kovar

{{Short description|Nickel–cobalt ferrous alloy}}

{{other uses}}

{{Use dmy dates|date=January 2024}}

File:Assortment of Kovar metal.jpg

Kovar (trademark of CRS Holdings, inc., Delaware{{cite web|last1=USPTO United States Patent and Trademark Office|title=Trademark Assignment Abstract|url=http://assignments.uspto.gov/assignments/q?db=tm&sno=71367381|accessdate=18 June 2014|year=1993}}) is a nickel–cobalt ferrous alloy compositionally identical to Fernico 1, designed to have substantially the same thermal expansion characteristics as borosilicate glass (≈{{val|5|ul=ppm|up=K}} between {{val|30|and|200|u=°C}}, to ≈{{val|10|u=ppm|up=K}} at 800 °C) to allow a tight mechanical joint between the two materials over a range of temperatures. It finds application in glass-to-metal seals in scientific apparatus, and conductors entering glass envelopes of electronic parts such as vacuum tubes (valves), X-ray and microwave tubes and some lightbulbs. {{citation needed|date=November 2020}}

Kovar was invented to meet the need for a reliable glass-to-metal seal, which is required in electronic devices such as light bulbs, vacuum tubes, cathode-ray tubes, and in vacuum systems in chemistry and other scientific research. Most metals cannot seal to glass because their coefficient of thermal expansion is not the same as glass; as the joint cools after fabrication the stresses due to the different expansion rates of the glass and metal cause the joint to crack.{{citation needed|date=November 2020}}

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 by 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.{{citation needed|date=November 2020}}

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.