When joining ceramic to metals a lot factors need to be considered… First is always the expected service environment, temperature and thermal cycling. Second is the ceramic and the metal materials being joined. And third, the size and geometry of the joint.
Ceramic:metal bonding does require careful analysis of the chemistry of the materials and the service conditions in order that the proper joining filler metal be selected. Two classes of metals are used in bonding 1) Solders (melting below 450˚C) and 2) Brazes (melting above 450˚C and the selection of one on the other as the bonding filler metal depends on service temperature and stress. For low temperature, hermetic seals, many times solders are selected. For higher service temperatures, typically over 300˚C, braze filler metals would be used. Solders in many cases are tin (Sn) or Indium (In) based since these metals have low vapor pressures and ductile and can be wet and join both metals and “prepared” ceramics. As described in the Bonding Processes part of this forum, ceramics need to have a pre-metallization on their surfaces to enable metal fillers to wet and adhere, this applies to solders and to brazes. However, recently S-Bond Technologies has developed S-Bond solders that with the additions of reactive and rare earth metals, permit direct solder wetting on ceramics.
Two classes of filler metals (solders and brazes) are non-reactive and active. On-reactive filler do not add any reactive elements while active filler metals normally have the common additions of Ti, Hf, and/or Zr which directly react with the ceramic surfaces via a local reduction of the ceramic and the formation of ceramic phases that incorporate the active elements. The majority of commercial ceramic:metal bonding work is accomplished with non-reactive braze fillers, many Copper-Silver (Cu-Ag) based. In this application of ceramic:metal bonding, the ceramics are plated, either the Mo-Mn process, discussed in the Bonding Processes of this forum, or vapor deposition of a Ti then Ni layer. Cu-Ag filler wet, adhere and react with nickel (Ni) layers readily and also wet and adhere to the metallic side of the joint. Cu-Ag braze filler metals generally braze from 800 – 900˚C and successfully bond ceramic to metal provided the joint size and design can tolerate the CTE mismatch on cooling from the brazing temperature. When ceramic:metal bonding processes can fracture the ceramic or bond, then solder filler metals may be considered if service temperatures permit. If not, then low CTE metals need to be substituted on the side opposite the ceramic:metal joint. Low CTE metals include Kovar®, Invar®, titanium or refractory metals. Note that solder filler metals may be used if service temperatures permit, to significantly reduce ceramic:metal bonding CTE related failures.
Active braze and solder filler metals are the other ceramic:metal bonding materials. Cu-Ag-Ti active braze fillers are the main commercial filler metal used and can, at 830 – 900˚C, in high vacuum bond directly to many ceramics to metals, again provided the CTE mismatch can be tolerated. The advantage of active brazing is it is direct and no premetallization is needed. The disadvantage is vacuum brazing is required and the ceramic:metal joint is less strain tolerant since the braze joint is made as a thin reaction zone as compared to a much thicker composite joint when using the Mo-Mn metallization and non-reactive brazing to Ni-layer.
This is just a short discussion on the topic of “choice” of filler metals for ceramic:metal bonding. Please feel free to contribute your knowledge, experiences and/or solutions to this forum.
If you have questions regarding ceramic:metal bonding, Contact Us.