Kovar Alloy

Kovar not only has thermal expansion similar to glass, but its nonlinear thermal expansion curve can often be made to match a glass, thus allowing the joint to tolerate a wide temperature range. Chemically, it bonds to glass via the intermediate oxide layer of nickel(II) oxide and cobalt(II) oxide. The proportion of iron oxide is low due to its reduction with cobalt. The bond strength is highly dependent on the oxide layer thickness and character. The presence of cobalt makes the oxide layer easier to melt and dissolve in the molten glass. A grey, grey-blue or grey-brown color indicates a good seal. A metallic color indicates lack of oxide, while black color indicates overly oxidized metal, in both cases leading to a weak joint.

The name Kovar is often used as a general term for Fe–Ni alloys with these particular thermal expansion properties. The related particular Fe–Ni alloy Invar exhibits minimum thermal expansion.

Applications:

Kovar is an iron-nickel-cobalt alloy with a coefficient of thermal expansion similar to that of hard (borosilicate) glass. This makes it especially suitable for uses which require a matched-expansion seal between metal and glass parts. Thus kovar finds wide usage in the electronics industry for metal parts bonded to hard glass envelopes for such devices as power tubes, x-ray tubes, etc., and other applications requiring glass-to-metal seals.