Active Brazing of SiC-Base Ceramics to High-Temperature Alloys

Active metal brazing of SiC-base ceramics and composites to Ti, Cu-clad-Mo, and nickel was carried out to screen promising commercial braze compositions with liquidus temperatures of 810-1040 °C. Microstructure, elemental composition, and microhardness were characterized using FESEM, EDS, and Knoop test in joints brazed using Ag-Cu-Ti alloys (Cusil-ABA and Ticusil) and a Ni-base alloy (MBF-20). Ti- and Si-rich interfacial layers developed in joints brazed using Ti-containing brazes, whereas Ni- and Si-rich layers formed in joints brazed using Ni-base brazes. Monolithic SiC/Mo and SiC/Kovar joints with Cusil-ABA had sound interfaces and compressive shear strengths of 25-30 MPa. In the case of composites, surface preparation influenced bond quality: ground SiC-SiC samples led to sound joints and unground samples led to interfacial decohesion at low thermal strains whereas joints cracked regardless of the surface preparation at large thermal strains. Among SiC-SiC/metal joints, SiC-SiC/Cu-clad-Mo had the best microstructure and bond quality. Microstructures of joints made using MBF-20 mixed with low-expansion Si-X (X: Y, Ta, Hf, Ti, B) eutectic powders to control the thermal expansion are also presented.