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

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.