| Summary: | <p>Nacre, a ceramic composite found in seashells, has baffled scientists due to its extraordinary mechanical properties. Consequently, nacre has been a main source of inspiration to create strong and tough ceramic composites that mimic nacre’s microstructure. The precise microstructural features responsible for its unique combination of high strength and toughness are however debated, and efforts to mimic and improve upon nature’s design have not led to commercial applications of ceramic nacre-like composites.</p>
<p>This dissertation details the replication of nacre’s microstructural architecture with technical ceramics and nanomaterials. Instead of nacre’s calcium carbonate building blocks, alumina was used, and instead of its organic phase, multi-walled carbon nanotubes (MWCNTs) were incorporated. To design a nacreous microstructure, freeze casting of alumina nanoparticles was used to template a microstructure of aligned lamellae over macroscopic length scales. Although successful, the fabricated lamellar microstructures’ macroscopic orientation was hard to consistently reproduce using this technique and the resulting composites, unlike nacre, fail in a completely brittle fashion. Therefore, alumina platelets that closely resemble the platelets in nacre were investigated. A recipe to homogeneously coat these platelets in situ with MWCNT was developed, and MWCNT growth and catalyst modification was studied in-depth. The resulting combined building blocks were sintered into a composite that displays a graceful failure mechanism like nacre does, with several toughening mechanisms observed to be active. In addition, nacre-like alumina/MWCNT composite materials have
been synthesised using a variety of MWCNT dispersion techniques. All studied nacre-like composites showed characteristic macroscopic interlocking behaviour and remain interlocked after crack initiation which prevented complete failure and gave these composites a residual strength. Pure nacre-like alumina specimens have also been tested in high strain rate and impact tests, where these materials showed a significant improvement over nontextured alumina. </p>
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