Machine Design

Metals and Ceramics

Many applications require joining metals to ceramics, such as sealing electrodes to glass enclosures for light bulbs. Such joints need to provide continuous chemical contact across the interface through chemical bonding to the mating materials. Several techniques are available to provide metal-to-ceramic bonds.

Solder glass: Seals between metals and glass can be made by collapsing a low-temperature glass around the glass/metal interface. As the intermediary glass is acting as a solder it is referred to as "solder glass."

Metallic bonding systems: There are four techniques for metallizing ceramics that rely on strong chemical bonding to create an adherent joint:

Refractory metallizations: These entail sintering molybdenum or tungsten powders to glassy phase-containing oxides. Often, bond-enhancing additives are used. The refractory metal powders simultaneously sinter both together and to the ceramic. The glassy phase in the ceramic will migrate into the metallic layer, filling in the interstices within the metal powder layer, thus chemically and mechanically interlocking with the metallic layer. Nickel-plating this layer achieves a solderable or brazeable surface. This technique has been used to produce seals and circuitry for alumina, beryllia, and mullite.

Active metal brazing: Adding titanium, zirconium, or hafnium to standard silver-based braze alloys forms an activated alloy. By firing such an alloy onto ceramic surfaces at 800 to 1,000°C in vacuum or argon, the braze will wet directly onto the ceramic through migration of the active metal toward the ceramic/metal interface accompanied by strong bonding potential of the active metal. This technique has been used for electronic sealing as well as for joining structural ceramics such as silicon nitride turbocharger blades to corresponding metal parts.

Direct chemical bonding: Tin directly bonds to various ceramics and metals if mixed with particular transition metals and fired to 580 to 1,000°C in carbon monoxide. Applications include general ceramic-to-metal soldering as well as very thermodynamically stable braze joints on standard alumina compositions, zirconia, and graphite. Theory suggests that a tin transition-metal phase forms within the tin matrix and then undergoes oxy-carbide formation on the surface. This oxy-carbide layer would then diffuse into the substrate to form a transition zone.

Direct bond copper: Copper's melting point is slightly lowered through eutectic phase formation with a small amount of oxygen. The surface of a copper shim is oxidized. It is fired in an inert or slightly oxidizing atmosphere between the eutectic temperature and the melting point of pure copper. This copper oxide then will melt and diffuse into most oxide substrates. On alumina, an intermediary reaction zone of copper aluminate forms.

Other metallization techniques include metal-filled frits (glassy phases) which, when fired onto the ceramic, establish bonding through glassy phase interaction. The metallic phase migrates toward the top of the coating and can be joined metallurgically. In treating the superconducting oxide YBa2Cu3O7-x, silver can be fired on the surface in a pure oxygen environment. Silver oxide forms which diffuses into the substrate and the rest reduces to pure silver upon cooling.

TAGS: Fasteners
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