Hot Pressed HfB2 and HfB2-20 vol%SiC Ceramics Based on HfB2 Powder Synthesized by Borothermal Reduction of HfO2

Phase pure hafnium diboride (HfB2) powder was synthesized by borothermal reduction of hafnium dioxide using amorphous boron at relatively low temperature (1600 degrees C) in vacuum. The synthesized HfB2 powder had an average particle size of 1.37 mu m with an equiaxed shape, and a low oxygen content of 0.79wt%. Using the as-synthesized HfB2 powder and a commercial SiC, HfB2 monolithic, and HfB2-20 vol%SiC composite were hot pressed at 2000 degrees C to relative densities of 95.7% and 99.2%, respectively. With the addition of SiC, the grain size decreased and the fracture behavior changed from intergranular to a mixed mode, which resulted in a high flexural strength of 993 +/- 90 MPa for the composite. Fracture toughness of the composite was 6.29 +/- 0.65MPam(1/2), which was significantly higher than that of the HfB2 monolithic and the reported values in literature.