Evaluation of brazed silicon nitride joints: Microstructure and mechanical properties

Sintered Si3N4 has been bonded to itself and to AISI 316 steel by the active-metal brazing route, A commercial Ag-35Cu-1.6Ti filler has been used with joining taking place during a 30 min hold at 850 degrees C under vacuum. Si3N4/Si3N4 joints have been produced with strength distribution (average bend strength = 773.5 MPa, Weibull modulus = 11.2) similar to that of the monolithic ceramic. Direct brazing of the Si3N4 to AISI 316 steel was unsuccessful. However, reliably strong (bend strength of 250-400 MPa) ceramic/steel joints with 20 x 20 mm(2) cross sections were fabricated by using Cu, Mo, or Nb interlayers. The most potent inter-layer used in this work was Mo, whose coefficient of thermal expansion matches best that of the ceramic.