Mechanical testing of limpet teeth micro-beams using FIB
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Mechanical testing of limpet teeth micro-beams using FIB Dun Lu1 and Asa H. Barber1 Department of Materials, School of Engineering and Materials Science, Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom 1
ABSTRACT Discrete volumes of material in the form of a beam have been isolated from a parent limpet tooth using Focused Ion Beam (FIB) techniques. Mechanical bending tests of individual beams are performed using atomic force microscopy (AFM). The relatively small volumes tested in this beam-bending configuration allow approximation of the limpet tooth structure to a uniaxial short fibre composite. This mechanical testing technique is superior to conventional micro-hardness indentation as a defined stress condition within a locally defined volume is examined. Composite theory is shown to be valid for describing the mechanical properties of limpet teeth at sub-micron lengths scales and used to determine the synergy between the nanomaterial constituents. INTRODUCTION Limpet teeth perform a mechanical function for feeding and can be considered as a biological composite where mineral crystals of an iron oxide-hydroxide (Goethite) are expected to behave as a reinforcing phase for a polymeric chitin matrix. The mineral forms regular fiberlike crystals, often with a diameter of a few tens of nanometers [1], which show a complex organization within the tooth geometry. However, at micron to sub-micron length scales the mineral fibers are highly aligned and approximate to a uniaxial short fiber composite. Mechanical studies of limpet teeth at these length scales present an opportunity to elucidate behavior in a structural biological composite and ascertain the influence of the nanomaterial constituents on resultant mechanical performance. The capability of Focused Ion Beam (FIB) to be used as a milling tool for producing micrometer-sized samples for mechanical testing has begun to attract increasing attention in the study of synthetic materials [2]. FIB has also proven suitable for site-specific milling of subsurface structures of biological samples for conventional electron microscopy examination [3]. The ability to select specific volumes using FIB is particularly important for the understanding the mechanical properties of limpet teeth material constituents, which is not possible at larger length scales where the shape and geometry of limpet tooth in addition to the inherent material properties define mechanical behavior of the tooth. The characterization of mechanical properties of materials at the sub-micro scale is typically carried out using atomic force microscopy (AFM) in contact mode. Mechanical testing using AFM has been previous shown to be suitable for measuring the mechanical properties of a number of nanomaterials and their composites [4, 5, 6]. Mechanical testing on limpet teeth is sparse, despite its potential as a high performance biological fibrous nanocomposite, but some work has examined macrohardness behavior [7]. While these macroscopic experiments are useful
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