Bio-inspired ceramic based composites

The progress of a wide range of strategic fields from aerospace, construction, transportation or medicine depends on the ability to develop new materials combining different structural and functional properties, from lightweight to high fracture resistance. One way of achieving this is by borrowing design principles found in Natural materials such as bone or nacre and integrating them into man-made materials. A common characteristic of these Natural composites is the combination of hard and soft phases in hierarchical structures with characteristic dimensions spanning multiple length scales. The process of adapting natural structural features to new technologies is called “bio-inspiration”. This PhD project, attempts to mimic some of the structural motifs found in natural materials to produce innovative lightweight ceramic structures and ceramic-based composites with high fracture resistance. Freeze casting (also known as ice-templating) was used to produce strong and lightweight SiC scaffolds using SiC particles with two different morphologies: spherical nanoparticles (400 nm in diameter) and micro-fibres (~18 μm long and ~1.5 μm in diameter). In both cases the optimal rheological behaviour for the freeze casting slurries along with optimal sintering conditions for the consolidation of the freeze cast scaffolds have been identified. Freeze casting of nanoparticle suspensions resulted in porous layered scaffolds with wavelengths (lamella wall plus pore space) ranging from ~60 to ~15 microns and porosity between ~50% and ~75%. The packing of fibres during freeze casting allowed the formation of highly porous networks with porosities as high as 98%. The mechanical (compressive and flexural strength) and functional properties (electrical conductivity, thermal conductivity) of the scaffolds have been measured and related to their structure. These scaffolds have been employed as preforms to produce SiC based composites through infiltration of the residual porosity with two different polymers: PMMA and Epoxy resin. The distinctive layered architecture of the composites enables a combination of high flexural strength and fracture resistance. The main toughening mechanisms (crack deflection, crack bridging, plastic deformation etc.) have been identified.To form metal-ceramic composites, layered freeze cast alumina preforms have been infiltrated with an aluminium alloy (Al-4Mg). The wettability of this alloy on alumina allows the infiltration of porous scaffolds without the application of external pressure. Therefore an extremely porous preform can be fully infiltrated without damaging the ceramic network. The microstructures of alumina preforms obtained from nanoparticles (400nm in diameter) or platelets (5-10μm in diameter and 300-500nm in thickness) have been compared. The use of platelets enables the fabrication of layered ceramic scaffolds with wall thickness of ~5-6μm. Mechanical tests revealed that the layered freeze casted composites exhibit a combination of high fracture resistance and flexural strength, with values that are superior to other metal-ceramic composites. Materials fabricated using alumina platelets can reach the strengths up to ~890MPa. All the materials exhibit stable crack propagation with a characteristic R-curve. An extensive study of crack interaction with the microstructural features has been performed to identify the key toughening mechanisms such as crack deflection, crack bridging, micro-cracking and plastic deformation. The results of this thesis suggest that the structure of natural materials can be used as a blueprint in the development of new advanced composites. This requires processing techniques able to implement, in practical dimensions, the design concepts found in natural materials. However, a deep understanding of the relationships between structure and mechanical response encompassing the influence of structural parameters acting at multiple length scales is still needed to guide this effort.Open Acces