Effect of the heating rate on the formation mechanism of zirconium diboride by reactive spark plasma sintering

The synthesis of ZrB2 from elemental reactants via reactive spark plasma sintering (SPS) process is markedly affected by the heating rate conditions adopted. Specifically, when the temperature during SPS is increased at 500 °C/min or faster, the synthesis reaction proceeds under the combustion regime. On the other hand, if heating rates equal or lower than 200 °C/min are considered, the process is governed by a gradual solid-state diffusion mechanism. Although the route involving the combustion synthesis event permits the obtainment of pure dense products at relatively milder conditions, the gradual evolution of the synthesis reaction is preferable. Indeed, the inconveniences encountered during the process (gas pressure increase, powder expulsion, product inhomogeneity, abrupt sample displacement, die/punches breakage) make its practical exploitation difficult. In contrast, safety conditions are preserved when sufficiently lowering the heating rates to suppress the combustion reaction. Correspondingly, 96 % dense monolithic products can be obtained at temperature levels of about 2,000 °C within 30 min total time.