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It’s tough to be strong: Advances in bioinspired structural ceramic-based materials Figure 4. Nacre microstructure types and schematics show different architectures. The middle row shows natural nacre microstructures. The bottom row shows artificial nacre microstructures. (a, e, i) brick-and-mortar; (b, f, j) mineral bridges; (c, g, k) surface asperities; and (d, h, l) organic matrix. Scale bars: (f) 500 nm; (g) 500 nm; (h) 1 μm; (j) 20 μm; (l) 1 μm. spired materials. Promising techniques for making composites with enhanced mechanical properties include infiltrating ceramic scaffolds with polymers or metals and aligning ceramic microstructures with external forces. Variations of polymer or metal infiltration techniques into ceramic scaffolds include melt immersion, solvent evaporation, in-situ polymerization, particle centrifugation, and chemical vapor deposition. Table 1 compares flexural strength and fracture toughness for crack initiation of several high-toughness Al2O3- based composites. Pressure sintering and infiltrating with PMMA were used to form freeze-cast Al2O3 scaffold composites that resemble the brick-and-mortar structure of nacre with features such as mineral bridges extending from one lamella to another and surface roughness (asperities) on the ceramic phase.33 Infiltrating molten Al-Si alloys into freeze-cast Al2O3 scaffolds increased flexural strength (up to 328 MPa) and toughness (up to 8.3 MPa·m1/2).34 However, polymeric or metallic phases in the Al2O3-based composites may be undesirable for certain applications, such as high-temperature enivronments. Therefore, freeze casting and densification with pressurized spark plasma sintering was used to develop Al2O3/SiO2/CaO composites. These composites exhibited the highest combination of strength (470 MPa) and toughness (6.2 MPa·m1/2) yet to be reported for a fully ceramic material.35 Capitalizing on freeze casting to control growth of ice crystals provides these materials microstructural alignment and outstanding combinations of strength and toughness. Similar to bone and nacre, these composites employ several fracture resistance mechanisms. Yet another technique used low magnetic fields to align Al2O3 platelets in polymer matrices.36,37 The aligned microstructures increased flexural modulus, strength, and fracture toughness significantly compared with identical composites without magnetic alignment.37 However, these composites are limited by the low achievable volume fraction and discontinuity of ceramics platelets embedded in a continuous polymer matrix. Inspirations from biological structures Figure 3 shows two artificial materials inspired by bone that mimic the microstructures of osteons (Figures 3(a) and (c)) and trabeculae (Figures 3(b) and (d)). Figure 3(e) shows an artificial osteon-like architecture produced by freeze casting in a mold with a patterned bottom surface to promote alignment of the ceramic particles. 38 The artificial trabecular scaffold shown in Figure 3f was also fabricated by freeze casting using a high viscosity water-based freezing vehicle. Figure 4 juxtaposes schematics and micrographs of Credit: (e) Wikipedia. (f, g, h, l) Porter; UCSD. (i) Deville and Bouville; LSFC. (j) Deville; Wiley. (k) Ritchie, LBL natural abalone nacre (Figures 4(a)– (h)) with several bioinspired materials that mimic the microstructural features of nacre (Figures 4(i)–(l)). In the first column of Figure 4 (parts (a), (e), and (i)), the strong and tough ceramics developed by Bouville et al.35 are compared with natural nacre. These ceramics had extremely high mechanical properties (Table 1) and mimicked almost all microstructural features of nacre on equivalent length scales, including the brick-and-mortar architecture, platelets, mineral bridges, and surface asperities. The next three columns of Figure 4 compare natural and artificial mineral bridges (Figure 4(b), (f), (j)), surface asperities (Figure 4(c), Credit: Porter; UCSD Figure 5. Microstructure alignment with applied magnetic field during freeze casting. (a) Freeze cast, (b) magnetic alignment, and (c) polymer infiltration to produce ceramic-based porous scaffolds and hybrid composites. (d, e) ZrO2 scaffolds and (f) ZrO2-epoxy composites fabricated by the respective techniques. All scale bars are 100 μm. 22 www.ceramics.org | American Ceramic Society Bulletin, Vol. 93, No. 5


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