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About Kovar

What is Kovar?

Kovar is a special alloy consisting primarily of iron (Fe), nickel (Ni), and cobalt (Co). Originally a registered trademark of Carpenter Technology Corporation, the name Kovar has now become generic.

Properties and Machinability of Kovar

Kovar's most notable feature is its coefficient of thermal expansion, which is close to that of hard glass and ceramics. This property prevents cracking and breakage when Kovar is used to join materials with different expansion coefficients. Kovar is airtight, corrosion-resistant, and remains stable in harsh environments.
Refined by vacuum melting, it exhibits uniform thermal expansion properties with little variation in chemical composition. Its low thermal expansion near room temperature and high heat resistance make it very useful for high-precision applications and for preventing thermal cracking during sealing, encapsulation, and contact. Because it can be welded or brazed to steel, copper, and nickel, Kovar is suitable for bonding dissimilar materials.


On the other hand, Kovar is difficult to process and cut due to its low thermal conductivity and highly viscous nature. In addition to the time required to remove burrs at the production site, care must be taken when procuring Kovar, such as choosing a larger size, as it may warp when obtained.

The following three points summarize why Kovar is difficult to cut:

(1) Heat Generation during Cutting
Due to Kovar’s low thermal conductivity, heat tends to accumulate in the cutting area. This causes the tool to deteriorate as its cutting edge temperature rises. Kovar's thermal conductivity is approximately 17 W/(m-K), close to that of Inconel (11.5–14.8 W/(m-K)), another nickel alloy.
(2) Work Hardening
Work hardening is the process by which a metal becomes harder when undergoing plastic deformation. This process can shorten the lifespan of cutting tools. As work hardening progresses, chipping (damage to the cutting edge) may occur, which can adversely affect machining accuracy and finish. Thus, cutting conditions that prevent work hardening should be established before working with Kovar.
(3) Chip Adhesion to Tools
Due to Kovar's high viscosity and ductility, chips generated during cutting stick easily to the tool. This can lead to tool damage, as well as machining and product defects. It is therefore important to select the right tools and review cutting conditions in advance.

A combination of the above problems can lead to a decrease in machining accuracy. For example, cutting tools or cutting edges could undergo microscopic chipping, or residual chips in the tool could cause excessive grinding. Therefore, careful attention must be paid in establishing conditions and selecting tools for each process.

Major Chemical Composition of Kovar (%)

Alloy name Chemical composition (%)
Kovar C P S Mn Si Cu Cr Mo Ni Co Fe
0.03% 0.02% 0.02% 0.5% 0.3% 0.2% 0.2% 0.2% 28.5~
29.5%		 16.8~
17.8%		 Bal.

Physical Properties of Kovar

Density 8.3～8.4 g/cm³
Curie temperature Approx. 450℃
Melting point Approx. 1480°C
Modulus of elasticity at 20℃ 8137 Gpa
Thermal conductivity at 20℃ 17 W/m.k
Specific heat 0.5 J/g-°C
Thermal expansion 5.5×10-6 °C
Electrical resistivity 1.0×10-6 Ω·m
Poisson's coefficient 0.3

*Data are for reference only.

Applications of Kovar

Due to its low thermal expansion coefficient, Kovar is mainly used in electronic components and transistor-related applications. In the manufacturing and assembly process of electronic components, dissimilar materials such as metals, glass, and ceramics must be bonded together. However, differences in the thermal expansion coefficients of these materials can cause problems such as thermal cracking.
Kovar, which has a thermal expansion coefficient similar to that of hard glass and ceramics, is an ideal bonding material for bonding dissimilar materials. Its properties are fully demonstrated in the production of optical communication components such as transistor lead caps, IC lead frames, high-power communication tube components, quartz crystal cases, and glass encapsulations for electronic tubes.

Measures to Be Taken When Cutting Kovar

Eliminate Thermal Problems
A prior review of cutting conditions is important, especially the optimization of cutting speed and feed rate. Although a slower cutting speed reduces the generation of machining heat, an appropriate balance is needed; a speed that is too slow may cause welding.
If reducing the cutting speed is not an option, the use of coolant is also effective. Coolant cools the cutting area and provides lubrication to promote chip evacuation.
Prevent Work Hardening
Use sharp tools, adjust the blade angle, and design a low-friction machining path (toolpath) to prevent work hardening, which is caused by plastic deformation and frictional heat generated during cutting.
Prevent Chip Welding
Kovar chips are highly viscous and difficult to remove, and these contribute to welding.The following measures are effective in preventing this:

  • 1）Select Tools with Large Rake Angles

    Tools with a large rake angle reduce the thickness of chips and facilitate their evacuation. However, too large a rake angle reduces the strength of the cutting edge, so an appropriate balance is necessary.

  • 2）Reduce Frictional Resistance of the Rake Surface

    Keeping the rake surface smooth prevents chip adhesion. When tools are worn and grinding marks appear, perform appropriate maintenance, such as grinding.

  • 3）Select Tools with Low Affinity for Kovar

    While cemented carbide tools are highly durable, they have a strong chemical affinity for Kovar and tend to adhere to it easily. Selecting tools with a low affinity for Kovar, such as cermet and coated tools, helps prevent welding.

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