Migration of iron, nickel, cobalt and chromium associated with silver brazing during ceramic-to-metal joining

Abstract

During the study of bond strengths of ceramic-to-metal seals under various brazing conditions, it was observed that the strength was affected by the length of time that the braze material was in the liquid state. Brazing times as short as 10 min sometimes adversely affected the strength of the seal. Photomicrographs of a typical weak bond clearly show silver braze penetration deep in the metallizing layer after 10 min at 20/sup 0/C above the melting point of the braze. A reaction layer was also observed in the photomicrographs. Considerable iron has migrated from the Kovar pin through the silver braze and formed an interface between the silver braze and the molybdenum metallizing material. Significant concentrations of cobalt and nickel were also found in the reaction zone, indicating a diffusion reaction of Kovar through the silver. Further investigations showed that while braze durations as short as 1 min at 20/sup 0/C above the liquid temperature gave much stronger bonds than 10 min did, significant migration of material was evident. In view of the significant migration of both the pin alloy and the braze material, it was concluded that brazing time and temperature should be minimized to achieve maximum bond strength.