Wetting behavior and interfacial microstructure of palladium- and silver-based braze alloys with C–C and SiC–SiC composites

High-temperature sessile-drop wettability tests were conducted on unpolished C–C and SiC–SiC composite substrates using commercial braze alloys Palco (Pd-35Co), Palni (Pd-40Ni), Cusil-ABA (63Ag–35.3Cu–1.75Ti), and Ticusil (68.8Ag–26.7Cu–4.5Ti). Observations revealed non-uniform, anisotropic spreading, copious braze infiltration of the composite substrates, particularly C–C composite, and Ti enrichment at the composite/braze interface together with dissolution of Si (from SiC–SiC composite) in braze and diffusion of Co (from Palco) in the composite. The droplet/composite contact region near the droplet center revealed intimate and microstructurally sound bonding. However, inter-laminar shear cracking within the SiC–SiC composite in contact with Ticusil, Palco, and Palni, and partial substrate/droplet de-cohesion near the edge of the droplet were also observed. In Palco and Palni droplets, fiber tows in the contact region de-laminated from the main body of the composite via inter-laminar shear cracking resulting in fiber flotation, segregation, and surface degradation. The study is one of the first empirical enquiries into the complex wetting and spreading behavior of brazes on commercial C–C and SiC–SiC composites.