Synthesis of Ultra-Fine Hafnium Carbide Powder and its Pressureless Sintering

Ultra-fine HfC powders with particle sizes of 380 or 225 nm were synthesized by carbothermal reduction of HfO2 with graphite or carbon black, respectively. The investigation of the process indicated that a vacuum level as high as 5 Pa was appropriate for decreasing the reaction temperature and consequently limiting the particle growth of HfO2 reactant. Type and size distribution of carbon sources demonstrated an obvious effect on the particle size of the HfC powders obtained. A model was proposed to explain this phenomenon. Carbon black with a characteristic particle size of 42 nm resulted in a HfC powder with a median particle size of 225 nm. Compared with the reported values in the literature, the synthesized HfC powder exhibited better sinterability than the commercial HfC powder. Using the synthesized powder, monolithic HfC ceramics pressureless sintered at 2400 degrees C reached a relative density of 98.4% and an average grain size of about 4 mu m.