Skip to content 
A wrong material choice on a hermetic seal part may not show up early. However, it can crack under thermal cycling. That’s where Kovar comes in. It expands and contracts at nearly the same rate as certain glasses and ceramics, so temperature changes don’t put stress on the seal.
But picking Kovar is only half the job. The other half is knowing what to put in your RFQ and what to ask a supplier before you commit to them.
In this guide, we’ll cover when Kovar makes sense for hermetic seal parts, how it compares to Invar and Alloy 42, and what to include in an RFQ when sourcing Kovar machining services.
When Is Kovar the Right Material for Hermetic Seal Parts?
The key advantage of Kovar is its coefficient of thermal expansion (CTE), which closely matches that of certain glasses and ceramics.
Some metals and glass expand at different rates, and temperature changes can put stress on the seal between them. Over time, that stress can crack the seal or break the bond.
Kovar is often a strong choice when sealing metal to compatible glass or ceramic in applications that experience temperature changes. Think vacuum tubes, sensor housings, and transistor packages where a cracked seal can fail the whole part.
However, not every project needs this level of matching. If your part doesn’t involve a glass to metal or ceramic to metal seal, or won’t face real temperature changes, Kovar’s main advantage doesn’t apply. In these cases, you may end up paying more without gaining anything.
There are some applications that can tolerate a bit of mismatch, so it’s better to ask if you need an exact CTE match.
Confirm three things before you specify Kovar. Does your part really need a glass or ceramic seal? Will it face real temperature changes? Does your application allow zero tolerance for seal failure? If the answer to all these questions is yes, Kovar is likely worth considering. Otherwise, a lower-cost alternative may meet your requirements.
Kovar vs. Invar vs. Alloy 42: A Quick Comparison
All three materials are nickel-iron alloys that control thermal expansion to some extent. However, they can’t be used interchangeably for hermetic seals due to some limitations.
Let’s compare Kovar, Invar, and Alloy 42 and see which one is best for a hermetic seal.
| Material | CTE Match with Glass | Glass Sealing Performance | Cost |
| Kovar | Excellent | Well suited to matched hermetic seals | High |
| Invar | Low expansion, doesn’t match glass | Not good for glass sealing | Medium |
| Alloy 42 | Good but not perfect | Suitable for selected sealing applications | Low |
The presence of cobalt in Kovar helps provide a close CTE match with certain borosilicate glasses. Invar is quite different. It barely expands at all, which makes it useful for precision instruments. But it is not a substitute for Kovar.
Alloy 42 is cheaper because it doesn’t contain cobalt. However, its CTE match is weaker, so it’s only suitable for parts where a less critical seal is acceptable.
Price shouldn’t be the only factor. If your part needs an excellent hermetic seal, Kovar is worth the extra cost. If a less critical seal will do, Alloy 42 can save you money.
What Makes Kovar Difficult to Manufacture?
Kovar is hard to machine for a few different reasons. It hardens as it’s cut, which wears down most cutting tools. That means more frequent tool changes and extra care at every step.
As an engineer, you may expect a higher cost per part and a longer lead time compared to more common materials. If you want the full technical breakdown, tooling choices, and cutting parameters, here is a detailed Kovar Machining guide.
What to Specify in Your RFQ
Your RFQ should include the following information:
Material Grade
Specify the exact material standard you need so the alloy meets your thermal expansion requirements. The standard industry specification for Kovar is ASTM F15.
Tolerance
Only a few dimensions typically need tight control. Clearly define which features matter most, such as a sealing surface or a mating diameter. You can reduce cost by keeping the remaining features at standard tolerance.
Sealing Surface and Finish
Clearly point out which surfaces need to seal against glass or ceramic. This tells your supplier exactly where precision and surface finish matter most.
Specify the surface finish needed for the seal to bond properly, and note if the part needs corrosion protection.
Traceability Documentation
Confirm whether your supplier provides material certificates and inspection reports. It’s important if you are working in aerospace, medical, or other regulated applications where you may need to prove material origin later.
How to Evaluate a Kovar Machining Supplier
Once your RFQ is ready, the next step is to figure out who can deliver on it. Here are a few things that you need to consider before choosing a Kovar machining supplier.
Does the Supplier Have Experience in Kovar Machining?
General CNC capability doesn’t mean a supplier can handle Kovar. Ask for real examples of finished Kovar parts and which industries they were made for. If a supplier has already worked on parts for your industry, they’re more likely to handle the task smoothly.
Who Handles Heat Treatment?
If your part needs annealing or stress relief, ask whether the supplier handles this in-house or coordinates it through an outside partner. Both can work, but you should understand the full supply chain either way. This affects your lead time and tells you who is accountable if something goes wrong at that step.
Who’s Responsible for Leak Testing?
Confirm exactly who owns the leak-testing step. This often falls on the customer after final assembly, since Kovar machining suppliers rarely offer it themselves. So ask directly instead of assuming.
What’s Actually Included in Their Scope of Supply?
Some suppliers only machine the part and deliver it to engineers. Only a few can support assembly, sealing, and final testing too. It does not necessarily matter whether they handle these processes directly or through qualified partners. However, the answer tells you whether you have to manage multiple vendors or can keep the work under one roof.
Ask these four questions early so you’ll know exactly what you’re getting, and what you’ll still need to handle yourself.
FAQs
1. Why is Kovar used for hermetic seals?
Kovar is commonly used for hermetic seals because its coefficient of thermal expansion closely matches that of certain glasses and ceramics. This helps reduce stress at the seal during temperature changes and lowers the risk of cracking or bond failure.
2. Is Kovar suitable for all glass-to-metal seals?
No. Kovar is a strong choice when its thermal expansion characteristics are compatible with the glass or ceramic being used. The required temperature range, sealing process, and application requirements should also be considered before selecting Kovar.
3. What is the difference between Kovar and Invar for hermetic seals?
Kovar is designed to provide a close thermal expansion match with certain glasses and ceramics, making it well suited to matched hermetic seals. Invar has very low thermal expansion and is more commonly selected for applications requiring dimensional stability.
4. What should be included in a Kovar machining RFQ?
A Kovar RFQ should clearly specify the material grade or standard, critical tolerances, sealing surfaces, required surface finish, and traceability or inspection documentation. These details help the supplier evaluate manufacturability, cost, and inspection requirements.
5. Why is Kovar difficult to machine?
Kovar can work-harden during machining, which increases cutting-tool wear and makes process control more important. As a result, machining Kovar may require more careful tooling and process planning than common engineering metals.
6. What should I look for in a Kovar machining supplier?
Look for experience machining Kovar parts, the ability to control critical tolerances and sealing surfaces, material traceability, and a clear understanding of any required heat treatment, assembly, sealing, or leak-testing responsibilities.
Right Kovar Choice Starts with a Complete RFQ
Kovar doesn’t have to be difficult, not if you make the right calls early on. Choose it for the right reason. Write a clear RFQ. Ask the right questions before picking a supplier. Do that, and your project runs smoothly.
Ready to move forward with a Kovar part? Send HRCCNC your specs. We’ll walk you through what we handle directly and where we bring in qualified partners. Machining, assembly, sealing, or final testing, we’ll tell you exactly who does what.
