Kovar Thermal Expansion Coefficient: The Complete Guide for Engineers & Designers

Introduction

If you’ve ever worked on a hermetic glass-to-metal seal or a ceramic-to-metal package, you’ve likely encountered Kovar. And if you’ve had a seal fail during thermal cycling, you’ve learned firsthand why the Kovar thermal expansion coefficient matters more than most other material properties.

Kovar is a nickel‑cobalt‑iron alloy (29% Ni, 17% Co, balance Fe) developed specifically to match the thermal expansion of borosilicate glasses and alumina ceramics. When we talk about the Kovar thermal expansion coefficient, we’re really talking about the alloy’s ability to expand and contract at nearly the same rate as the mating glass or ceramic over a wide temperature range—typically from room temperature up through the sealing temperature (around 450°C to 600°C, depending on the glass).

In this guide, I’ll walk you through the exact CTE values, how they vary with temperature, the mechanical and physical properties that support sealing, and the practical considerations we use at KELTRYN when machining Kovar parts for hermetic applications. Whether you’re designing a power tube feedthrough, an optoelectronic package, or a medical implant housing, understanding the thermal expansion behavior of Kovar is the first step toward a reliable, leak‑tight assembly.

What Is the Coefficient of Thermal Expansion (CTE)?

Let’s start with the basics. The coefficient of thermal expansion (CTE) is a material property that describes how much a material expands or contracts per degree of temperature change. It’s most often expressed as a linear expansion coefficient—the fractional change in length per °C (or per °K).

There are two common ways to quote CTE:

Instantaneous CTE – the expansion coefficient at a single temperature (or a very narrow range). This can change with temperature, especially near phase transitions or magnetic ordering changes.
Mean CTE – the average coefficient over a specified temperature range. In Kovar specifications, you’ll almost always see mean CTE values over ranges like 30°C to 450°C or 30°C to 600°C. This is what matters for sealing because the entire temperature excursion from sealing to room temperature (and back in service) must be accommodated.

Why does CTE matching matter? Imagine sealing a metal pin through a glass insulator. Heat the assembly to ~450°C; the glass softens and wets the metal. As you cool it down, both materials shrink. If the metal shrinks faster (higher CTE), it will pull away from the glass, creating gaps or tensile stresses that crack the glass. If the metal shrinks slower (lower CTE), the glass will be put into tension and may fracture. The ideal is a near‑perfect match over the entire cooling range, so that residual stresses are minimized.

Kovar’s composition is engineered to achieve that match with the most common borosilicate glasses (e.g., Corning 7056, 7052, 7720) and with alumina ceramics when used with appropriate metallization. That’s why it’s the go‑to alloy for high‑reliability hermetic seals.

Kovar’s Thermal Expansion Coefficient: Exact Values and Temperature Ranges

Now let’s get to the numbers. The standard specification for Kovar is ASTM F15. Under this specification, the mean CTE for annealed material must fall within the following ranges:

| Temperature Range (°C) | Mean CTE (×10⁻⁶ /°C) |

|------------------------|-----------------------|

| 30 – 450 | 5.0 – 5.5 |

| 30 – 500 | 5.2 – 5.8 |

| 30 – 600 | 8.5 – 9.0 |

The jump in CTE above about 450°C is due to the Curie transition (magnetic ordering change) at around 435°C. Below the Curie temperature, the alloy has a lower expansion; above it, the expansion rate increases. This behavior is deliberately tailored: during the sealing process, the glass typically sets (vitrifies) somewhere in the 400–500°C range. Kovar’s expansion up to that point is low and linear, matching the glass. Above the setting temperature, the glass is still soft and can accommodate a higher metal expansion without cracking.

Important: These values apply to material in the annealed condition. Cold‑worked Kovar may have slightly different CTE, especially if residual stresses are not relieved. For sealing applications, always use annealed material.

Other relevant standards include MIL‑I‑23011 (for Kovar used in electron tubes) and MIL‑STD‑1275 (for hermetic seals). They all reference essentially the same composition and CTE requirements.

Let’s look at a more granular table comparing CTE at various ranges, based on typical test data from a certified lot:

| Temperature Range | Mean CTE (×10⁻⁶ /°C) |

|-------------------|-----------------------|

| −60 to +100°C | 5.1 – 5.4 |

| 25 to 300°C | 5.0 – 5.3 |

| 30 to 450°C | 5.0 – 5.5 |

| 30 to 500°C | 5.2 – 5.8 |

| 30 to 600°C | 8.5 – 9.0 |

| 30 to 700°C | 9.5 – 10.5 |

At KELTRYN, we always request CTE certification from our material suppliers for each heat we machine. Even within ASTM F15, small variations in trace elements can shift the expansion curve. Critical applications—especially those with large‑diameter seals or thin glass walls—require tight CTE control.

Mechanical and Physical Properties That Complement CTE

Kovar’s thermal expansion is its star property, but a successful seal also depends on its mechanical strength, thermal conductivity, and magnetic behavior. Here are the key numbers for annealed Kovar:

Density: ~8.17 g/cm³
Tensile Strength (UTS): 550–620 MPa
Yield Strength (0.2% offset): 340–410 MPa
Elongation: 30–35%
Hardness: ~155–200 HV (annealed)
Modulus of Elasticity: ~138 GPa
Thermal Conductivity: ~17 W/m·K at 25°C (decreases with temperature)
Electrical Resistivity: ~0.49 µΩ·m at 25°C
Curie Temperature: ~435°C
Thermal Expansion (mean 30–450°C): 5.0–5.5 ×10⁻⁶ /°C

The moderate thermal conductivity (~17 W/m·K) is a sweet spot. It’s high enough to conduct heat away from internal electronics in a power tube or IC package and low enough that it doesn’t cause rapid thermal gradients during sealing (which could crack the glass). For comparison, copper has ~400 W/m·K—too high for sealing because it would chill the glass locally. Stainless steel 304 has ~15 W/m·K, similar to Kovar, but its CTE (~17 ×10⁻⁶ /°C) is far too high for glass sealing.

The magnetic properties are relevant for applications like sensors, relays, and electron beam devices. Kovar is ferromagnetic at room temperature and loses its magnetism above the Curie point (~435°C). If your device requires a non‑magnetic material at operating temperature, that can be a concern. For high‑frequency RF packages, magnetic permeability can also affect electrical performance, so alternative alloys (like copper‑tungsten) may be specified, but Kovar remains the standard for most hermetic applications.

Kovar Chemistry and Composition (ASTM F15)

The precise balance of nickel and cobalt is what gives Kovar its unique expansion curve. The nominal composition per ASTM F15:

| Element | Weight % |

|---------|----------|

| Nickel | 29.0 |

| Cobalt | 17.0 |

| Manganese | ≤0.50 |

| Silicon | ≤0.20 |

| Carbon | ≤0.04 |

| Aluminum | ≤0.10 |

| Magnesium | ≤0.10 |

| Zirconium | ≤0.10 |

| Iron | Balance |

Alloying with 29% Ni and 17% Co lowers the thermal expansion of pure iron to the point where the alloy matches borosilicate glass from room temperature up to about 450°C. The ratio is critical: too much nickel raises the CTE; too little cobalt reduces the Curie point and changes the expansion slope.

The trace elements (Mn, Si, C, Al, Mg, Zr) are added for deoxidation and grain refinement. They do affect CTE consistency—especially carbon, which can form carbides that alter the local expansion in the microstructure. At KELTRYN, we always verify the mill certificate for every batch used in sealing parts. A 0.01% variation in cobalt can shift the CTE by 0.1×10⁻⁶ /°C, which might be acceptable for a wide‑tolerance seal but disastrous for a thin‑wall optical feedthrough.

How Kovar Compares to Other Low‑Expansion Alloys

A common question we get is “What is Kovar equivalent to?” Or “Can I substitute Invar or Alloy 42?” The short answer is: only if you fully understand the temperature range of your sealing process and service conditions.

Here’s how some common low‑expansion alloys compare:

|-----------------|-------------------------------|-------------------------------|-------------------------------|

Key differences:

Invar has a famously low CTE at room temperature (around 1.2×10⁻⁶ /°C), but its expansion increases sharply above 200°C due to its Curie point (~230°C). So it cannot match borosilicate glass through a 450°C sealing cycle. Invar is excellent for precision instruments that operate near room temperature, not for hermetic seals.
Alloy 42 has a CTE close to Kovar up to about 300°C, but it rises faster above that. It is used for compression seals with certain glasses, but it’s not a drop‑in replacement for Kovar in a full sealing cycle.
Alloy 52 matches higher‑expansion glasses (like lead glass) and is not suitable for borosilicate.

Trademarked equivalents include Nilo K (Special Metals), Rodar (Carpenter), and Dilver P1 (Ed Fagan). They all meet ASTM F15. At KELTRYN, we often machine parts to these cross‑references because our customers have legacy prints calling out “Nilo K” or “Rodar.” We always confirm the material meets ASTM F15 chemistry and CTE before proceeding.

How Strong Is Kovar? Mechanical Strength in Sealing Applications

Another frequent question: “How strong is Kovar?” The answer depends on its condition.

Annealed: UTS 550–620 MPa, yield 340–410 MPa, elongation 30–35%. This is the standard condition for sealing. It has enough ductility to absorb some differential thermal stress without cracking.
Cold‑drawn or hardened: UTS can reach 700–800 MPa or more, but elongation drops significantly. Cold‑drawn Kovar is sometimes used for pins or lead wires that need higher strength, but it must be stress‑relieved before sealing, or the high residual stresses can cause the glass to crack.

Strength matters in sealing because during cooling, the metal wants to contract more than the glass (even with good matching). The metal puts the glass into compression (which is good for ceramics and glasses), but if the metal too strong, it can generate excessive stress that the glass cannot handle. Kovar’s moderate strength and good ductility provide a safety margin.

At KELTRYN, when we machine Kovar parts for hermetic packages—like flanges with thin walls (<0.5 mm)—we have to manage both the material’s work hardening and the risk of burrs. Kovar is gummy in the annealed state, so we use sharp tooling, appropriate feeds, and coolant to avoid tearing. For high‑strength applications, we can specify cold‑drawn rod but always recommend a final stress‑relief anneal before any sealing operation.

Thermal Conductivity of Kovar and Its Role in Sealing

You asked: “What is the thermal conductivity of Kovar?” At room temperature, it’s about 17 W/m·K. That’s roughly the same as stainless steel 304 (~15 W/m·K) and far lower than aluminum (~200) or copper (~400). Why does this matter?

During glass‑to‑metal sealing, the assembly is heated to 450–600°C. The metal part must heat up evenly so that the glass wets uniformly. Too high thermal conductivity (like copper) would conduct heat away from the seal area quickly, creating a cold spot and preventing proper wetting. Too low conductivity (like ceramics) would cause huge temperature gradients. Kovar’s moderate conductivity is just right: it heats up and cools down at a rate compatible with the glass.

In operation, Kovar components in power tubes or IC packages must dissipate heat. 17 W/m·K is modest—it’s not a heat sink material. But for a Kovar lid on a ceramic package, the thermal path is usually short, so the conductivity is adequate. If you need much better heat dissipation, you might design with a copper core under a Kovar flange, or use a composite like CuW (copper‑tungsten). But for the sealing interface itself, Kovar’s CTE match is non‑negotiable.

Top Applications Where Kovar’s CTE Is Essential

The Kovar thermal expansion coefficient makes it indispensable in these fields:

Hermetic Seals in Power & Microwave Tubes – Glass‑to‑metal seals in magnetrons, klystrons, and traveling‑wave tubes. The vacuum integrity depends on CTE match over the entire processing temperature range.
Integrated Circuit & Semiconductor Packaging – Kovar lids (often with a Ni‑Au finish) seal ceramic or metal‑ceramic packages. The lid’s expansion must match the ceramic substrate to prevent lid‑lift during solder or brazing.
Photonics & Optoelectronics – Laser diode packages, optical fiber feedthroughs, and sensor windows. Any misalignment due to thermal expansion destroys optical coupling efficiency.
Medical Devices & Implantables – Battery casings, sensor housings, and hermetic feedthroughs for pacemakers and neurostimulators. Leak‑tight seals are life‑critical, and Kovar’s biocompatibility (when coated) is well established.
Aerospace & Defense Electronics – Connectors, relays, transducers, and microwave modules that must survive wide temperature cycles. Kovar also has low outgassing, critical for space applications.
Sensors & Vacuum Equipment – Pressure sensors, vacuum feedthroughs, and thermocouple connectors. The seal must hold vacuum or inert atmosphere for years.

At KELTRYN, we see a steady stream of orders for Kovar housings, lids, sealing rings, flanges, and frames for these exact applications. Each part has its own critical dimensions, often with flatness and parallelism tolerances of 0.025 mm or less. The CTE of the material determines the final dimensions at room temperature after the cooling step. We always hold to the customer’s specified dimensions at 20°C, which requires us to account for the CTE when machining—especially if the part will be subsequently brazed or sealed.

How to Select the Right Kovar Grade and Heat Treatment

Kovar is available in many forms: sheet, strip, rod, wire, tubing, and bar. Each form can affect final CTE uniformity due to differences in grain size and texture from the rolling or drawing process. For critical seals, we recommend:

Use annealed material – The mill‑annealed condition (typically at 850–900°C in a hydrogen atmosphere) gives the most consistent CTE and ductility.
Avoid over‑annealing – Holding Kovar above 1000°C can cause excessive grain growth. While large grains don’t change CTE, they reduce strength and can cause surface roughness issues during machining or etching.
Check the heat lot – Request a CTE certification from the supplier. For hermetic seals, we often ask for the mean CTE measured over 30–450°C on an actual sample from the lot.
Surface preparation – For glass‑to‑metal sealing, Kovar parts are typically pre‑oxidized (a controlled oxide layer). The oxidation step must be controlled in thickness and adhesion. Machining must leave a clean, burr‑free surface to avoid trapping gases or causing leaks.

At KELTRYN, we also see customers who need Kovar machined to thin walls (0.3–0.5 mm). This is tricky because annealed Kovar is soft and tends to deform. We use specialized workholding and light cuts. After machining, we always deburr and inspect for any torn material that could interfere with sealing.

Frequently Asked Questions About Kovar Thermal Expansion

What is the coefficient of thermal expansion of Kovar?

The mean CTE for annealed Kovar (ASTM F15) is 5.0–5.5 × 10⁻⁶ /°C over 30–450°C, and 8.5–9.0 × 10⁻⁶ /°C over 30–600°C.

What is Kovar equivalent to?

Kovar is equivalent to ASTM F15, UNS K94610, Nilo K (Special Metals), Rodar (Carpenter), Dilver P1 (Ed Fagan). All meet the same chemistry and CTE requirements.

What is the thermal conductivity of Kovar?

~17 W/m·K at 25°C. It decreases with temperature but remains in the range of 15–20 W/m·K up to 500°C.

How strong is Kovar?

Annealed: UTS 550–620 MPa, yield 340–410 MPa, elongation 30–35%. Cold‑drawn can reach higher strength but with reduced ductility.

Can Kovar be welded?

Yes, Kovar can be welded by TIG, electron beam, or laser. However, it is susceptible to hot cracking if filler material is used incorrectly. For hermetic seals, electron beam or laser welding is preferred to minimize heat input and avoid micro‑cracks. We often weld Kovar housings to Kovar flanges; post‑weld stress relief is recommended.

Does Kovar corrode?

Kovar has good oxidation resistance up to about 500°C in air. In humid or saline environments, it can rust (since it’s mostly iron). For long‑term corrosion protection, Kovar is usually plated with nickel or gold—especially in medical or aerospace applications.

Conclusion

Understanding the Kovar thermal expansion coefficient isn’t just a technical detail—it’s the foundation of reliable hermetic sealing. From the exact CTE values across temperature ranges to the alloy composition that makes it possible, every element matters. When you’re designing a package that must hold vacuum for a decade, or a sensor that must survive thermal shock from −55°C to +125°C, you need a material that moves with its glass or ceramic partner.

At KELTRYN, we machine Kovar components day in and day out. We know how the material behaves under the tool, how heat treatment affects final dimensions, and how to hold the tight tolerances your sealing process demands. Whether you need a prototype Kovar lid or a production run of 500 flanges, we’re here to help.

Ready to discuss your Kovar machining project? Contact our engineering team at info@keltryn.com](mailto:info@keltryn.com) or visit [www.keltryn.com to request a quote. We’ll review your drawing, provide DFM feedback, and deliver parts with certified material traceability and CTE data.

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This article was written by a senior application engineer at KELTRYN. For technical data sheets or to speak with a Kovar specialist, reach out to us directly.