In-situ synthesis and densification of HfB2 ceramics by the spark plasma sintering technique

Hafnium diboride (HfB2) ceramics were in-situ synthesized and densified by the spark plasma sintering (SPS) method using HfO2 and amorphous boron (B) as starting powders. Both synthesis and densification processes were succesfully accomplished in a single SPS cycle with one/two step heating schedules, which were designed by considering thermodynamic calculations made by Factsage software. In two step heating schedule, soaking at 1000 °C, which was supposed to be the synthesis temperature of HfB2 particles, caused a creep like behaviour in final ceramic microstructures. A single step synthesis/densification schedule at 2050 °C with a 30 min hold time under 60 MPa uniaxial pressure leads to obtain monolithic HfB2 ceramics up to 94% of it's theoretical density. Considering the literature, low hardness values (max. 12 GPa) were achieved, which were directly attributed to the low bonding between HfB2 grains in terms of the residual stresses occurred during the synthesis and cooling steps. Samples produced by applying one step heating schedule showed transgranural fracture behaviour with a, fracture toughness of 3.12 MPa m1/2. The fracture toughness of the samples produced by applying two step heating schedule was higher (5,06 MPa m1/2) and the fracture mode changed from transgranular to mixed mode.