequence of the small protein weight fraction, the Young’s modulus remains almost identical. Moreover, it has been shown that additional factors besides * To whom correspondence should be addressed. E-mail: zburghard@ mf.mpg.de. † University of Stuttgart. ‡ Max-Planck-Institute for Metals Research. 10.1021/nl902324x CCC: $40.75 Published on Web 11/06/2009
2009 American Chemical Society
the ordered layer structure contribute to nacre’s extraordinary mechanical properties which exceed the simple additive rule for composites. These include the submicrometer thickness of the aragonite platelets,5 the presence of nanoasperities on the platelets6 and of mineral bridges between them,4 as well as the spatial variation of the Young’s modulus associated with the alternating inorganic/organic structure.7 Moreover, it has been observed that the aragonite platelets are composed of nanosized particles with an average size of 30 nm, which undergo rotation and deformation when nacre is subjected to external stress, thereby contributing to the energy dissipation in the material.8 Although it is clear that all these factors act synergistically, their relative importance has not yet been clarified. During the past few years, increasing effort has been devoted toward exploiting nacre’s structural design principle in the synthesis of novel nanocomposites. In these activities, a major focus has been on layered nanocomposites composed of polymers (>50 wt %) reinforced by clay platelets with thickness of ∼1 nm.9-11 These hybrid films reach strength comparable to that of nacre and an elastic modulus approaching that of bone.11 Their mechanical properties could be further improved via chemical cross-linking of the polymer, providing access to higher strength and stiffness compared to natural nacre.12 It has furthermore been demonstrated that strong polymer reinforcement can be achieved through incorporation of a low volume fraction of high-
Figure 1. Multiscale hierarchical structure governing the mechanical performance of nacre, and schematic illustration of the nacre-like hybrid films. (a) Polarized light microscope image of cross-sectional Red Abalone shell specimen. The upper mosaic-like region is a calcite layer, while the bottom region comprises the nacreous layer. The inset shows an AFM image acquired within the nacre part. The thickness of the aragonite platelets is 400-500 nm and that of the protein layers 40-50 nm. In panel b, the strength, Young’s modulus, and fracture toughness of natural nacre is compared to values of their inorganic (aragonite, CaCO3) and organic (protein) components, on the basis of data taken from refs.4 The cross-sectional structure of the investigated hybrid films, composed of alternating layers of TiO2 and organic polyelectrolytes, is presented in (c). The polyelectrolyte layer shown in the enlarged has the composition (PEI/PSS)(PAH/PSS), corresponding to a total thickness of 10 nm.
strength platelets with similar size and aspect ratio like those of the aragonite platelets in nacre.13 In comparison, only little work has been performed on nanocomposites wherein the inorganic layers constitute the major component and that more closely resemble natural nacre. These thin films were composed of submicrometer thick calcium carbonate14,15 silicate16 structures, or layers of the inorganic oxides TiO217 and ZnO.18 While their detailed mechanical properties remain to be explored, first hints of improved performance of these layered composites have been gained.16-18 In addition, attempts to make a bulk alumina based nacre-like composite have been made.19 However the direct transfer of nacre’s architecture to an artificial inorganic material has not been achieved yet. In particular, for the scaling of the structural components, the low content of the organic phase (
