Synthesis & Fundamental Formation Mechanism Study of High Temperature & Ultrahigh Temperature Ceramics

Abstract

Borides and carbides of tantalum and hafnium are of great interest due to their ultrahigh temperature applications. Properties of these ceramics including oxidation resistance and mechanical properties might be further improved through solid solution/composite formation. Synthesis of single-phase TaxHf1-xC and TaxHf1-xB2 solid solution powders including nanopowders via carbothermal reduction (CTR) is complicated due to noticeable difference in reactivity of parent oxides with carbon, and also the low solubility of those oxides in each other. Moreover, for TaC-HfC system the solid solution may go through phase separation due to the presence of a miscibility gap at temperatures below 887°C.In this study, a method of low-cost aqueous solution processing followed by CTR was used to synthesize TaxHf1-xC and TaxHf1-xB2 solid solution powders. In fact, method was first used to synthesize boron carbide (B4C) powders as it paves the way for a detailed study on the synthesis of TaxHf1-xC and TaxHf1-xB2 solid solutions powders considering the fact that B4C contains both carbon and boron in its structure. Particular emphasis was given to investigate the influences of starting compositions and processing conditions on phase separation during the formation of both carbide and boride phase(s). It was found that individual TaC-HfC and TaB2-HfB2 phases always form quickly but separately during the CTR process (e.g., at 1600 °C within a few minutes). Those carbides and borides remain phase-separated unless heated to much higher temperatures for long time due to the slow inter-diffusion between them. It was also found that for TaxHf1-xC applying a DC electric field through the use of spark plasma sintering (SPS) system significantly accelerates the inter-diffusion of Ta and Hf leading to formation of a single-phase TaxHf1-xC solid solution at 1600 °C for 15 minutes. On the other hand, for borides alkali metal reduction reaction (AMR) method appears to be an excellent alternative to CTR-based method for formation of a single-phase TaxHf1-xB2 solid solution. In this method, chlorides of tantalum and hafnium are directly reduced using sodium borohydride (NaBH4) giving rise to formation of a single-phase Ta0.5Hf0.5B2 solid solution nanopowders in one step at much lower temperatures (e.g., 700 °C) by avoiding the oxides formation and the associated phase separation of individual borides as observed in the CTR-based process.