TiC/metal nacrous structures and their fracture toughness increase
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TiCyymetal nacrous structures and their fracture toughness increase C. H. Liu Department of Physics, Tsinghua University, Beijing 100084, China
Wen-Zhi Li and Heng-De Li Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China (Received 7 December 1995; accepted 12 April 1996)
Multilayers of TiC and a series of metals have been fabricated by ion beam sputtering deposition to simulate nacre. The individual layer thickness varies from 1 to 10 nm, and the total thickness of the multilayers is about 1 mm. Transmission electron microscopy (TEM), low-angle x-ray diffraction (LXRD), and high-resolution electron microscopy (HREM) show their periodicity and lattice images. A particular method is devised to evaluate the relative toughness of this artificial pearlite. It is shown that the toughness of these nanocomposite materials can be tremendously improved. A maximum of toughness appears at a certain modulation. Metals with high plasticity such as Al and Cu can produce a particularly spectacular effect on increasing the toughness of these multilayers.
I. INTRODUCTION
Biomaterials are receiving more and more attention from the materials science community. Among them nacre from mollusk shells is perhaps one of the most thoroughly investigated,1–7 although details involved remain largely unknown. The fine and orderly microstructure of nacre and its biological process of formation are both fascinating and intriguing. In essence, nacre consists of a brick-and-mortar microstructure on the nanometer scale. Aragonite crystals of some 400 nm in thickness are imbedded in a thin (about 40 nm) network of biopolymeric matrix. Because of the special combination of the two phases and their highly ordered arrangement, nacre possesses considerably higher fracture toughness than ordinary monolithic ceramics, while also possessing high strength. This is one very important reason why materials scientists are so interested in nacre. From these natural biomaterials we can learn some guidelines to design and construct new and tough ceramic composites. We noticed that some efforts 8–10 have been made to prepare the pearl-like composites. However, so far the attempts have been made only on the macro scale to construct some illustrating models. The layer thickness of the ceramic phase in these model systems was generally about 100–400 mm, and the processing often required high temperatures. This was far from the fine scale of pearlite. A multifunction piece of equipment developed in this laboratory provides a versatile means to prepare various systems of multilayers on the nanometer scale at room temperature. In this study, we have constructed multilayers of TiC (ceramic) and a series of metals J. Mater. Res., Vol. 11, No. 9, Sep 1996
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(Fe, Cr, Co, Ni, Cu, Al, etc.) to mimic the nacrous structure on the nanometer scale. In processing, this microassembling is also similar to the moving front growth process of na
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