Oxidation of TaSi-Containing ZrB-SiC Ultra-High Temperature Materials

ABSTRACT

Hot pressed coupons of composition ZrB₂ -20 v% SiC -5 v% TaSi₂ and ZrB₂ -20 v% SiC -20 v% TaSi₂ were oxidized in stagnant air at temperatures of 1627 and 1927°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 ZrB₂ -20 v% SiC -20 v% TaSi₂ sample oxidized for five 10-minute cycles at 1927°C. Oxidation kinetics and product formation were compared to ZrB₂ -20 v% SiC with no TaSi₂ additions. It was found that the 20 v% TaSi₂ composition exhibited improved oxidation resistance relative to the material with no TaSi₂ additions at 1627°C. However, for exposures at 1927°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 TaSi₂ content to 5 v% were unsuccessful. In addition, the enhanced oxidation resistance at 1627°C due to 20 v% TaSi₂ 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 TaSi₂-containing ZrB₂-SiC material system. TaSi₂-additions to ZrB₂-SiC at any level are not recommended for ultra-high temperature (>1900°C) applications due to excessive liquid phase formation.

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