Using Invar as Metal Optic Material

Invar is a 36% nickel-iron alloy which has the lowest thermal expansion among all metals and alloys in the range from room temperature up to approximately 230°C. The Invar alloy is ductile and easily weldable, and machinability is similar to austenitic stainless steel.

Durability, Material and Composition

FeNi36 (64FeNi in the US) aka Invar 36, is a popular selection for metal optics as it offers a unique and considerably lower coefficient of thermal expansion (hence the name source, invariable). With workability that is ductile, weldable and easy to machine, similar to austenitic stainless steel, Invar has maintained its popularity over the years.

Machined optical components can be readily created when using Invar when the final product requires drilled and/or tapped holes, or extensive machining for the final, correct configuration. Because of Invar’s highly desirable properties, it typically ranges 5x in cost when compared with 304 stainless steel.

Invar also is readily available for shipping from many different suppliers when availability for timed completion of projects is key.

Thermal Expansion

4.9 ppm/ºF, 8.9 ppm/ºC

Thermal conductivity

72.6 Btu-in/ft2/hr/ºF

Typical linear coefficient of thermal expansion (cm per cm)

1.72 x 10 (-6)/ºC

Temperature coefficient of electrical resistivity

0.0011 per ºC

Curie Temperature

535 ºF, 279 ºC

Yield Strength

40 ksi

When compared with stainless steel (304) it is similar overall, however, when comparing Young’s Modulus and Specific Stiffness, the Invar properties of microyield strength and thermo conductivity are both lower, making it more desirable for optics manufacturing.

Fabrication Process

With CMM Optic’s forming and diamond-turning machining processes, Invar is an important material as it can be used for high-volume production. The CarTech Super Invar product is a popular selection for optical components, assemblies, and laser instrumentation.

As Invar is readily available in a wide variety of base sizes and forms from blocks, rods, bars, and sheets that can be easily custom machined into your final optic.

Using precision lathes and CNC machines to turn base materials into custom precision plano mirrors, complex polynomials, off-axis mirrors or parabolic optics. The onsite DMG MORI 5-axis machine gives CMM the ability to create and deliver production orders on time, and with incredible precision whether for a prototype model or large volume order.

Invar can be difficult to machine, but because of CMM Optic’s experience in handling this particular material, our machinists are able to create exacting parts using a heat treatment process, knowing when to change cutting tools and using the correct cutting speed.

Applications for Usage

Since the alloy’s discovery and development in 1920, resulting in a Nobel Prize, InvarFeNiCo alloys are also popular for manufacturing metal optics, as they have a similar expansion rate as borosilicate glass and are able to withstand harsh elements for military, security and space usage. Commercial usage can be found in semiconductors and television manufacturing.

Dispersive optics created from INVAR alloys have been used to create a high-pressure X-ray absorption spectroscopy (XAS) technique which allows detection of very small signals.

Mirror finishes on Invar substrates can be achieved by nickel plating and subsequent diamond machining.  When compared to other materials, Invar substrates are preferred if ease of engineering and cost are key factors when compared to other materials for stability, cooling and bending.  

Invar for Opto-Mechanical Uses

As the key property of Invar is low expansion rate under heat, it is an ideal material for creating opto-mechanical systems that are stable under a wide range of temperature conditions.

Since the visible light wavelength for optical systems is approximately 0.5μm, and system requirements typically demand that optical elements be held to some tolerance near this value, a structural metal with a low CTE is highly valued.  High dimensional stability is desirable for keeping lens housings and specific alignments without change or introducing errors.

When looking for optical mirror manufacturers that offer single point diamond turning to develop freeform optics in Troy, Michigan, please contact CMM Optic for more information on how our experience can benefit your next optical manufacturing projects.