Flash sintering of tungsten carbide

Binderless tungsten carbide (BTC) ceramics are inherently difficult to process and very brittle. Most consolidation techniques for processing pure WC powder require long sintering times and intense energy consumption. High-T pressureless and pressure-assisted sintering processes often lead to low-quality and coarsened microstructures, thus limiting the use of WC ceramics to few niche applications. Field-assisted sintering techniques (FAST), like spark plasma sintering (SPS), significantly improve the densification of fine and ultrafine WC powders. However, SPS requires high current outputs and expensive apparatus. SPS ceramics still lack adequate toughness to extend the use of BTC components in heavy-duty applications requiring reliable load-bearing capability and/or resistance against rapid and unexpected impacts or temperature drops. This research work explored a new consolidation route capable of boosting the mass transport phenomena (accelerated sintering) and, simultaneously, introducing new microstructural features. The process called flash sintering (FS) offers great potential in accelerating diffusion phenomena and altering the crystallographic and/or the defect chemistry of the sintered ceramics. Many scientific studies reported structural alterations, enhanced plastic flow and material softening by introducing “out-of-equilibrium” characteristics. Currently, FS technology requires, for its activation, a negative dependence of the electrical resistivity with temperature (NTC) of the material to be sintered. This is a universal requirement for the flash event to occur thus theoretically inhibiting the flash sintering of conductive materials with a positive temperature coefficient for resistivity (PTC), like metals or WC. In the present work, we reported how during electrical resistance sintering (ERS) experiments conducted on pure WC nanopowders, a flash event was triggered during the first seconds of the process. This was demonstrated to occur thanks to the different evolution of the electrical properties of a granular compact with temperature. WC powders possess an initial NTC behaviour which can activate a transitory thermal runaway phenomenon which makes the activation of a flash event in these materials possible, intense enough to allow ultrafast densification in less than 10 s. This breakthrough allows to verify whether and how the flash event modifies the final sintered material. FS and SPS sintered ceramics were compared in their microstructural, physical and mechanical properties, thus pointing out how some peculiar modifications are exclusively present in the flash-sintered material. FS can stabilize the WC1-x metastable phase after cooling to room temperature, and this was demonstrated to alter the high-temperature deformation of WC micropillars during compression. In addition, FS BTC are inherently softer with respect to SPS ones, resulting in higher fracture toughness and slightly lower hardness. Even if not final, the results indicate how the flash sintering of WC can be explored further to process engineered BTC ceramics with an optimized hardness/toughness ratio and an enhanced deformability.