3.2 Materials

In vacuum technology, the requirements placed on any material are very diverse. Depending on the application, ambient conditions and the vacuum to be achieved, it must be tested which requirements the materials must meet.
The following lists important requirements that must be considered:

Sufficient mechanical strength throughout the entire operational temperature range:
in addition to the structural integrity, it must be ensured that the deformation of the functional surfaces does not impact the functionality. Example: The atmospheric air pressure on evacuated chamber components is approximately 10 N/cm². With an area of 1 m², this results in a force of 100,000 N.

High gas tightness:
Each material is basically gas permeable. The overall process of gas permeability is called permeation. It depends on the material, the type of gas and the ambient conditions – especially the temperature. With the use of elastomer seals, their permeation must be considered. Example: For an FKM (fluoroelastomer) seal of DN 500 ISO-K, the permeation rate for atmospheric air with 60 % humidity is about 4 · 10^-7 Pa · m³/s. Therefore typical vacuum systems with FKM seals rarely achieve a working pressure of more than 1 · 10^-8 hPa.

Low intrinsic vapor pressure, high melting and boiling point:
Intrinsic vapor pressure that is too high limits the final vacuum pressure. In addition to the vacuum compatibility of oils and grease, the intrinsic vapor pressure of metals or their partial pressure in alloys must be considered in alloys. Example: For brass, the partial pressure of the zinc is limited to a maximum permissible temperature in a high vacuum at about 100°C.

Clean surfaces, low content of foreign gases, easy degassing:
Clean surfaces are a prerequisite. However, any surface that was exposed to the ambient air, is coated with an adsorbed layer. Chemically or physically adsorbed molecules on the surface or in the volume of the material represent a source of gas once they desorb (when they detach from surface). To achieve a low final pressure, materials with low desorption rates must be used. Example: A monolayer of adsorbed gas roughly equals a gas quantity of 4 · 10^-2 Pa · m³/m². Considering a pipe, closed at both ends, with a diameter of 50 cm and a length of 100 cm (surface area of about 2 m², volume approx. 200 l), the release of the monolayer results in a pressure rise of about 0.4 Pa or 4 · 10^-3 hPa. This does not take into account the fact that the surface is always larger than the geometric area.

Good thermal shock resistance, adjusted expansion behavior:
Example: The different thermal expansion limits the maximum permissible temperature for the combination of aluminum seal and stainless steel flanges to about 150°C. Often after too high temperatures, a deterioration in the sealing effect occurs during cooling.

Corrosion resistance, chemical resistance:
Example: A lot of coating processes require chemically active process gases. It is therefore necessary to consider whether the fluids used affect components or seals. In particular, thin-walled components such as metal bellows are vulnerable to corrosion. If necessary, their service life should be determined in tests.
Special applications may also place further demands on the materials.
As a general rule: the lower the desired working pressure, the greater the demands placed on the material and the smaller the selection of possible materials. Therefore, especially in UHV (ultra-high vacuum) technology, the choice of materials is of great importance.

3.2.1 Metallic materials

Many metals and metal alloys have high strength, are easy to process and clean, temperature resistant and less sensitive to mechanical influences than glass. The consideration of economic aspects, such as the material price or availability, leads essentially to the selection of stainless steel, carbon steel and aluminum alloys as materials for mechanical components.

3.2.1.1 Stainless steel

Stainless steel is the preferred material for the construction of chambers or components in vacuum technology. Stainless steel has sufficient strength for flange connections – even in bake-out processes. It can be welded so that it is vacuum-tight, its surface is sufficiently well passivated and thus provides sufficient protection for many applications. The following tables list the chemical composition and properties of the commonly used stainless steels in vacuum technology.

European steel numbers are frequently used interchangeably with comparable material designations of AISI (American Iron and Steel Institute), such as 1.4301 with 304, 1.4307 and 1.4306 with 304 L, 1.4404 and 1.4435 with 316 L and 1.4429 with 316LN. The materials however are only approximately comparable. The differences for vacuum applications are mostly marginal. However, for special requirements, the interchangeability must be assessed in each individual case. Example: If material 1.4301 is required for a component, it must generally be supported by the associated material certificate. If the certificate only shows the material 304, the requirement is not met. The designation in the certificate is important here. If the respective material specifications are met, manufacturers can also certify semifinished products with several designations. If a material is certified as being 1.4301, 1.4307, 304 and 304L, for instance, the uses are more diverse.

To avoid problems, for example, during the acceptance of a system, the demands placed on the materials and their certification must be predefined when requesting semifinished products or components. Subsequent certification is often not possible, especially for special requirements: for example, due to special mechanical properties, a restriction in the chemical composition or the certification according to AD 2000 W2 (Working Group for Pressure Vessels 2000 date sheet W2, “Materials for pressure vessels”) or the ASME (American Society of Mechanical Engineers).

Stainless steel 1.4301: the most frequently used chromium-nickel steel. Excellent cold forming, welding and polishing properties. Sufficient corrosion resistance for many applications. Suitable for vacuum applications. Used, for example, for flanges, pipe components and chambers.

Stainless steel 1.4305: Variant of 1.4301 with sulfur content to improve machinability (machining steel). Lower corrosion resistance than 1.4301. Non-weldable. Sufficiently well-suited for vacuum applications. Use in part for turned and milling parts, such as centering rings.

Stainless steel 1.4307, 1.4306: low carbon variant of 1.4301. Due to the low carbon content it is weldable, without being susceptible to intergranular corrosion. Slightly lower strength than 1.4301. Highly suitable for vacuum applications. For uses requiring a very low carbon content, e. g. for CF flanges. 1.4307 is increasingly replacing 1.4306, as the benefits associated with the higher chromium and nickel content are often not sufficient to justify the higher purchase costs.

Stainless steel 1.4401: excellent cold forming properties. Good weldability and polishability. Due to the molybdenum additive it is more resistant than 1.4301 to non- oxidizing acids and chloride ion-containing media. Well suited for vacuum applications. Used for example for valve housings, in areas which require greater protection against corrosion, or for domestic drinking water systems.

Stainless steel 1.4404: low carbon variant of 1.4401. Due to the low carbon content it is weldable, without being susceptible to intergranular corrosion. Highly suitable for vacuum applications. It is used when a very low carbon content or greater corrosion resistance is required, for example, for pipes and flange components in the semiconductor industry.
Stainless steel 1.4435: the higher nickel content compared to 1.4404 stabilizes the austenitic structure, reduces the formation of delta ferrite and is therefore barely magnetic even in the welded seam area. Due to the increased molybdenum additive it is more resistant than 1.4404 to non-oxidizing acids and chloride ion-containing media. Highly suitable for vacuum applications. It is often used in the pharmaceutical industry, also according to the Basel Standard 2 (BN2), which sets tougher analytical limits and defines the permissible ferrite content.

Stainless steel 1.4429: similar characteristics as 1.4435, however, greater strength due to the high nitrogen content. This also stabilizes the austenitic structure, thereby minimizing the formation of delta ferrite and thus the magnetization. Highly suitable for vacuum applications. It is used for CF flanges, especially when vacuum annealing for cleaning or demagnetization occurs at high temperatures. The availability for tubes and sheet metal from 1.4429 is low. Therefore, flanges from 1.4429 for chambers or components are often combined with semifinished products made of 1.4404 or 1.4307.

Stainless steel 1.4429 ESR: Same properties as for 1.4429, but improved microstructure and higher purity due to the ESR (electroslag remelting process) treatment. Exceptionally well suited for vacuum applications. It is used as a premium quality for CF flanges that exhibit great strength and minimal magnetizability, combined with the high chemical purity and homogeneity of the structure.

Stainless steel 1.4571: classic “V4A” steel with high availability. Stabilized with titanium and therefore weldable without being susceptible to intergranular corrosion. Similar properties to 1.4401, however, due to the titanium carbides in the structure, it is only moderately polishable and not suited for electro-polishing. Well suited for vacuum applications. Used for example for piping and apparatus construction, where greater corrosion resistance is required.

ESR (electroslag remelting process): through the ESR process, dense and low-segregation stainless steels, with high chemical and structural purity are produced under controlled, reproducible conditions. The block of a primary melt is electrically remelted in the ESR furnace. One electric pole is on the primary block, the opposite pole on the bottom of the water-cooled crucible. The slag is located between the poles, which is heated up by resistance heating above the melting temperature of the stainless steel. Metal droplets, which are cleansed of nonmetallic impurities when they come in contact with the liquid slag, are constantly released from the bottom of the primary block. Coarse inclusions disappear almost completely when passing through the slag. The remaining inclusions are small and distributed nearly uniformly over the secondary block. The stainless steel cleaned by the ESR process is characterized by an extremely high density and homogeneity.

Austenitic steels have good weldability and are non magnetic as fully austenite. In the annealed condition, they are characterized by very high toughness values that are maintained even at extremely low temperatures. They tend to have a high work hardening ability, in particular with higher carbon content. Parts of the microstructure can transform into deformation martensite. Fully austenitic steel is susceptible to hot crack formation during welding. In many austenitic materials, the chemical composition is adjusted in such a way that a delta ferrite content of up to 10 % is created within the weld metal, which reduces the susceptibility to hot cracking. Therefore, many steels described as austenitic can contain ferritic or martensitic content in the structure, depending on their mechanical or thermal treatment.

Magnetizability: A fully austenitic microstructure is not magnetic. Through the previously described conversion of components of the microstructure into deformation martensite or the formation of delta ferrite, even steels designated as austenitic can become slightly magnetic. For martensite and ferrite are both magnetizable. Through the solution annealing the cold work hardening, and thus the martensite, can be reduced or even reversed. The components of the delta ferrite in the microstructure are essentially dependent on the ratio of the ferrite formers chromium, molybdenum, silicon and niobium to the austenite formers nickel, carbon, nitrogen and manganese. The ferrite content can degrade partially through heat treatment and thereby reduce the magnetizability. Since stainless steels with the same material designation can have different chemical compositions within the described limits, their magnetizability is not a fixed quantity. By plotting the nickel equivalent of the austenite formers against the chrome equivalent of the ferrite formers in a diagram according to DeLong, the austenite and ferrite contents can be seen. In Figure 3.3, for some stainless steels the areas of chromium and nickel equivalents are shown (colored rectangles) and their mean equivalents are given as icons.

Stabilized steels contain titanium and niobium, which binds the outgoing carbon during welding and thus prevents the formation of chromium carbides. The formation of chromium carbides would lead to chromium depletion at the grain boundaries and can make the material susceptible to intergranular corrosion. For welded structures, from sheet thicknesses of approx. 6 mm or above, low-carbon (C ≤ 0.03 %) or stabilized stainless steels should be used. The titanium carbides severely restrict the polishability.

Heat treatments: For austenitic stainless steels, the temperature for solution annealing is about 1,050°C. Due to the risk of chromium carbide formation, which primarily takes place in the temperature range between 600°C and 800°C, and the resulting impairment compared to intergranular corrosion, the temperature range between 900°C and 500°C must be passed through quickly. Finished vacuum components can be annealed under vacuum at a temperature range of 950°C to 1,100°C. The surfaces are purged, in particular, of residual hydrocarbons (purification annealing), while hydrogen bonded in the volume is outgassed (low hydrogen annealing) and the magnetizability decreases (demagnetisation annealing). In addition, any existing chromium carbides dissolve (solution annealing) and stresses in the material resulting from processing are relieved (stress free annealing). However, the heat treatment also however reduces mechanically advantageous hardening. With metal sealed flanges, annealing can lead to an undesirable reduction in the material hardness in the cutting area. The knife edges can then collapse if metal seals are used and lose their function. We therefore recommend flange material 1.4429 ESR for annealing treatments. Its exceptional hardness ensures sufficient hard knife edges.

Corrosion: Corrosion depends on various factors, therefore information on resistance has only an indicative value and is intended for general information. It should facilitate the selection of stainless steel, but does not constitute a guarantee, as it is not readily applicable to the actual operating conditions. For example, increased temperature and concentrations as well as mechanical stress and damage to the surface, all have a corrosion accelerating effect. Moreover, the absence of oxygen prevents the re-formation of the passivating chromium oxide layer, so it lacks corrosion protection. Furthermore, impurities can promote corrosion. In practice, it is usually chlorine ions and other halide ions that cause pitting, crevice and stress cracking corrosion. The passive layer is locally ruptured as a result and the corrosion continues locally. In particular, thin-walled components such as metal bellows are vulnerable to these types of corrosion. If necessary, the service life should be determined by testing. In addition, cooling water presents a risk for components which should not be underestimated. The surfaces surrounded by water must be sufficiently passivated and the cooling water must show the characteristics specified by the manufacturer.

3.2.1.2 Carbon steel

Carbon steel is used in vacuum technology, as long as pressures of less than about 1 · 10^-5 hPa do not need to be created and maintained and corrosion protection is not required. Compared to stainless steel it is a relatively low-priced building and construction material, which has good weldability and is easy to process. However the continuous outgassing of CO and the tendency to corrode from air must be taken into account when this type of steel is used. On the atmosphere side corrosion protection can be provided by painting, while on the vacuum side this can be provided by nickel plating. For tank construction, the steel grades used must be chosen carefully, especially where weldability and tightness are concerned. Boiler construction methods are only transfer- able to a limited extent to vacuum vessel construction. When dimensioning, the stress caused by the external atmospheric pressure must be considered and welding must ensure a vacuum-tight seal. In addition, the tools used must be strictly separated from those that are used for processing stainless steel, to avoid contaminating the stainless steel. The same applies for storing and transporting mild steel and stainless steel. Mild steel is often used for fasteners for flange connections, where the surfaces are zinc, nickel or chrome plated to protect them from corrosion.

3.2.1.3 Aluminum

Aluminum is mainly used in the low and high vacuum range, usually as an alloy, in special cases it is also used as pure aluminum. Components such as ISO-KF pipe components are often made of cast aluminum alloys with reworked flange faces. When selecting the material, shrinkage and porosity must be taken into consideration. For centering and supporting sealing rings, the components are made from bar stock. For metallic seals in the form of profile seals or wires, annealed aluminum-silicon alloys are preferred.

The vapor pressure of aluminum is low, and is only about 6 · 10^-9 hPa at the melting point of 660°C. The large thermal expansion, the high thermal conductivity and stable aluminum oxide layer make it difficult to weld aluminum. There is a risk of the formation of pores and cracks, combined with a with great distortion. Uniform heating before welding reduces these risks. However, in practice this is often not possible.

Aluminum is not magnetizable. Aluminum flange connections can only be used for metallic sealing UHV connections to a limited extent, as their hardness is often too low. Although special bimetal flanges, consisting of an aluminum base and a stainless steel plating or aluminum flanges with hardened sealing surfaces, have been developed, their use often fails due to the relatively high price, critical processability or the limited application possibilities.

To increase their abrasion resistance, e. g. for use in cleanrooms or to increase the corrosion protection, the aluminum surfaces are often anodized. This results in a several µm thick, porous oxide layer, which is of only limited suitability for vacuum applications. Gas molecules increasingly become deposited on such surfaces leading to high desorption rates. In addition, gas molecules on the surface area can tunnel under seals and create leaks. There are various anodic treatments available. In deciding whether and, if so which method, to use, the limitations must be considered and weighed against the benefits.

3.2.2 Sealing materials

3.2.2.1 Elastomer seals

Elastomer seals are basically gas permeable. The overall process of gas penetration is called gas permeation and is dependent on the material, the type of gas and the ambient conditions - especially the temperature. In addition, elastomers are subject to outgassing. In vacuum-compatible elastomers, the outgassing rate decreases and after a sufficiently long evacuation time, the permeation dominates and with constant ambient conditions, a constant gas rate penetrates the seal. Permeation and outgassing are dependent on the diffusion. A high gas tightness causes slow outgassing, leading to a long period of time until a steady permeation gas flow sets in. The time period can be as long as several hundred hours, which can be greatly accelerated by the baking out method. If the vacuum system has no major sources of gas, for example, due to desorption or leaks, elastomeric seals can significantly determine the final pressure. Example: the material FKM (fluoroelastomer) has a low gas permeability for air. For a seal with a nominal diameter of DN 500 ISO-K, the permeation of atmospheric air with 60 % humidity is approximately 4 · 10^-7 Pa · m³/s. In vacuum systems with FKM seals, an operating pressure greater than 1 · 10^-8 hPa is therefore rarely achieved.

Elastomer seals can be reused several times, if properly used. They require a contact force of several N/mm², which is already exceeded by sufficiently large O-ring seal diameters due to the atmospheric air pressure acting on the flange.

Elastomer seals can change their properties due to poor storage conditions or improper handling. Factors that must be avoided are: UV radiation, oxygen, ozone, heat, moisture, solvents or excessively large deformations. To preserve their properties, we recommend the following ambient conditions during storage:



During use, they lose their elasticity partially due to cyclic loads, long service life in the deformed state, high temperature or aging. If they are used for too long they can even become brittle. Therefore, elastomer seals have to be replaced regularly. Since the operating conditions are very diverse, no general statement can be made regarding the durability. If there is no empirical data available for the elastomers used under the respective process conditions, their service life may need to be determined by experiments.

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.