Brazing of silicon nitride with reactive filler metals

Silicon nitride braze joints were produced in a single step by reactive bra zing at elevated temperatures without premetallizing. A filler metal of com position Co-10Ti was employed, with joining taking place during a 15-minute hold at 1300 degrees C under a high vacuum. The reaction layer that develo ped was not uniform. Zn those locations where the reaction layer was thick, the silicon nitride remained stable. But regions that showed a thin reacti on layer showed some degradation. Regardless of the reaction layer thicknes s, a gap was observed in places between the braze metal and the ceramic sub strate. This was caused by the premature isothermal solidification of the b raze metal as it got depleted of titanium. Titanium acts as a melting tempe rature depressant of the cobalt-rich alloy. Ni-13-Cr-7Hf was another high t emperature filler metal developed that showed good wetting. During brazing with this filler metal at 1250-1275 degrees C, hafnium did not diffuse read ily towards the silicon nitride and thus prevented the substrate from decom posing. Ultimately these joints were fabricated after premetallizing the si licon nitride.