4. Ceramic-to-metal seals

4.1 How metal or alloys bond to ceramics

• Hard metallization

  A metal deposit is first firmly attached to the ceramic surfaces to be sealed, and then brazed on in the same way as a metal part.

• Active metal sealing

  In this case, the bond is obtained by using a number of special metal alloys that are chemically highly reactive in the liquid state, and capable of reducing the surface of the oxides making up the ceramic with which they are in contact, thus forming an interfacial compound firmly bonded to the ceramic.

• Enamel sealing

  We have just seen (§