Enthalpy driving force and chemical bond weakening: the solid-solution formation mechanism and densification behavior of high-entropy diborides (Hf1-x/4Zr1-x/4Nb1-x/4Ta1-x/4Scx)B2

First-principles demonstration of band filling-induced significant improvement in thermodynamic stability and mechanical properties of Sc[Formula: see text]Ta[Formula: see text]B[Formula: see text] solid solutions

Mixtures of different metal diborides in the form of solid solutions are promising materials for hard-coating applications. Herein, we study the mixing thermodynamics and the mechanical properties of AlB[Formula: see text]-structured Sc[Formula: see text]Ta[Formula: see text]B[Formula: see text] solid solutions using the first-principles method, based on the density functional theory, and the cluster-expansion formalism. Our thermodynamic investigation reveals that the two diborides readily mix with one another to form a continuous series of stable solid solutions in the pseudo-binary TaB[Formula: see text] [Formula: see text]ScB[Formula: see text] system even at absolute zero. Interestingly, the elastic moduli as well as the hardness of the solid solutions show significant positive deviations from the linear Vegard's rule evaluated between those of ScB[Formula: see text] and TaB[Formula: see text]. In case of Sc[Formula: see text]Ta[Formula: see text]B[Formula: see text], the degrees of deviation from such linear trends can be as large as 25, 20, and 40% for the shear modulus, the Young's modulus, and the hardness, respectively. The improvement in the stability as well as the mechanical properties of Sc[Formula: see text]Ta[Formula: see text]B[Formula: see text] solid solutions relative to their constituent compounds is found to be related to the effect of electronic band filling, induced upon mixing TaB[Formula: see text] with ScB[Formula: see text]. These findings not only demonstrate the prominent role of band filling in enhancing the stability and the mechanical properties of Sc[Formula: see text]Ta[Formula: see text]B[Formula: see text], but also it can potentially open up a possibility for designing stable/metastable metal diboride-based solid solutions with superior and widely tunable mechanical properties for hard-coating applications.

Investigation of the Effect of Supersonic Flow of Dissociated Nitrogen on ZrB2-HfB2-SiC Ceramics Doped with 10 vol.% Carbon Nanotubes

The method of fabricating dense ultra-high temperature ceramic materials ZrB2−HfB2−SiC−CCNT was developed using a combination of sol-gel synthesis and reaction hot pressing approaches at 1800 °C. It was found that the introduction of multilayer nanotubes (10 vol.%) led to an increase in the consolidation efficiency of ceramics (at temperatures > 1600 °C). The obtained ZrB2−HfB2−SiC and ZrB2−HfB2−SiC−CCNT materials were characterized by a complex of physical and chemical analysis methods. A study of the effects on the modified sample ZrB2−HfB2−SiC−CCNT composition speed flow of partially dissociated nitrogen, using a high-frequency plasmatron, showed that, despite the relatively low temperature established on the surface (≤1585 °C), there was a significant change in the chemical composition and surface microstructure: in the near-surface layer, zirconium−hafnium carbonitride, amorphous boron nitride, and carbon were present. The latter caused changes in crucial characteristics such as the emission coefficient and surface catalyticity.