Microstructural evolution during the brazing of Al2O3 ceramic to kovar alloy by sputtering Ti/Mo films on the ceramic surface

Alumina (Al₂O₃) ceramics were metallized by magnetron sputtering Ti/Mo bilayer films on the surface with a subsequent high temperature sintering and were brazed to Kovar alloy (Fe-Ni-Co) using Ag-Cu eutectic alloy. The Ti/Mo metallization film and the brazing seam microstructures were investigated and the correlation between the Al₂O₃/Kovar joining strength and the microstructures of the brazing seam was discussed. The results show that the joining strength is related to the thickness of the Ti/Mo adhension layer which depends on the holding time during brazing. The mutual diffusion of the elements at the interface firstly increases the thickness of the adhension layer as the holding time increases and the Mo film acts as a barrier layer to block the diffusion of Ti atoms into the seam. The optimal brazing joining strength of 72.6±5.0 MPa could be achieved at a brazing temperature of 810 °C for 14 min However, if the holding time is further prolonged, Mo atoms will diffuse into the (Ni, Cu) solid solution, resulting in the diffusion of Ti atoms and the adhension layer becoming indistinguishable. Therefore, the intermetallic Ni₃Ti forms in the seam and the titanium oxide changes from TiO to Ti₂O₃ or Ti₃O₅, which leads to the joint strength decreasing.