TEM study on the morphology and interfacial structure of Nb-coated C-f/SiC composite

Nb-coated C-f/SiC composites are promising lightweight structural materials with wide applications in the aerospace field. The strength of interfacial bonding between Nb coating and C-f/SiC composite has significant influence on the material performance. In this study, we carefully investigated the interfacial structure between Nb coating and C-f/SiC composite by electron microscopy. The Nb coating on C-f/SiC composites was prepared by chemical vapor deposition. Cross-section imaging by SEM and TEM showed that layered structures were formed, the order is reaction zone, fine equiaxed grained Nb zone and coarse columnar grained Nb zone from the interface to the top of Nb. A sandwich-like interfacial reaction zone (an intermittent Nb5Si3 layer between two NbC layers, denoted as NbC vertical bar gamma-Nb5Si3 vertical bar NbC) can be formed between Nb coating and C-f/SiC composite. Additionally, nanoscale NbC particles were observed near the interface of the C-f/SiC composite, while needle-shaped gamma-Nb2C in the Nb coating, which is due to the interdiffusion of Nb and C elements. And an orientation relationship was identified between gamma-Nb2C and Nb matrix. More importantly, hydrogen was confirmed to be existed in the NbC phase and the micro-cracks at the interface by ToF-SIMS analysis, which have a negative effect on the interfacial bonding properties. The room-temperature shear strength of the joint is 37.75 MPa. The formation mechanisms of the interfacial structures in Nb-coated C-f/SiC composite were discussed based on thermodynamics and kinetics.