Ceramics are usually metallised to permit wetting by braze alloys, allowing a successful bond to be formed with another ceramic or metal. If the metallisation was not present (or defective) the braze would 'ball-up' and/or produce an inadequate bond.
The most common metallisation method, known as the moly-manganese process, is described below.
The surface of the ceramic is cleaned thoroughly and then coated with a primary metallising layer consisting of a refractory metal and glass. The most commonly used material is a 'paint' containing molybdenum, manganese oxide, glass frit, and solvent.
The paint is then fired, in a reducing atmosphere of hydrogen, to a temperature of approximately 1500°C. The thickness of this layer is up to 50µm. An alternative choice of refractory metal in the primary layer is tungsten, which is used when gold is to be plated onto the surface without an intermediate layer of nickel.
A secondary metallisation layer is then deposited by electrolytic or electroless plating, using metallic materials such as nickel, copper, gold, tin or lead. In general, the plating thickness of this layer is 2-4µm. Finally, another heat treatment (800°C for nickel or gold) in a reducing atmosphere diffuses this layer into the refractory metal of the primary layer.
This surface can then be used for bonding using brazing or soldering.
Further information
Sorry, but some information is only available to Industrial Members of TWI.
Joining ceramics - a guide to best practice
Brazing of ceramics
Brazing - a guide to best practice