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

Several compositions of ZrB(2)- and HfB(2)-based Ultra-High Temperature Ceramics (UHTC) were oxidized in stagnant air at 1627degreesC in ten minute cycles for times up to 100 min. These compositions include: ZrB(2)-20 vol% SiC, HfB(2)-20 vol% SiC, ZrB(2)-20 vol% SiC-20 vol% TaSi(2), ZrB(2)-33 vol% SiC, HfB(2)-20 vol% SiC-20 vol% TaSi(2), and ZrB(2)-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 1927degreesC 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 ZrB(2)-20 vol% SiC-20 vol% TaSi(2) showed the lowest oxidation rate at 1627degreesC, 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. (C) 2004 Kluwer Academic Publishers.