Spark plasma sintering of Zr- and Hf-borides with decreasing amounts of MoSi2 as sintering aid

ZrB2- and HfB2-based composites containing 1, 3, and 9 vol.% MoSi2 as sintering aid were densified by spark plasma sintering at temperatures ranging from 1700 to 1950 °C, with an applied pressure of 100 MPa. As received ZrB2 and HfB2 powders were also sintered at temperatures in the range of 2100–2200 °C. Fully dense samples were obtained between 1750 and 1850 °C when MoSi2 was added. As received ZrB2 powder was sintered to 98% at 2100 °C. The microstructural features were investigated by means of SEM–EDS technique. Silicon carbide was detected in all the doped compositions along with significant amounts of oxide species (Hf/ZrO2 and SiO2). Vickers hardness and fracture toughness were measured by indentation technique on polished sections. The effect of the MoSi2 content on densification, microstructure and mechanical properties is discussed.