Nanoscale Morphology and Indentation of Individual Nacre Tablets from the Gastropod Mollusc Trochus Niloticus
- PDF / 3,162,486 Bytes
- 20 Pages / 612 x 792 pts (letter) Page_size
- 5 Downloads / 163 Views
H.J. Qia) and M.C. Boyce Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
R. Panas, K. Tai, L. Frick, and C. Ortizb) Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 27 January 2005; accepted 2 May 2005)
The inner nacreous layer of gastropod mollusc Trochus niloticus is composed of ∼95 wt% planar arrays of polygonal aragonite-based tablets (∼8 m wide, ∼0.9 m thick, stacked ∼40 nm apart) and ∼5 wt% biomacromolecules. High-resolution tapping mode atomic force microscope images enabled nanoscale resolution of fractured tablet cross-sections, the organic component, and deformation of individual nanoasperities on top of tablet surfaces. Nanoindentation was performed on individual nacre tablets and the elastic modulus E and yield stress y were reduced from elastic-plastic finite element simulations yielding E ⳱ 92 GPa, y ⳱ 11 GPa (freshly cleaved samples) and E ⳱ 79 GPa, y ⳱ 9 GPa (artificial seawater soaked samples). Images of the indents revealed extensive plastic deformation with a clear residual indent and surrounding pileup.
I. INTRODUCTION
Nacre is an extraordinary example of a hierarchical biological nanocomposite and is found in the interior of many mollusc shells.1,2 Though nacre is composed of exceedingly weak constituent materials, its unique and highly organized design at multiple length scales (Fig. 1) enables outstanding mechanical performance including an excellent combination of stiffness, strength, impact resistance, and toughness.1,3–6 The structure of nacre has been reviewed previously;7,8 it is a composite material with a so-called “brick-and-mortar” structure [Fig. 1(a)] consisting of alternating layers of mineral tablets separated by thin layers of a biomacromolecular “glue.” Nacre is composed of ∼95 wt% pseudo-hexagonal, polygonal, or rounded aragonite tablets (a mineral form of CaCO3, orthorhombic, Pnma9) which have dimensions of ∼5–20 m (plane with normal defined by the crystallographic c[001]-axis) and ∼0.3–1.5 m in thickness (vertically parallel to the c[001]-axis)7,8 as shown in
a)
Present address: Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309 b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0273 2400
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 9, Sep 2005 Downloaded: 13 Mar 2015
Fig. 1(b). For gastropod nacre, demineralization and scanning electron microscopy imaging suggests that each tablet is subdivided by radial vertical organic membranes into a varying number (2–50) of sectors [Fig. 1(c)], which have been interpreted as polysynthetically twinned crystalline lamellae.10 The surface of nacre tablets [Fig. 1(d)] from California red abalone (Haliotis rufescens) possesses nanoasperities (∼30–100 nm diameter and ∼10 nm in height)3,11 and mineral “bridges” [Fig. 1(b)] (∼25–34 nm in size, ∼91–116 m−2 surface density)12–16 between sheets which pass throu
Data Loading...
