Effect of heating rates in brazing on fracture strength of Si3N4/Ni joints

Silicon nitride ceramics were brazed to nickel sheet by active brazing with nickel-copper-titanium solder under various heating rates. Properties of the reacted products were analyzed by using a scaning electron microscope, an electron probe microanalyzer, an X-ray diffraction meter, and a transmission electron microscope. Joining strength was measured by the 4-point bending test. Joining strength was improved with increasing heating rate. While fracture occured at the whole interface between ceramic and metal, fracture partially propagated into the ceramic in the case of the highest heating rate. The fracture occurred in the reacted products formed at the interface between silicon nitride and the nickel-copper-titanium solder. As a result of electron probe microanalysis of the reacted products on the metal-side fracture surface, silicon was detected with aluminium and oxygen, which serve as the sintering aid of silicon nitride. The silicon intensity of the reacted products increased with increasing joining strength. However, as a result of X-ray diffraction, titanium nitride in the form of a crystalline phase was detected, but no silicon compounds were detected. These results suggest that silicon in the reacted products exists as the amorphous phase with aluminium-oxide in the grain boundary of silicon nitride. It is considered that the titanium nitride and the silicon-aluminium oxide amorphous phase are both formed as the reacted products between silicon nitride and nikel-copper-titanium solder, and the silicon-aluminium oxide amorphous phase substantially controls joining strength.