Design of Lightweight and High-Strength Functionally Graded Materials Inspired by the Hard Shells of Plants

Functionally graded materials (FGMs) are novel composite materials with gradual variations in their structures and chemical compositions throughout their bodies, so that FGMs possess locally tailored properties. Since FGMs consist of continuous structures without borders of physical property changes, conflicting physical properties within a material can be maintain not to break in their bodies. In contrast to isotropic bulk materials, the structures and chemical compositions of FGMs can be accurately designed to create tailored multifunctional properties. Noted to the natural world, there are many kinds of FGMs such as bones, teeth, bamboos, shells, and crustacean shells. They continuously change their microstructures and chemical compositions, with corresponding smooth changes in mechanical properties such as hardness, stiffness, and toughness. These natural materials mentioned above are composed of elements lighter than iron, such as carbon, silicon, and calcium. They are both lightweight and strong with limited resources by placing these elements in the necessary quantities and in the necessary places. In recent years, much research has been conducted to mimic these natural materials to reduce the weight and increase the strength of materials. In this study, we focused on the grass species, Job’s tears (Coix lacryma-jobi). The seeds of Job’s tears are covered with the involucre, a thin, lightweight, hard shell containing a high amount of silica. This shell is only about 740 μm thick, but is strong enough to withstand a load of about 280 N. We analyzed the spatial distribution of porosity, chemical composition, and hardness of these shells to investigate the factors that contribute to their lightweight and high-strength. Then, FGMs mimicking the shells of Job’s tears were fabricated based on the analysis results. We used scanning electron microscopy (SEM) to observe the cross-sectional microstructures of the shell of Job’s tears and energy dispersive X-ray spectroscopy (EDX) to analyze their chemical compositions. The inner layer of the shell was composed of tubular plant cells with a diameter of approximately 2 μm. The plant cells became smaller and more isotropic toward the outer layer and were densely distributed. The outermost layer of the shell was deposited with fine particles less than 100 nm in diameter. Therefore, the porosity of the shell decreased toward the outer layer. EDX analysis showed that the shells contained more silica toward the outer layers.<br/>We measured the change in hardness from outer to inner layer in the cross-section of the shell by Vickers hardness test and nanoindentation test. The Vickers hardness test revealed that the outer layer of the shell was about four times harder than the inner layer, and the nanoindentation test showed a gradual decrease in elastic modulus from the outer to the inner layer of the shell. In addition, we analyzed the stiffness characteristics of the shells by performing three-point bending tests in different loading directions. The shells withstood three times more strain and twice as much load when bent from the outer layer of the shell compared to when bent from the inner layer.<br/>These results indicate that the outer layer of the shell is harder and the inner layer softer due to spatial gradients in both shell porosity and chemical composition. This material property would allow the shell to withstand loads from outside the shell.<br/>Based on our analyses, we fabricated FGMs mimicking the shells of Job’s tears. We printed porous materials with a spatial gradient of porosity using fused deposition modeling 3D printer and polyurethane filaments whose extent of foaming and hardness changes with heating temperature. By impregnating the porous materials with glass or silicone resin, we expressed the spatial gradient in chemical compositions and hardness. This study may provide design guidelines for lightweight, high-strength organic/inorganic hybrid FGMs.