Effects of carbon on the synthesis and densification of tantalum carbide powder

Tantalum carbide (TaC) powder was synthesised at 1500 ºC by sol-gel and carbothermal reduction processes using tantalum pentachloride (TaCl5) and phenolic resin as the starting materials. The effects of the C/Ta ratios in the Ta-containing precursor on the reaction yield, microstructure, chemical composition, and sinterability of the powders were investigated. The results showed that a high C/Ta ratio was favourable for the formation of TaC powder. With an increase in the C/Ta ratio, the oxygen content of the powder decreased, whereas the free carbon content increased. Consolidated TaC ceramics with high relative density (> 97 %) were obtained at 1900 ºC for 5 min under 80 MPa after sintering the powder synthesised at C/Ta ratios of 4.00 and above. However, the hardness and fracture toughness of the TaC ceramics were slightly reduced when the C/Ta ratio exceeded 4.00, owing to weak interface bonding caused by excessive free carbon in the powder. It was found that sintering TaC powders prepared at a C/Ta ratio of 4.00 produced dense TaC ceramics, with a Vickers hardness and fracture toughness of 16.54 GPa and 3.72 GPa∙m1/2, respectively.