Microstructure and mechanical properties of SiC/SiC joints brazed with Ni-based high-temperature filler

The low-cost Ni-based high-temperature brazing filler was used for brazing of SiC ceramics. The microstructure and phase composition of the joints were investigated by SEM, EDS, and TEM. The influences of brazing parameters on the mechanical properties and microstructure of SiC/SiC joints were systematically studied. The correspondence between mechanical properties and microstructure of the joints was clarified by analyzing the fracture morphologies of SiC/SiC joints. Finally, the high-temperature mechanical properties of SiC/SiC joints were evaluated. The results showed that the SiC/SiC joint consisted of three regions: the reaction zone, the central zone and the matrix zone. The reaction zone was mainly composed of Ni₂Si and different amounts of graphite; while the central zone contained complex carbides Ni₃Mo₃C and Ni₂Si. Increasing the brazing temperature and prolonging the holding time of graphite could rise up the thickness of the reaction zone in the SiC/SiC joints, and a number of graphite phases were formed, which significantly affected the flexural strength of the joints. When the thickness of the reaction zone and the amount of graphite formed were suitable, the maximum flexural strength of 179 MPa ± 7 MPa was obtained at 1300 ℃ for 40 min. Also, the SiC/SiC joints had good high-temperature stability. When the serving temperature was 700 ℃, the flexural strength retention rate of 97.2% was obtained, which showed a promising application prospect in high-temperature environments.