NbC in-situ formation behavior and effect in high-temperature brazing of C/C composite using Nb-containing Fe based active brazes

To reduce the contact angle and clarify the effect of adding strong (Nb) and medium (Cr) carbide forming elements in Fe-based braze, Nb was added into Fe (Fe-1/3/5/7Nb, wt.%) and Fe-Cr based brazes (SUS630-1.5Si-1/3/5Nb, wt.%) for high temperature brazing (HTB) C/C. Especially, NbC in-situ formation was observed only within bond seam. All the Fe-based active braze showed 36-80° contact angle, which is much smaller than 147° of pure Fe braze. However, Fe-Nb binary braze showed very poor strength (< 5 MPa), Fe-Cr-Nb multi brazes showed the highest shear strength of 19 MPa for 1 wt.% Nb addition. For Fe-Cr-Nb multi braze, Nb consumed excessive dissolved carbon and in-situ formed NbC particle of 2∼5 μm in Fe-Cr-C carbide ((Fe,Cr) 3 C 2 ) bond seam, rather than at interface due to higher activity of liquid Nb and liquid carbon, reducing coefficient of thermal expansion (CTE) and hardness of bond seam. Cr was detected in partially dissolved zone (PDZ) and bond seam matrix, where it reacted with dissolved C to form M 3 C 2 , connecting C/C and improving wettability . The joints fractured within weak PDZ. Fe-Nb brazes showed NbC particle reinforced Fe-(11-22 at.%)C bond seam, but without PDZ and infiltrated zone. With increasing Nb content, the carbon content in Fe-C matrix decreased, and the volume fraction of in-situ NbC particles increased, until NbC became matrix for Fe-7Nb braze. The critical size to crack of NbC was much higher than that of (Fe,Cr) 3 C 2 . The present study showed the difference of Nb and Cr reaction with C/C, providing a useful idea for designing Fe-based active braze.