Brazing metals to ceramics is a key technology in the fabrication of biomedical devices such as the feedthroughs and casings manufactured for cardiac, cochlear and neurostimulation implants. Brazing consists of assembling a ceramic with a metal part by means of a filler metal. For the medical applications developed at SCT, the metal part is made of Titanium, the filler metal is made of Gold and the ceramic is Alumina or Zirconia. In most cases, a preliminary metallization of the ceramic is required to increase the wettability of the filler metal. This metallization layer is deposited by PVD on the ceramic to allow the gold to properly wet the ceramic.  For the sake of miniaturization and long-term reliability, one of the key points is to develop the brazing process on ceramics with a mechanical strength higher than Alumina (to make thinner walled parts) and better hydrothermal resistance than Zirconia. Alumina-Zirconia composites combine the advantages of both ceramics to produce materials that are more resistant and less sensitive to hydrothermal aging compared to respectively Alumina and Zirconia. In addition to the mechanical properties, the brazing ability of Alumina-Zirconia composites must be investigated. This study aims to understand the diffusion and oxydo-reduction phenomena occurring during brazing, as it is key to produce hermetic joints of high quality.
In all cases, Au-Ti intermetallic compounds are presents near the titanium. However, differences are observed in the reaction layer close to the ceramic. For Zirconia and Alumina-Toughened Zirconia (ATZ), the bond layer (a titanium oxide) is only few microns thick. This layer is porous and cracked. For Alumina and Zirconia-Toughened Alumina (ZTA), the bond layer is not well-defined and oxide particles of a few hundred nanometers can be seen in the gold brazing near the ceramic. One of the hypotheses currently explored is that the oxygen in Zirconia might be more susceptible to diffuse and to form an oxide with Titanium, than the oxygen in Alumina.