Brazing of carbon/carbon composite beams using gold filler with tungsten trioxide and tungsten carbide coatings

Carbon/Carbon (C/C) composites are expected to be used as space components due to their excellent specific strength and high heat resistance. This study investigates a reliable brazing technique for C/C composites, aimed at their application as beam members in lunar geodesic dome structures. To improve the wettability of pure gold filler metal and prevent its excessive penetration, the end surfaces of the carbon fibers and the graphite matrix were coated with tungsten trioxide and tungsten carbide. The coating process successfully formed a protective layer on both the carbon fibers and the matrix, facilitating effective gold wetting. In three-point bending tests of the brazed specimens, the average maximum flexural stress reached 48.5 MPa. The fracture occurred as a tensile failure accompanied by interlaminar shear. These results indicate that the joint strength was greater than the strength of the C/C composite base material. Microstructural analyses confirmed that tungsten coatings formed around the fibers. When the C/C composite was placed with an oblique carbon fiber orientation to the joining surface for use as a beam material, the gold filler penetrated along the carbon fiber at an oblique angle. Furthermore, the alternating fiber orientation of the composite contributed to stress redistribution during loading, enhancing joint strength. These findings demonstrate that tungsten-based coatings and fiber architecture are critical to improving the mechanical performance of brazed C/C composite joints.