KOVAR alloy is an Iron-Nickel-Cobalt alloy with a density of 8.36g/cm3 and a linear expansion coefficient between 20 and 500℃ equal to that of glass and ceramics. Most metals cannot be sealed with glass because their thermal expansion rate is different from that of glass and the joints are liable to break when cooled. The nonlinear thermal expansion curve of KOVAR alloy can usually be matched with glass. It combines with glass through the intermediate oxide layer of nickel oxide and cobalt oxide, and the bonding strength depends on the thickness of the oxide layer. The reduction of cobalt makes the proportion of iron oxide lower and the oxide layer easier to melt and dissolve in the molten glass, allowing the joint to withstand a wide range of temperatures.
KOVAR alloy provides a tight mechanical connection between hard glass (e.g., borosilicate glass) and ceramics within a certain temperature range. Judging from the color of the interface, gray, gray-blue or grey-brown means a good seal, and black indicates that the alloy has been over-oxidized and weak bonding. Kovar alloy is suitable for glass/ceramic-metal seals in scientific equipment such as vacuum tubes (valves), microwave tubes, X-ray and microwave tubes, transistors, diode integrated circuits, etc.
Commercial brands for Invar alloy
|Special Metals||Nilo® K|
|French||Aperam Alloys Imphy|
|German||VDM Metals||Pernifer® 2918|
|Japan||NIPPON YAKIN||NAS 29CO|
Standards for Invar alloy
|ASTM F15 K94610||DIN 17745 Ni-Co 29 18|
|YB/T 5231 4J29|
Perm invar alloy composition
Typical linear coefficient of expansion（4J29）
|Temperature||Average coefficient of linear expansionā，10-6/℃|
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