Dual-phase medium-entropy diboride-carbide ceramics with metal element exchange during sintering

Multiphase composition design is a strategy to optimize the microstructures and properties of ceramic materials through mutual inhibition of grain growth, complementary property improvement, or even mutually reinforcing effects. More interesting phenomena can be expected if chemical interactions between the constituent phases exist. In this study, spark plasma sintering was used to prepare fully dense dual-phase (Zr, Hf, Ta)B₂-(Zr, Hf, Ta)C ceramics from self-synthesized equimolar medium-entropy diboride and carbide powders. The obtained ceramics were composed of two distinct solid solution phases including the Zr-rich diboride phase and the Ta-rich carbide phase, indicating metal element exchange occurred between the starting equimolar medium-entropy diboride and carbide during sintering. Owing to the mutual grain-boundary pinning effect, fine-grained dual-phase ceramics were obtained. The chemical driving force originating from the metal element exchange during the sintering process is considered to promote the densification process of the ceramics. The metal element exchange between the medium-entropy diboride and carbide phase significantly increased Young’s modulus of the dual-phase ceramics. The dual-phase medium-entropy 50 vol.% (Zr, Hf, Ta)B₂-50 vol.% (Zr, Hf, Ta)C ceramics with the smallest grain size exhibited the highest hardness of 22.4 ± 0.2 GPa. It is inferred that optimized comprehensive properties or performance of dual-phase high-entropy or medium-entropy ceramics of diborides and carbides can be achieved by adjusting both the volume content and the metal element composition of the corresponding starting powders of diborides and carbides.