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Figure 1. Cross sectional image of the aboral spine of Phyllacanthus imperialis based on µCT data visualizing the core–shell structure with its differentiated substructures. 1: Cortex, 2: Radiating layer, 3: Medulla.
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Figure 2. Schematic overview of the freeze-casting process with four main processing steps: preparation of the suspension, solidification via freezing, sublimation and sintering.
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Figure 3. Schematic illustration of the mold designs for the freeze-casting process. (A) The conventional setup for producing unidirectional pore systems consists of a polytetrafluorethylene (PTFE) tube that was fixed on a copper plate. The mold design in (B) was used to manufacture a structural graded ceramic inspired by the microstructure of spine of Phyllacanthus imperialis. The freezing direction is indicated by the black arrows. 1: PTFE-mold, 2: particle suspension, 3: Al2O3 particle, 4: ice crystal, 5: copper plate, 6: cold plate, 7: frozen suspension with unidirectional ice crystals, 8: copper film, 9: Teflon film, 10: silicon film.
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Figure 4. Spine of Phyllacanthus imperialis. Turquoise box illustrates the scanned area via μCT. A complete reconstruction of the scanned spine area can be seen on the right-hand side.
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Figure 5. Microstructure of the spine of Phyllacanthus imperialis. (A) Z-projection of the microstructure of the spine shows the three structural units: the cortex, radiating layer and medulla. Various sections (yellow boxes) of the cortex microstructure are displayed in (B).
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Figure 6. Porosity, Φ, of the cortex and the size of the pore channels within the cortex of the spine of Phyllacanthus imperialis. The porosity of the µCT sections of the cortex is displayed in (A). The average porosity of each μCT section is displayed in (B). The assignment of the µCT sections (2, 3, 4) is displayed in Figure 5B.
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Figure 7. A stacked µCT reconstruction of the inner microstructure of the spine of Phyllacanthus imperialis. The turquoise and green boxes visualize different cross-sectional planes of the structure of the radiating layer. The inner mesh of the center of the spine is shown in the yellow boxes (polygonal passages, pore rows). The largest and the smallest pore diameter are indicated by a1 and a2, respectively.
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Figure 8. Characterization of the pore sizes and porosities of the microstructural units (cortex, radiating layer and medulla) in the spine of Phyllacanthus imperialis. The pore axis length is shown by a1 and a2, and of the structures forming the radiating layer, the medulla and the cortex are displayed in (A). The location within the spine microstructure is given as numbers. The affiliation of the numbers is shown in Figure 5B and Figure 7, respectively. The plot in (B) represents the average porosities of the cortex, radiating layer and medulla.
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Figure 9. (A) µCT reconstruction of the spine of Phyllacanthus imperialis demonstrating the calculated strut thickness distribution of the meshes (stereom layers and polygonal meshes in the spine center) and a superstructure. The stereom layers (marked in green) running directly from the cortex to one sidearm of the spine center (xy-plane). Thus, a wedge-like structure (dashed line in green) is formed, which is a superstructure. (B) Cross sections, parallel to the z-axis, of the stereom layer and spine center. The color scheme in (A,B) indicates the strut thickness.
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Figure 10. Quantity of the cortex in the microstructure of the spine of Phyllacanthus imperialis. (A) The quantity of the cortex is expressed as a/t ratio. The a/t ratio is composed of the radius of the spine segment, a, which was normalized by the cortex thickness, t. The quantity of the a/t ratio and the porosity at specific positions in the spine is demonstrated in (B) as black squares and red circles, respectively.
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Figure 11. Plots in (A,B) show the maximum compressive strength and Young’s modulus of the spine segments of Phyllacanthus imperialis as a function of the porosity, Φ. The color bar indicates the structural factor of the spine segments, the a/t ratio. A change of the failure modes was observed at a porosity of approximately 55% being highlighted in the plots as light-yellow line.
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Figure 12. Spine segments of Phyllacanthus imperialis under uniaxial compression. Typical stress (σ)-strain (ε) curves of failure mode I and II are demonstrated in (A,B), respectively. A detailed view of the initial stress–strain curve of a spine segment undergoing failure mode II is displayed in (C). The red letters belong to the microphotographs presenting the fracture behavior at a specific stage of uniaxial compression. *: ‘Quasi-ductile’ regime. **: Densification. ***: Linear elastic regime with a brief alignment period. ****: Dismantling of the cortex.
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Figure 13. Specific energy absorption per volume unit, UV(ε), and the energy efficiency, η(ε), of the spine segments of Phyllacanthus imperialis. The relation of the average values of the UV(ε) and η(ε) to the strain, ε, is displayed in (A,B), respectively. FM I: Failure mode I, FM II: Failure mode II.
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Figure 14. Cross sectional BSE image of the spine’s core of Phyllacanthus imperialis, which was uniaxial loaded until the first acoustic signal occurred. Initial crack propagation at the medulla-radiating layer interface is displayed in high magnification in the red box. The advanced stage of crack propagation is demonstrated on the left-hand side: the crack is deflected along the stereom sheet.
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Figure 15. Structural graded ceramic inspired by the microstructure of Phyllacanthus imperialis. The microstructure of the ceramic and its division in different zones is displayed in (A) that illustrates the cross-sectional area perpendicular to the freezing direction. The orientation of the cell channels is given in (B). The red arrow demonstrates the freezing direction. A higher magnification of the graded structure is displayed in (C). The area is marked as red rectangle in (A).
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Figure 16. Freeze-casted cellular ceramics inspired by the strut arrangement of the spine’s radiating layer and medulla. Both ceramic types have the solid loading of 14.4 vol.% in common and differentiate in terms of the used gelatin concentration in the suspension. A gelatin concentration of 3.5 vol.% has been used to manufacture the cellular oblate cell structure, (A), being highly anisotropic, (B), in their pore shape. Polygonal cellular cell shapes, (C), can be manufactured with a gelatin concentration of 6.8 vol.% showing rather a foam-like character, (D).
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Figure 17. The plots in (A,B) show the average Young’s modulus and maximum compressive strength of the freeze-casted ceramics as a function of the porosity, Φ.
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Figure 18. The stress (σ)-strain (ε) curves and the corresponding microphotographs of the fracture behavior of the freeze-casted ceramics. The ceramics including the polygonal cells compensate stress by a progressive interplay of crumbling and flaking (a–e, white letters). An intensive lath-like segmentation (a–e, red letters) can be observed for the ceramics being characterized by cellular oblate anisotropic cells.
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Figure 19. Comparable display of BSE images of the spine microstructure of Phyllacanthus imperialis and of the structural graded ceramic manufactured via freeze-casting. The cross section (perpendicular to the z-axis) of the spine microstructure is displayed in (A) demonstrating the radiating layer and medulla. Part of the structural graded freeze-cast ceramic with its cross section is presented in (B).
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Figure 20. BSE images of the cross sections of the freeze-casted ceramics (perpendicular to the freezing-direction). The cavities of the ceramics were filled with epoxy resin (=white areas). Therefore, the varying degrees of interconnectivity of the struts in the ceramics is demonstrated in (A,B) depending on the gelatin concentration.
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