Hot-pressed ZrB2–SiC composite ceramics: Effect of various Ta-containing additives on the microstructure and mechanical properties

The ZrB₂–SiC composites have been commercially used at ultrahigh temperatures, but it often failed due to their poor toughness. In order to solve this problem, four types of Ta-containing additives (Ta, TaC, TaB₂ and TaSi₂) were used as the “third phase” to regulate the microstructure, so as to enhance the mechanical properties of hot-pressed ZrB₂–20SiC-based ceramics (in vol. %). The incorporation of the additives generated a core–shell structure, which comprised of a ZrB₂ core and a (Zr, Ta)B₂ solid solution shell. The additives helped refine the ZrB₂ grains in addition to the metallic Ta and release the internal stress field generated by the thermal misfit. The interfacial structure was modified by the formation of the coherent core/shell interface and the semi-coherent interface of adjacent ZrB₂ grains and the semi-coherent ZrB₂/(Zr, Ta)C interface in the TaC-additive composite. The addition of TaB₂ or TaC hardened the ZrB₂–20SiC ceramics, whereas the addition of Ta or TaSi₂ reduced the hardness. The fracture toughness was enhanced by the formation of the Ta-containing phases. These phases reduced the stress intensity factor of the crack tip, which was proportional to the intrinsic residual stress. However, the crack-propagation mechanism would be changed by the incorporation of various Ta-containing additives. The decrease in the crack deflection, which was induced by the stronger interfacial bonding force and the significant consumption of SiC, resulted in relatively low toughness in the Ta- and TaC-included samples. The weaker interfacial bonding force in the TaB₂- and TaSi₂-included samples caused an increase in deflection and generated branching, which enhanced the toughness of the TaSi₂-included composites to ～4.72 MPa?m^1/2.