Thermochemical model on the carbothermal reduction of oxides during spark plasma sintering of zirconium diboride

Carbon was used to reduce oxides in spark plasma sintered ZrB 2 ultra‐high temperature ceramics. A thermodynamic model was used to evaluate the reducing reactions to remove B 2 O 3 and ZrO 2 from the powder. Powder oxygen content was measured and carbon additions of 0.5 and 0.75 wt% were used. A C–ZrO 2 pseudo binary diagram, ZrO 2 –B 2 O 3 –C pseudo ternaries, and Zr–C–O potential phase diagrams were generated to show how the reactions can be related to an open system experiment in the tube furnace. Scanning transmission electron microscopy identified impurity phases composed of amorphous Zr–B–O with lamellar BN and a Zr–C–O ternary model was calculated under SPS sintering conditions at 1900°C and 6 Pa to understand how oxides can be retained in the microstructure.