Effect of 20 mol% Al addition on phase formation and mechanical properties of TaCx (x=0.5, 0.55, 0.6 and 0.7) ceramics

Abstract TaCx (x = 0.5, 0.55, 0.6 and 0.7) ceramics with 20 mol% Al addition were prepared by hot pressing TaC, Ta and Al powder mixtures. Effects of the overall C:Ta ratios and the Al addition on constituent phases, microstructures and mechanical properties of the consolidated materials were examined. Results showed that O impurity contained in the starting powders was collected in form of Al2O3 due to strong affinity of O with Al. The TaCx-Al composites were easy to densify due to presence of molten Al at its melting point (Tm = 660oC) and a transient Al-Ta eutectic liquid at higher temperatures. After densification, the metallic liquids were eliminated ultimately by formation of refractory Ta4AlC3 via reaction of Al, Ta and TaC. Reactions between the remnant Ta and TaC after Ta4AlC3 formation resulted in ζ-Ta4C3-z or h-Ta2C depending on the initial C:Ta ratio. The Ta4AlC3 compound was compatible with the tantalum carbides. Microstructures of the TaCx-Al (x=0.5-0.7) composites were consisted of ζ-Ta4C3-z and Ta4AlC3 fine lamellar grains to benefit the mechanical properties of the composites. The TaC0.7-20mol% Al composition had satisfying mechanical properties (flexural strength 613±28 MPa and fracture toughness 11.2±0.8 MPa·m1/2). TG-DSC evidenced an increase in oxidation resistance of the TaCx ceramics by the addition of 20 mol% Al.