Oxidation of TaSi sub(2-Containing ZrB) sub(2)-SiC Ultra-High Temperature Materials

Hot pressed coupons of composition ZrB2 -20 v% SiC -5 v% TaSi2 and ZrB sub(2 -20 v% SiC -20 v% TaSi) sub(2) were oxidized in stagnant air at temperatures of 1627 and 1927[deg]C for one, five and ten 10-minute cycles. The oxidation reactions were characterized by weight change kinetics, x-ray diffraction, and SEM/EDS. Detailed WDS/microprobe quantitative analyses of the oxidation products were conducted for the ZrB sub(2 -20 v% SiC -20 v% TaSi2 sample oxidized for five 10-minute cycles at 1927[deg]C. Oxidation kinetics and product formation were compared to ZrB) sub(2) -20 v% SiC with no TaSi sub(2 additions. It was found that the 20 v% TaSi) sub(2) composition exhibited improved oxidation resistance relative to the material with no TaSi sub(2 additions at 1627[deg]C. However, for exposures at 1927[deg]C less oxidation resistance and extensive liquid phase formation were observed compared to the material with no TaSi2 additions. Attempts to limit the liquid phase formation by reducing the TaSi) sub(2) content to 5 v% were unsuccessful. In addition, the enhanced oxidation resistance at 1627[deg]C due to 20 v% TaSi sub(2 additions was not achieved at the 5 v% addition level. The observed oxidation product evolution is discussed in terms of thermodynamics and phase equilibria for the TaSi) sub(2)-containing ZrB sub(2-SiC material system. TaSi) sub(2)-additions to ZrB sub(2-SiC at any level are not recommended for ultra-high temperature (>1900[deg]C) applications due to excessive liquid phase formation.)