Reactive carbothermal reduction of ZrC and ZrOC using Spark Plasma Sintering

Zirconium carbide and oxycarbide bulks were produced via Reactive Spark Plasma Sintering (RSPS) using zirconia and carbon powders. The effects of dwell temperature, uniaxial pressure and heating rate on final chemical composition/stoichiometry were investigated. Heating rates (20–200°C min–1) had no significant effect on RSPS. The carbothermal reduction was hindered by increasing the sintering pressure. This was because of gases entrapped within the closed porosity. Under pressure-less conditions, the reaction onset started at ∼1300°C and the reduction was completed within 10 min resulting in an oxygen content as low as 1.47 at.-%. Under 16 MPa applied pressure, the reaction onset was shifted to 1830°C. Sub-micrometric zirconia powder (<0.1 µm) impacted negatively on the carbothermal reaction because of enhanced ZrO2 sintering preceding the carbothermic reaction. The RSPS reaction mechanism began with zirconia particle densifications between 900 and 1400°C followed by nucleation and growth of an intermediate O-rich ZrOC phase, between 1800 and 2100°C almost all zirconia was consumed. The ZrOC phase then reacted out its oxygen while simultaneously sintering. The results suggest that an optimised RSPS involve a two-step process employing a pressure-less SPS synthesis followed by a pressure-assisted sintering.