Low temperature densification mechanism and properties of Ta1-xHfxC solid solutions with decarbonization and phase transition of Cr3C2

As a novel member of the ultra-high temperature ceramic family, which have extremely high melting points and remarkable hardness, Ta1-xHfxC solid solution ceramics are promising for applications in thermal protection systems. Ta1-xHfxC (x = 0, 0.2, 0.5, 0.8, and 1.0) with 2 vol% Cr3C2, were densified up to 98.8% at 2000 °C using a two-step spark plasma sintering process. Effect of Cr3C2 on the linear shrinkage of Ta1-xHfxC was investigated. Possible ‘eutectic’ reaction within TaCCr3C2 ceramic was inferred to contribute to the shrinkage at 1462 °C. High-angle annular dark-field scanning transmission electron microscopy combined with energy-dispersive spectroscopy was employed to further confirm the mutual diffusion between rock-salt structured ‘CrCx’ and TaC. Flexural strength, fracture toughness and Vicker’s hardness of Ta1-xHfxC ceramics were in the range of 439–492 MPa, 4.0–5.8 MPa∙m1/2 and 14.9–19.9 GPa respectively. The coefficient of thermal expansion (in the temperature range of 25–1000 °C) and thermal conductivity (at 1000 °C) of Ta1-xHfxC varied from 7.17 to 7.51 × 10−6 K−1 and 31.9–42.9 W/m·K, respectively. The high-temperature strength of Ta0.5Hf0.5C decreased to 165 MPa up to 1600 °C, approximately 34% of room-temperature strength, and a ‘zig-zag’ load-displacement curve was observed.