3.2.2.2 Metal seals

For applications with high or low temperatures, very long service life, high radiation loads and wherever very low permeation rates are important, metal seals must be used instead of the elastomer seals. Materials that are often used for metal seals are copper and aluminum, in some cases even silver and gold and primarily indium in cryotechnology . While gold, silver, and indium are mainly used as wire seals, aluminum is also used as a profile seal. There are also resilient metal-coated seals.

Metal seals require high contact pressures. During assembly they cause plastic deformation and can therefore only be used once. Their hardness should be less than that of the flange, so that they adapt to their microstructure to create a metallic ultra-high vacuum-tight connection. The screws or fasteners must be dimensioned in such a way that the specific contact forces of up to 600 N/mm seal length are maintained in each operating mode.

For ISO-KF and ISO-K/ISO-F-flanges made of stainless steel there are metal seals with a diamond cross-section (aluminum edged seals) available. Aluminum-silicon alloys are used, which are annealed and will require a contact force of about 100 N / mm seal length. The different thermal expansion of aluminum and stainless steel limits the maximum operation temperature to about 150°C. After excessively high temperatures, a deterioration of the sealing effect can occur during cooling.

Copper has about the same thermal expansion as austenitic stainless steel. Copper gaskets are used as flat seals (CF flanges) or wire seals (COF flanges). They must be oxygen-free, i.e. copper quality OF (Oxygen Free) or OFHC (Oxygen Free High Conductivity) is used. If oxygen-free copper is not used, the available oxygen can react with hydrogen during heat treatments (e.g. during baking out). This leads to the so-called “hydrogen brittleness” in which the resulting water can lead to leaks by breaking up the microstructure. Flat copper seals for CF flanges require a contact pressure of at least 200 N/mm seal length. Their maximum allowable operation temperature for air is 200°C. With a silver coating it increases to a maximum of 450°C. Annealed copper seals require a lower contact pressure. They should be used in particular for viewports, to keep the tension during assembly as small as possible.