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The chemical composition is well controlled so as to provide low, uniform, thermal expansion property for this alloy. The following sections will discuss in detail about super alloy KOVAR.
Kovar ASTM F15, UNS K94610 Bar is a controlled expansion alloy. It is a Nickel Iron alloy with 29% Nickel, 17% Cobalt and the remaining balance is Iron. The expansion characteristics of Nilo k (Kovar) match both borosilicate (or Pyrex) glasses and alumina ceramics, making it one of the most popular of the controlled expansion alloys for hermetic sealing applications (Glass to Metals Sealing). Nilo K (Kovar) is produced to ensure good properties for machining and deep drawing, and is a porosity free product.
Kovar (Nilo K) Bar not only has thermal expansion similar to glass, but its nonlinear thermal expansion curve can often be made to match a glass, thus allowing the joint to tolerate a wide temperature range. Chemically, it bonds to glass via the intermediate oxide layer of nickel oxide and cobalt oxide; the proportion of iron oxide is low due to its reduction with cobalt. The bond strength is highly dependent on the oxide layer thickness and character.The presence of cobalt makes the oxide layer easier to melt and dissolve in the molten glass. A grey, grey-blue or grey-brown color indicates a good seal. A metallic color indicates lack of oxide, while black color indicates overly oxidized metal, in both cases leading to a weak joint.
| Kovar Chemical Composition (Basic Density 8.36) | |||||||||||||
| Weight % | Ni | Fe | Co | C | Mn | Si | Al | Cr | Mg | Zr | Ti | Cu | Mo |
| Alloy K Kovar |
29.0 | 53.0 | 17.0 | 0.04 max | 0.50 max | 0.20 | 0.10 max | 0.20 max | 0.10 max | 0.10 max | 0.10 max | 0.20 max | 0.20 max |
| Material | Condition | Yield Strength (0.1% Offset) | Tensile Strength | Elongation % | ||
| psi | MPa | psi | MPa | |||
| Alloy K Kovar |
Annealed | 49,000 | 340 | 75,000 | 520 | 42 |
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HASTELLOY, HAYNES and C-22 are registered trade names of Haynes International, Inc., MONEL & INCONEL are registered trade names of INCO family of companies., NITRONIC is a registered trade name of Armco Inc., CARPENTER & 20Cb-3 are registered trade names of Carpenter Technology Corporation., FERRALIUM is a registered trade name of Langley Alloys, LTD.
