Simulation of nacre with tin/pt multilayers and a study of their hardness
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Simulation of nacre with TiNyyPt multilayers and a study of their hardness J. L. He, W. Z. Li, and H. D. Li Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China (Received 30 May 1996; accepted 22 May 1997)
TiNyPt multilayers with individual thicknesses between 1 and 8 nm were prepared by ion beam sputtering deposition to simulate the micro-laminated architecture of nacre. Multilayer hardness and the laminated structure were investigated. It was found that sharp but incoherent interfaces were formed between individual layers. The multilayer hardness had strong dependence on layer arrangement. The range of layer thickness appropriate for high hardness was experimentally determined. Hardness enhancement of 30 –70% was generally observed. With the layer thickness properly adjusted, the multilayer can even be harder than the hard component (TiN). Annealing experiments indicated that the hardness enhancement was an intrinsic property of the TiNyPt multilayers.
I. INTRODUCTION
Biological systems can be a great source of inspiration for the design and processing of novel synthetic materials. Among them, nacre is perhaps one of the most interesting and thoroughly studied.1–4 Composed of alternating nanoscale layers of proteins and CaCO3 platelets, nacre possesses considerably high mechanical properties, such as fracture toughness and strength. The unique “brick and mortar” structure, which efficiently integrates the hardness of aragonite crystals and the plasticity of biopolymetrics, offers material researchers a promising access to new composites with outstanding mechanical properties. While great efforts have been made to simulate nacre with multilayers of ceramics and metals,5–8 many were conducted at the micron/above level, much coarser in comparison with the nanostructure of nacre. Improvement of fracture toughness or fracture strength was usually the main consideration of this kind of simulation. Recently, however, there have been increasing interests in the hardness study of dissimilar material nano-multilayers. Hardness enhancement was widely observed when the composition modulation wavelength (bilayer repeat length) was reduced below about 10 nm.9–13 Extensive research has been conducted on metal/metal9,10,14,15 and ceramic/ceramic systems.11,16–18 From an application point of view, however, hardness enhancement in ceramic/metal systems should be more interesting and more promising because fracture toughness or strength can be simultaneously improved. Excellent comprehensive properties can thus be obtained. In the literature, there were already reports about hardness enhancement in ceramic/metal superlattices.13,19,20 Hardness behavior versus modulation wavelength was also investigated.15,18,21 Nevertheless, much further work is still required to clearly understand 3140
http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 11, Nov 1997
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the effects of layer arrangement on hardness behavior and the
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