Brazing of Silicon Nitride with Reactive Filler Metals

Silicon nitride braze joints were produced in a single step by reactive brazing at elevated temperatures without premetallizing. A filler metal of composition Co-10Ti was employed, with joining taking place during a 15-minute hold at 1300C under a high vacuum. The reaction layer that developed was not uniform. In those locations where the reaction layer was thick, the silicon nitride remained stable. But regions that showed a thin reaction layer showed some degradation. Regardless of the reaction layer thickness, a gap was observed in places between the braze metal and the ceramic substrate. This was caused by the premature isothermal solidification of the braze metal as it got depleted of titanium. Titanium acts as a melting temperature depressant of the cobalt-rich alloy. Ni-13-Cr-7Hf was another high temperature filler metal developed that showed good wetting. During brazing with this filler metal at 1250-1275C, hafnium did not difluse readily towards the silicon nitride and thus prevented the substrate from decomposing. Ultimately these joints were fabricated after premetallizing the silicon nitride.