17.1 Introduction

This chapter examines a relatively new paradigm for tailoring the mechanical properties of composite materials. As has often happened throughout history, biological systems have been the inspiration. Recent studies of biological ceramic composites (bone and teeth, mollusk shells, etc.) have generated a number of new ideas on how structure can affect material properties (Weiner and wagner, 1998; Weiner and Addadi, 1997; Aksay and Weiner. 1998: Wang, et al., 1997; Wang, 1998; Berman, et al., 1993, 1990, 1988; Smith, et al., 1999).

Figure 17.1: Examples of biological structures that have inspired new materials, with (a) abalone, (b) teeth, and (c) radiolaria.

The most important idea to emerge, and the focus of much of this chapter, is the notion of hierarchical structure. Simply stated, hierarchical structure refers to the existence of structural organization at multiple length scales (often nanometer to micrometer, to millimeter) within a given material. Examples of such structures are shown in Fig. 17.1. Of course, all materials possess some degree of organized structure at multiple length scales; the key is that this organization occurs in such a way as to cooperatively enhance a given property. In the case of biological ceramic composites, organic phase controls the structure of the mineral phase at multiple length scales in a manner that leads to dramatic improvements in the mechanical toughness over bulk mineral.

The nacreous layer of mollusk shells is one example of this phenomenon. Mollusk shells...