Characterization of Ultra-high Temperature Ceramics via Transmission Electron Microscopy - ZrB 2 and HfB 2 composites containing Hi-Nicalon SiC fibers -

The materials described in the present report were produced aiming at improving the fracture toughness of ZrB 2 and HfB 2 ceramics, through the introduction of Hi-Nicalon TM SiC short fibers. Different kinds of sintering additives were chosen for the densification of the borides, namely Si 3 N 4 , MoSi 2 and ZrSi 2 . For selected compositions various sintering cycles were testes to study the progressive fiber evolution as a function of the temperature. All the materials produced were examined through x-ray diffraction (XRD), to identify the crystalline phases formed upon sintering, 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 formation of new phases at the triple junctions and to the study of the interfaces in high resolution mode. The mechanical properties, such as fracture toughness and flexural strength were measured and compared to the baseline unreinforced materials to evaluate the effective increment. Local measurement of hardness and Young's modulus was performed by nanoindentation. This study allowed understanding what is the interaction mechanism among fiber, matrix and sintering additive, how this last influences the fiber evolution and in which terms the mechanical properties depend on the sintering additive or on the fibres properties. This activity also pointed out that, besides a discrete toughness improvement (25-50%), Hi-Nicalon fibers have some limits, in particular two important conclusions were drawn: - first, the need to carry out specific surficial treatments on these fibers in order to reduce the strength of the fiber/matrix bonds, - second to change type of fibres and chose more refractory and inert ones.