A new use for electron beam brazing was recently developed by NASA Lewis Research Center's Manufacturing Engineering Division. This work was done to fabricate a fiber-optic probe (developed by Sentec Corporation) that could measure high temperatures (<600 °C) of vibrating machinery, such as in jet engine combustion research. Under normal circumstances, a sapphire fiber would be attached to platinum by a ceramic epoxy. However, no epoxies can adhere ceramic fibers to platinum under such high temperatures and vibration. Also, since sapphire and platinum have different thermal properties, the epoxy bond is subjected to creep over time. Therefore, a new method had to be developed that would permanently and reliably attach a sapphire fiber to platinum.
The fiber-optic probe assembly consists of a 0.015-in.-diameter sapphire fiber attached to a 0.25-in.-long, 0.059-in.-diameter platinum shell. Because of the small size of this assembly, electron beam brazing was chosen instead of conventional vacuum brazing. The advantage of the electron beam is that it can generate a localized heat source in a vacuum. Gold reactive braze was used to join the sapphire fiber and the platinum. Consequently, the sapphire fiber was not affected by the total heat needed to braze the components together.
First, a copper fixture that acted as a holding and centering device was fabricated. Then two 0.015-in.-diameter rings of gold braze were placed around the sapphire fiber, and the fiber was inserted into the platinum shell. The fixture was then placed in an electron beam welder with a vacuum atmosphere of 10-4 Torr. A power setting of 1 mA at 55 kV was used to create a broad beam sufficient to melt and cause the gold wire to flow into the joint. The total time required for the braze was approximately 5 seconds.
Lewis contacts: Dr. Margaret L. Tuma, (216) 433-8665,
Margaret.L.Tuma@grc.nasa.gov;
Walter A. Wozniak, (216) 433-2405,
Walter.A.Wozniak@grc.nasa.gov;
and David Kowalski, (216) 433-2099,
David.Kowalski@grc.nasa.gov
Authors: Roger C. Forsgren and Thomas Vannuyen
Headquarters program office: OASTT
Programs/Projects: Aeronautics Base R&T, Fiber Optic
Temperature Sensor
Previous articleLast updated April 16, 1998, by Nancy.L.Obryan@nasa.gov
Responsible NASA Official:
Gynelle.C.Steele@nasa.gov
216-433-8258
Point of contact for NASA Glenn's Research & Technology reports:
Cynthia.L.Dreibelbis@nasa.gov
216-433-2912
SGT, Inc.
Web page curator:
Nancy.L.Obryan@nasa.gov
216-433-5793
Wyle Information Systems, LLC
