Tin-based reactive solders for ceramic/metal joints
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I.
INTRODUCTION
R E C E N T advances in ceramics and the increasing use of ceramic materials have provided an impetus for studying new methods of joining metals and ceramics. Examples o f ceramic-metal components include structural ceramic-metal components (such as heat engine components), wear parts, tool materials, electrical feedthroughs, and metal contacts for ceramic superconductors. Among the various joining processes (such as diffusion bonding, plasma spraying, brazing, as well as physical and chemical vapor deposition), brazing is expected to emerge as an important joining technique. The most important step in using brazing for ceramics is the design of the filler metal. Hence, this area has received considerable attention. [~-19] The critical problem in designing filler metals for brazing and soldering of ceramics is the poor wettability of conventional filler metals on ceramics. To overcome this problem, reactive metals are added to the filler metal. [HS1 These reactive metals promote flow by decomposing a thin layer of the ceramic. Recent efforts in the literature have been directed toward design of suitable filler metals, as well as toward investigating the oxidation behavior of such filler metals. [2~ These investigations have centered on designing filler metals for structural ceramics/metal joints that are expected to operate at high temperatures (>600 ~ Hence, the filler metals designed possessed a liquidus temperature that exceeded 800 ~ The brazing temperature was expected to be in the 800 ~ to 1000 ~ range. Brazing with fillers that possess a liquidus in the 800 ~ to 1000 ~ range leads to severe residual stresses from the mismatch in the thermal expansion coefficients of the ceramic and metalJ z~'221 These stresses are often severe enough to cause cracking on cooldown from the joining temperature. The popular solution to this problem is the use of interlayer materials such as Mo, Cu, Kovar, and Invar, which possess a thermal expansion coefficient intermediate to the ceramic and the metal, t22+ Thus, the
RAKESH R. KAPOOR, Postdoctoral Associate, and THOMAS W. EAGAR, Leaders for Manufacturing Professor of Materials Engineering, are with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted March 6, 1989. METALLURGICAL TRANSACTIONS B
final joint is often very complex and possesses several intermediate layers. There are several ceramic components (including some for ceramic heat engines) where the service temperature of the ceramic-metal joint is much lower than 600 ~ Examples of these would include electrical feedthroughs (expected operating temperature
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