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 degrees 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 TaCeCr3C2 ceramic was inferred to contribute to the shrinkage at 1462 degrees 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 439e492 MPa, 4.0e5.8 MPa center dot m(1/2) and 14.9e19.9 GPa respectively. The coefficient of thermal expansion (in the temperature range of 25e1000 degrees C) and thermal conductivity (at 1000 degrees C) of Ta1-xHfxC varied from 7.17 to 7.51 x 10(-6) K-1 and 31.9e42.9 W/m$K, respectively. The high-temperature strength of Ta0.5Hf0.5C decreased to 165 MPa up to 1600 degrees C, approximately 34% of room-temperature strength, and a 'zig-zag' load-displacement curve was observed. (C) 2020 The Chinese Ceramic Society. Production and hosting by Elsevier B.V.