! "# $% &"’ (## )! $% *# $% +, - ) .../ 0 1%$ 1 0 1. 02 34 ) ...#! Introduction Biological materials, such as hard tissue, bone, shell, present optimized structures and excellent properties mainly thanks to their hierarchical structural patterns originating at the nanometre scale. The understanding of these structures is of great scientific interest for applications to man-made materials (biomimetics). In particular, the development of advanced bio- inspired materials is expected to be fundamental for the manufacture of future military equipment, such as hard and tough lightweight body armour. The aim is to deposit a hierarchical impact resistance coating (external) inspired by nacre (mother of pearl) and to develop an expandable auxetic foam to create “smart airbags” (internal) to minimise behind armour blunt trauma (BABT) (Fig. 1). Nacre is a ceramic laminate composite made of aragonite platelet layers separated by thin layers of organic material; this organized structure produce a significant increase in the aragonite mechanical properties. Acknowledgements The authors would like to thank EPSRC and Dstl for the financial support. ATL and Renesy for the deposition of the coatings. References [1] Materials Research to Meet 21st Century Defense Needs, National Materials Advisory Board, The National Academies Press, (2003). [2] Bruet B. J. F. et al. (2008) Nature Materials 7 748-756. [3] Evans K. E. et al. (2000) Adv. Mat. 12:617-628. Hierarchical coatings Fig. 2 demonstrates that laminated hierarchical biological structures such as nacre can be mimicked by micro-laminated ceramic-metal composites producing a significant improvement in mechanical properties [1]. Therefore B 13 C 2 - TiVZr and TiVZr - TiVZr nitride multilayered coatings will be deposited by chemical and physical vapour deposition (CVD and PVD). The composition, microstructure and thickness of the coatings will be varied to optimize impact energy dissipation through crack propagation, plastic deformation and elastic responses. The structure of the coating will be inspired by biological systems, for instance the ancient fish armour described in [2]; where the hard outer layer of fish scales transfers the impact load to the underlying softer layers that disperse the energy via plasticity and also defend from deeper penetrations. Fig. 1 - Schematic of the combined outer ceramic laminate composite and inner auxetic foam layer. Fig. 2 - Mechanical properties of natural and synthetic materials. B 4 C/Al composites (produced to mimic nacre) show significant improvement in properties over single-phase B 4 C [1]. Substrates A range of substrates will be chosen from available military materials, such as bulk ceramics, aluminium alloys, armour grade steel and Kevlar depending on the multilayer deposition conditions and their impact performance. Auxetic foams Auxetic materials expand when stretched and become thinner when compressed due to their negative Poisson's ratio (Fig. 3). A re-entrant cell (Fig. 4) or buckled rib thermoplastic polymer auto- expanding foam will be made. The numerous branching substructures generate a complex load redistribution which affects the structure deformation. This strain induced expansion could be useful for minimising BABT. Fig. 3 - Difference in the deformation profile (exaggerated) of non-auxetic and auxetic materials [3]. Fig. 4 - Schematic deformation of a re-entrant honeycomb network applying a tensile load [3]. Non- auxetic Auxetic Inner layer: an auxetic foam structure Armour substrate Outer layer: a laminate composite structure