Layered ceramic architectures with alternating material layers with tailored microstructures have been demonstrated as effective approach for optimizing the mechanical behaviour of advanced ceramics. Due to the different coefficients of thermal expansions within the combined multi-material layer regions, alternating tensile as well as compressive residual stresses are generated after cooling down from the sintering temperature. For instance, in designs where the compressive residual stresses are located within the top surface layer regions, a significant increase in the strength can be achieved. In cases where damage tolerance is pursued, the in-plane compressive residual stresses may be embedded within the ceramic architecture aiming to provide “crack arrest” of propagating surface cracks. In addition to the architectural design approach for layered ceramic systems (laminates), recent research has demonstrated that orienting the microstructure (“texturing”) along the [0001] basal directions within specific layer regions may further improve the fracture resistance of alumina-based laminates through energy-dissipating deflection mechanisms during the fracture process. This approach resembles some natural systems, as in seashells and is often referred to as bioinspired design concept.
The fracture behaviour and the underlying mechanisms of such textured alumina-based ceramic laminates have not been fully understood yet. In this thesis, the micro-scale fracture toughness of individual textured alumina grain and grain boundaries is investigated through micro-cantilever bending tests. The investigation on the micro-scale level may be used for better understanding the macroscopic fracture behaviour of layered alumina-based laminates with textured microstructures. Furthermore, the Hertzian contact damage behaviour of alumina-based laminates with internal textured regions is assessed and the corresponding surface as well as sub-surface damage are studied in detail. In addition, the high-temperature fracture behaviour of layered alumina ceramics with textured microstructures are explored by performing (uniaxial) bending tests up to temperatures of 1200 °C, which may be paramount for assessing their potential for high-temperature applications.
Another important aspect is that although such alumina-based 2D-architectures (planar structures fabricated through tape casting) may show high potentials in designing mechanical resistant or damage-tolerant systems, the application of the multi-material design concepts with residual stresses on more complex-shaped components has not been investigated yet. Therefore, the potentials of designing alumina-based 3D-multi-material architectures through stereolithographic printing and their mechanical performance is studied. Firstly, the mechanical strength of 3D-printed alumina is tailored by embedding alumina-zirconia layers between outer pure alumina surface layers with significant compressive residual stresses. Secondly, the thermal shock behaviour of 3D-printed multi-material ceramics with embedded alumina (protective) regions under compressive residual stresses is explored. Based on the damage-tolerant design concept, a first demonstrator component (i.e. ceramic turbine blade) with complex shapes is designed through the multi-material design approach and its effectiveness for arresting thermal shock cracks is demonstrated.