Boakye-Yiadom, SolomonDuntu, Solomon Hanson2023-12-082023-12-082023-12-08https://hdl.handle.net/10315/41749The demand for tough materials in extreme conditions has grown due advanced technology requirements, such as high pressures (> 90GPa), elevated temperatures (> 2000°C), and radiation exposure, has grown substantially. Advanced ceramics, particularly alumina (Al2O3), with their low weight, hardness, and chemical resistance, are promising candidates. However, their inherent brittleness (low fracture toughness) has limited their applications. To address this, researchers have incorporated sub-micron and nano-scale reinforcements like zirconia (ZrO2), graphene (GN), and carbon nanotubes (CNTs) into alumina to create composite materials. However, challenges remain in achieving consistent mechanical properties and minimizing trade-offs between fracture toughness (KIC) and strength. This study investigates the impact of single and combined nano-scale reinforcements (ZrO2, GN, and CNTs) on the microstructure, mechanical properties, toughening mechanisms, tribological performance, and functional attributes of monolithic alumina. The nanocomposites were fabricated by uniformly dispersing selected optimal amounts of ZrO2 (4wt% and 10wt%), GN (0.5wt%), and CNTs (2wt%) through a colloidal mixing process, followed by hot-press sintering. The results revealed a uniform distribution of additives within the alumina matrix, leading to significant matrix grain size reduction (up to 80%) in the Al2O3-10wt%ZrO2-0.5wt%GN-2wt%CNTs multi-material nanocomposite compared to pure alumina. Microhardness increased by up to 48% in the Al2O3-10wt%ZrO2-0.5wt%GN-2wt%CNTs multi-material nanocomposites due to refined grain structures and effective load transfer capabilities. Furthermore, fracture toughness (KIC) improved by up to 160%, and bending strength increased by up to 46% in Al2O3-10wt%ZrO2-0.5wt%GN-2wt%CNTs multi-material nanocomposite, due to synergistic toughening and strengthening mechanisms involving pull-outs, crack arrest, and crack bridging by GN and CNTs. This nanocomposite also exhibited up to a 93% reduction in wear rate compared to pure alumina, attributed to wear resistance mechanisms such as micro-crack bridging and intergranular fracture restriction during sliding. Further, the incorporation of GN and CNTs improved the electrical conductivity of monolithic alumina from 10-13 S/m up to 102 S/m (increase up to 15 orders of magnitude), with the Al2O3-10wt%ZrO2-2wt%CNTs nanocomposite registering the highest conductivity value. This was ascribed to the intrinsic electrical properties of carbon nanostructures, percolation effect and the refined grain structure of the nanocomposite which enhances electron mobility by forming continuous networks and pathways.Author owns copyright, except where explicitly noted. Please contact the author directly with licensing requests.Materials ScienceMechanical engineeringElectronic Thesis or Dissertation2023-12-08AluminaZirconiaGrapheneCarbon NanotubesNanocompositesToughening MechanismSynergyHall-PetchGrain SizeFracture ToughnessTribology and WearX-Ray Diffraction (XRD)Functional propertiesCeramic matrix composites (CMC)Scanning electron microscopy (SEM)Transmission electron microscopy (TEM)Focused ion beam (FIB) millingBending strengthCoefficient of friction (COF)Crsytallite sizes