Ceramic-to-metal brazed assemblies

Microcertec S.A.S has extended its range of technical capabilities and products through a partnership with Bacmi a French company specialized in ceramic-to-metal and metal-to-metal technologies.

Using Microcertec machined ceramics, we can offer custom-made ceramic-to-metal assemblies from prototype to medium-size production quantities.

Not all technical ceramics can be brazed. Those that are suited for brazing are mainly :

• aluminium oxide (alumina)
• sapphire and ruby
• and to a lesser extent, aluminium nitride, zirconia and silicon nitride.

The metal that is directly joined to the ceramic must have a close coefficient of thermal expansion so that the risk of residual stress in the seal that would lead to rupture is minimized. Nickel-iron alloys - Kovar, FN48, FN52… - and titanium are best suited.

But it is also possible to design ceramic-to-metal brazed assemblies using stainless steel, molybdenum, tungsten, niobium, platinum…

The braze alloy to be used will depend upon various parameters : final temperature of use of the product, nature of the metal to be joined, several metals of different kinds to be brazed. The following alloys Ag-Cu, Ag-Cu-Pd, Ag-Cu-Au, Au-Cu, Au-Ni, Ag, Au, Cu are typical and cover a range of temperatures from 780°c to 1100°c.

An hermetic ceramic to metal assembly can be done in various ways, but all of them require that a metal interlayer be deposited so that the adhesion of the subsequent brazing will be possible :

• sintered thick layer of molybdenum-manganese, only with aluminium oxide ceramics
• thick layer of a reactive metal that chemically reacts
• thin layer of W-Ti for instance deposited by PVD process.