Mohammad Mirkhalaf

The quest for stiff, strong and tough hybrid materials: an exhaustive exploration.

How to arrange soft materials with strong but brittle reinforcements to achieve attractive combinations of stiffness, strength and toughness is an ongoing and fascinating question in engineering and biological materials science. Recent advances in topology optimization and bioinspiration have brought interesting answers to this question, but they provide only small windows into the vast design space associated with this problem. Here, we take a more global approach in which we assess the mechanical performance of thousands of possible microstructures. This exhaustive exploration gives a global picture of structure–property relationships and guarantees that global optima can be found. Landscapes of optimum solutions for different combinations of desired properties can also be created, revealing the robustness of each of the solutions. Interestingly, while some of the major hybrid designs used in engineering are absent from the set of solutions, the microstructures emerging from this process are reminiscent of materials, such as bone, nacre or spider silk.

Nacre-like materials using a simple doctor blading technique: Fabrication, testing and modeling

The remarkable mechanical performance of biological materials such as bone, nacre, and spider silk stems from their staggered microstructure in which stiff and strong reinforcements are elongated in the direction of loading, separated by softer interfaces, and shifted relative to each other. This structure results in useful combinations of modulus, strength and toughness and therefore is increasingly mimicked in bio-inspired engineering composites. Here, we report the use of a simple and versatile technique based on doctor-blading to fabricate staggered composites of microscopic alumina tablets with high alignment in a chitosan matrix. Tensile tests on these nacre-like materials show that the modulus and strength of the composite films are enhanced by the incorporation of ceramic tablets, but only up to 15vol% after which all properties degrade. This phenomenon, also reported in the past for most of nacre-like materials, composed of micro/nano tablets, obtained from different techniques, has been limiting our ability to produce large volumes of high-performance nacre-like materials. Examination of the structure of the films revealed that at lower tablet concentrations the tablets are well-aligned and well dispersed thorough the volume of the film. At 15vol% and beyond, we observed tablet misalignment and clustering. In order to investigate the impact of these imperfections on material performance we developed large scale finite element models representative of the structure of the composite films. These models show that the mechanical performance significantly degrades with tablet misalignment, and especially at high tablet concentrations. The simulations along with the SEM images therefore quantitatively explain the experimental trends, e.g. the degradation of mechanical properties at high tablet contents.

Biomimetic Hard Materials

Materials such as bone, teeth, and seashells possess remarkable combinations of properties despite the poor structural quality of their ingredients (brittle minerals and soft proteins). Nacre from mollusk shells is 3,000 times tougher than the brittle mineral it is made of, a level of toughness amplification currently unmatched by any engineering material. For this reason, nacre has become the model for bio-inspiration for novel structural materials. The structure of nacre is organized over several length scales, but the microscopic brick-and-mortar arrangement of the mineral tablets is prominent. This staggered structure provides a universal approach to arranging hard building blocks in nature and is also found in bone and teeth. Recent models have demonstrated how an attractive combination of stiffness, strength, and toughness can be achieved through the staggered structure. The fabrication of engineering materials that duplicate the structure, mechanics, and properties of natural nacre still present formidable challenges to this day.

Simultaneous improvements of strength and toughness in topologically interlocked ceramics

Significance Topologically interlocked materials (TIMs) use frictional sliding to generate large deformations and build toughness in otherwise all brittle components. TIMs can be up to 10 times more impact resistant than their monolithic form, but this improvement usually comes at the expense of static strength. Here we report a TIM design based on octahedral blocks which is not only much tougher (50×) than monolithic plates of the same material, but also stronger (1.2×). With no evidence of upper bounds for strength and toughness, TIMs have a tremendous potential as high-performance structural materials. Based on our experiments we propose a nondimensional “interlocking parameter” which could guide the exploration of new TIM designs and other new architectured systems. Topologically interlocked materials (TIMs) are an emerging class of architectured materials based on stiff building blocks of well-controlled geometries which can slide, rotate, or interlock collectively providing a wealth of tunable mechanisms, precise structural properties, and functionalities. TIMs are typically 10 times more impact resistant than their monolithic form, but this improvement usually comes at the expense of strength. Here we used 3D printing and replica casting to explore 15 designs of architectured ceramic panels based on platonic shapes and their truncated versions. We tested the panels in quasi-static and impact conditions with stereoimaging, image correlation, and 3D reconstruction to monitor the displacements and rotations of individual blocks. We report a design based on octahedral blocks which is not only tougher (50×) but also stronger (1.2×) than monolithic plates of the same material. This result suggests that there is no upper bound for strength and toughness in TIMs, unveiling their tremendous potential as structural and multifunctional materials. Based on our experiments, we propose a nondimensional “interlocking parameter” which could guide the exploration of future architectured systems.

Hierarchical structure, mechanical properties and fabrication of biomimetic biomaterials

Abstract: This chapter investigates the concept of hierarchy widely found in biological materials. First, natural hierarchical materials are explored in terms of their high order structures formed from universal building blocks. Hierarchical arrangement is claimed to give rise to remarkable mechanical properties of biological structures. Therefore at the next step, the significance of hierarchical structuring on mechanical properties is investigated through available analytical models. Finally, fabrication methods which could potentially lead to artificial hierarchical structures are briefly reviewed in the domain of biomimetics.