Kovar spotlight

Following the success of Invar 36, Signicast expands its offerings with Kovar, an iron-nickel-cobalt alloy. Kovar, sharing the "Invar Effect" with its predecessor, has low and linear thermal expansion below the Curie Point, then expands significantly above it. While Invar prioritises a minimal coefficient of thermal expansion (CTE) for temperature stability, Kovar's "co-variant" nature ensures its CTE closely aligns with borosilicate glasses and certain alumina ceramics.

What are the benefits and applications of Kovar?

Kovar's thermal expansion rate is the key property that makes this superalloy useful. Compared with other alloys, Kovar expands at a variable rate as temperature increases, with very little dimensional change. This makes Kovar an excellent choice for seals and glass-to-metal bonded assemblies with minimal locked-in pressure.

It is used in applications requiring a hermetic seal between the metal alloy and the glass or ceramic component. The metal-to-glass seal is created by performing an oxidising treatment on the parts after they have been annealed. The oxide layer acts as an intermediary, bonding both to the glass and to the alloy substrate on which it was formed. The hermetic seal creates a bond free from the problems and failures caused by dirt, dust, moisture, fungus or low pressure at high altitudes. Applications for this alloy include X-ray tubes, microwave tubes and light bulb ends, among many others.

Physical and mechanical properties

By varying the amounts of the major alloying elements, the CTE behaviour of these alloys can be tailored to match different ceramics and glasses.

Kovar's typical physical and mechanical properties are as follows:

• Density: .302 lb/in3

• Curie Temperature: 435 ºC

• Melting Point: 1450 ºC

• Specific Heat:

• .105 cal/gm/ ºC at 0 ºC

• .155 cal/gm/ ºC at 430 ºC

• Heat of Fusion: 64 cal/gm

• Thermal Conductivity: 17.3 W/m · K

• Electrical Resistivity: 490 microhm/mm

• Shear Modulus: 7.5 · 106

• Modulus of Elasticity: 20 · 106

• Ultimate Tensile Strength: 75,000 psi

• Yield Strength: 50,000 psi

• Elongation: 30%

• Hardness (Rockwell B): 78

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