Dynamic nanomechanical response of nacre
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Nacre, the shiny inner layer of mollusk shells is a model biomimetic nanocomposite system. Its exceptional mechanical properties have been the inspiration for materials scientists for several decades. Nacre exhibits a layered brick and mortar structure. It is composed of 95% inorganic (aragonitic CaCO3) phase and 5% organic (mainly proteins and polysaccharides) phase that are arranged in interlocked brick and mortar architecture with the mineral as bricks and organics as the mortar. In the current work, we describe the dynamic nanomechanical behavior of nacre using dynamic nanoindentation (nano-DMA) experiments. Two sets of loads were applied to obtain the dynamic response from varying depths in nacre. These tests were performed at three different frequencies (25, 50, and 100 Hz) to study the effect of frequency on the dynamic properties of nacre. The loss modulus (E⬙) and the loss factor (tan ␦) were measured. Both of these parameters were observed to increase with increase in depth. Significant increase in tan ␦ was observed with the increase in frequency. Photoacoustic Fourier transform infrared spectroscopic studies on nacre indicate the presence of water in nacre. This water may be present at nanograin interfaces in nacre platelets, at organic–inorganic interfaces, and also in the organic phase in nacre. We believe that water is one of the significant contributors to the viscoelasticity of nacre. Our results indicate that the aragonite platelets in nacre may also contribute to viscoelasticity.
I. INTRODUCTION
Nacre, the shiny layer of many seashells is a model biomimetic nanocomposite that has inspired many researchers around the world for the design of nextgeneration advanced materials. The literature shows extensive study on nacre for several decades because of its highly organized microarchitecture and the enhanced mechanical properties. Nacre exhibits much higher fracture toughness than that of monolithic ceramics, and it is 3000 times tougher than aragonite, which is the major constituent.1–3 Nacre is composed of 95 wt% aragonitic CaCO3 and 5 wt% organic material primarily composed of proteins and polysaccharides. The structure of nacre is often referred to as brick and mortar structure and more recently, as interlocked bricks.4 Here, polygonal aragonite bricks 200–500 nm thick and the organic 20–50 nm thick are highly organized.5–7 This highly organized microarchitecture and many other details of the nanostructure contribute to the enhanced mechanical properties of nacre. In the literature, extensive studies on nacre can be
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0247 J. Mater. Res., Vol. 21, No. 8, Aug 2006
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found that are being conducted to investigate the facts behind its enhanced properties such as toughness, impact resistance, and strength. Different kinds of experiments and computer-based simulations like finite element analysis (FEA) are being performed to decipher the nanostructu
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