Reactive consolidation and high-temperature strength of HfB₂–SiB₆

During the spark plasma sintering at 1900 °C, SiB₆ decomposes into cubic silicon carbide and boron carbide, owing to the reducing environment of the furnace. For the HfB₂-SiB₆ ceramic improvement in hardness (24.5 ± 0.7 GPa) was attributed to the formation of the B₁₂(C,Si,B)₃. Fracture toughness by indentation (6.8 ± 2.4 MPa·m^1/2), single-edge notched bend specimens (4.6 ± 0.4 MPa·m^1/2) and room-temperature strength (513 ± 21 MPa) of the HfB₂–SiB₆ composite produced by spark plasma sintering was higher or on the same level as the HfB₂–SiC ceramics. The high-temperature flexural strength tests suggested that the strength would decrease monotonically with an increase in temperature. At or below 1600 °C, only a linear stress-strain response was observed, and resulted into a mean strength of ~320 MPa. During the tests at 1800 °C, we observed a nonlinear deformation indicating ongoing plastic deformation which led to a strength decrease down to 230 ± 30 MPa.