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Kovar Alloy

Kovar Alloy

Kovar is an alloy composed of nickel and cobalt similar to Fernico 1, possessing thermal expansion characteristics nearly identical to those of borosilicate glass (~5×10^−6/K between 30-200°C and 10×10^−6/K at 800°C), forming a secure mechanical connection over a wide temperature range.

Equivalent grades

• Kovar Alloy
• ASTM F15
• UNS K94610
• FeNi29Co17
• Nilo-k

Chemical Composition Reference


Physical Property Reference

• Density: 8.3 g/cm³

• Youngs Modulus: 138 GPa

• Yield Strength: 270 MPa

• Tensile strength: 518 MPa

• Thermal Conducitivity: 17 W/K·m

• Specific Heat: 0.46 J/g·K

Stock & Dimensions

Our warehouse has a large number of stock to provide you with a variety of products in different sizes, and can also customize the size according to your needs.
• Bar: 0.5~600mm Diameter
• Coil: 0.5~150mm Thick
• Plate: 0.5~200mm Thick
• Pipe: 6~630mm External Diameter; 0.5~40mm Wall Tick
• Available in specific sizes

Product Forms

We provide you with a variety of product forms, including but not limited to:
• Bar/Rod
• Pipe/Tube
• Coil/Strip
• Plate/Sheet/Circle
• Wire
• Fitting (Flange, Elbow, Tee...)
• Customize

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Kovar Alloy

Essential details
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.


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.

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