Oxidation of ZrB2- and HfB2-based ultra-high temperature ceramics: Effect of Ta additions

Several compositions of ZrB2- and HfB2-based Ultra-High Temperature Ceramics (UHTC) were oxidized in stagnant air at 1627°C in ten minute cycles for times up to 100 min. These compositions include: ZrB2-20 vol% SiC, HfB2-20 vol% SiC, ZrB2-20 vol% SiC-20 vol% TaSi2, ZrB2-33 vol% SiC, HfB2-20 vol% SiC-20 vol% TaSi2, and ZrB2-20 vol% SiC-20 vol% TaC. The weight change due to oxidation was recorded. The ZrB2-20 vol% SiC-20 vol% TaSi2 composition was also oxidized in stagnant air at 1927°C and in an arc jet atmosphere. Samples were analyzed after oxidation by X-ray diffraction, field emission scanning electron microscopy, and energy dispersive spectroscopy to determine the reaction products and to observe the microstructure. The ZrB2-20 vol% SiC-20 vol% TaSi2 showed the lowest oxidation rate at 1627°C, but performed poorly under the more extreme tests due to liquid phase formation. Effects of Ta-additions on the oxidation of the diboride-based UHTC are discussed.