The bio – inspired “brick and mortar” concept has proven to be an effective way to enhance damage tolerance in ceramic – based materials. Based on the phenomenon of crack shielding, bifurcation and deflection, caused by the preferred orientation of the microstructure, energy is dissipated during crack propagation. This results in an improved fracture behavior.
This thesis focusses on the fabrication and characterization of textured alumina (TA) composites, aiming to understand how a second phase influences the development of texture as well as the physical and mechanical properties of the composite material.
Monolithic textured alumina with SiO2 + CaO as sintering additives, was fabricated utilizing the tape casting process for grain alignment, followed by templated grain growth (TGG) during sintering. In addition, a variety of compositions of textured alumina containing 0.5 vol% – 20 vol% of monoclinic zirconia (m-ZrO2) were prepared. Both the reference and composite materials were characterized according to their microstructure, physical and mechanical properties. The degree of texture was determined using microstructural and XRD analysis. The hardness was measured after Vickers and the fracture toughness by using the Single Edge V – Notched Beam method (SEVNB).
In order to better understand the effects of texture on their damage tolerance behavior in ceramic materials, monolithic textured alumina samples were tested perpendicular as well as parallel to the basal surface of the oriented direction grain basal surface. Anisotropic fracture behavior was observed with slightly higher toughness for the testing orientation perpendicular to the basal planes. The composite materials were only tested perpendicular to the grain orientation. Whereas for pure TA and TA with small contents (0.5 vol%) of second phase, intergranular fracture was observed, increasing zirconia content (beyond 1 vol%) led to transgranular fracture. It was found that with increasing volume fraction of the second phase, the templates are hindered from growing, resulting in the loss of texture. High quality of preferred grain orientation was only achieved for compositions containing up to ≈ 2 vol% m-ZrO2. Furthermore, higher relative density was obtained with increasing second phase, however with a detrimental effect on fracture resistance and a change in Vickers hardness. The finding of this thesis opens the possibility of fabricating textured alumina with incorporation of a second phase, aiming to tailor density, hardness and fracture resistance, based on the final application pursued.