Abalones are members of a large class of mollusks that have one-piece shells. The shell is thin, lightweight, and for millions of years, has protected abalones from many large and powerful natural marine predators. The interior surface of the shell is mother-of-pearl, or nacre, which is iridescent, hard, strong, and tough. Scientists have found that nacre is made of micron-sized tiles stacked like bricks.
Recently, it was discovered that each tile is further composed of nanometer-sized crystals (~30nm in diameter), with a very thin organic layer (~1 nm) intercalated between crystal boundaries. While nacre is 95-99% by volume calcium carbonate, surprisingly, it is 20 times stronger and 3000 tougher (in terms of energy required to fracture it) than calcium carbonate crystal, the same material that makes chalk, limestone, marble and travertine.
The macroscopic properties of nacre, like many natural structural or armor materials, are the product of hierarchical microstructural organization over multiple length scales. Nature makes very different systems out of similar building blocks. We believe that nature’s strategy in making materials with drastically different properties out of same constituents is the materials microstructural architecture.
The challenge for understanding the effect of microstructural architecture is that current experimental techniques cannot differentiate the contribution of different structural features. Although mechanical properties of millimeter- to meter-sized materials can be measured experimentally, interpretations of the data are difficult.
The development of tools – from optical microscopy to electron microscopes – has advanced our understanding of the microstructures and microstructural architectures in nature’s materials. These scientific tools can provide information on the topography, morphology, composition, and crystallographic information of a material. To identify nature’s design strategies in materials with microstructural architecture, we believe that we need an advanced simulation tool to complement our experiment tools. This way, we can establish the structure-property relationships and providing insights on the deformation mechanisms.
Our research on nature materials was supported by the National Science Foundation. Through a recent grant made by the Defense Advanced Research Projects Agency (DARPA) Young Faculty Award Program, our group will study the hierarchically-structured nacre with the goal to establish the effect of the microstructural architecture on its mechanical properties. For this project, we will first establish a theoretical and computation tool that can predict the properties of materials from their fundamental constituents, i.e., atoms and molecules; we will then build a structural model of nacre from the atoms of calcium carbonate and nacre proteins for reproducing the mechanical properties of nacre in computer.
Based on our findings, we believe we will be able to make ceramic materials that are stronger and tougher than both nacre and existing man-made ceramics. We hope this research will contribute to improve the Department of Defense capability in prediction and design of new materials for defense applications; in particular, we look forward to working with DARPA and DOD communities for the development of lightweight ceramic armor materials.