Influence of thermal cycling on the properties and microstructure of alumina/kovar brazed joints
The present work was undertaken in order to study the effect of controlled thermal cycles on both vacuum tightness and microstructure of Al{sub 2}O{sub 3}/Kovar joints brazed with Ag-26.7% Cu-4.5%Ti alloy (TICUSIL - Wesgo). Joints were obtained by heating the alumina and Kovar samples at7{degrees}C/min, maintaining 850{degrees}C during 10 min and cooling to 250{degrees}C at 3{degrees}C/min. A vacuum better than 5.5 x 10{sup -5} mbar was kept throughout the process. Vacuum tightness measurements were performed using a He mass spectrometer, while for the microstructural studies, a Zeiss scanning electron microscope equipped with an EDX detector was employed. From the above results, it can be concluded that both thermal cycling and long exposure at not very high temperatures can be harmful to brazed joints. Cracks and loss of vacuum tightness were a general occurrence for water-cooled samples (4{degrees}C), while when this cooling medium was kept at 20{degrees}C, only half the samples were affected. Leakage and cracks were not observed in air-cooled samples. Microstructural studies showed migration of Ti-rich phases moving only within the Ag-rich portion of the filler and forming a reaction layer adjacent to the original one. This phenomenon is certainly responsible for the loss of mechanical properties and correlated defects such as cracks and vacuum leakage. These results show that even for materials having similar thermal expansion coefficient, such as alumina and Kovar, thermal mismatch stresses are sufficient to impair mechanical and functional properties.
- OSTI ID:
- 115515
- Report Number(s):
- CONF-9404233--
- Country of Publication:
- United States
- Language:
- English
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