Annealing of a (Hf0.2Ta0.2Ti0.2Nb0.2Zr0.2)C high-entropy ceramic up to 2100 °C: In-situ removal of oxide impurities and microstructural modification

A (Hf0.2Ta0.2Ti0.2Nb0.2Zr0.2)C high-entropy ceramic (HEC) prepared by the combination of self-propagation high-temperature synthesis (SHS) and spark plasma sintering (SPS), was annealed at 2000 degrees C and 2100 degrees C in order to remove the impurities and modify the microstructure. The microstructure and phase composition of the HECs were characterized by X-ray diffraction, scanning electron microscopy, and transmission electronic microscopy. Moreover, the mechanical properties were examined by nanoindentation. It is found that the as-prepared HEC contains the impurities of (Hf, Zr)O2 and amorphous carbon. The oxide and amorphous carbon can be in-situ removed by further carbothermal reduction reaction during the annealing, followed by dissolution of the reduction products into the HEC phase. In other words, the heat treatment can significantly improve the microstructure of the HEC by the removal of impurity and uniform distribution of the compositional elements. In addition, the annealing at 2000 degrees C and 2100 degrees C enhance the mechanical properties of the HEC. The nano-hardness, elastic modulus and fracture toughness are 34.04 +/- 1.49 GPa, 525.82 +/- 14.51 GPa and 3.60 +/- 0.55 MPa m1/2, respectively, for the HEC annealed at 2100 degrees C for 1 h, which are 1.13-2.08 times of the as-prepared HEC.