Grain refinement of V-containing high-entropy borides during spark plasma sintering

In general, the grains of ceramic materials tend to grow and coarsen with the prolonging of holding time at high temperatures due to grain boundary migration. In this study, high-entropy diboride ceramics composed of (Hf 0.2 Zr 0.2 Ta 0.2 V 0.2 Nb 0.2 )B 2 were prepared using spark plasma sintering (SPS) at 30 MPa. Notably, grain refinement of the high-entropy diboride ceramics with increased holding times during the SPS process was unexpectedly observed. Specifically, the grain sizes of the (Hf 0.2 Zr 0.2 Ta 0.2 V 0.2 Nb 0.2 )B 2 were refined from approximately 6 µm (synthesized at 1900°C for 5 min) to around 2 µm (synthesized at 1900°C for 10 min). This intriguing phenomenon is likely attributable to the intrinsic lattice distortion observed in high-entropy ceramics. The introduction of VB 2 was found to significantly enhance lattice distortion, playing a crucial role in the grain refinement of the (Hf 0.2 Zr 0.2 Ta 0.2 V 0.2 Nb 0.2 )B 2 . Notably, the grain refinement resulted in significant improvements in flexural strength and fracture toughness. This work offers a novel strategy for tailoring the microstructure and mechanical properties of high-entropy ceramics.