Low temperature sintering of Hf_0.95Nb_0.05B₂-based ceramics with submicron-scaled grains and enhanced mechanical properties

Hf0.95Nb0.05B2 ceramics and their composites containing 20 vol% SiC were prepared via high-pressure spark plasma sintering in the study. The densification, microstructures, and mechanical properties of the prepared materials were then investigated. It is challenging to achieve full densification of HfB2 ceramics, even with markedly refined Hf0.95Nb0.05B2 solid solution powder under the sintering conditions of 2000 degrees C/30 MPa. However, under the sintering conditions of 1700 degrees C/200 MPa, a dense microstructure of Hf0.95Nb0.05B2 ceramics was achieved. Moreover, the Hf0.95Nb0.05B2-20 vol% SiC composite was densified at a lower temperature (1500 degrees C) and exhibited ultrafine grains (300 nm) and high-density defects, including stacking faults, LomerCottrell locks, and twins, thus resulting in exceptional comprehensive mechanical properties, such as ultrahigh hardness (32 GPa) and significantly improved fracture toughness (5.2 MPa.m1/2).