Graduate Student, University of California, Berkeley
I create and study ceramics inspired by mother-of-pearl, which is actually ~95% (by volume) chalk!
The next generation of structural materials will be inspired by designs found in nature. We use nacre, or the mother-of-pearl found in abalone shells, as a model material to create bioinspired structural ceramics with high toughness. Natural materials are fascinating because they combine multiple desirable properties that are rarely seen in synthetic materials, such as sensing and self-healing capabilities. Compared to natural materials, there are no current synthetic materials that display both high strength and high toughness.
Toughness is a measure for how sensitive a material is to flaws, while strength is a measure for how much force a material can take without permanently deforming. To illustrate this, think of how easy it is to tear a paper with a small flaw, like a rip, compared to a fresh sheet of paper. This is because the paper has high strength but low toughness, similar to engineering ceramics. This limits current applications because this high sensitivity to flaws make ceramics likely to catastrophically shatter when in use. On the other hand, metals have low strength compared to ceramics, but they have high toughness. You can observe this in how easily you can bend a paper clip without it breaking in half. These examples shows the tradeoff found in current synthetic materials. Strong materials are brittle, but ductile materials are weak. Natural materials provide inspiration for tough ceramics by using well-defined architectural concepts within the microstructure. They display toughening mechanisms that are rarely found in synthetic materials.
Nacre displays 3000 times higher toughness (in energy terms) compared to its constituents, which is 95% chalk. This dramatic increase in toughness is a direct result of its brick-and-mortar microstructure. By mimicking this microstructure, we can create a tough and strong ceramic that can be used in structural applications. It can be used in energy applications to replace the heavy metals used in gas turbine blades, enabling them to generate more power with the same amount of fuel. We replicate this structure by coating premade alumina platelets with nickel through heterogenous precipitation. These platelets are aligned using slip casting before consolidating it using spark plasma sintering. The final materials illustrate the need to account for how the composition of the slurry and sintering conditions can affect the strength and fracture toughness of the bulk ceramic materials.
Abstract: Nacre is the iridescent layer found inside a large number of mollusk shells. This natural composite has a very high mineral content, which makes it hard and stiff. However it is the toughness of nacre which is the most impressive: it is three orders of magnitude tougher than the mineral it is made of. No manmade composite material can boast such amplification in toughness, and for this reason nacre has become a biomimetic model material. The mineral in nacre comes in the form of microscopic polygonal tablets, which have the ability to 'slide' on one another in large numbers when the material is loaded in tension. This key mechanism makes nacre a quasi-ductile material, which in turn greatly increases its toughness and makes it damage tolerant. Numerous 'artificial nacres' were developed in the past but none of them can truly duplicate the remarkable mechanism of tablet sliding. In this work selected structural features of nacre were implemented in a PMMA-based composite, which for the first time could replicate the collective tablet sliding mechanism. This material demonstrates that the powerful toughening mechanism operating in natural nacre can be duplicated and harnessed in engineering materials.
Pub.: 24 Aug '10, Pinned: 01 Jul '17
Abstract: Nacre, also known as mother-of-pearl, is a hard biological composite found in the inside layer of many shells such as oyster or abalone. It is composed of microscopic ceramic tablets arranged in layers and tightly stacked to form a three-dimensional brick wall structure, where the mortar is a thin layer of biopolymers (20–30 nm). Although mostly made of a brittle ceramic, the structure of nacre is so well designed that its toughness is several order of magnitudes larger that the ceramic it is made of. How the microstructure of nacre controls its mechanical performance has been the focus of numerous studies over the past two decades, because such understanding may inspire novel composite designs though biomimetics. This paper presents in detail uniaxial tension experiment performed on miniature nacre specimens. Large inelastic deformations were observed in hydrated condition, which were explained by sliding of the tablets on one another and progressive locking generated by their microscopic waviness. Fracture experiments were also performed, and for the first time the full crack resistance curve was established for nacre. A rising resistance curve is an indication of the robustness and damage tolerance of that material. These measurements are then discussed and correlated with toughening extrinsic mechanisms operating at the microscale. Moreover, specific features of the microstructure and their relevance to associated toughening mechanisms were identified. These features and mechanisms, critical to the robustness of the shell, were finely tuned over millions of years of evolution. Hence, they are expected to serve as a basis to establish guidelines for the design of novel man-made composites.
Pub.: 02 Mar '07, Pinned: 01 Jul '17
Abstract: Nacre possesses a remarkable combination of mechanical properties. Its high stiffness, strength and toughness are attributed to a highly aligned structure of aragonite platelets "glued" together by a small fraction (∼5vol%) of polymer; theoretically it can be described by a shear-lag model of staggered tensile elements between which loads are transferred via shear. Despite extensive research, it has not been possible yet to manufacture this aligned structure as a bulk material of considerable volume with a fast and easy production process. Particularly porous materials would benefit from enhanced wall material properties to compensate for performance loss due to their high porosity. An important application for such porous materials are tissue scaffolds for bone substitution. Bone, like nacre, exhibits excellent mechanical properties, particularly an exceptionally high toughness, because of its composite structure of hydroxyapatite platelets aligned in a ∼35vol% polymer matrix. Through the freeze casting process, which results in a fast and straightforward self-assembly of platelet-shaped particles during directional solidification, highly porous bulk materials with nacre-like cell walls can now be created. This porous nacre outperforms by a factor of 1.5-4 in terms of stiffness, strength and toughness materials that have the same amount of porosity but do not exhibit the nacre-like microarchitecture. The self-assembly process presented in this study thus has tremendous potential for the creation of highly porous, yet mechanically strong tissue scaffolds for low or medium load bearing bone substitute materials. Due to the versatility of the freeze casting process, materials with a self-assembled cell wall structure can be created from high-aspect ratio particles of all material classes. This enables material optimization for a great variety of applications such as impact protection, filtration, catalysis, energy generation and storage, in addition to those with excellent mechanical properties at high porosity.
Pub.: 15 Jan '13, Pinned: 01 Jul '17
Abstract: The development of composite materials that are simultaneously strong and tough is one of the most active topics of current material science. Observations of biological structural materials show that adequate introduction of reinforcements and interfaces, or interphases, at different scales usually improves toughness, without reduction in strength. The prospect of interphase properties tuning may lead to further increases in material toughness. Here we use evaporation-driven self-assembly (EDSA) to deposit a thin network of multi-wall carbon nanotubes on ceramic surfaces, thereby generating an interphase reinforcing layer in a multiscale laminated ceramic composite. Both strength and toughness are improved by up to 90%, while keeping the overall volume fraction of nanotubes in a composite below 0.012%, making it a most effective toughening and reinforcement technique.
Pub.: 28 May '16, Pinned: 01 Jul '17
Abstract: Sintering and accompanying microstructural evolution is inarguably the most important step in the processing of ceramics and hard metals. In this process, an ensemble of particles is converted into a coherent object of controlled density and microstructure at an elevated temperature (but below the melting point) due to the thermodynamic tendency of the particle system to decrease its total surface and interfacial energy. Building on a long development history as a major technological process, sintering remains among the most viable methods of fabricating novel ceramics, including high surface area structures, nanopowder-based systems, and tailored structural and functional materials. Developing new and perfecting existing sintering techniques is crucial to meet ever-growing demand for a broad range of technologically significant systems including, for example, fuel and solar cell components, electronic packages and elements for computers and wireless devices, ceramic and metal-based bioimplants, thermoelectric materials, materials for thermal management, and materials for extreme environments. In this study, the current state of the science and technology of sintering is presented. This study is, however, not a comprehensive review of this extremely broad field. Furthermore, it only focuses on the sintering of ceramics. The fundamentals of sintering, including the thermodynamics and kinetics for solid-state- and liquid-phase-sintered systems are described. This study summarizes that the sintering of amorphous ceramics (glasses) is well understood and there is excellent agreement between theory and experiments. For crystalline materials, attention is drawn to the effect of the grain boundary and interface structure on sintering and microstructural evolution, areas that are expected to be significant for future studies. Considerable emphasis is placed on the topics of current research, including the sintering of composites, multilayered systems, microstructure-based models, multiscale models, sintering under external stresses, and innovative and novel sintering approaches, such as field-assisted sintering. This study includes the status of these subfields, the outstanding challenges and opportunities, and the outlook of progress in sintering research. Throughout the manuscript, we highlight the important lessons learned from sintering fundamentals and their implementation in practice.
Pub.: 16 May '17, Pinned: 01 Jul '17
Abstract: Biological materials are complex composites that are hierarchically structured and multifunctional. Their mechanical properties are often outstanding, considering the weak constituents from which they are assembled. They are for the most part composed of brittle (often, mineral) and ductile (organic) components. These complex structures, which have risen from millions of years of evolution, are inspiring materials scientists in the design of novel materials. This paper discusses the overall design principles in biological structural composites and illustrates them for five examples; sea spicules, the abalone shell, the conch shell, the toucan and hornbill beaks, and the sheep crab exoskeleton.
Pub.: 01 Jul '06, Pinned: 01 Jul '17
Abstract: Living organisms are adept at resisting contact deformation and damage by assembling protective surfaces with spatially varied mechanical properties, i.e., by creating functionally graded materials. Such gradients, together with multiple length-scale hierarchical structures, represent the two prime characteristics of many biological materials to be translated into engineering design. Here, we examine one design motif from a variety of biological tissues and materials where site-specific mechanical properties are generated for enhanced protection by adopting gradients in structural orientation over multiple length-scales, without manipulation of composition or microstructural dimension. Quantitative correlations are established between the structural orientations and local mechanical properties, such as stiffness, strength and fracture resistance; based on such gradients, the underlying mechanisms for the enhanced protective role of these materials are clarified. Theoretical analysis is presented and corroborated through numerical simulations of the indentation behavior of composites with distinct orientations. The design strategy of such bioinspired gradients is outlined in terms of the geometry of constituents. This study may offer a feasible approach towards generating functionally graded mechanical properties in synthetic materials for improved contact damage resistance.
Pub.: 05 Aug '16, Pinned: 01 Jul '17
Abstract: Parallel and concentric alignment of microscopic building blocks into several orders of magnitude larger structures is commonly observed in nature. However, if similarly aligned structures are artificially produced their thickness is generally limited to just about one or two orders of magnitude more than the dimensions of the smallest element. We show that sedimentation provides a promising approach to manufacture solid materials consisting of well-aligned platelet-shaped particles while being more than 30,000 times thicker than the individual particle. Such sediments contain up to 28 vol% of particles without any further treatment and can be densified to 67 vol% particle fraction by subsequent unidirectional pressing. The degree of orientation of the platelet-shaped particles within the sediments was tracked by high-energy X-ray diffraction measurements. The Hermans orientation parameter, a statistical measure of the quality of alignment, was determined to be 0.63 ± 0.03 already for as-sedimented samples while the standard deviation of the orientation distribution of particles, another measure of average misalignment, was found to be (21.5 ± 1.4)°. After pressing, these values further improved to (0.81 ± 0.01) and (14.6 ± 0.4)°, respectively. Such quality of alignment competes with, if not even exceeds, values reported in the literature.
Pub.: 20 May '15, Pinned: 01 Jul '17