Characterization of Ultra-high Temperature Ceramics via Transmission Electron Microscopy - ZrB 2 composites containing different kinds of fibers -

The materials described in the present report were produced aiming at improving the fracture toughness of ZrB 2 ceramics, through the introduction of various kinds of short fibers. The fibers introduced into the ZrB 2 matrix are based on carbon or silicon carbide (Tyranno SA3, Tyranno ZMI coated with pyrolytic carbon, Hi-Nicalon S fibers coated with Si-doped BN). Different kinds of sintering additives were chosen for the densification of the boride, namely ZrSi 2 , Si 3 N 4 , TaSi 2 and MoSi 2 . For each composition various sintering cycles were tested to identify the best temperature allowing densification of the matrix and minimize the fiber degradation. All the materials produced were examined through 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 evolution of the fiber morphology and to the study of the interfaces with the matrix in high resolution mode. This study allowed understanding of the fiber chemical stability towards different sintering additives and ZrB 2 matrix. Varying the sintering temperature it has also been possible to evaluate how the fiber morphology and chemistry change with temperature and additive. TEM analyses revealed to be essential to achieve this goal. The mechanisms leading to the final microstructure have been proposed and the mechanical properties have been correlated to the microstructural features. At the state of the art, the highest fracture toughness achieved for ZrB 2 composites containing short fibers does not exceed 6 MPa·m 1/2 . The introduction of BN- or C- coated fiber did not increase the toughness owing to a detachment or to a degradation of the coating.