Nacre-inspired Composites Research Articles

Fabrication and characterization of nacre-inspired metal/ceramic composites containing AlN nanowhiskers

Nacre-like composites prepared by infiltrating ice-templated porous ceramic-scaffolds with molten metal phases have successfully imitated the strengthening and toughening mechanisms of nacre at different lengthscales from macro to micro, but have not yet been able to replicate the nanoscale toughening mechanisms of nacre, which limits the further improvement in mechanical properties of the nacre-inspired composites. To resolve this issue, we prepared nacre-inspired Al/SiC composites containing AlN nanowhiskers by in-situ growth of whiskers during the spontaneous infiltration process of Al-10wt.%Mg alloy into an ice-templated porous SiC skeleton. It was found that the oxidation of the SiC skeleton had a significant impact on the growth of AlN nanowhiskers, as well as the phase composition and mechanical properties of the composites. When the skeleton was oxidized at 1100 ° C in air for 2 hours, the composite exhibited a large number of AlN whiskers and the optimal phase composition, while the flexural strength and fracture toughness of the composite reached their maximum values. The strengthening and toughening mechanisms, such as plastic deformation, crack deflection, multiple cracking, crack bridging, crack branching, and delamination, caused by the lamellar structure were observed. It was also found that the pull-out and bridging of the AlN whiskers provided strengthening and toughening effects at the nanoscale for the nacre-like composites.

A Strong, Tough, and Stable Composite with Nacre-Inspired Sandwich Structure.

Improving the fracture resistance of nacre-inspired composites is crucial in addressing the strength-toughness trade-off. However, most previously proposed strategies for enhancing fracture resistance in these composites have been limited to interfacial modification by polymer, which restricts mechanical enhancement. Here, a composite material consisting of graphene oxide (GO) lamellae and nanocrystalline reinforced amorphous alumina nanowires (NAANs) has been developed. The structure of the composite is inspired by nacre and is composed of stacked GO nanosheets with NAANs in between, forming a sandwich-like structure. This design enhances the fracture resistance of the composite through the pull-out of GO nanosheets at the nanoscale and GO/NAANs sandwich-like coupling at the micro-scale, while also providing stiff ceramic support. This composite simultaneously possesses high strength (887.8MPa), toughness (31.6MJm-3), superior cyclic stability (1600 cycles), and long-term (2 years) immersion stability, which outperform previously reported GO-based lamellar composites. The hierarchical fracture design provides a new path to design next-generation strong, tough, and stable materials for advanced engineering applications.

Sustainable liquid metal-induced conductive nacre

Nacre has inspired research to fabricate tough bulk composites for practical applications using inorganic nanomaterials as building blocks. However, with the considerable pressure to reduce global carbon emissions, preparing nacre-inspired composites remains a significant challenge using more economical and environmentally friendly building blocks. Here we demonstrate tough and conductive nacre by assembling aragonite platelets exfoliated from natural nacre, with liquid metal and sodium alginate used as the “mortar”. The formation of GaOC coordination bonding between the gallium ions and sodium alginate molecules reduces the voids and improves compactness. The resultant conductive nacre exhibits much higher mechanical properties than natural nacre. It also shows excellent impact resistance attributed to the synergistic strengthening and toughening fracture mechanisms induced by liquid metal and sodium alginate. Furthermore, our conductive nacre exhibits exceptional self-monitoring sensitivity for maintaining structural integrity. The proposed strategy provides a novel avenue for turning natural nacre into a valuable green composite.

Fracture behaviors of nacre-like composites via phase-field fracture modeling

Nacre-inspired composites, composed of hard and soft phases, exhibit exceptional combinations of stiffness, strength, and fracture toughness. However, the fracture behaviors of these composites are significantly influenced by the geometric factors and material properties of their constituents. In this study, we systematically investigate the fracture behaviors of nacre-like composites by employing a phase-field fracture model considering the effects of oblique angle between loading and platelet directions, a critical factor often overlooked in previous studies, coupled with the effect of other geometric and material parameters, such as the aspect ratio, Young's modulus and fracture toughness ratio between hard and soft phase. Our findings highlight the significant influence of the oblique angle on the mechanical properties and fracture behaviors of these composites. Particularly, it is found the maximum Young’s modulus and strength are obtained when the direction of hard platelet is aligned with the tension direction, while the maximum toughness is achieved at a moderate oblique angle, especially in cases of large aspect ratios, small Young's modulus and fracture toughness ratios between the hard and soft phases. Besides, the maximum fracture toughness is more likely to be obtained at a small aspect ratio and large Young's modulus ratio between hard and soft phases. The insights gained from our study would offer valuable guidance for the design and practical application of nacre-mimetic composites.

Determining the role of microstructural topology on the mechanical performance of nacre-inspired composites using a phase-field model

Bio-inspired composites are celebrated for their remarkable strength and fracture toughness, often surpassing the properties of their constituent materials. A prime example of this inspiration is the nacre which is approximately three thousand times tougher than its components and showcases not only high toughness but also high strength. This extraordinary performance is largely attributed to its intricate microstructure. However, engineered composites have faced challenges in achieving similar levels of toughness. Attempts to replicate the “brick–mortar” microstructure of nacre revealed that mere imitation falls short. Recent studies emphasize the critical influence of specific parameters, such as volume fraction, aspect ratio, and tablet waviness angle, on the microstructure effectiveness. Prior investigations, whether experimental, numerical, or analytical, underlined the importance of these parameters in shaping composite mechanical properties. However, many of these studies relied on commercial software with some inbuilt fracture criteria, often overlooking the potential benefits of incorporating a phase-field model for a more comprehensive parametric analysis. Our research delves into the impact of these parameters on the mechanical performance of nacre-inspired composites, utilizing a phase-field model based on Griffith’s fracture criteria for complex crack propagation modeling. We notably reveal the superior performance of geometrically interlocking architecture compared to non-interlocking and rectangular architecture, marking a pioneering parametric exploration of this microstructure using a phase-field model. We validate our findings through experimental and numerical data. In summary, our work demonstrates that by strategically adjusting microstructural parameters, one can achieve optimal performance enhancements in nacre-inspired engineered composites. For instance, even a slight alteration of the waviness angle while maintaining constant volume fraction and aspect ratio can yield a remarkable 16.23% increase in fracture energy. Our open-source finite element codes, implemented using the Julia-based Gridap package, offer a “virtual” experimental tool for designing bio-inspired composites with high toughness and reasonable strength. This approach also provides valuable insights into creating exceptionally tough multilayered composites with diverse constituent combinations, holding significant promise for applications in automotive, aerospace, armor, and beyond.

A data-driven simple design of single-level nacre-inspired composites with high strength and toughness

Achieving a good strength–toughness match in nacre-inspired composites (NICs) is a difficult issue since it involves an optimum combination of many microstructural parameters. A multi-parameter design rule for the strengthening and toughening of NICs, which is based on extensive finite element (FE) data analysis and experimental verification, is given in this paper. Extensive FE calculations of NICs under uniaxial tension are firstly performed, with which the effects of the aspect ratio of hard phase (ARHP), the volume fraction of soft phase (VFSP) and the hard/soft-phase modulus ratio (HSMR) on the strength and toughness of composites are analyzed. Combinatorial conditions of these parameters for NICs with low strength and low toughness, high strength and low toughness and well-matched strength and toughness are determined, respectively, by comparisons of FE data samples. Furthermore, 3 D-printed NIC specimens consistent with FE models are prepared and tested by tensile experiments. The experimental results agree well with the FE results, both of which disclose the microscopic mechanism behind the strength and toughness of composites. It is found that a mixed failure mode including tensile fracture of hard phase and shear failure of soft phase can be realized by the combination of a large ARHP, a moderate VFSP and a large HSMR, which consequently leads to an excellent strength–toughness match in NICs. Compared with existing nacre-inspired composites with the same hard phase but more complex microstructures such as interlocking interfaces, deflected hard phases and graded soft phases, the present composite not only shows superior strength and toughness but is also easier to produce. Therefore, a highly-efficient and simple method can be provided for the design of strong and tough biomimetic composites.

Multi-objective Bayesian optimization for the design of nacre-inspired composites: optimizing and understanding biomimetics through AI.

The hierarchical structures found in biological materials lead to an outstanding balance of multiple material properties, and numerous research studies have been initiated to emulate the key concepts for the designing of engineering materials, the so-called bioinspired composites. However, the optimization of bioinspired composites has long been difficult as it usually falls into the category of 'black-box problem', the objective functions not being available in a functional form. Also, bioinspired composites possess multiple material properties that are in a trade-off relationship, making it impossible to reach a unique optimal design solution. As a breakthrough, we propose a data-driven material design framework which can generate bioinspired composite designs with an optimal balance of material properties. In this study, a nacre-inspired composite is chosen as the subject of study and the optimization framework is applied to determine the designs that have an optimal balance of strength, toughness, and specific volume. Gaussian process regression was adopted for the modeling of a complex input-output relationship, and the model was trained with the data generated from the crack phase-field simulation. Then, multi-objective Bayesian optimization was carried out to determine pareto-optimal composite designs. As a result, the proposed data-driven algorithm could generate a 3D pareto surface of optimal composite design solutions, from which a user can choose a design that suits his/her requirement. To validate the result, several pareto-optimal designs are built using a PolyJet 3D printer, and their tensile test results show that each of the characteristic designs is well optimized for its specific target objective.

Micromechanics of engineered interphases in nacre-like composite structures

Although the composition of polymer matrix is only 5% by weight, the presence of polymer as an interlayer between the brittle aragonite bricks significantly enhances the toughness characteristics of the natural nacre. In this study, the mechanics of nacre-like brick-mortar structures is investigated through numerical simulations. The shear stress distribution in the matrix of nacreous composites is found to be non-homogeneous primarily due to: (i) elastic properties mismatch between brick and mortar and (ii) the periodic microstructure. This non-homogeneous stress distribution is successfully reduced by grading the elastic modulus of the matrix material so as to considerably enhance the performance of the nacreous composite. A framework has been developed here to design nacre-inspired composites incorporating a functionally modulus graded interphase material. The different parameters influencing the non-uniform stress distribution, such as: interphase thickness, elastic modulus and overlap length are studied, elucidating how such parameters can be effectively controlled to reduce the non-homogeneous stress distribution and reduce the peak stresses. The peak stresses in the interphase are observed to exponentially increase up to 100%, when overlap length is 10% of the brick length. The strength and peak stresses in the interphase are observed to be higher for thin interphases, where a 50% decrease in thickness resulted in a 40% increase in the peak shear stress, and an 80% reduction yielded 150% increase. On the other hand, the elastic modulus is observed to scale with the strength, for instance, when the modulus is increased by 20% and 50%, the increase in peak shear stresses in the interphase are observed to be 10% and 22%, respectively. Furthermore, the shear stresses in the interphase are made uniform by varying the parameters such as: interphase thickness, elastic modulus and overlap length. The developed methodology has been extended to design a nacre-like structure based on the material combination used in metal matrix composites, where the shear strength of the proposed nacre-like composite structure is found to be 32% higher than the natural nacre. The results provide a guideline for the design of nacreous composites.

A novel approach to the fabrication of lamellar Al2O3/6061Al composites with high-volume fractions of hard phases

Freeze casting has been widely used in the preparation of nacre-inspired composites because of its prominent advantages in controlling the fine structure of ceramic scaffolds. However, the ceramic-phase content in composites prepared by this method is much lower than that in nacre due to the limitations of the dispersion and viscosity of the ceramic slurry. In this work, we combined the ideas of freeze casting and in-situ reaction to prepare nacre-inspired lamellar composites with high hard-phase contents. We first prepared the TiO2-−SiO2 porous scaffold with an initial solid loading of 20 vol% via unidirectional freeze casting and then infiltrated it with a 6061Al alloy. During the reaction process, the TiO2-SiO2 layers were progressively transformed into Al2O3 layers, and the Al layers were transformed into Al3Ti. Composites with different hard-phase volume fractions (approx. 29%, 38%, 52% and 65%) and different interfacial bonding strengths could be obtained by altering the infiltration temperature. Increases in the hard-phase content and the interfacial bonding strength substantially enhanced the bending and compressive strengths of the composites, with maximum values of 350 MPa and 854 MPa, respectively, achieved in the composites with approx. 65 vol% hard phases. However, the highest fracture toughness of 42.9 MPa m1/2 and maximum fracture work of 3.6 kJ/m2 appeared when the hard-phase content in the composite was approx. 38 vol%, sharing an appropriate interfacial bonding strength. A robust interface favors stress transfer but impedes crack deflection, while a weak interface fails to bear loads, even though it is conducive to crack deflection. A moderately-strong interface together with a considerable amount of layered soft phase contributes to the generation of multiple-cracking, and thereby greatly toughens the metal−ceramic composite.

Nacre-inspired Hard and Tough Materials.

Nature fabricates materials with properties that are difficult to reproduce with manmade counterparts. For example, nacre, composed of layers of CaCO₃ crystals that are interspaced with small quantities of organic components, is one of the toughest known biomaterials. To produce materials with such fascinating proper- ties, nature has established processes that offer an excellent control over their structure and local composition. Inspired by nacre, a lot of work has been devoted to the fabrication and characterization of composites with similar structures that nevertheless display distinctly different mechanical properties. The first part of this review summarizes methods used to produce nacre-inspired layered composites, their influence on the composition, structure, and mechanical properties. A key difference between the formation of nacre and that of nacre-inspired materials is the mechanism and kinetics of the formation of the inorganic components. In an endeavor to gain a better control over the mechanical properties of the inorganic platelets contained in nacre-inspired composites, the second part of this review describes methods to control the shape, structure, and orientation of CaCO₃ formed in organic scaffolds.

Bioinspired Design and Fabrication of Polymer Composite Films Consisting of a Strong and Stiff Organic Matrix and Microsized Inorganic Platelets.

Intensive studies on nacre-inspired composites with exceptional mechanical properties based on an organic/inorganic hierarchical layered structure have been conducted; however, integrating high strength, stiffness, and toughness for engineering materials still remains a challenge. We herein report the design and fabrication of polymer composites through a hydrogel-film casting method that allow for building uniformly layered organic/inorganic microstructure. Alginate (Alg) was used for an organic matrix, whose mechanical properties were controlled by Ca2+ cross-linking toward the simultaneously strong, stiff, and tough resultant composite. Alumina (Alu) microplatelets were used for horizontally aligned inorganic phase, and their alignment and interactions with the organic matrix were improved by polyvinylpyrrolidone (PVP) coating on the platelet. The composite film exhibits well-balanced elastic and plastic deformation under tensile stress, leading to high stiffness and toughness, which have not been generally achieved in microplatelet-based composite films developed in previous studies. The synergistic effect of Ca2+ cross-linking and PVP-coated Alu platelets on the mechanical properties improved polymer-platelet interfacial interactions, and platelet alignment is clearly demonstrated through mechanical tests and Fourier transform infrared and X-ray diffraction analyses. We further demonstrate that the reinforcing effect of the Alu platelet and PVP-coated platelet on the mechanical properties is dependent on humidity. Such effects are maximized at highly dry conditions, which is consistent with the model estimation. Furthermore, a thick bulk composite was produced by laminating thin films and showed high mechanical properties under flexural stress. Our design and fabrication strategies combined with the understanding of their mechanism yield an alternative approach to produce engineered composite materials.

Shear Strength and Interfacial Toughness Characterization of Sapphire–Epoxy Interfaces for Nacre-Inspired Composites

The common tensile lap-shear test for adhesive joints is inappropriate for brittle substrates such as glasses or ceramics where stress intensifications due to clamping and additional bending moments invalidate results. Nevertheless, bonding of glasses and ceramics is still important in display applications for electronics, in safety glass and ballistic armor, for dental braces and restoratives, or in recently developed bioinspired composites. To mechanically characterize adhesive bondings in these fields nonetheless, a novel approach based on the so-called Schwickerath test for dental sintered joints is used. This new method not only matches data from conventional analysis but also uniquely combines the accurate determination of interfacial shear strength and toughness in one simple test. The approach is verified for sapphire-epoxy joints that are of interest for bioinspired composites. For these, the procedure not only provides quantitative interfacial properties for the first time, it also exemplarily suggests annealing of sapphire at 1000 °C for 10 h for mechanically and economically effective improvements of the interfacial bond strength and toughness. With increases of strength and toughness from approximately 8 to 29 MPa and from 2.6 to 35 J/m2, respectively, this thermal modification drastically enhances the properties of unmodified sapphire-epoxy interfaces. At the same time, it is much more convenient than wet-chemical approaches such as silanization. Hence, besides the introduction of a new testing procedure for adhesive joints of brittle or expensive substrates, a new and facile annealing process for improvements of the adhesive properties of sapphire is suggested and quantitative data for the mechanical properties of sapphire-epoxy interfaces that are common in synthetic nacre-inspired composites are provided for the first time.

Hierarchically roughened microplatelets enhance the strength and ductility of nacre-inspired composites

Rough interfaces featuring nanoscale asperities are known to play a major role in the mechanics of nacre. Transferring this concept to artificial bioinspired composites requires a detailed understanding about the effect of the surface topography of reinforcing elements on the mechanical performance of such materials. To gain further insights into the effect of asperity size, hierarchy and coverage on the mechanics of nacre-inspired composites, we decorate alumina microplatelets with silica nanoparticles of selected sizes and use the resulting roughened platelets as reinforcing elements (15vol%) in a commercial epoxy matrix. For a single layer of silica nanoparticles on the platelet surface, increased ultimate strain and toughness are obtained with a large roughening particle size of 250nm. On the contrary, strength and stiffness are enhanced by decreasing the size of asperities using 22nm silica particles. By combining particles of two different sizes (100nm and 22nm) in a hierarchical fashion, we are able to improve stiffness and strength of platelet–reinforced polymers while maintaining high ultimate strain and toughness. Our results indicate that carefully designed hierarchically roughened interfaces lead to a more homogeneous stress distribution within the polymer matrix between the stiff reinforcing elements. By enabling the deformation of a larger fraction of the polymer matrix, this design concept improves the mechanical response of bioinspired composites and can possibly also be exploited to enhance the performance of conventional fiber–reinforced polymers.

Influence of initial flaws on the mechanical properties of nacre

Nacre is a bio-composite made up of hard mineral and soft protein, and has excellent mechanical properties. This paper examines the effect of naturally occurring defects (initial flaws) in nacre on its mechanical properties such as toughness and strength. A random fuse model is developed incorporating initial flaws. Numerical simulations show that initial flaws affect different mechanical properties at different rates. The variation in the experimentally obtained mechanical properties of nacre reported in the literature is shown to be due to initial flaws. The stress in the mineral and protein increases due to initial flaws, but by different amounts. The results obtained in this study are useful for gaining insight into the failure of nacre and development of nacre-inspired composites.