Effect of Composite Substrate Properties on the Mechanical Behavior of Brazed Joints in Metal-Composite System

Advanced composite components are being considered for a wide variety of demanding applications in aerospace, space exploration, and ground-based systems. A number of these applications require robust integration technologies to join dissimilar materials (metal/composites) into complex structural components. In this study, three types of ceramic composites (C-C, C-SiC, and SiC-SiC) were vacuum brazed to commercially pure Ti using the Cusil-ABA (63Ag-35.3Cu-1.75Ti) active metal braze alloy. Composite substrates with as-fabricated and polished surfaces were used for brazing. The joint microstructure examined using scanning electron microscopy (SEM) showed sound metallurgical bonding in Ti/C-C and Ti/SiC-SiC joints. In Ti/C-SiC joints, interlaminar shear failure within the composite was observed although the joint interfaces appeared to be sound. The joint interface examination using the energy dispersive x-ray spectroscopy (EDS) showed Si and Ti enrichments at the Ti-side of the joints in all composites. No significant difference in the Knoop microhardness profiles in the Ti near the joint region was observed between polished and unpolished samples. However, the butt-strap tensile (BST) tests on bonded specimens at room temperature revealed a higher shear strength in joints made using polished SiC-SiC composite. These observations are discussed in the light of the wetting properties of the braze alloy, substrate composition, and interlaminar properties.