Multi-phase (Zr,Ti,Me)B2 solid solutions: preparation and microstructure evolution

Abstract

ZrB2 is widely recognized as the most prominent ultra-high temperature ceramic for aerospace applications, in view of its melting point above 3000°C, and despite it exhibits lower oxidation and ablation resistance as compared to HfB2, it has a much lower density. The addition of TiB2 further lowers the overall weight, which is a relevant factor for materials intended to flight, but it also worsen the oxidation resistance. In this work, different Mecompounds, where Me = Nb, Hf, Cr, V, are added to the ZrB2-TiB2 system to study their effect on the densification, microstructure and thermo-mechanical properties. By adjusting the processing and sintering cycles, fully dense multi-phase ceramics with density in the 5.3-5.7 g/cm3 range and hardness close to 24 GPa have been obtained. A common feature to all materials, is the formation of solid solutions and microstructural details obtained by x-ray diffraction, scanning and electron microscopy are highlighted. Particularly, we explored the nanotexturing of the shell within micron-sized boride grains of the matrix, which resulted from the preferential precipitation of Me-compounds with poor solubility within ZrB2 or TiB2 lattice. Preliminary bending strength and oxidation behavior of these intricate bulk multiphase ceramics are also provided.