High vacuum grease is a specialized sealing and lubricating compound engineered for exceptionally low volatility. In a vacuum, ordinary greases evaporate, contaminating the chamber and raising base pressure. A high vacuum grease is formulated to stay put, holding a seal at ground-glass joints, O-rings, and rotary fittings without outgassing into the system.
The Defining Property: Low Volatility
Every material releases trapped and dissolved gases when the surrounding pressure drops. A conventional grease contains light hydrocarbon fractions and additives that boil off quickly under vacuum, coating optics and sensors and preventing the system from reaching a deep vacuum.
High vacuum grease starts from stable, high-molecular-weight base fluids, most often silicone or, for solvent resistance, perfluorinated polyether. The product is refined to strip volatile fractions, so its vapor pressure at room temperature is extremely low and its rate of mass loss under vacuum is negligible over long service intervals.
Key Properties Beyond Volatility
- Wide thermal range: silicone-based vacuum greases typically hold their consistency from around minus 40 degrees Celsius to over 200 degrees, so a joint stays sealed through bake-out and cool-down.
- Chemical inertness: the base fluids resist reaction with common process gases, acids, and bases, and perfluorinated grades add resistance to oxygen service and aggressive solvents.
- Electrical insulation: most vacuum greases are strong dielectrics, useful for sealing electrical feedthroughs.
- Water repellency: a thin film fills microscopic surface roughness where leaks originate and blocks moisture ingress.
Where High Vacuum Grease Is Used
Vacuum systems are the primary application: sealing demountable joints, lubricating stopcocks and valves, and dressing O-rings on chamber doors and flanges to fill surface imperfections. It is also used on scientific instruments, where moving parts must be lubricated without contaminating a sensitive beam path or detector, and in semiconductor and coating equipment, where any deposited residue can spoil a process run.
Outside vacuum work, the same low-volatility, wide-temperature, chemically inert film makes these greases useful for valves and fittings in chemical processing and for sealing and protecting connectors in equipment that sees drastic temperature changes.
Email Us with your base pressure, temperature range, and chemical exposure for guidance on selecting a grade.
Choosing the Right Grade
Start with the vacuum level. For rough and medium vacuum a general silicone grease is adequate; for high and ultra-high vacuum, choose a grade with a published vapor pressure low enough to protect your base pressure, and plan for a bake-out procedure.
Match the temperature range to the full cycle the joint sees, including any bake-out. Confirm chemical compatibility with process gases and cleaning solvents, since a silicone grease attacked by a solvent will migrate and fail. Where the sealed joint also experiences thermal cycling, differential expansion between a glass or ceramic part and a metal fitting works the grease film, a stress mechanism related to how a CTE mismatch causes bonded and sealed joints to fail.
For assemblies that combine vacuum service with high external temperatures, a high-emissivity ceramic coating on the housing exterior can manage surface heat independently of the seal.
High Vacuum Grease Versus Other Sealing Options
Grease is not the only way to seal a demountable joint, and choosing it deliberately matters. An elastomer O-ring in a well-machined groove seals without any grease and is preferred for joints that are opened often, with a trace of grease used only to lubricate and protect the elastomer. Metal seals such as copper gaskets are used for ultra-high vacuum and bake-out temperatures that would degrade any grease.
High vacuum grease earns its place on ground-glass joints, stopcocks, rotary feedthroughs, and irregular or lightly loaded joints where an O-ring cannot be fitted, and as a thin dressing on O-rings to fill surface roughness and ease assembly. It is also the practical choice for temporary or frequently reconfigured laboratory apparatus.
The failure mode to avoid is using grease as a substitute for a proper sealing surface. If a joint leaks without grease because it is scratched or out of round, repair the surface rather than packing it with compound.
Application in Brief
Clean both mating surfaces of old grease and debris. Apply a thin, translucent film, not a thick layer, since excess grease traps air, attracts particulates, and increases the outgassing load. Assemble with a gentle twisting motion to spread the film evenly, then wipe away any grease squeezed out of the joint.
Key Takeaways
High vacuum grease is a low-volatility sealing and lubricating compound built on stable silicone or perfluorinated base fluids. Its purpose is to seal demountable joints and lubricate moving parts without evaporating into the vacuum and contaminating the system.
Select by vacuum level, temperature range, and chemical compatibility, apply it as a thin film, and keep the quantity minimal. Used correctly, it protects both the integrity of the vacuum and the life of the seals.
To match a vacuum grease to a specific system, Contact Our Team.
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