Time Dependent Nanomechanical Response of Nacre
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Time Dependent Nanomechanical Response of Nacre Bedabibhas Mohanty, Devendra Verma, Kalpana S. Katti, and Dinesh R. Katti Civil Engineering, North Dakota State University, Fargo, ND, 58105 ABSTRACT Nacre, the shiny inner layer of seashells, is a model biomimetic system composed of 95% of inorganic (aragonite) phase and 5% of organic phase (mainly proteins and polysaccharides). Nacre exhibits an interlocked layered ìbrick and mortarî structure where the bricks are made up of aragonite and mortar is the organic phase. We have performed nanoindentation and dynamic nanoindentation tests to study the nanomechanical and dynamic nanomechanical response of nacre. The indentation experiments performed at low loads indicate an elastic modulus of about 15 GPa for the organic phase. The low load, low penetration experiments appear to be better indicators of nanomechanical behavior. Dynamic nanomechanical response of nacre was studied using dynamic nanoindentation (nano-DMA). Significant increase in the values of tan δ was observed with increase in frequency. Also, the dynamic nanoindentation experiments indicate that nacre exhibits viscoelastic behavior. Further, fourier transform spectroscopy experiments on nacre in innate and undisturbed state indicate the presence of water in nacre. The nanograin structure of nacre platelets, as well as the entrapped and adsorbed water, are two important contributors to the viscoelastic response of nacre. Atomic force microscopy experiments also indicate a very high force to separate organic material from the aragonite in nacre. These experiments provide important insight into nanomechanical response of nacre, its constituents and also interfaces. INTRODUCTION Nacre, the inner iridescent layer of molluscan seashells is a natural bionanocomposite with excellent engineering properties. The major part of its composition is aragonitic CaCO3 with a small amount of organic material in the form of proteins and polysaccharides (~ 2-5% by weight). Nacre is widely investigated in the literature because of its exceptional mechanical properties despite being composed of a brittle and a very soft phase [1-11]. Nacre exhibits a very high fracture toughness which is 3000 times that of the pure aragonite [2]. The morphology of nacre is highly hierarchical and it has a very well organized micro-architecture. The structure of nacre is often called a ìbrick and mortarî structure [1-3] where the bricks of the different layers are interlocked with one another [4]. The bricks are made up of aragonite and are polygonal in shape with a thickness of about 250-500 nm. These bricks are separated by a 20 nm thick layer of organic material that acts as the mortar. Due to its simple composition, highly organized microstructure and high toughness, nacre is a source of inspiration for designing novel biomimetic nanocomposites.
In our previous studies, we have investigated the effect of microstructure and the molecular interactions on the overall properties of nacre using computer-based simulations and exper
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