Brazes, thermal expansion coefficients

A simpler (one-step) and more economical joining process is direct brazing in a furnace under a vacuum or inert gas atmosphere through the use of active filler metals [Mizuhara Cl al., 1989]. An active element such as the commonly used Ti in the filler metal forms a true alloy with the base metal. The difference in the thermal expansion coefficients between the ceramic membrane and the metal housing can lead to high stress at the... 

Certain alloys of iron, nickel, and cobalt (Kovar, Fernico, etc.) have thermal expansion curves which nearly match those of borosilicate glasses, and a good bond may be formed between the two. Kovar is similar to carbon steel in its chemical properties. For example, it oxidizes when heated in air and is not wet by mercury. It may be machined, welded, copper brazed, and soft soldered. Silver solders should not be used with Kovar since they may cause embrittlement. At low temperatures Kovar undergoes a phase transformation, and the change in expansion coefficient below this temperature may be sufficient to cause failure of a glass-to-Kovar seal. The transformation temperature usually is below... 

Table 4 Liquidus (TL) and brazing (TB) temperatures of the braze alloys, along with selected room-temperature mechanical properties of the braze foils, and the room-temperature coefficient of thermal expansion (GTE).

The Joints that cracked on cooling from the brazing temperature had either a high yield strength braze or were high temperature brazes and therefore were subjected to large thermal residual stresses. The coefficients of thermal expansion of these brazes are not known as a function of temperature, but they are expected to be approximately 15 x 10" / C near room-temperature. 

When two materials are bonded with a high-temperature material, such as a solder or a braze, the material structure has zero stress when the bonding material is in the molten state. When the structure is cooled, the bonding material solidifies and stress is produced by the mismatch in coefficients of thermal expansion among the materials being bonded and the bonding material. 

Glasses may be joined to metals and other glasses by fusion. Often, glass seals must be graded through several glass compositions from one material to another due to differences in their thermal coefficients of expansion. Ceramics may be metallized and then brazed to other ceramics or metals to form hermetic joints. A ceramic-based adhesive that it is possible to use to 150°C is Ceramabond 552 . The adhesive cirres at 120°C however, the cirred material tends to be porous. 

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