Thermal grease is often treated as a simple supporting material in electronics—something that “just fills the gap” between a CPU and a heatsink. But in vacuum or low-pressure environments, this seemingly simple compound becomes a far more complex engineering material.
In reality, thermal grease is not just a heat conductor. It is a chemically formulated composite, usually based on silicone oil, synthetic base fluids, and thermally conductive fillers. And under vacuum conditions, those base components can behave in ways that directly affect system reliability.
This is where the concept of volatilization and outgassing becomes critical.
What Happens to Thermal Grease in a Vacuum?
In a normal atmosphere, Wärmeleitpaste remains stable because surrounding air pressure helps suppress evaporation. But once pressure drops—such as in aerospace, satellites, vacuum chambers, or high-end optical systems—the physics changes.
Under vacuum, low-molecular-weight compounds inside the grease begin to escape. This phenomenon is known as Ausgasen, and it is essentially a slow release of volatile molecules from the material into the environment.
Scientific studies and industrial references consistently show that:
- Vacuum environments accelerate evaporation of volatile components
- Silicone-based materials often contain trace low-molecular-weight siloxanes
- These molecules migrate and evaporate more easily under low pressure conditions
- The result is contamination and long-term performance degradation
Vereinfacht ausgedrückt: thermal grease does not “boil” like water, but it slowly releases internal compounds into the vacuum space.
Outgassing vs Volatilization: What’s the Difference?
These two terms are often confused, but in vacuum engineering they are distinct:
- Volatilization: the physical transition of molecules into gas phase (evaporation-like behavior)
- Ausgasung: the broader process of trapped or embedded molecules being released from a material over time
In thermal grease, both occur simultaneously.
Vacuum grease and thermische Grenzflächenmaterialien (TIMs) are specifically evaluated based on their vapor pressure, which determines how easily they release molecules. If vapor pressure is too high, the material effectively becomes a contamination source inside the vacuum chamber.
Why Silicone-Based Thermal Grease Is Especially Sensitive
Most conventional thermal greases use silicone oil as the base fluid because it provides:
- Good thermal stability
- Wide operating temperature range
- Non-conductivity
- Low cost and easy processing
However, silicone chemistry has a known weakness in vacuum systems: the presence of low-molecular-weight siloxanes.
These small molecules can:
- Evaporate under heat and vacuum
- Condense on optical surfaces (forming haze or film)
- Contaminate electrical contacts
- Increase maintenance cycles in sealed systems
This is why aerospace-grade materials often undergo vacuum distillation, post-curing, or special formulation control to reduce volatile fractions.
Why Vacuum Makes Everything Worse
In atmospheric conditions, even if a small amount of grease evaporates, it is quickly diluted by air.
In vacuum conditions, there is no such buffer.
Stattdessen:
- Molecules released from grease freely travel
- They migrate toward colder surfaces
- They condense on sensitive components
- They accumulate over time
This creates what engineers often call a “contamination loop” inside sealed systems.
A well-documented issue in vacuum engineering is that once contamination begins, it can gradually degrade:
- Optical clarity (lenses, sensors, cameras)
- Electrical reliability (contact resistance increase)
- Thermal performance (surface film reduces heat transfer efficiency)
Thermal Grease vs Low-Outgassing Materials
Not all thermal greases behave the same way in vacuum environments.
Advanced formulations now exist specifically for aerospace, semiconductor, and precision optical industries.
These materials aim for:
- Extremely low Total Mass Loss (TML)
- Minimal Collected Volatile Condensable Materials (CVCM)
- Reduced siloxane content
- High cross-link density silicone or non-silicone bases
Some engineered TIMs even replace silicone oils entirely. Non-silicone thermal greases, for example, are designed to eliminate silicone volatilization risks while maintaining conductivity.
This shift is significant because it directly addresses the root problem: molecular mobility under vacuum conditions.
Does Thermal Grease “Evaporate” Completely?
A common misconception is that thermal grease disappears in vacuum.
It does not—at least not in a short timeframe.
Instead, the behavior is gradual:
- Light volatile compounds leave first
- Base oil migration occurs over time
- Thickener and filler remain relatively stable
- Performance slowly degrades rather than failing instantly
So the correct interpretation is:
Thermal grease does not fully evaporate in vacuum, but it continuously loses volatile components that can compromise system cleanliness and reliability.
Engineering Implications in Real Applications
The practical impact depends heavily on the application.
Unterhaltungselektronik
Vacuum is not a concern. Outgassing is negligible in normal use.
Industrial Electronics
Moderate concern in sealed enclosures or high-temperature operation.
Aerospace and Space Systems
Critical concern. Outgassing control becomes a design requirement, not an option.
In fact, NASA and aerospace-grade specifications exist specifically to qualify materials for vacuum compatibility, ensuring they meet strict outgassing thresholds.
Why Material Selection Matters More Than Thickness
A common misconception in thermal design is that performance depends only on thickness or conductivity.
In vacuum environments, chemical stability matters just as much—if not more—than Wärmeleitfähigkeit.
Even a highly conductive grease can become problematic if it:
- Releases volatile oils
- Condenses on optical sensors
- Alters vacuum pressure stability
- Forms insulating films over time
This is why low-outgassing TIMs are often preferred even if their thermal conductivity is slightly lower.
HakTak Perspective: Engineering for Stable Thermal Interfaces
Unter HakTak, thermal interface materials are designed with both thermal performance and environmental stability in mind. In vacuum-related applications, formulation control is not optional—it defines system reliability.
Key design considerations include:
- Reducing low-molecular-weight silicone fractions
- Selecting thermally stable filler systems
- Improving cross-linked polymer networks
- Controlling batch-level volatile residue
These adjustments may seem subtle, but they determine whether a material performs reliably in normal electronics—or survives in aerospace-grade vacuum conditions.
Kurzes Fazit
Thermal grease does not instantly evaporate in vacuum, but it does outgas, releasing volatile components over time. This process can lead to contamination, reduced thermal performance, and reliability risks in sensitive systems. In low-pressure environments, material formulation becomes just as important as thermal conductivity, making low-outgassing or vacuum-compatible TIMs essential for long-term stability.
FAQs
Does thermal grease fully evaporate in vacuum?
No. It gradually releases volatile compounds but does not fully disappear quickly.
Why is outgassing a problem?
Because released molecules can contaminate optics, electronics, and vacuum systems.
Is silicone thermal grease safe in vacuum?
Only specially formulated low-outgassing grades are suitable.
What is the main cause of outgassing?
Low-molecular-weight oils and additives inside the grease.
How can outgassing be reduced?
By using low-outgassing materials, post-curing, and vacuum-compatible formulations.