Kovar (UNS K94610 / ASTM F15): A Controlled Thermal Expansion Alloy for Glass-to-Metal Matching
Alloy Definition and Chemical Composition
Kovar is a nickel-iron-cobalt based alloy whose coefficient of thermal expansion has been specially controlled. It is used in applications where hermetic (gas tight) sealing with glass and ceramic materials is required. The alloy has an expansion coefficient similar to that of various ceramics, above all borosilicate glass.
The chemical composition of Kovar is as follows:
- Iron (Fe): Balance
- Nickel (Ni): 29%
- Cobalt (Co): 17%
- Manganese (Mn): 0.3% maximum
- Silicon (Si): 0.2% maximum
- Carbon (C): 0.04% maximum
This alloy has been formulated so as to offer a constant coefficient of thermal expansion especially in the 20°C–400°C range. This property allows it to match glass or ceramic and thereby makes possible the thermomechanical compatibility needed for sealing.
KOVAR Standards and Certifications
The Kovar alloy has been defined in many industrial and military standards. The most widespread international specifications are as follows:
- ASTM F15 – Alloy definition for hermetic glass-to-housing joints
- UNS K94610 – Unified Numbering System code
- DIN 1.3981 / W.Nr. 1.3981
- MIL-I-23011 Class 1
- SAE AMS I-23011
- AFNOR-NF A54-301
- ISO 4829-1
These standards are used with the aim of guaranteeing both the chemical purity and the mechanical behaviour of the Kovar alloy. ASTM F15 in particular is the most important norm accepted as a reference in electronic encapsulation operations.
KOVAR Trade Names and Equivalent Materials
- Kovar® – Registered brand of Carpenter Technology and other producers
- Fernico, Rodar, Dilver P – Equivalent trade names used in Europe and Asia
- Nilo K – Special Metals brand
- Alloy 29-17 – Refers to the composition of 29% nickel and 17% cobalt
- UNS K94610, 1.3981 – Universal designation codes
Commercially the name “Kovar” is most often used for all low expansion alloys of the Fe-Ni-Co system, but technically only those consistent with ASTM F15 are accepted as genuine Kovar.
Areas of Use and Engineering Applications
Kovar offers a unique engineering solution in precision systems where sealing with glass or ceramic is required. It is preferred especially in systems exposed to heat or operating in vacuum environments.
1.Electronics Industry
*Transistor, diode and integrated circuit packages
*Metal housing connectors such as TO-3 and TO-5
*Microwave component sealing elements
2.Aerospace
*Hermetic subsystem housings
*Glass-to-metal joints of vacuum tubes and laser tubes
3.Medical Devices
*Pacemakers, implantable device bodies
*Sensors with hermetic glass covers
4.Optoelectronic Systems
*Fibre optic connector housings
*Laser housing systems
5.Research and Nuclear Energy
*Experimental vessels with a controlled atmosphere
*Nuclear sensor sealing elements
In these applications thermal stability, magnetisability control, high strength and expansion compatible with glass are to the fore.
Advantages
- Thermal Expansion Matching: Thanks to a coefficient of thermal expansion at the level of 4.6 x 10⁻⁶/°C, compatible with borosilicate glass, no cracking or sealing problem is experienced at glass-to-metal transitions.
- Hermetic Sealing Capability: Thanks to its surfaces machined to tight tolerances it provides sealing in vacuum or gas environments. In encapsulation systems a leak value below 10⁻⁹ mbar·L/s is possible.
- Microstructure Control Through Heat Treatment: Internal stresses can be relieved by annealing; the magnetic and mechanical properties can be adjusted with controlled heat treatment.
- Weldability and Formability: It is suitable for TIG, laser and resistance welding. It is amenable to wire drawing, sheet rolling and press forming.
- High Dimensional Stability: It preserves its dimensional stability over a long period in precision-dimensioned optoelectronic components and micromechanical applications.
Disadvantages
- High Material Cost: Because of the cobalt content and the precision production process it is expensive compared with other structural steels.
- Magnetic Properties: Because of the high nickel and cobalt content it is ferromagnetic. In some applications this may not be wanted.
- Tendency to Oxidation: Oxidation may occur on the surface at high temperature. For this reason the surfaces joining the glass should be kept clean and heat treatment should be carried out in a controlled atmosphere.
- Machinability Difficulties: Because of its hardness and elastic springback it requires special cutting tools for precision machining.
Conclusion and Engineering Evaluation
Kovar is a strategic material playing a critical role in special engineering applications thanks to its thermal expansion matching, sealing capability and dimensional stability. It is one of the few alloys that can be used together with glass and ceramic materials in electronic, optoelectronic and nuclear systems.
ASTM F15 — Kovar produced according to technical norms such as this one is, however costly it may be, unrivalled in precision systems thanks to the superior sealing and stability performance it offers. For product designers, Kovar is a technically ideal selection, especially in systems requiring CTE matching.
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#UNSK94610
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#SAE AMS I-23011
#AFNOR-NF A54-301
#ISO 4829-1
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#Fernico
#NiloK
#Alloy29-17
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We supply this material
We supply Kovar (1.3981) — to the ASTM F15 standard, in the dimensions and forms required. Contact us for stock availability and pricing.
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