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NASA NTRS1996

Graphite Fiber Textile Preform/Copper Matrix Composites

Graphite fiber reinforced/copper matrix composites are candidate materials for critical heat transmitting and rejection components because of their high thermal conduction. The use of textile (braid) preforms allows near-net shapes which confers additional advantages, both for enhanced thermal conduction and increased robustness of the preform against infiltration and handling damage. Issues addressed in the past year center on the determination of the braid structure following infiltration, and the braidability vs. the conductivity of the fibers. Highly conductive fibers eventuate from increased graphitization, which increases the elastic modulus, but lowers the braidability; a balance between these factors must be achieved. Good quality braided preform bars have been fabricated and infiltrated, and their thermal expansion characterized; their analytic modeling is underway. The braided preform of an integral finned tube has been fabricated and is being prepared for infiltration.

Filatovs, G. J.↗

NASA NTRS1995

Graphite fiber textile preform/copper matrix composites

Graphite fiber reinforced/copper matrix composites have sufficiently high thermal conduction to make them candidate materials for critical heat transmitting and rejection components. The term textile composites arises because the preform is braided from fiber tows, conferring three-dimensional reinforcement and near net shape. The principal issues investigated in the past two years have centered on developing methods to characterize the preform and fabricated composite and on braidability. It is necessary to have an analytic structural description for both processing and final property modeling. The structure of the true 3-D braids used is complex and has required considerable effort to model. A structural mapping has been developed as a foundation for analytic models for thermal conduction and mechanical properties. The conductivity has contributions both from the copper and the reinforcement. The latter is accomplished by graphitization of the fibers, the higher the amount of graphitization the greater the conduction. This is accompanied by an increase in the fiber modulus, which is desirable from a stiffness point of view but decreases the braidability; the highest conductivity fibers are simply too brittle to be braided. Considerable effort has been expended on determining the optimal braidability--conductivity region. While a number of preforms have been fabricated, one other complication intervenes; graphite and copper are immiscible, resulting in a poor mechanical bond and difficulties in infiltration by molten copper. The approach taken is to utilize a proprietary fiber coating process developed by TRA, of Salt Lake City, Utah, which forms an itermediary bond. A number of preforms have been fabricated from a variety of fiber types and two sets of these have been infiltrated with OFHC copper, one with the TRA coating and one without. Mechanical tests have been performed using a small-scale specimen method and show the coated specimens to have superior mechanical properties. Final batches of preforms, including a finned, near net shape tube, are being fabricated and will be infiltrated before summer.

Gilatovs, G. J.↗