Bulk Ceramic-Based Biologically Inspired Composites: Design, Fabrication and Testing

Strength and toughness are mutually exclusive mechanical properties; an increase in one result in the decline in the other. Accordingly, ceramics with superior strength have a very low toughness; likewise, metals with similar density have relatively lower strength but higher toughness. However, biological systems design lightweight materials, circumventing this limitation of conventional materials, by aggregating various multiscale toughening mechanisms. In challenging habitats, organisms evolve to produce remarkable multifunctional material systems that improve their "fit" and "survivability". Unlike traditional materials, natural materials employ special arrangements of structural elements into cellular, gradient, fibrous, layered, or overlapped "architected composites". These natural material systems are "architected" to delocalize damage and prevent defect coalescence, to avoid catastrophic failure, even though they are mainly composed of brittle building blocks (>90 vol% mineral content). Consequently, the study of natural materials has attracted the attention of scientists as the benchmark for the development of new synthetic materials. With the advent of additive manufacturing technology, the design and assessment of architected composites with bio-inspired motifs have become increasingly feasible. In this dissertation, I use multi-step fabrication methods with additive manufacturing as a key step to produce and study different biologically inspired architectures. With control over the design parameters of the architectural features, an in-depth understanding of the organization is accomplished. The case studies are primarily focused on bulk composite material systems with multiple phases and motifs inspired by various biological material systems. This dissertation aims to reveal the structure-property relationships of these structural motifs and the trade-offs to the mechanical robustness due growth-related constraints.

With the help of stereolithographic additive manufacturing technique and centrifugal infiltration, we propose a bio-inspired method for preparing ceramic-metal composites. The approach allowed for flexible design, scalability, and dimensional control of individual phases. The ceramic-metal composites were fabricated with structures simplified from the mollusk shell architectures, exhibiting specific strength up to 169% higher than the base metal. The crack growth toughness of up to 12.9 MPa m1/2 was recorded, with crack deflection at ceramic-metal interfaces. Additionally, using tomographic analysis we show that the high porosities of 9% and 15% for green and sintered 3D printed parts, if improved, could further enhance the strength and fracture toughness of these composites.

The outer protective layer of a bivalve mollusk exoskeleton, called the prismatic layer, is composed of normally oriented prismatic building blocks separated by soft organic matrix. The growth of the prismatic layer is regulated by the thermodynamic boundary conditions of the habitat and is directed from the exterior to the interior of the shell. A consequence of growth is a graded structure with a fine side (higher grain count with smaller grain size) and a coarse side (higher grain count with smaller grain size), however, the presence of grading results in asymmetry. Using mechanical testing we reveal that the organisms' selection of fine side as the loading face is "not the most optimized arrangement for templating". In fact, opting for the coarse side over the fine side as the loading side simultaneously enhances mutually exclusive properties such as stiffness, strength, and energy absorption. We further show that the curved prism motifs in the proximal parts of the Ostrea edulis shells result in a significant reduction in mechanical robustness due to the growth-related restrictions arising from the simultaneous normal and lateral growth of shells. Moreover, we show that although the addition of a nacre-like backing layer reduces the effects of axial directional asymmetry, the resistance of the prismatic layer to initiate damage in a coarse side-loaded hybrid composite is superior to the fine side-loaded counterpart. This part of the research highlights the need for caution when directly mimicking structural designs found in biological systems. Biological material systems are typically multifunctional, tailored to specific habitats and organism-specific needs, and often constrained by growth requirements and economic limitations.

The shells of the pteropods – pelagic gastropod species, are comprised of helical or as posited by certain researchers "S-shaped" aragonite mineral motifs. These helical motifs are remarkably close packed in an organic matrix without noticeable spaces. We develop a biological process mimicking image processing technique called the "Bottom-up Sectional Morphing" to model perfectly closed packed structures with control over the radius and pitch of the helical motifs. With the developed composites we attempt to characterize the effect of the helix radius of individual motifs on the global mechanical properties. With the help of compressive tests, we characterize the delocalization of load as the radius of the helical motifs is increased. With the help of slab-shaped samples, we study the puncture resistance and interlocking behaviors due to increased helical radius. Using standardized fracture toughness tests, the toughness of the composites is determined. Additionally, the R-curve behaviors as a function of helical radiuses is characterized. On average, the fracture strength of the composite doubled as the radius of the helical motifs increased from 0 mm to 3.9 mm. Remarkably, the fracture toughness of helical composites was as high as 12-times the rule-of-mixtures estimated values. We summarize the extrinsic toughening mechanisms within the composites compared them to the mechanisms reported for helicoidal (twisted plywood) composites. Additional interlocking due to the uneven orientation of major axes in double basket weave pattern helical system are reported. Using explicit finite element simulations, we show that the curved motifs in comparison to normally oriented prisms, can help in developing localized high stress pockets, thus delocalization of damage that can help in increasing energy absorption during the progression of damage.

Also, taking cues from fish scale ultrastructures, we design three-phase ceramic-epoxy-fiber composites. The fish scales feature gradient architectures with varying biomineralization extents from the distal to proximal regions (with respect to the fish body). From exterior to interior the mineralization content reduces, however, the collagen fiber count subsequently increases. To mimic the design approach, we use a 3D printed gradient ceramic lattice embedded in an epoxy matrix and backed using Kevlar fibers. With high-speed impact tests (73.5 ± 2.5 ms-1) we show that, although functionally graded composites (without Kevlar backing) show larger impact signatures compared to the similar density uniform density composites (without Kevlar backing) but absorb 35.7% higher energy during the process. High rebound velocity (22 ± 2.46 m/s) was observed for variable density composites with Kevlar backing. Additionally, using micro computed tomographic analysis of variable density composites with Kevlar backing we demonstrate that pre-stretching of fibers helps in the suppression crack. The results from this study were used in the design of polymer-elastomer composites with functionally graded material and fiber distribution.

Interweaving fibers with hard solid lattices becomes challenging when one of the planar surfaces of the lattice is closed because of the functional grading. To overcome this challenge, I propose a new lattice interweaving method called "Warp-Assisted Binder-Tugging (WABT)", that can interweave the lattice using only one of the planar faces. Using WABT we refine the 3-phase composites design by incorporating strategically placed internal reinforcements. Cured photopolymer thermoset plastics are intrinsically brittle materials with mechanical properties like that of epoxy. Therefore, we choose this material along with urethane elastomer to prepare polymer-elastomer (hard-soft) composites, with and without reinforcements. We demonstrate the efficacy of strategic material distributions using dynamic puncture tests and projectile impact tests. The results show that concentrating brittle plastics towards the loading side improves energy absorption ability by 30.29% and puncture strength by 21.47%. A further 61.76% and 35.12% improvement in the energy absorption and puncture strength is recorded for slabs with backing and reinforcements. We show the response of the as-prepared composites under high speed projectile impact tests with incident projectile speeds of 151.5 ± 2.5 ms-1. The μ-CT characterization of damaged samples revealed the load delocalization and crack suppression behaviors due to the material distributions and reinforcements.