Effects of adhesive composition on bond strength of joined silicon nitride ceramics

Pressureless sintered silicon nitride ceramics were joined at 1600°C for 30 min under an external pressure of 5 MPa by using mixed powders slurries composed of Y2O3, Al2O3, SiO2 and Si3N4. The effects of Si3N4/(Y2O3+Al2O3+SiO2) ratio of the adhesive on the bond strength of the joints were investigated. The results showed that the bond strength improved with increasing Si3N4/(Y2O3+Al2O3+SiO2) ratio of the adhesive, because the thermal expansion coefficient of the adhesive was reduced and the nitridation reactions in the adhesive were accelerated by the addition of silicon nitride powders. However, when the silicon nitride content further increased, the bond strength decreased due to the increasing viscosity of the adhesive which had negative effects on the wetting and spreading characteristic of the adhesive. A maximum bond strength of 550 MPa was obtained by joining with an adhesive with the ratio of 0 under the experimental conditions, the grain size of the acicular β-Si3N4 grains grown in the joint was smaller than those in the joined ceramic.