What Is Kovar Alloy ?
2025/9/26
Kovar alloy is an alloy primarily composed of iron (about 54%), nickel (about 29%), and cobalt (about 17%) that was developed to enable sealing with hard glass by closely matching the alloy’s coefficient of thermal expansion to that of the glass.
Thanks to its low thermal expansion near room temperature and stable expansion over a wide temperature range, Kovar can be reliably sealed, enclosed, or brought into contact with hard glass. The name “Kovar” is a registered trademark of Carpenter Technology Corporation. Its high nickel content gives Kovar toughness and excellent heat resistance, making it suitable for demanding applications.
Kovar is used as an essential material for bonding metals to glass or ceramics, transistor lead caps, optical communication components, vacuum tube materials, and glass-sealed electronic components.
Characteristics of Kovar
Kovar is a tough metal with low thermal conductivity and high heat resistance. Its low thermal conductivity can cause it to stick to cutting tools, leading to tool chipping. The material is also prone to work hardening, and its machinability is similar to SUS304 stainless steel, which is why Kovar is classified as difficult to machine.
Removing burrs can be time-consuming, and processes beyond cutting may require significant effort. Additionally, the material may already be warped upon delivery, so careful consideration, such as selecting slightly larger sizes when sourcing, is recommended. Its low thermal expansion near room temperature ensures that dimensions remain stable within typical operating temperature ranges.
Density (g/cm³) | Melting Point (°C) | Electrical Resistivity at 20°C (μΩ·cm) | Tensile Strength at 20°C (MPa) | Yield Strength (MPa) | Elongation (%) | Young’s Modulus (GPa) | Thermal Conductivity at 20°C (W/(m·K)) | Magnetic Transformation Temperature (°C) |
|---|---|---|---|---|---|---|---|---|
8.35 | 1450 | 49 | 540 | 270 | 35 | 159 | 17 | 430 |
Machinability of Kovar
Kovar is difficult to machine for three main reasons:
Prone to work hardening
Poor thermal conductivity
High affinity with the tool material
When these conditions coincide, problems such as chips sticking to the cutting tool and overcutting due to the chips not detaching properly are likely to occur. This leads to a deterioration in surface finish and a significant reduction in machining accuracy. Selecting the right cutting tools and machining conditions is essential when working with Kovar to ensure efficiency and prevent tool wear.
Applications of Kovar
Kovar is primarily used for sealing with hard glass or ceramics. To bond metal to glass, three conditions must be satisfied, all of which Kovar meets:
The metal’s melting point is higher than the working temperature of the glass.
When the metal and glass come into contact, they have a strong attraction to each other and spread across each other’s surface (mutually wettable).
The thermal expansion coefficient below the annealing temperature matches between the metal and the glass.
In recent years, with increased integration and higher conductivity requirements, copper alloys are increasingly used as alternatives, especially with the rise of resin sealed components. Wires and tubes made from Kovar take advantage of its low and consistent thermal expansion coefficient, as well as its close match to hard glass, making it ideal for hermetic seals in IC lead frames and sealing terminals for electronic components.
Machining Records of Kovar
At Yumoto Electric, our primary experience is with aluminum alloys and stainless steel for industrial machinery, so Kovar machining is still limited. Most of our Kovar work has focused on small, precise components, such as parts with an outer diameter of 7 mm, inner diameter of 3 mm, and length of 15 mm.
Kovar’s low thermal conductivity causes heat to build up during machining, and its high toughness makes cutting challenging, classifying it as a hard-to-machine material. Despite this, we can efficiently handle Kovar in small-batch, fast-turnaround production, just as we do with aluminum alloys and other materials.
Key Takeaways
Kovar is a metal with a low coefficient of thermal expansion near room temperature.
Kovar is difficult to machine and is classified as a hard to machine material.
Step Up Your Project with Yumoto Electric
Yumoto Electric delivers high-quality components fast, worldwide. We provide precision machining for a wide range of metals using milling, turning, and specialized metal processing. We have experience with over 200 materials including aluminum alloys, stainless steel, molybdenum, and titanium.
If you’re unsure about material selection or machining methods, we offer complimentary consultations to help optimize your design and production. For any questions, please feel free to contact us.
