Efficient Technologies for the Fabrication of Dense TaB₂-Based Ultra-High-Temperature Ceramics

A 20 min ball-milling treatment (ball to powders ratio equal to 1) was demonstrated to be a valuable method for mechanochemically activating Ta, B₄C, Si, and graphite to prepare TaB₂−SiC and TaB₂−TaC−SiC ultrarefractory composites by self-propagating high-temperature synthesis (SHS). The resulting completely converted powders were spark plasma sintered at 1800 °C for 30 min, thus obtaining products about 96% dense. TGA characterization of bulk materials confirmed the beneficial effect of SiC in the resistance to oxidation behavior of the composite materials, while the presence of TaC appears to be unfavorable from this point of view. The obtained mechanical properties are in general comparable to those ones of Ta-based monolithic and composite bulk ceramics densified in previous works, although fracture toughness is significantly higher for TaB₂−SiC. This outcome holds also true when the comparison is extended to the TaB₂−TaC−SiC sample sintered in this work. The possible explanation is based on the occurrence of several toughening mechanisms (crack bridging, crack deflection, frictional interlocking, and crack branching) involved when TaB₂−SiC samples are subjected to the prescribed indentation conditions, whereas crack propagation is facilitated by the relatively finer and more homogeneous microstructure exhibited by the ternary system.