Synthesis and growth mechanism of highly crystalized multi-branched HfB2 microrods with self-toughening effect

[Display omitted] •A method utilizing sol–gel assisted carbothermal reduction was developed to facilitate the synthesis of multi-branched HfB2 with high crystallinity and aspect ratio.•By modulating the atomic molar ratio of Hf: B: C in the reactants, the aspect ratio of HfB2 microrods can be tailored from 9.52 to 12.68.•Oriented attachment and reactive templated growth of the growth mechanisms of HfB2 microrods would guiding significance for one-dimensional growth of ceramic powders.•The fracture toughness of the sample was significantly enhanced upon the addition of 6 wt% HfB2 microrods, increasing from 3.98 MPa m1/2 to 7.24 MPa m1/2. Development of strategies to toughen HfB2 ceramic, improve their mechanical properties is the key to promote their applications in extreme thermal environments. This study explores a kind of self-toughening method of HfB2 ceramic by incorporation the highly crystalized multi-branched HfB2 microrods. The multi-branched HfB2 microrods were synthesized by sol–gel process and carbothermal reduction, exhibiting length of 6–13 μm and diameter of 0.60–1.50 μm, respectively. Detailed analysis of the formation mechanism of HfB2 microrods reveals that the HfB2 multi-branched structure is a collaborative consequence of orientation attachment mechanism and reactive templates growth, in which process monoclinic HfO2 crystals transform into crystalized HfO2 microrods at ∼1000 °C, and worked as template to induce one-dimensional HfB2 microrods. Furthermore, spark plasma sintering experiments show that the optimal mechanical properties were attained by 6 wt% addition of HfB2 microrods, comparing with HfB2 ceramics free of self-toughening agent, the hardness and fracture toughness of self-toughened HfB2 ceramic improved by 8.82 % and 81.91 % respectively, demonstrating the effectiveness of self-toughening of the microrods without sacrificing the hardness of HfB2 ceramic. Thus, it can be seen that the work has important reference value for the synthesis of HfB2 powder with different morphologies and the toughening of HfB2 ceramics.