A technique for holding sensor lead wires in place on substrates made of SiC-based monolithic ceramic or SiC/SiC fiber-reinforced ceramic-matrix composite materials involves routing the wires through semicircular hoops that have been reaction-bonded to the substrates. These hoops are made of SiC-based materials similar or identical to the substrate materials. This technique was devised to prevent the detachment of lead wires from surface-mounted thin-film sensor systems (e.g., thermocouples and strain gauges) during testing of the substrates (panels, subcomponents, etc.) at temperatures >1,000 °C in the presence of high-speed gas flows.

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Semicircular SiC Hoops that have been reaction-bonded to SiC/SiC composite substrates are used to hold wires in place. The substrates shown here are flat, but the concept has also been demonstrated on curved substrates.
The reaction-bonding process used to join the hoops to the substrates is called "ARCJoinT" (which signifies "Affordable, Robust Ceramic Joining Technology"). This process was described in several previous NASA Tech Briefs articles, including "Joining of SiC-Based Ceramic and Fiber-Reinforced Composite Parts" (LEW-16405), Vol. 22, No. 5 (May 1998), page 54; "Reaction-Forming Method for Joining SiC-Based Ceramic Parts" (LEW-16661), Vol. 23, No. 3 (March 1999), page 50; and "Reaction-Forming Method for Joining SiC-Based Parts" (LEW-16827), Vol. 24, No. 4 (April 2000), page 59. To recapitulate: A carbonaceous mixture is applied between the parts to be joined. The parts are heated to a temperature of 115±5 °C for 10 to 20 minutes. This action cures the mixture, bonding the parts with moderate strength. Next, silicon or a silicon alloy in tape, paste, or slurry form is applied to the joint regions. The parts are heated to a temperature between 1,250 and 1,425 °C for 5 to 10 minutes, causing the silicon to melt, infiltrate the joints, and react with carbon. The finished, full-strength joints contain silicon carbide with minor amounts of silicon and other phases. The joints are expected to retain mechanical strength and integrity at temperatures up to 1,350 °C in air.

Once hoops have been joined to a substrate via this approach, the sensor lead wires can be slipped through the hoops (see figure) and connected to the sensors. Any excess space between the lead-wire insulation and the hoop can be filled with a refractory cement or another nonreactive material, if necessary, to prevent the wires from moving.

As an alternative to SiC as a starting material, hoops could be made initially of carbon — more specifically, graphite. If carbon hoops are used, then additional silicon is applied to the joints and the carbon is converted to silicon carbide during the bonding process. The advantage of this approach is the relative ease of machining graphite (vs. machining SiC).

This work was done by J. Douglas Kiser, Jih-Fen Li, and Lisa C. Martin of Glenn Research Center and Mrityunjay Singh of Dynacs Engineering Co. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Glenn Research Center,
Commercial Technology Office,
Attn: Steve Fedor,
Mail Stop 4—8,
21000 Brookpark Road,
Cleveland, Ohio 44135.

Refer to LEW-17009.

NASA Tech Briefs Magazine

This article first appeared in the May, 2001 issue of NASA Tech Briefs Magazine.

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