Guides / Materials

Invar vs Zerodur and ULE Glass: Choosing a Low-Expansion Material

September 20, 2026

When an optical or precision structure needs expansion close to zero, the choice is not always between different metals. Zerodur and ULE, both low-expansion glass ceramics, compete directly with Invar and Super Invar for the same applications, and the right choice depends on geometry, cost and how the part will be made, not just which material has the lowest number on a datasheet.

What Zerodur and ULE are

Zerodur, a glass ceramic developed for astronomical telescope mirrors, and ULE, Corning’s ultra-low expansion titania silicate glass, are both engineered to hold an expansion coefficient very close to zero across a specified temperature range, tighter than Invar 36’s typical 1.2 to 1.6 ppm per degree C and closer to or below Super Invar’s typical 0.3 to 0.6 ppm per degree C. They achieve this through their material composition rather than through a metallic alloy structure, which is also why they behave very differently in fabrication.

Fabrication differences

This is where the comparison usually gets decided in practice, more than the expansion number alone. Invar is machined with conventional cutting tools, turned, milled, drilled and tapped much like any metal, following the processes covered in Invar 36 machining parameters. Zerodur and ULE are brittle glass ceramics, shaped primarily through grinding, polishing and specialized optical fabrication techniques rather than conventional machining. Complex features like threaded holes, thin walls or intricate mounting geometry that are routine in machined Invar are difficult or impractical in a glass ceramic.

This makes Invar and Super Invar the practical choice for structural elements, mounts and fixtures with complex geometry, while Zerodur and ULE dominate for the optical elements themselves, mirror substrates in particular, where their fabrication process is specifically built around producing precision optical surfaces.

A comparison at a glance

	Invar 36 / Super Invar	Zerodur / ULE
Typical CTE	1.2 to 1.6 / 0.3 to 0.6 ppm/°C	Near zero at design temperature
Fabrication	Conventional machining	Grinding and optical polishing
Complex geometry	Practical	Difficult, limited to simpler forms
Typical role	Structures, mounts, fixtures	Mirror substrates, optical elements
Relative cost	Lower	Higher

Where Invar remains the better choice

For any part that needs threaded holes, brackets, complex mounting features or a shape that would be impractical to grind, Invar and Super Invar are the practical option regardless of how their expansion number compares with a glass ceramic. This covers most structural and mounting hardware in optical systems, discussed more broadly in Invar for optical benches and telescope structures, even when the optical element it supports is made from Zerodur or ULE.

Designing a mixed-material assembly

It is common for an optical system to use both materials together, an Invar or Super Invar mount holding a Zerodur or ULE mirror. The design has to account for the CTE mismatch at the interface between the two materials, even though both individually hold low expansion, since a small mismatch across a temperature range can still introduce stress at the bond or mechanical interface if not designed with enough compliance. This is a design engineering consideration that sits upstream of the machining process, but it affects how the Invar mounting features should be toleranced and how any bonded interface is specified.

When to specify a glass ceramic instead of Invar

If your part is itself an optical surface, a mirror substrate or an etalon, a glass ceramic is usually the appropriate material regardless of the machining tradeoffs, since that is what the material and its fabrication process are built for. If your part is a structural or mounting element, even one with tight tolerance requirements, Invar or Super Invar is almost always the more practical choice.

Getting a quote for the Invar side of the assembly

For the mounts, structures and fixtures in a mixed-material optical system, the same sourcing process applies as for any precision Invar part.

Send your drawing through our RFQ form with your interface requirements to any glass ceramic components noted, and we match you with shops experienced in precision optical structural hardware.

Frequently asked questions

Keep reading

• Machining Invar Layup Molds and Mandrels for Composite Parts
• Machining Invar for Cryogenic and Vacuum Applications