Kovar vs Invar: Which Low-Expansion Alloy Do You Need?

Both control expansion, but they answer different questions. Kovar is designed to seal to glass; Invar is designed to stay put in free space.

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Side-by-side

Comparison

Property	Kovar	Invar 36
Coefficient of expansion	~5.1 ppm/K	~1.2 ppm/K
Designed to match	Borosilicate glass, ceramics	Optical benches, precision frames
Thermal expansion vs steel	Much lower	~1/5 of steel
Magnetic	Ferromagnetic	Ferromagnetic
Typical use	Glass-to-metal seals, hermetic packages	Laser/optical mounts, metrology, satellite hardware

Decision guide

How to decide

Choose Kovar when the part must seal to glass or ceramic and stay hermetic across temperature cycles.
Choose Invar 36 when a structure must hold its dimensions as the environment warms and no glass seal is involved.
Both are ferromagnetic - if magnetic behaviour matters, neither is the neutral choice.

FAQ

Frequently asked questions

Can Kovar and Invar be used interchangeably?

No. Kovar's expansion is matched to specific glasses and ceramics; Invar is chosen for open structures that must stay dimensionally stable. Substituting one for the other usually breaks the seal or the stability.

Which is the more difficult material to machine?

Both work-harden and conduct heat poorly, so both are demanding. Neither is a material to treat as ordinary steel.

Are they magnetic?

Yes, both are ferromagnetic, which matters in sensors, magnetic shielding and instrumentation.

Keep reading

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