Ceramic-to-metal seals (typically alumina brazed to Kovar or stainless steel) offer superior mechanical strength, higher temperature capability, and better resistance to thermal cycling than glass-to-metal seals. Glass-to-metal seals (borosilicate or matched glasses bonded to Kovar or other alloys) are less expensive, well-characterized electrically, and adequate for moderate-temperature and pressure environments. For UHV systems, harsh environments, and applications requiring repeated thermal cycling, ceramic-to-metal seals are generally preferred.
How Ceramic-to-Metal Seals Are Made
Ceramic-to-metal seals are fabricated by metalizing the surface of a ceramic (most commonly 94–96% alumina) using a molybdenum-manganese (Mo-Mn) sintering process, and then brazing the metalized ceramic to a metal component with a filler metal such as copper, silver-copper, or a titanium-based alloy. The result is a fully metallurgical joint with no organic material in the seal path.
The thermal expansion coefficients of the ceramic and metal must be closely matched or carefully engineered. Kovar (a nickel-iron-cobalt alloy) is widely used as the metal component because its thermal expansion coefficient closely tracks that of alumina over a broad temperature range. Stainless steel is used in some designs, particularly where corrosion resistance or compatibility with UHV bakeout is the priority.
How Glass-to-Metal Seals Are Made
Glass-to-metal seals bond a glass directly to a metal by heating both to the glass softening temperature and allowing the glass to wet and adhere to the metal surface. Two primary approaches are used: matched seals, in which the glass and metal have nearly identical thermal expansion coefficients, and compression seals, in which the metal shrinks more than the glass on cooling, placing the glass in compression and making the seal robust under internal pressure.
Borosilicate and aluminosilicate glasses are commonly used. Kovar and other controlled-expansion alloys are typical metal partners. Glass-to-metal seals are a mature, cost-effective technology used in vacuum tubes, electronic packages, sensors, and some feedthrough designs.
Key Technical Differences
Mechanical Strength
Ceramic is significantly stronger in flexure and compression than most glass compositions. Ceramic-to-metal joints can withstand higher bolt-up forces, vibration loads, and pressure differentials. Glass-to-metal seals are more susceptible to fracture under mechanical shock or if bolt torque is applied unevenly.
Temperature Capability
Ceramic-to-metal seals can survive continuous operating temperatures exceeding 400°C and short excursions to higher temperatures in inert environments. Glass-to-metal seals are generally limited to lower service temperatures, and the glass itself can become a conduction path at elevated temperatures. For UHV bakeout at 250°C or higher, ceramic-to-metal designs are the more robust choice.
Thermal Cycling Resistance
Repeated cycling from cryogenic temperatures to elevated operating temperatures stresses every dissimilar-material joint. Ceramic-to-metal brazed joints tolerate more thermal cycles before fatigue crack initiation than glass-to-metal bonds, which can develop micro-cracks at the glass-metal interface under repeated cycling.
Electrical Properties
Both seal types provide high electrical isolation between conductors and the metal shell. Glass typically offers volume resistivities above 1012 ohm·cm at room temperature. High-purity alumina ceramic has similar or higher resistivity and maintains better isolation at elevated temperatures where glass begins to conduct. For RF feedthroughs or high-frequency applications, the dielectric properties of the specific ceramic or glass formulation must be evaluated against frequency and temperature requirements.
Outgassing
Both alumina ceramic and borosilicate glass have low outgassing rates when properly fired and cleaned. Ceramic components are typically baked at higher temperatures during manufacturing, which can drive off more adsorbed species and result in a cleaner surface for UHV. Glass can contain trace amounts of water that are released slowly, which may affect base pressure in the most demanding XHV applications.
Cost
Glass-to-metal seals are generally less expensive to fabricate than ceramic-to-metal brazed assemblies, which require precision metallization, controlled-atmosphere brazing, and tighter dimensional tolerances. For high-volume or cost-sensitive applications with moderate temperature and mechanical requirements, glass-to-metal remains a practical choice.
Application Guidance
- UHV systems with bakeout above 200°C: ceramic-to-metal preferred.
- Cryogenic systems with repeated thermal cycling: ceramic-to-metal preferred.
- Defense, aerospace, and space hardware: ceramic-to-metal preferred for qualification against MIL-STD-883, MIL-PRF-28861, and related military and space standards.
- Cost-sensitive industrial sensors and instrumentation at moderate vacuum: glass-to-metal acceptable.
- High-voltage feedthroughs in harsh environments: ceramic-to-metal for better dielectric performance at temperature.
Choosing the Right Seal for Your Application
The right choice depends on your operating temperature, number of thermal cycles, mechanical load, vacuum level, and budget. MPF Products manufactures both ceramic-to-metal and glass-to-metal hermetic feedthroughs and can advise on material compatibility, leak rate qualification, and the appropriate seal type for your specific pressure, temperature, and environmental requirements.