Novel refractory high-entropy ceramics: Transition metal carbonitrides with superior ablation resistance

Highlights: • Novel ultra-high-temperature-ceramic (UHTC) as well as high-entropy material with superior ablation resistance was proposed. • Special micro-segregation is found in the HECN, where Ta, Hf, Zr, Ti elements aggregated inside of grains and Nb segregated at the grain boundary. • There is the compact multi-oxide scale that involves Hf-Zr-rich grains skeleton and continuous molten-like phase with Ta-Ti-Nb segregation. A novel high-entropy material as well as ultra-high temperature ceramic (Ta_0.2Hf_0.2Zr_0.2Ti_0.2Nb_0.2)C_0.8N_0.2 was synthesized from ten kinds of transition metal carbides and nitrides, via high-energy ball-milling and spark plasma sintering. Besides, the ablation behavior of high-entropy ceramic was first studied by oxy-acetylene torch system at 2500 K. The results showed that optimized nitrogen substituted in sublattice could remarkably improve the ablation resistance by an impressive 57±1 % and 72 ±1 % in mass ablation rate and linear ablation rate compared to high-entropy carbides, since the highly dense multiple oxides scale with large Hf-Zr-rich grains skeleton surrounded by continuous secondary-phase in the boundaries that was able to retard the diffusion of oxygen.