Development of refractory composites for extreme aerospace environments

Materials for new generation space crafts have to satisfy a strict set of requirements due to the high thermo-mechanical loads to which they will be subjected during the various phases of the flight. Zirconium diboride (ZrB2), belonging to the ultra-high temperature ceramic (UHTC) class, is one of the most refractory material possessing interesting engineering and physical properties. It is often sintered with 10-30 vol% SiC particles, which improve mechanical strength and oxidation resistance. However, these composites possess low fracture toughness, 3-4 MPa·m½, low thermal shock resistance and the operating temperature is limited to about 1650°C, i.e. when SiC starts to suffer active oxidation in subsurface scale. Another feature of ZrB2-composites is their relatively high density, around 6 g/cm3. If these materials are intended to be parts of flying vehicles, another issue to address is to decrease their specific weight, without jeopardizing their thermo-mechanical properties. In this work, functionally graded (FG) composites based on ZrB2 and containing SiC chopped fibers from 5 to 50 vol% were produced by powder metallurgy and hot pressing. An additional MoSi2-ZrB2 outermost layer was topped to improve the oxidation resistance. To protect SiC fibers from detrimental reactions with MoSi2 during sintering, a ZrB2-based buffer layer was sandwiched between the fiber-rich bulk and the outermost MoSi2-containing scale. The fibers in the optimized architecture were efficaciously protected during the sintering stage and effectively toughened the matrix. The best FG composite had a density as low as 3.65 g/cm3, nearly halved compared to pure ZrB2, and fracture toughness from 5 to 7 MPa??m, doubled as compared to monolithic ZrB2. These composites displayed excellent oxidation resistance at 1650°C in air thank to the protective action exerted by the MoSi2-ZrB2 outermost layer.