Carbon-carbon (C-C) composites exhibit unique properties including a high strength-to-weight-ratio, and ability to withstand very high temperatures, making them suitable for a variety of high-tech applications, such as turbine engine components, thermal protection for the space shuttle, and carbon brakes.  As the number of applications for C-C composites increases, technology for joining C-C components will need to be developed in order to produce a wider variety of sizes and geometries.  More specifically, C-C joining technology would also be of great benefit to existing industries that manufacture C-C components.  The method, for example, would allow a second refurbishment of carbon brakes by joining a new thin C-C element to a worn C-C brake “core” or even to design an entirely new generation of carbon brakes.                

Combustion Synthesis (CS) has generated significant interest due to its ability to produce a variety of advanced materials that include ceramics, composites and functionally graded structures [1]. However, this approach is also gaining more and more attention as a tool for joining various materials. Different experimental schemes on CS-welding of super alloys, refractory metals (e.g. Mo, Ta) and ceramics have been reported [2, 3].            

We have developed a novel approach for joining C-C composites, employing high temperature heterogeneous combustion reactions.   The general concept of this approach can be described as follows.  A layer of highly exothermic reactive mixture is placed between two surfaces of C-C composite that are to be joined.  The stack is held in place between two electrodes that are connected to a high current DC power supply.  A pneumatic system applies an initial load to the stack, holding it in place.  Next, DC current is passed through the electrodes in order to preheat the stack and initiate a reaction in the reactive mixture layer.  Note that, typically, the electrical conductivity of the heterogeneous mixture is lower than the C-C composite, thus most of the joule heating occurs in this layer.  Once the ignition temperature has been reached, the combustion reaction is self-sustained and proceeds rapidly at temperatures on the order of 3000K.  After a predetermined delay time, the load applied to the stack is rapidly increased to promote interaction between the reactive layer and the C-C composites, enhancing the mechanical properties of the joint.  A programmable logic controller monitors operational conditions and controls the pneumatic and electrical systems.             

Along with such attractive features as low energy consumption and low cost equipment, this approach has some additional advantages.  First, the liquid intermediate reaction product interacts with solid refractory materials at an extremely high combustion temperature and forms physically and chemically bonded junctions.  Second, the final joint is also a refractory material, which is especially important for high operational temperature applications.  Third, this method gives unique opportunities to produce functionally graded structures.  The latter is critical in solving the problem of coefficient of thermal expansion mismatch between the weld and the joined materials.

By utilizing a novel computer-assisted joining apparatus, we have explored the influence of different processing parameters on the properties of the joint.  For example, reactive mixtures of varying compositions, and thus different reaction kinetics, are under investigation.   Also, the effects of several characteristic times on the microstructure of the joint are being investigated.  Specifically, these times include the duration of preheating before the mixture ignition, time for the reactive layer to produce a “melt” and time of interaction between the melt and composites.  Varying the reactive layer composition and thickness, electrical current applied, the timing and magnitude of the applied load, we seek to optimize the combustion reaction to produce joints with high strength that exhibit similar refractory properties to the C-C composite bulk material.  In this work, our recent results toward this goal are presented and analyzed.

References:

1.     Varma, A., Rogachev, A.S., Mukasyan, A.S., and Hwang, S. Adv. Chem. Eng., 24 (1998) 79-226.

2.     Miyamoto Y., Nakamoto T., Koizumi M., Yamada O.  J. Mater. Res., 1 (1986) 7-9.

3.     Sherbakov V.A., Shteinberg A.S., Int. J. SHS, 2 (1993) 357-369.