Joining of carbon carbon composite materials

An investigation of the fundamental considerations for joining of carbon carbon composite materials was performed. The joining method utilized in this study was the resistance brazing process. The workpiece configuration consisted of a metallic foil or powder interlayer sandwiched between carbon carbon composite materials. The geometry of the workpieces employed was selected to localize the flow of current during resistance brazing to channel the heat into the interlayer region and produce volumetric rather than areal type joints. The conditions (tap setting/duration of current flow) required to generate the heat necessary to melt the interlayer were determined by trial and error. A repetitive heating technique was used to provide melting of the interlayer. Interlayer selection was based on wettability, adhesion, melting temperature and linear coefficient of thermal expansion (CTE). The carbon carbon composite material was qualitatively evaluated by Scanning Electron Microscopy/ Energy Dispersive Spectrometry (SEM/EDS) techniques to investigate the presence of antioxidants and the fiber bundle orientation which influences the wetting and adhesion of the molten interlayer on the carbon carbon composite material and affects brazement strength. The wetting and adhesion behavior of the interlayer was evaluated prior to joining by simulating (by induction heating) the heating cycles experienced during resistance brazing to melt the interlayer on the surface of the composite material. Visual examination and planimetric surface area measurements of the solidified interlayer on the surface of the composite material were then performed to characterize the wetting behavior. Alloying high melting temperature pure metallic interlayers with low melting temperature elements was employed to lower the melting temperature of the pure element and affect wettability. Interlayers with CTE similar to carbon were chosen for joining of the carbon carbon composite materials. Thus, the inherent thermal expansion problem between the metallic interlayer and the carbon carbon composite material is reduced during the heating and cooling cycles which limits the formation of defects in the joint. The joint strength was evaluated using a torsional shear type testing method. The strength was expressed as a ratio of the torsional shear strength of the brazement to the virgin carbon carbon composite material. An evaluation of the torsional shear tested fracture surfaces showed that the wetting behavior varied across the joint, and that fiber pull out occurs. Evaluation of brazement cross sections showed different joint characteristics ranging from areal to volumetric. The joint characteristics were categorized for each of the interlayers. Titanium-Copper alloy interlayers showed the best potential for wetting and produced a volumetric type joint which had the highest torsional-shear strength ratio of all the metallic interlayers and approximately 50% of the strength of the virgin material. The resistance brazing method can potentially produce brazements with higher strengths than those produced by furnace heating due to the steeper temperature gradient which localizes the flow; penetration; wetting and adhesion of the interlayer producing localized volumetric joints instead of areal type joints. Increased interaction (wetting/adhesion) between the interlayer and load bearing fiber bundles of the carbon carbon composite material is characteristic of volumetric type joints and aids in increasing strength.