Compositionally complex diborides: Competing solubility during sintering and properties

An ultra-high temperature ceramic based on ZrB2, TiB2, and SiC was hot pressed to full density at 1850 degrees C. Addition of 5 vol% of different metal-compounds, in the form of HfC, VC, NbC or CrB2, increased the densification temperature to 1910 degrees C. The resulting compositionally complex ceramics had homogeneous microstructures with boride grains exhibiting core-shell features. The shell was a solid solution containing variable amounts of the three metals. Notably, TiB2 remained as a discrete phase in the reference material and in the presence of the V- and Cr- based additions, whilst it dissolved into the main ZrB2-based grains for other additives. Thermodynamic simulations and atomic size factors were exploited to explain the different solubility in the various systems. The compositionally complex diborides exhibited excellent properties at room temperature, with hardness up to 25 GPa and strength up to 800 MPa, which was preserved up to 1500 degrees C. However, increasing the testing temperature to 1800 degrees C resulted in plastic deformation owing to residual carbide phases. Electrical resistivity ranged between similar to 13 and 70 mu Omega<middle dot>cm, with higher values in those ceramics where TiB2 remained as a discrete phase. The observed overall properties improvements in compositionally complex borides pave the way for tailored design of UHTC materials with multication non-equiatomic composition for applications in extreme environments.