Mechanical, thermal, and oxidation properties of refractory hafnium and zirconium compounds

The thermal conductivity, thermal expansion, Youngs Modulus, flexural strength, and brittle-plastic deformation transition temperature were determined for HfB2, HfC0.98, HfC0.67, and HfN0.92 ceramics. The oxidation resistance of ceramics in the ZrB2-ZrC-SiC system was characterized as a function of composition and processing technique. The thermal conductivity of HfB2 exceeded that of the other materials by a factor of 5 at room temperature and by a factor of 2.5 at 820 degrees C. The transition temperature of HfC exhibited a strong stoichiometry dependence, decreasing from 2200 degrees C for HfC0.98 to 1100 degrees C for HfC0.67 ceramics. The transition temperature of HfB2 was 1100 degrees C. The ZrB2/ZrC/SiC ceramics were prepared from mixtures of Zr (or ZrC), SiB4, and C using displacement reactions. The ceramics with ZrB2 as a predominant phase had high oxidation resistance up to 1500 degrees C compared to pure ZrB2 and ZrC ceramics. The ceramics with ZrB2/SiC molar ratio of 2 (25 vol% SiC), containing little or no ZrC, were the most oxidation resistant. Published by Elsevier Science Limited.