Effects of Al addition on densification, microstructure and mechanical properties of TaC-Al ceramics

Tantalum carbide ceramics containing 0.25-3.40 wt% Al as densifying aid were prepared by hot pressing at 1900 °C under 30 MPa for 60 min. Effects of Al addition on densification, phase composition, microstructure and mechanical properties of the TaC-Al ceramics were investigated. Al was an effective oxygen getter which reacted with volatile oxygen impurities such as Ta2O5 on surfaces of TaC particles to form non-volatile minor phase of Al2O3, thus slowed TaC grain growth via inhibiting the non-densifying matter transport by evaporation-condensation. When Al content was larger than the value that was needed for complete impurity reaction of 10Al + 3Ta2O5 = 6Ta + 5Al2O3, presence of Al liquid at high temperatures improved densification of the TaC-Al ceramics because Al liquid spread along the grain boundaries and wet the TaC grains. The TaC ceramic with 0.75, 1.50 and 3.40 wt% Al addition reached a relative density value of 91.6, 97.5 and 100%, respectively, with a similar average grain size of 3.5-4.8 μm. Fe impurity dissolved in Al and formed Al6Fe; slight reaction of Al with TaC formed a thin layer of Ta-Al-C compound around TaC grains. The TaC ceramic with 1.50-3.40 wt% Al additions showed good Vickers hardness, Young's modulus and flexural strength in the range of 12.4-13.2 GPa, 413-479 GPa and 532-673 MPa, respectively. Increase in oxidation resistance of TaC ceramics was observed at 600 °C in air with a few percentages of Al additions.