Application of Ceramic Fiber-Filled Metal O-Rings in High-Temperature Sealing Technology

Introduction

O-rings, as a common static sealing element, are widely used in flange connections, valves, and pressure vessels. At room temperature, rubber or polymer O-rings suffice; however, in high-temperature (>500°C) or extreme environments (such as vacuum, high pressure, or corrosive media), metal O-rings are required. Metal O-rings are typically hollow structures (e.g., C-type or E-type cross-sections) to provide necessary elastic deformation and resilience. Nevertheless, performance degradation of pure metal structures at ultra-high temperatures (>800°C) has become a bottleneck.

To address this issue, the industry has introduced ceramic fiber filling technology. This composite design fills high-purity ceramic fibers (such as alumina-silicate fibers) inside a metal shell, forming a “hard shell + soft core” structure. It retains the corrosion resistance and shape stability of the metal while leveraging the high-temperature elasticity and low creep of ceramic fibers to significantly enhance overall sealing performance. This article analyzes its core mechanisms and technical advantages in depth.

Limitations of Pure Metal O-Rings

Pure metal hollow O-rings (e.g., made of high-temperature alloys such as Inconel 718 or Hastelloy C-276) rely on the elastic modulus and yield strength of the metal itself to maintain sealing stress. However, under high-temperature conditions, metal materials face the following challenges:

1. Creep and Stress Relaxation: At high temperatures, atomic diffusion in metals intensifies, leading to creep. Sealing stress decays over time; typically, Inconel alloys exhibit creep rates >10^{-5}/h at 700–900°C, causing permanent deformation and leakage risk.
2. Resilience Decay: The Young’s modulus of metals decreases with rising temperature. For example, stainless steel retains only about 50% of its room-temperature modulus at 1000°C, preventing the O-ring from recovering its original shape during thermal cycling and resulting in uneven contact on the sealing surface.
3. Poor Adaptability to Surface Irregularities: Under low bolt preload, pure metal O-rings struggle to fill microscopic defects on flange surfaces (e.g., roughness Ra > 3.2 μm), especially prone to gas leakage in vacuum environments.
4. Limited Temperature Upper Limit: Most pure metal O-rings have a continuous operating temperature not exceeding 900°C. Beyond this range, oxidation, grain coarsening, and fatigue failure accelerate.

These limitations are particularly pronounced in extreme conditions (e.g., rocket engine combustion chambers or nuclear reactor cooling systems), prompting the development of composite material solutions.

Principle and Performance Improvements of Ceramic Fiber Filling

The core of ceramic fiber-filled metal O-rings lies in compactly filling high-purity ceramic fibers (e.g., Al₂O₃-SiO₂ composite fibers, fiber diameter 5–10 μm, density 2.5–3.0 g/cm³) inside a tubular metal shell. The shell is typically made of high-temperature alloys (e.g., Inconel X-750), with a thickness of 0.5–1.0 mm, providing mechanical protection and shape constraint. Filling is achieved via high-pressure forming or vacuum impregnation to ensure uniform fiber distribution.

Working Principle

During installation, the O-ring is compressed, and the internal ceramic fibers provide the primary elastic support. The sealing stress can be approximately described by:

$\sigma_s = \frac{F_p}{A_c} + k_f \cdot \delta$

where $\sigma_s$ is the sealing stress, $F_p$ is the preload force, $A_c$ is the contact area, $k_f$ is the effective fiber stiffness, and $\delta$ is the compression deformation. Compared to pure metal, ceramic fibers maintain a more stable $k_f$ at high temperatures, as their glass transition temperature (Tg) exceeds 1400°C with virtually no creep.

Key Performance Improvements

1. High-Temperature Resilience Maintenance: The elastic modulus of ceramic fibers remains >100 GPa even at 1200°C, while the metal shell plays only an auxiliary role. Even if the shell softens, the fiber core provides continuous recovery force, achieving resilience rates >95% after thermal cycling.
2. Extended Temperature Upper Limit: The composite O-ring supports continuous operation at 1100–1400°C, far exceeding pure metal. The low thermal conductivity of the fibers (<1 W/m·K) helps reduce thermal bridging and improves thermal insulation.
3. Enhanced Adaptability: Fibers offer 20–40% compressibility, effectively filling surface defects. At low preload (<10 MPa), leakage rates can be controlled below 10^{-9} Pa·m³/s, suitable for highly deformed flange systems.
4. Creep Suppression: Fiber creep rate at high temperature is <10^{-8}/h, extending the stress relaxation time constant of the overall assembly to thousands of hours.
5. Vacuum and Media Compatibility: In ultra-high vacuum (<10^{-6} Pa) or corrosive gas environments (e.g., HF, Cl₂), fiber filling reduces gas permeation paths and improves seal integrity.

Additionally, the design offers vibration and impact resistance, suitable for dynamic sealing applications.

Material Selection and Manufacturing Considerations

Material Selection

• Metal Shell: Prefer Inconel 625 or 718 (oxidation resistant, strength >1000 MPa at 800°C).
• Ceramic Fiber: High-purity Al₂O₃ (>99%) fibers, temperature resistance >1300°C; avoid boron-containing fibers for nuclear radiation compatibility.
• Fill Density: 80–90% volumetric fill rate to ensure elasticity without excessive stiffness.

Manufacturing Process

1. Metal tube forming: Precision extrusion or welding into hollow rings.
2. Fiber filling: High-pressure injection or winding method.
3. Surface treatment: Silver or gold plating to enhance conductivity and corrosion resistance (suitable for semiconductor vacuum furnaces).
4. Testing standards: Refer to API 6A or ASME B16.20, including helium leak testing and thermal cycling validation.

Potential challenges include fiber fracture risk (requires optimized filling pressure) and higher cost (composite O-rings cost 2–3 times more than pure metal).

Application Scenarios and Performance Comparison

Ceramic fiber-filled metal O-rings have been validated in multiple high-end fields. The table below compares performance of different O-ring types under typical parameters:

For example, in SpaceX’s Raptor engine, such seals are used in combustion chamber flanges to ensure no leakage in oxidizing environments >1000°C. In nuclear power, they are applied in high-temperature gas-cooled reactor (HTGR) cooling loops, significantly reducing maintenance frequency.

Conclusion

Ceramic fiber-filled metal O-rings effectively compensate for the elastic deficiencies of pure metals at ultra-high temperatures through composite material design, achieving revolutionary improvements in sealing performance. This technology not only extends the temperature limit but also enhances system reliability and adaptability. With advancements in materials science (e.g., nano-reinforced fibers), its applications will further expand to even more extreme environments. Engineers should consider operating conditions, cost, and compatibility when selecting to optimize design solutions.

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