Modification of Thermo-Chemical Properties of Hot-Pressed ZrB2-HfB2 Composites by Incorporation of Carbides (SiC, B4C, and WC) or Silicides (MoSi2 and CrSi2) Additives

ZrB₂-HfB₂ composites allow us to obtain materials characterized by the high chemical resistance characteristic of HfB₂ while reducing density and improving sinterability due to the presence of ZrB₂. Since boride composites are difficult-to-sinter materials. One way to achieve high density during sintering is to add phases that activate mass transport processes and, after sintering, remain as composite components that do not degrade and even improve some properties of the borides. The following paper is a comprehensive review of the effects of various and the most commonly used sintering aids, i.e., SiC, B₄C, WC, MoSi₂, and CrSi₂, on the thermo-chemical properties of the ZrB₂-HfB₂ composites. High-density composites with a complex phase composition dominated by (Zr,Hf)B₂ solid solutions were obtained using a hot pressing method. The tests showed differences in the properties of the composites due to the type of sintering additives used. From the point of view of the thermo-chemical properties, the best additive was silicon carbide. The composites containing SiC, when compared to the initial, pure borides, were characterized by high thermal conductivity λ (80-150 W/m·K at 20-1000 °C), a significantly reduced thermal expansion coefficient (CTE ~6.20 × 10^-6 1/K at 20-1000 °C), and considerably improved oxidation resistance (up to 1400 °C).