Representative samples of yttria stabilized zirconia (7YSZ) co-doped with varying concentrations of TiO2 were fabricated using plasma spraying. Samples were sintered in order to minimize porosity and to simulate the bulk material properties. After sintering, porosity levels of less than 1.25% were achieved. Both as-sprayed and sintered samples with 5, 10 and 15 wt% TiO2 addition levels were microstructurally characterized using SEM, XRD and optical image analysis methods. Vickers hardness, Young’s modulus, and fracture toughness were measured using nano and macroindentation methods. Microstructural analysis revealed that sintering of the TiO2 doped samples was required to achieve a homogeneous composition distribution, with TiO2 predominantly residing in solid solution within the ZrO2 matrix. Sintering for 325 hs at 1200 °C resulted in sufficient diffusion of TiO2 into the 7YSZ. The addition of TiO2 stabilized more tetragonal phase as revealed by XRD measurement. Sintering also showed significant improvements in fracture toughness in all co-doped samples. Fracture toughness values calculated using load-independent equations provided a clear trend in fracture toughness improvement with TiO2 addition. Ferroelastic toughening of the tetragonal phase was believed to have played an effect. There was also a reduction in monoclinic phase content with TiO2 addition, which may have limited microcrack formation and consequently increased the fracture toughness. With the addition of 10 wt% TiO2, the fracture toughness was improved by over 50%; however, this improvement started to decline at 15 wt% TiO2 addition. Volumetric porosity measurements also revealed significant improvements in fracture toughness with respect to lowering the porosity content as observed in all sintered samples.
Effect of Crack Bridging on the Toughening of Ceramic/Graphene Composites
