Particle size/ grain size correlation and mechanical properties of spark plasma sintered 8Y-ZrO2, MgAl2O4, and Al2O3 based composites
The primary goal of this dissertation is to obtain relationship between the two fundamental ceramic processing parameters of particle size and grain size distribution in order to describe grain growth in ceramic oxides. Commercial powders of cubic zirconia (8Y-ZrO2), Spinel (MgAl2O4), and Alumina (Al2O3) were characterized for particle sizes by using dynamic light scattering. Agglomeration of powder particles was avoided using surface chemistry and planetary ball milling. Lower average sizes by as much as 50% along with narrower distribution were obtained as compared to the as-received information.
Using the advanced technique of Spark plasma sintering highly dense fine-grained commercial ceramic oxides were fabricated grain sizes varying from 150 nm to >10 µm. Average grain size and size ratio calculated using powder particle sizes were used to empirically model grain growth tendencies among ceramic oxides at different sintering temperatures (950-1300℃). Activation energy of grain growth was also calculated for all the sintered composites. Spinel sintered composites exhibited slowest grain growth (range of grain size = 1.7-2.3 µm) amongst the single-phase ceramics while α-Alumina specimens exhibited grains in the range of 10.5-16 µm. For multiphase ceramics, 8Y-ZrO2/Al2O3 exhibited uniform grain size compared to spinel-based binary composites while the novel three-phase composite were also studied. It was concluded that multiphase composites exhibit limited grain growth tendency, as indicated by a 10x smaller size ratio when compared to single phase composites. For each ceramic oxide, sintering experiments were performed using powders with two different particle sizes (~100 and ~200 nm). Results indicate that larger-sized powders result in slower grain growth but eventually achieve higher grain size at temperatures >1200°C.
An extensive analysis of hardness properties was analyzed by plotting Hall-Petch relationship for the sintered single/multiphase ceramic oxides. A strong correlation of hardness-grain size relationship was observed for single phase ceramic oxides and ternary composite (R2~0.8). On the other hand, fracture toughness values for most of the ceramic oxide samples sintered contained a scatter of results for microstructures with different grain sizes; thereby no correlation was obtained. Fractal micrography study of sintered single/multiphase composites highlighted the presence of primary inter-granular/trans-granular or mix modes of failure. The effects of different heating rates on the microstructure and sintering kinetics via master sintering curves of single, binary and ternary phase composites were also studied