Carbon-carbon (C/C) composites are used in a number of aerospace and ground-based applications, such as the nose cone and leading edges of the space shuttle, rocket nozzles, exit cones, heat shield, and aircraft braking systems. A number of applications of C/C require the composite to be integrated (joined or attached) to metals or other substrates. One particular area of current interest is the utilization of C/C composites in thermal management applications. A number of such applications utilize copper-clad molybdenum (Cu-clad Mo) because of its tailorable thermal conductivity and thermal expansion properties. However, the high density of Cu-clad Mo has limited its use in lightweight heat-rejection systems. C/C composites containing high-conductivity carbon fibers provide excellent heat dissipation and low expansion properties at considerable reduced weight. Acting in combination, Cu-clad-Mo and C/C can provide excellent heat dissipation and some weight advantage over the heavier Cu-clad Mo. By controlling the clad layer thickness in Cu-clad-Mo, the coefficient of thermal expansion mismatch between C/C and Cu-clad Mo can be designed to minimize residual stresses during joining and service while maintaining acceptable levels of thermal conductivity needed for thermal management applications.
Researchers have brazed chemical vapor infiltrated C/C composite and resin-derived C/C composites to Cu-clad Mo using five commercial copper-silver (Cu-Ag) active braze alloys with good thermal conductivity and ductility: Cu-ABA, Ticuni, Ticusil, Cusil-ABA, and Cusin-1ABA. The presence of Cu as a cladding on Mo and as an alloying additive in brazes ensures chemical compatibility and enhanced ductility at the joint interfaces. The joints were vacuum brazed under a load of 0.3 to 0.4 N at 15 to 20C above the braze liquidus. Joints were characterized using optical and scanning electron microscopy, energy dispersive x-ray spectroscopy, and Knoop microhardness measurements across the joint.
The joints displayed intimate physical contact and were free of structural imperfections such as interfacial microvoids, shrinkage porosity, and microcracking. Extensive braze infiltration of interfiber regions in the CVI C/C composites occurred regardless of the carbon fiber orientation at the mating surface. Titanium- bearing braze alloys displayed good spreading on C/C, with Ticusil exhibiting better coverage than Cusil-ABA. Some dissolution and interdiffusion of elements across the joint were observed. In particular, the joint interfaces were enriched in Ti, which is consistent with the high chemical affinity of Ti towards C and the large negative change in the Gibb’s free energy for TiC formation via Ti + C →TiC. The Cu cladding on Mo did not melt at the joining temperatures, which were below the melting point of Cu; however, some dissolution of Cu occurred in the braze. In joints made using resin-derived C/C composites, there was evidence of some cracking within the C/C composite owing to the low interlaminar shear strength of the composite. The Knoop microhardness profiles displayed gradients in the joint region with an abrupt rise in hardness in the vicinity of the braze region and a sharp decline in the adjoining Cu-clad Mo and C/C regions.