A typical ceramic material, such as alumina, consists of aluminum-oxide powder granules embedded in a glassy-matrix binder system of calcium oxide and silicon dioxide, among others. It is to this glassy matrix that any bonding reaction actually occurs.
In order to achieve this bonding of a brazing filler metal (BFM) to an alumina ceramic, titanium (Ti) is usually added to the particular BFM being used, such as to the AWS BAg-8 alloy (the silver-copper eutectic composition, 72Ag-28Cu). Additions of up to about 5% Ti are used.
The titanium, at brazing temperature, will react quickly with the oxides of calcium and silicon to preferentially form titanium oxide and other reacted compounds in the joint area due to the presence of the Ti, resulting in a high-strength bond between the filler metal and the ceramic surface as well as with the base metal being joined. When a BFM contains Ti additions to assist in the brazing process, the BFM is commonly called an "active brazing alloy" (ABA). They are commercially available from a number of sources.
Please note that to keep the Ti addition "active" during this brazing process, a vacuum chamber is typically used for brazing so that the titanium will only react at high temperature with the oxides of the glassy matrix in the ceramic rather than reacting early on with any oxygen in the atmosphere in the brazing chamber. Therefore, this joining process is typically limited to vacuum brazing furnaces.
Be careful when considering this process because although the bonding may be very successful, the difference in the thermal-expansion characteristics between the ceramic and the metal to which it is being joined may cause premature cracking to develop in the brazed joint upon cooling or during subsequent use in service.
It is very important to try to match the expansion characteristics of the ceramics to the metals so that huge stresses in the joints are not built up. Thus, it is often a common practice to braze the ceramic first to a low-expansion metal such as Kovar, and then later use the Kovar as the base to which to join other metal components later on.