Reactive hot pressing route for dense ZrB₂-SiC and ZrB₂-SiC-CNT ultra-high temperature ceramics

The in-situ exothermic reactions between ZrC_0.8, B₄C and Si have assisted densification and allowed to obtain fully dense ZrB₂-31 wt.%SiC ultra-high temperature ceramics within 6 min at 1750 °C. The use of zirconium carbide instead of metallic zirconium in the green body obviated the possibility of in-situ SHS process and allowed to apply the pressure at low temperatures. The latter provided a first densification stage just above 1050 °C. A slight carbon excess was created in the green body to preserve the carbon nanotubes. The developed reactive hot pressing route (1830 °C, 3 min, 30 MPa) has been successfully used to obtain ZrB₂-SiC ceramics containing 8 vol.% of multi-wall carbon nanotubes (MW-CNT). The carbon nanotubes survived the thermal cycle and could be clearly observed in the sintered ceramics. The CNT addition improved the fracture toughness of the composite from 4.3 MPa m^1/2 for ZrB₂-31 wt.%SiC to 6.8 MPa m^1/2 for ZrB₂-29 wt.%SiC-CNT.