Choosing the Right Vacuum Materials

Considerations to ensure a successful system design. Know the outgassing rates of selected materials…

Approximate outgassing rates to use for choosing vacuum materials or calculating gas loads

(All rates are for 1 hour of pumping)

A fundamental challenge in vacuum technology is the vacuum chamber. The materials used must minimize gas load while maintaining sufficient strength to resist external atmospheric pressure. While increasing wall thickness or incorporating additional internal or external supports can address strength concerns, the primary challenge lies in evaluating potential gas loads. Vacuum chambers are typically made from metals, glass, ceramics, or plastics. A common factor among these materials is the presence of sorbed water molecules on the internal surfaces, which must be desorbed during the pumping process.

The majority of water molecules tend to adhere to one another in a cluster, making the choice of base material relatively insignificant until a sufficient amount of water has desorbed, leaving only a single monolayer on the surface. Now, the requirements shift. It becomes essential to select a material that does not form strong bonds with the water molecules. This consideration eliminates many plastic options, but more critically, it necessitates an evaluation of gas emissions from the material’s bulk. This phenomenon, along with surface desorption, is referred to as outgassing. Additionally, there is the permeation of gases from the surrounding atmosphere through the walls of the chamber, which includes sealant and gasket materials like elastomer O-rings.

In addition to evaluating strength and permeation, the same standards should be applied to any material subjected to a vacuum environment. While permeation is a phenomenon that affects most materials, its impact is often negligible. For instance, the minimal amount of atmospheric helium that seeps into a Pyrex bell jar through the glass’s minute micropores is insignificant at pressures of 10^-6 to 10^-7 torr, but it could pose a significant issue in a Pyrex system designed to function at 10^-11 torr.

Generally, the assessment of materials in a vacuum primarily focuses on the outgassing rate. Furthermore, gas loads can result from the vaporization of the material itself or its components. Materials with high vapor pressure present a clear risk of contaminating the vacuum environment and reducing the ultimate pressure obtainable.

Metals are the most prevalent vacuum chamber materials, with stainless steel (SS) far ahead of other metals such as mild steel (MS) or aluminum (Al) alloys. Mild steel is usually used only for systems that require moderate vacuums of 10^-6 torr. The obvious choice is between SS or Al. Other considerations when deciding to use SS doesn’t mean any and all SS alloys. Free-machining alloys such as 303 SS contain sulfur (S), but the vapor pressure of sulfur is too high for high vacuum systems. Better choice is 304 SS. For ultrahigh vacuum (UHV) applications, usually requires the low-carbon 304L alloy as a best choice.

The material should have a surface finish designed to for smoothness to result in the lowest desorption rate per unit area. Then the surface cleaning needs to ensure that all organic contamination is removed and that no porous welding scale remains. This is absolutely required to assure that the minimum gas load is contributed from the material. If Al is chosen, make sure the surface hadn’t been anodized since the oxide film absorbs large quantities of water vapor and then slowly desorbs them into the vacuum environment.

The choice of sealing materials and gaskets are another extremely important consideration because the gas loads from elastomer O-rings can be greater than those from the chamber’s surface. This means using vacuum-baked O-rings that have been carefully handled with lint-free gloves. Do not use solvents to clean elastomer. Solvents are absorbed and cause swelling. Swelling increases outgassing and atmospheric permeation of the elastomer. If feasible, consider using metal gaskets and avoiding the O-rings entirely.

Other materials to be used in a vacuum might include ceramics, glasses and even plastic substrates. All have to be evaluated to ensure that the gas loads are as small as possible.

Ceramics are generally considered to be good vacuum materials, but only if high-density sintered materials are used. This differentiates between the insulator in a UHV-rated feedthrough and others. Porous materials contain massive amounts of gas. Even normally acceptable metals such as Al need to be looked at carefully. Avoid household Al foil. This material can be coated with peanut oil used as a lubricant and it is virtually impossible to remove with solvent cleaning.

In addition to gas load effects, the physical properties of materials need consideration. Internal arrays are often assembled with SS nuts and bolts that are likely to become heated by the process. This results in extreme surface galling that can make it impossible to disassemble the pair, but a thin coating of Milk of Magnesia (unflavored) brushed on the threads before assembly makes disassembly easy. Sliding surfaces can also cause galling or sticking problems with pairs such as SS-to-SS, but a coating of molybdenum disulfide can act as a vacuum-compatible lubricant. Vented hardware is also available to release trapped gas from threads. These are only a few of the many examples of the materials problems that need to be considered, and a successful system requires a full analysis.

There are a number of materials that need to be avoided whenever possible. High vapor pressure metals can be a problem, and they can sneak in easily if close attention isn’t given. Zinc and cadmium-plated nuts and bolts are a prime example. If these materials become heated during the process, they can sublime within the system to cause metallic contamination. Additionally, they can form thin oxide coatings that sorb large quantities of water vapor. The zinc content of brass is often a problem in any but the most non-stringent requirements. Any material, then, that might vaporize under vacuum needs to be treated with suspicion. This includes many plastic materials.

Check out our online store for a variety of stainless steel (SS) and aluminum vacuum component products.

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