Ortiz Research Group Fall 2008 MRS Abstracts

TITLE: Functionally-graded sandwich design of the armor protecting a hot-vent gastropod AUTHORS (FIRST NAME, LAST NAME): Haimin Yao1, Ming Dao1, Timothy Imholt2, Subra Suresh1, Christine Ortiz1 INSTITUTIONS (ALL): 1. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambirdge, MA, USA. 2. Raytheon, Inc, Marlboro, MA, USA. ABSTRACT BODY: Recently, a gastropod mollusk was discovered near the Kairei deep-sea hydrothermal vent field of the Central Indian Ridge, where it experiences a harsh environment characterized by extreme pressure (~250 atm), lack of light, high temperatures (~350-400°C) and temperature gradients (~350°C/cm). In addition, since this snail lives a relatively sedentary life, it can suffer attacks from predators such as Phymorhynchus snails and Brachyuran crabs. In this study, we have discovered that the microstructure of the shell of this hot-vent gastropod mollusk is an exceedingly unique multilayered, functionally graded "sandwich" structure possessing a compliant organic layer embedded in between two stiffer mineralized layers that is beneficial for both mechanical and thermal protection. Backscattered electron microscopy, Energy Dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) analyses indicate that the armor possesses an outer nodular mineralized layer (~20 μm thick) embedded with iron sulfide (greigite, Fe3S4) particulates, followed by a graded composite transition region (~10 μm thick) to an inner, relatively thick (~150 μm thick) organic layer, followed by another graded interphase region (~50 μm thick) to the inner thicker (~200 μm thick) non-nacreous, highly calcified, aragonitebased layer. Instrumented indentation across the cross-section of the shell yielded indentation moduli and hardness values as follows; the outer nodular iron-based layer, the middle organic layer, and the inner calcified layer have moduli 28.84±6.89 GPa, 8.04±0.38 GPa, and 98.89±6.63 GPa respectively and hardness values of 5.4±0.6 GPa, 0.46±0.03 GPa and 1.73±0.57 GPa, respectively. The properties of the gradient layers are intermediate to those of the neighboring homogeneous layers. Based on these experimental data, the multilayered microindentation finite element computational model of the entire shell "sandwich-like" structure was created and compared to various monolayered structures. This multilayered model was predicted to have an excellent combination of high stiffness and high energy absorptivity, suggesting superior resistance to both static and kinetic mechanical attacks. Moreover, a comparative simulation study on the thermal response to a transient, external impact of high temperature (100 °C) was carried out. Among different structures for comparison, the multilayered shell structure again stands out in terms of mitigating the impact of high temperature. The maximum internal temperature experienced by this structure is only about 64 °C, while for other control structures it reaches up to 91.7 °C. The fundamental concepts discovered here may be able to provide guidelines for the design of human synthetic engineered armors.