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Abstract
The thecosomes are a group of planktonic pteropods with thin, 1 mm-sized aragonitic shells, which are known to possess a unique helical microstructure consisting of interlocking nanofibres. Here we investigate the detailed hierarchical structural and mechanical design of the pteropod Clio pyramidata. We quantify and elucidate the macroscopic distribution of the helical structure over the entire shell (~1 mm), the structural characteristics of the helical assembly (~10-100 μm), the anisotropic cross-sectional geometry of the fibrous building blocks (~0.5-10 μm) and the heterogeneous distribution of intracrystalline organic inclusions within individual fibres (<0.5 μm). A global fibre-like crystallographic texture is observed with local in-plane rotations. Microindentation and electron microscopy studies reveal that the helical organization of the fibrous building blocks effectively constrains mechanical damages through tortuous crack propagation. Uniaxial micropillar compression and cross-sectional transmission electron microscopy directly reveal that the interlocking fibrous building blocks further retard crack propagation at the nanometre scale.
Highlights
The thecosomes are a group of planktonic pteropods with thin, 1 mm-sized aragonitic shells, which are known to possess a unique helical microstructure consisting of interlocking nanofibres
Three-dimensional reconstructions of the shell from C. pyramidata based on X-ray micro-computed tomography data are shown in Fig. 1c–e and clearly illustrate the shell’s bilateral symmetry
We explored the smaller length scale mechanical behaviour of the interlocked fibrous building blocks through uniaxial compression of micropillars prepared from C. pyramidata shells using FIB milling (Fig. 8)
Summary
The thecosomes are a group of planktonic pteropods with thin, 1 mm-sized aragonitic shells, which are known to possess a unique helical microstructure consisting of interlocking nanofibres. Driven by the need for new lightweight composites with improved mechanical properties and cost-effective and environmentally friendly manufacturing strategies[1,2], many engineers are turning to nature for design insight into the fabrication of damage tolerant and hierarchically ordered structural materials[3,4,5]. Biomineralized composites, such as bone, sponge spicules and mollusc shells, for example, demonstrate excellent mechanical properties, especially considering their relatively soft/weak organic and stiff/brittle ceramic constituents[6,7,8,9,10]. The mechanistic understanding of the enhanced fracture resistance in this complex material system holds great potential in the development of bio-inspired engineering materials with improved mechanical properties
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