Natural composite materials are renowned for their mechanical strength and toughness: despite being highly mineralized, with the organic component constituting not more than a few per cent of the composite material, the fracture toughness exceeds that of single crystals of the pure mineral by two to three orders of magnitude. The judicious placement of the organic matrix, relative to the mineral phase, and the hierarchical structural architecture extending over several distinct length scales both play crucial roles in the mechanical response of natural composites to external loads. Here we use transmission electron microscopy studies and beam bending experiments to show that the resistance of the shell of the conch Strombus gigas to catastrophic fracture can be understood quantitatively by invoking two energy-dissipating mechanisms: multiple microcracking in the outer layers at low mechanical loads, and crack bridging in the shell's tougher middle layers at higher loads. Both mechanisms are intimately associated with the so-called crossed lamellar microarchitecture of the shell, which provides for 'channel' cracking in the outer layers and uncracked structural features that bridge crack surfaces, thereby significantly increasing the work of fracture, and hence the toughness, of the material. Despite a high mineral content of about 99% (by volume) of aragonite, the shell of Strombus gigas can thus be considered a 'ceramic plywood' and can guide the biomimetic design of tough, lightweight structures.

The crossed-lamellar microarchitecture (microstructure) of the shell of Strombus gigas, the giant Queen conch native to Caribbean habitats, is the most common of the several shell microarchitectures known in the mollusk family. We have studied tissue regeneration in juvenile S. gigas conchs and compared the microstructure in this regenerated tissue to the microstructure of wild S. gigas shells. The regenerated hard tissue was of two types: hard tissue grown during wound repair, and so-called "flat pearls" which are hard tissue grown on abiotic substrates inserted between the mantle and the outer covering. In both cases, the crossed-lamellar microstructure is observed after formation of a transition structure consisting of a large quantity of matrix and aggregates of aragonite crystallites.

Carbon nitride films have been prepared by ion beam assisted deposition with high N-ion R U X ~S up to the order of 1 mA Both Auger elecmn spectroscopy and Rutherford backscattering spectroscopy measurements show that an average nitrogen concentration of up to 45 aL% has been incorporated into the film. The nitrogen concentration in some local areas reaches as high as 59 at.% examined by energy-dispersive x-ray analysis. In such a high nitrogen content area, Lane spots of the predicted PC3Nd smcNre have been observed for the first time by transmission electron microscopy. Under continuous electron irradiation, B-CsNd gains refine to dimensions of < 0.1-0.4 pm in diameter.Recently, considerable effort has been directed towards the synthesis of the covalent solid carbon nitride B-CgN.4. This effort was initiated following a theoretical prediction by Liu and Cohen [l] that p-C3N4 would have excellent properties, such as a hardness comparable to or greater than that of the hardest known material, diamond. Further attention stems from the fact that six electron diffraction rings of the predicted p-C3N4 phase have been observed in the~C-N thin film prepared by pulsed laser ablation [2]. In addition, the potential applications of this new material will be extensive, for example, Si is lattice-matched to B-C3N4 131, which can promote a layer by layer growth of pC3N4 on Si.So far various approaches of material synthesis have been used in attempts to prepare pC3N4, such as pulsed laser ablation of graphite targets combined with an atomic nitrogen beam [2], N-['4,51 or NH3-[6] ion beam assisted deposition (BAD), electron cyclotron resonance plasma assisted vapour deposition [7], rf diode sputtering [SI, N-ion implantation into carbon [9] and dc magnetron sputtering [3], etc. None of these efforts have succeeded in synthesizing carbon nitride films with fhe stoichiometry of pC3N4. In most of the films the concentration ratio C/N is about 0.7 or below. Based on x-ray photoelectron spectroscopy (XPS) analysis for binding states of carbon and nitrogen atoms, Marton et a1 found that their carbon nitride films, deposited by three different methods, were composed of two phases that have sp3 and sp2 bonded structures, respectively [4]. As far as the growth of crystalline B-C3N4 is concerned, the direct evidence reported is rare. Most films were found to be amorphous except that a few reports show the existence of crystalline p-CsN.4 by presenting several electron diffraction rings from the tiny pC3N4 grains embedded in the matrix of the films [2,6,8]. In the literature [2,6] the grain size of B-C3N4 crystallites was c10 nm. In [8], the j?-C3Nd crystallites were reported to have typical dimensions of M 0.5 p m and occupy less than 5% of the film volume.

Hard tissue regeneration (i.e. shell repair) is an important biomineralization process in mollusks. Rapid regeneration is important in avoiding loss of fluids and preventing attacks by predators. We have studied such tissue regeneration in the Queen conch (Strombus Gigas) by inserting an abiotic glass cover slide between the mantle tissue and the shell. The glass cover slides were removed after mineralization periods extending from 6 hours to 4 days, and the deposited materials on the glass substrates (“flat pearls”) analyzed by X-ray diffraction, and scanning and transmission electron microscopy.Although the CaCO3 in native Queen conch shell is exclusively aragonite, calcite was detected in the regenerated materials grown on glass substrates. Calcite formation occurred only during the very early stage of mineralization and the initial minerals formed were soon overgrown by aragonite. The initial aragonite overgrowth had a spherulitic morphology, and was thus relatively poorly oriented; after this spherulitic transient, the microstructure was recognizable as the crossed-lamellar structure of the natural shell.Thus, shell regeneration on abiotic substrates differs from shell formation during growth of conchs only in the very early stage. Once the crossed-lamellar microstructure has formed, further hard tissue development is identical to that occurring during natural shell growth.

Microindentation using a hard indentor is a very useful tool to probe the local mechanical properties (plastic flow, fracture, toughness, creep, etc.) of hard solids. We have used a depth sensitive indentation (DSI) version of this technique to study the shell of the conch Strombus gigas.In particular, we prepared polished sections in which the third order lamellae, the basic building block of the structure, could be indented along the three orthogonal directions; the indentation behavior along these three directions is quite different. Besides the normal information such as the hardness H from some typical dimension (typically the diameter of a pyramid-shaped indentation produced by a so-called Vickers diamond indenter) after indenting with a given load, the DSI technique also provides much additional information available from the loading and unloading curves. As can be seen from Fig. 1, the loading and unloading curves for the three orientations are quite different, as is the depth of the residual impression.