Transition metal diborides belong to the group of ultra-high temperature ceramics (UHTC) characterized by very high-melting point (T_m > 3000 °C) and exceptional thermophysical properties (hardness, chemical inertness, ablation resistance) at high temperature^1,2. Among them, ZrB₂ features the lowest density, which makes it the best candidate for aerospace applications. However, UHTC are intrinsically hard to sinter, due to the covalent nature of their bonds which is responsible for their extraordinary properties but also of extremely low self-diffusion.
In the last decade, flash sintering (FS) has been used to produce dense ceramic sample in less than a minute³ and has been proven to be efficient also for metal-like compounds⁴. Moreover, the extreme sintering conditions (flash event, very high heating rate, current flow) often result in peculiar microstructures leading to unexpected mechanical properties⁵. In the present work, the applicability of electrical resistance flash sintering to commercial ZrB₂ powders (Höganäs AB Grade B) was explored.
The untreated ZrB₂ powder exhibited surprisingly high electric resistance, making the incubation time for the flash event extremely variable and non-reproducible, leading to poor final densities. This effect is likely correlated with the surface oxide layer, which is also known to favour coarsening over sintering⁶. The sinterability of the powder was therefore improved by either removing the surface oxides with a reduction treatment (prior to sintering) or doping with sintering aids such as WC (or B₄C).
In both cases, the incubation time was shortened and uniformed to a few seconds. Despite this, however, the reduced ZrB₂ powder sintered to high density only when some Mo contamination from the electrodes penetrated the sample. The addition of WC, instead, led to effective densification and a multiphasic microstructure involving ZrB₂ and WC grains combined with at least two intermediate phases. The mechanical properties of dense (above 95% TD) ZrB₂-based materials were tested by microindentation, showing performances comparable to what reported in the literature in spite of the simpler, time- and energy-saving process.

 
¹ Telle, R., Sigl, L. S. & Takagi, K. Boride-Based Hard Materials. in Handbook of Ceramic Hard Materials 802–945 (Wiley-VCH Verlag GmbH).
² Chamberlain, A. L., Fahrenholtz, W. G., Hilmas, G. E. & Ellerby, D. T. High-Strength Zirconium Diboride-Based Ceramics. J. Am. Ceram. Soc. 87, 1170–1172 (2004).
³ Biesuz, M. & Sglavo, V. M. Flash sintering of ceramics. J. Eur. Ceram. Soc. 39, 115–143 (2019).
⁴ Mazo, I., Molinari, A. & Sglavo, V. M. Electrical resistance flash sintering of tungsten carbide. Mater. Des. 213, 110330 (2022).
⁵ Mazo, I., Molina-Aldareguia, J. M., Molinari, A. & Sglavo, V. M. Room temperature stability, structure and mechanical properties of cubic tungsten carbide in flash sintered products. J. Mater. Sci. 58, 1829–1848 (2023).
⁶ Guo, S.-Q. Densification of ZrB₂-based composites and their mechanical and physical properties: A review. J. Eur. Ceram. Soc. 29, 995–1011 (2009).