Intermediate and final stage sintering kinetics of spark plasma sintered hafnium diboride

This study investigates the sintering kinetics during the intermediate and final stages of spark plasma sintering (SPS) of HfB2 over a temperature range of 1750-1950 degrees C under uniaxial pressure of 50 MPa. During the intermediate stage (T < 1850 degrees C), a creep deformation-based model was utilised for studying the densification kinetics. The evaluated stress exponent (n) similar to 1 and an activation energy (Q(D)) of similar to 888 kJ/mol suggest lattice diffusion to be the dominant densification mechanism during this stage. At 1850 degrees C, a transition to grain boundary sliding was observed, with n similar to 1.5, as confirmed by microstructural analysis. In the final stage of sintering (T > 1850 degrees C), a grain growth-based model with grain growth exponent (m) similar to 3 and an activation energy (Q(g)) of similar to 475 kJ/mol supported the dominance of lattice diffusion during grain growth. At 1850 degrees C, a relative density of similar to 97 % was achieved, marking a shift from interface-controlled deformation via grain boundary sliding to lattice diffusion-driven grain growth. Thus, this study highlights the sintering mechanisms of HfB2 ceramics at various SPS temperatures, plausibly aiding in microstructure optimization for improved performance of ultra-high temperature ceramics (UHTCs) in extreme environments.