Investigation of Refractory Carbide Behavior in Flowing Hydrogen at Very High Temperatures Relevant for Nuclear Thermal Propulsion Applications

Nuclear thermal propulsion (NTP) is an in-space propulsion technology capable of high specific impulse (850 – 1100 s) and thrust (10 – 250 klbf). Due to their high melting temperature and favorable nuclear properties, refractory carbides (RCs) are attractive matrix candidates for NTP applications. In this thesis, the performance of SiC, TiC, and ZrC in NTP relevant environments (high temperature, flowing hydrogen) was investigated through thermodynamic modelling and hot hydrogen testing. Intrinsic RC compatibility with hot hydrogen was investigated through testing of high purity sample coupons in unpressurized, flowing hydrogen at relevant temperatures (2000 – 2500 K) and time scales (<120 minutes). Nano-infiltrated transient eutectic (NITE) SiC samples were tested to identify deviations in corrosion behavior due to relevant manufacture parameters required for fabrication.Thermodynamic calculations predicted ZrC to be most stable, followed by TiC, and SiC. Experimental observations confirmed this trend and active attack of all materials observed. SiC exhibited acceptable hydrogen compatibility up to 2150 K. NITE SiC exhibited greater weight loss than pure SiC, due to preferential attack of sintering additives (Al₂O₃ and Y₂O₃). High purity TiC and ZrC coupons exhibited acceptable hydrogen compatibility for all temperatures. Use of SiC, produced with current NITE manufacture technology, as a fuel matrix should be limited to temperatures below 2250 K due to high temperature incompatibility of sintering aids and the matrix. Identification of alternative sintering aids capable of higher temperature compatibility or development of TiC or ZrC matrices derivative of current manufacture technologies can enable higher performing NTP systems.