Toughening mechanisms in abalone shell

Abstract

Abalone shell (Haliotis Rufescens) is a naturally ocurring ceramic/polymer composite material. The system displays a unique laminated structure of calcium carbonate (aragonite) crystals in a matrix of biological macromolecules. The CaCO3 crystals and the organic matrix are arranged in a miniature “brick and mortar” structure referred to as nacre. Figure 1 is a TEM bright field micrograph illustrating the high degree of order observed in this microstructure.Although the nacre region of the shell is more than 95% CaCO3 by volume, the natural matrix material and the arrangement of the microstructure lead to a substantial increase in the observed mechanical properties. Mechanical tests performed on the nacre region show a fifty-fold increase over that of pure bulk CaCO3 (Fig. 2), which also compares with other ceramic and cermet systems.Vickers microhardness testing was performed on samples polished for optical microscopy. Crack propagation features were observed by standard SEM techniques and analyzed in an attempt to identify the possible toughening mechanisms that are operating in the nacre structure. The cracks generally travel by a tortuous path, often displaying microcracks and crack branching. However, these mechanisms alone are not sufficient to account for the observed mechanical properties.