ZrB2 and HfB2 toughened with Hi Nicalon SiC chopped fibers

ZrB2 and HfB2 are candidate materials for use in aggressive environment, owing a unique combination of favourable properties of high temperature stability and excellent engineeristic properties. This class of materials is raising always more interest for future generation space vehicles, as wing leading edges and nose tips, as well as propulsion system elements. The most investigated system is based on ZrB2-SiC, owing to a high strength, up to 1 GPa, high hardness, around 20 GPa and oxidation resistance at least up to 1600°C. The major weak point remains the low fracture toughness, 3 to 5 MPam1/2. It has been recently demonstrated that the introduction of elongated secondary phases and the choice of the proper sintering additive, can lead to almost twofold increase of the fracture toughness. This work presents the last developments of ZrB2 and HfB2 ceramics toughened with HI Nicalon® SiC chopped fibers. The effect of various sintering additives, MoSi2, Si3N4, ZrSi2 and TaSi2, is investigated in relationship to the microstructure evolution upon sintering, the fiber interaction, the resulting interface with the matrix and to the high temperature behaviour. Scanning and transmission electron microscopy was used to investigate the microstructure modification at nanoscale level. The fiber morphology resulted notably modified depending on the sintering temperature and the sintering additive; in particular, silicides had a more aggressive behavior toward the fiber, which developed a multilayered aspect. Nanoindentation was employed to characterize hardness and Young's modulus of each scale. As for the mechanical properties, flexural strength, with the 4-point method, and toughness, measured by the CNB technique, were compared to values of reference unreinforced materials to assess the effective improvement. Oxidation tests in a bottom-up loading furnace box were also performed on selected composites in the temperature range 1200-1700°C.