The presence of bubbles in low-hydrogen silicone oil during use is a common but unavoidable problem, especially in space-grade applications where reliability is paramount. Bubbles can severely impact performance, and their root cause must be systematically analyzed and completely eliminated.
The dangers of bubbles
In space applications, the presence of bubbles is fatal:
Impacting lubrication and mechanical performance: At lubrication points, bubbles can cause discontinuous oil films, resulting in "dry friction" and localized overheating, increased wear, and even seizure of moving parts.
Reducing dielectric strength: While bubbles themselves are excellent insulators, their presence significantly reduces the overall dielectric strength of silicone oil, potentially causing localized discharge or electrical breakdown, threatening spacecraft electronic systems.
Altering physical properties: Bubbles can significantly alter the volume and compressibility of silicone oil, affecting its accuracy and stability as a damping fluid or pressure transmission medium.
A vicious cycle in vacuum environments: In the vacuum of space, entrained gases rapidly escape, causing not only bulk contamination but also potentially boiling and spraying the silicone oil, leading to complete performance failure.
Main Causes and Solutions for Bubble Generation
The key to solving bubble problems lies in "prevention first, elimination second."
1. Air Entrainment During Operation (Mechanical Entrainment)
Cause: This is the most common cause. Vigorous mixing, pouring, filling, and especially pumping can easily entrain air, causing it to shear and disperse into tiny bubbles, which can then be mixed into the viscous silicone oil.
Solution:
Optimize operation procedures: Adopt slow, steady mixing and filling methods. Avoid pouring from high altitudes. It is recommended to use a cannula to slowly inject along the container wall.
Use Specialized Equipment: Use vacuum filling equipment. During the filling process, simultaneously evacuate the oil storage tank and the components to be filled (such as gearboxes and dampers). Filling is completed under negative pressure, which fundamentally eliminates air entrapment.
Design Proper Flow Paths: The piping system should be designed to flow smoothly, avoiding sharp bends and narrow areas to minimize turbulence.
2. Trace Moisture and Low-Boiling Compounds in the Material or Container
Cause: The silicone oil itself may contain trace amounts of moisture, or moisture may be adsorbed on the surfaces of containers and tools. Under vacuum or high temperature conditions, this moisture and low-boiling compounds vaporize, forming numerous tiny bubbles.
Solution:
Pre-treat the silicone oil: Before use, vacuum-degas the silicone oil. Place the silicone oil in a vacuum drying oven at a high temperature (e.g., 80-100°C, below its flash point) and high vacuum for several hours to fully remove dissolved and entrained trace gases, moisture, and volatile components.
Ensure a dry environment and tools: All containers, tools, and components that come into contact with the silicone oil must be thoroughly dried in an oven to remove adsorbed water. The operating environment should maintain low humidity.
3. Chemical Reaction Gas Generation
Reason: Although low-hydrogen silicone oil is chemically stable, under extreme conditions (such as high temperatures or in the presence of metal impurities), the Si-H bond may react with trace amounts of water or oxygen, producing hydrogen.
≡Si-H + H₂O → ≡Si-OH + H₂↑
≡Si-H + O₂ → ... → Gas generation may occur
Solution:
Strictly control raw material purity: Use high-purity space-grade silicone oil.
Avoid contamination: Ensure a clean working environment and prevent contact with incompatible chemicals (strong acids, strong bases, and certain catalysts). Strictly isolate the fluid from air and moisture during storage and handling.
Choose appropriate materials: Avoid using metals such as copper and zinc that may catalyze Si-H bond reactions as contact parts or containers.
Post-processing: If bubbles have already formed
If bubbles already exist in silicone oil, it must not be used directly in space components. Rework is necessary:
Standing: For larger bubbles, allowing them to slowly rise and burst can be achieved by standing for a long time. However, this method is inefficient and ineffective for smaller bubbles.
