High-Temperature Interdiffusion of Tantalum and Niobium with SiC for Processing Hybrid Metal/CMC Components

To ensure the leak tightness of SiC/SiC composites cladding, niobium and tantalum have been retained as liner/coating materials for their high melting point, ductility and weldability; however, their chemical compatibility at high temperatures towards SiC remains to be assessed. In the literature, large discrepancies in the composition of the reaction zone and the kinetics were noticed between some metallic liners and SiC. In this work, diffusion couple experiments between Nb and Ta with SiC and SiC/SiC were conducted at high temperatures (1050-1500 degrees C) to determine the diffusion paths and the reaction kinetics in order to estimate the lifetime of such coatings in nominal conditions. A detailed analysis of the interaction area was conducted as a function of temperature by a combination of experimental characterizations and thermodynamic calculations. No significant difference in the sandwich cladding materials was observed. The interfacial reactivity was found to be strongly higher than expected from literature data. C and Si were evidenced as the main diffusing species in the Nb/SiC and Ta/SiC systems. From the reaction layer thickness extrapolation in gas-cooled fast reactor operating conditions, niobium but especially tantalum have been approved as liner material in hybrid CMC/metal cladding materials from a chemical compatibility point of view.