(180b) Novel Approach for Joining Carbon-Carbon Composites Using High-Temperature Heterogeneous Combustion Reactions

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.