Characterization of Ultra-high Temperature Ceramics via Transmission Electron Microscopy - In situ toughened ZrB 2 composites through SiC platelets -

The materials described in the present report were produced aiming at improving the fracture toughness of ZrB 2 -SiC composites, through the activation of effective mechanisms, such as crack deflection and crack bridging. The concept behind this work is based on the irreversible SiC ?->? transformation, occurring at temperature above 1900°C. Two were the routes covered. The first method consists in the sintering of the composite and simultaneous SiC transformation from rounded particle to elongated platelet in a pressureless furnace. To investigate the effect of the sintering additive and the secondary phases formed upon sintering, we used either MoSi 2 or Si 3 N 4 . Both these additives enhance the densification through formation of small amount of liquid phases, which in turn favour the SiC growth along the c axis. The second route foresees the hot pressing of the composite containing La 2 O 3 -MgO as sintering additives and the subsequent annealing at higher temperature in the same pressureless furnace mentioned above in order to crystallize the glassy phase. In this case, several annealing cycles were explored to identify the most promising microstructure, displaying the right compromise between SiC platelets development and ZrB 2 unavoidable grains coarsening. All the materials produced were examined through x-ray diffraction (XRD), to identify the crystalline phases formed upon sintering or after the annealing cycle, scanning electron microscope (SEM), to study the distribution of the secondary phases, and transmission electron microscope (TEM), to analyse the microstructure at nanoscale level, with particular attention to the presence of crystallized pockets at the triple junctions and to the study of the interfaces by high resolution mode. The mechanical properties, such as fracture toughness and flexural strength, were measured too to evaluate the goodness of the composition and of the thermal cycle. Fracture toughness values as high as short fiber-reinforced ZrB 2 composites were obtained pressureless sintering a ZrB 2 -SiC composite after addition of MoSi 2 . On the other hand, the annealing cycle performed on the La 2 O 3 -MgO containing ceramic, effectively promoted SiC elongation and the crystallization of the glassy phase into La-silicide at the triple junction. This phase and residual amorphous phase promoted intergranular fracture, but also showed diffused microcracking, resulting in no appreciable fracture toughness increase as compared to the as sintered material.