Vacuum-Compatible Materials — Metals
Estimated time: 20–25 minutes
Learning Outcome: Identify the primary metals used in vacuum systems; explain why each is chosen and what its limitations are. Competency: M04-COMP-01 (M04-IND-01.01), M04-COMP-02 (M04-IND-02.01)
Orient
A vacuum chamber must do several things at once: hold vacuum against atmospheric pressure (roughly ten tonnes per square metre of surface), resist corrosion, release minimal gas under vacuum, and be cleanable to molecular standards. Not many materials meet all these requirements.
Core Content: The Big Three Metals
Stainless Steel (Type 304 and 316)
Why it's the standard: Stainless steel is the workhorse of vacuum technology. It's strong, corrosion-resistant, weldable, and — critically — it can be polished to a smooth surface that minimises gas adsorption.
Vacuum properties:
- Low outgassing rate after proper cleaning and bake-out
- Can be electropolished — an electrical process that smooths metal surfaces at a microscopic level, like wet-sanding with an invisible abrasive controlled by electricity — to reduce surface area and water adsorption
- Withstands bake-out temperatures (up to 450°C for UHV applications)
- Non-magnetic (important for some scientific applications)
Where you'll find it: Chamber bodies, flanges, tubing, valve bodies, internal fixtures. On R1-A, the chamber (R1-CH) and the valve bodies (R1-V-ISO, R1-V-VENT) are stainless steel.
Limitations: Heavy. Expensive compared to aluminium. Type 304 can be sensitised (lose corrosion resistance) if improperly welded — Type 316 is more resistant but costs more.
Key insight for diagnostics: A stainless steel chamber that has been properly cleaned and baked out will have very low outgassing. If you see elevated outgassing from a stainless steel system, suspect contamination (fingerprints, cleaning residue, oil) rather than the metal itself.
Aluminium (6061 and 6063 alloys)
Why it's popular: Aluminium is lightweight, easy to machine, and relatively inexpensive. For rough vacuum and moderate high-vacuum applications, it's an excellent choice.
Vacuum properties:
- Higher outgassing rate than stainless steel (aluminium forms a porous oxide layer — a surface full of microscopic holes that acts like a sponge, trapping water molecules inside)
- Cannot be baked above ~150°C without softening (limits its use in UHV)
- Good thermal conductivity (useful for temperature-controlled applications)
- Easy to machine complex shapes (lowers manufacturing cost)
Where you'll find it: Rough vacuum chambers, KF fittings, lightweight fixtures, vacuum frames, optical tables. In many teaching and prototyping environments, aluminium chambers are standard.
Limitations: The porous aluminium oxide surface adsorbs water aggressively — aluminium systems take longer to pump down than equivalent stainless steel systems. Not suitable for ultra-high vacuum without special surface treatments.
Key insight for diagnostics: If an aluminium system consistently shows extended pump-down times compared to specification, the cause may be the material itself (water trapped in the oxide layer) rather than a leak or contamination.
Copper and Copper Alloys
Why it's used: Copper has excellent thermal conductivity and is used for specialised applications — most notably as the gasket material in CF (ConFlat) flanges, where it provides a metal-to-metal seal.
Vacuum properties:
- Very low outgassing when clean
- Soft enough to deform and create a metal seal under compression (critical for CF gaskets)
- Excellent thermal conductivity (used in thermal management and cryogenic applications)
Where you'll find it: CF gaskets (OFHC copper — Oxygen-Free High-Conductivity copper, an ultra-pure copper with virtually no oxygen impurities; the purity matters because even trace oxygen would compromise sealing reliability), thermal straps, cryogenic components, and some specialised chamber liners.
Limitations: Not strong enough for structural applications. Oxidises readily in air (copper gaskets must be handled carefully to avoid contamination).
Checkpoint — What You've Gained So Far
You can now identify the three primary vacuum metals (stainless steel, aluminium, copper) and explain why each is chosen for specific applications. The comparison table below summarises their key properties side by side.
Materials Comparison
| Property | Stainless Steel | Aluminium | Copper |
|---|---|---|---|
| Outgassing (after cleaning) | Low | Moderate (oxide layer) | Very low |
| Max bake-out temp | 450°C | ~150°C | 450°C |
| Strength | High | Moderate | Low |
| Weight | Heavy | Light | Heavy |
| Cost | Moderate–High | Low–Moderate | Moderate |
| Primary vacuum use | Chambers, flanges, valves | Rough-vac chambers, fittings | CF gaskets, thermal |
Visual Reference: Common Vacuum Materials
The following photograph grid illustrates the three metals described above. Pay particular attention to surface finish differences — the electropolished stainless steel surface appears mirror-smooth, while the machined aluminium shows visible tool marks and a matte oxide layer.
[VIS-M04-001] Textbook Reference
See Basic Vacuum Practice, Ch. 6 (Vacuum Materials & Hardware), pp. 149–175
Flange types and fitting assemblies — KF exploded view, ISO-K, and CF flange cross-sections with component identification
Notice that the stainless steel surface is far smoother than the aluminium — this reduced surface area is precisely why stainless steel adsorbs less water and outgasses less under vacuum. The copper gasket's soft, uniform surface is what allows it to deform under knife-edge compression — a knife-edge is a sharp ridge machined into the flange that bites into the soft copper, creating a metal-to-metal seal tight enough for ultra-high vacuum — to create a UHV-grade seal.
What You Can Now Do
By the end of this section, you can:
- Identify stainless steel, aluminium, and copper as the primary vacuum metals
- Explain why stainless steel is the standard for high-performance vacuum systems
- Describe the outgassing trade-offs of aluminium (lighter and cheaper, but more water adsorption)
- Explain why copper is used for CF gaskets (soft, low outgassing, deforms under compression)