Joining of SiC ceramics using the Ni-Mo filler alloy for heat exchanger applications

In order to meet the sealing demands of SiC heat exchanger, the Ni-Mo filler alloy was designed, prepared and employed to braze SiC ceramics. Wetting behavior of the Ni-Mo filler alloy on SiC ceramics and interfacial microstructure of the brazed joints were systematically characterized using optical observation furnace and XRD, SEM, EDS, TEM, respectively. Flexural strengths of the brazed joints at room temperature and high temperature were measured with four-point flexural strength method. HCl immersion test was performed to evaluate the corrosion resistance of the joints. The Ni-Mo filler alloy exhibited excellent wettability on SiC ceramics. During the process of brazing, SiC reacted with element Ni of the Ni-Mo filler alloy, resulting in the formation of Ni₂Si + graphite reaction layer adjacent to the SiC substrate. Ni₃Mo₃C and Ni₂Si compounds were precipitated at the center of brazing seam. When the brazing temperature increased from 1250 ℃ to 1400 ℃, the thickness of Ni₂Si + graphite layer increased gradually. The maximum room-temperature flexural strength of 174 ± 33 MPa was obtained when brazed at 1300 ℃ for 40 min. The joints also exhibited stable high-temperature strength and acid corrosion resistance. When the test temperature was 700 ℃, 800 ℃, 900 ℃, the joints gave the strength retention rate of 92.5 %, 79.8 %, 67.2 %, respectively. It was believed that the formation of high melting point phases played an important role. Residual strength of the joints after HCl corrosion exceeded 130 MPa, revealing a good potential for applications in corrosion environment.