What is kovar alloy?

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

 

American	Carpenter	Kovar®/Rodar®
	Special Metals	Nilo® K
French	Aperam Alloys Imphy (Imphy Alloys)	Dilver P1®
German	VDM Metals	Pernifer® 2918
Japan	NIPPON YAKIN	NAS 29CO

 

Standards for Invar alloy

 

American	German	China
ASTM F15 K94610	DIN 17745 Ni-Co 29 18 1.3981	YB/T 5231 4J29

 

Perm invar alloy composition

Material	Fe	Ni	Co	C	Si	Mn	P	S	Cr	Cu
1.3981	Rest	29.0	17.0	≤0.04	≤0.2	≤0.5	/	/	≤0.2	≤0.2
K94610	Rest	28.0~30.0	16.0~18.0	≤0.05	≤0.3	≤0.5	/	/	/	/
4J29	Rest	28.5~29.5	16.8~17.8	≤0.03	≤0.3	≤0.5	≤0.02	≤0.02	≤0.2	≤0.2

 

 

Typical linear coefficient of expansion（4J29）

Temperature	Average coefficient of linear expansionā，10-6/℃
20℃~200℃	5.9
20℃~300℃	5.3
20℃~400℃	5.1
20℃~450℃	5.3
20℃~500℃	6.2
20℃~600℃	7.8
20℃~700℃	9.2
20℃~800℃	10.2

 

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