Transient liquid phase sintering of tantalum carbide ceramics by using silicon as the sintering aid and its effects on microstructure and mechanical properties

Tantalum carbide composites with 0.76-8.85 wt.% elemental silicon as a sintering aid were fabricated by spark plasma sintering (SPS) at 1700 °C and 30 MPa for 5 min. The transient-liquid-phase sintering behavior, the microstructures and the mechanical properties of the tantalum carbide composites were investigated. Oxide impurities present on the surfaces of the tantalum carbide particles were eliminated by reactions with the elemental silicon in a temperature range from 1271 °C to 1503 °C to benefit densification. Then the silicon melted at its melting point temperature of 1413 °C to facilitate rearrangement of the tantalum carbide particles. By the end of the densification, the elemental silicon transformed into more refractory TaSi₂ and SiC in the consolidated ceramics by reactions with the tantalum carbide at temperatures lower than 1773 °C. Both TaSi₂ and SiC particles improved densification by physically pinning growth of the tantalum carbide grains. Further densification was resulted from creep flow of the silicides after brittle-to-ductile transformation of the silicides at temperatures <1650 °C. Due to the good effects of using elemental silicon as the sintering aid, all the compositions reached densities >96.7% theoretical. The average grain sizes in the consolidated materials decreased with the silicon addition from about 19 μm in the 0.76 wt.% Si composition to about 9 μm in the 8.85 wt.% Si composition. A good flexural strength up to ∼709 MPa was reached in the 8.85 wt.% Si material due to full density and fine microstructure.