The wetting, reaction and bonding of silicon nitride by Cu-Ti alloys

The wettability and reactivity of pressureless sintered Si3N4 by powdered Cu-Ti alloy were investigated using sessile drop tests conducted in a vacuum. Bonding of Si3N4 to itself was also carried out and joint strength was evaluated by compressive shear testing. The correlation of wetting behaviour with reaction and bond strength was interpreted. The wettability of Cu-Ti alloys on Si3N4 was improved greatly by addition of titanium up to 50 wt%. However, the reaction-layer thickness was increased up to 10 wt% and thereafter decreased up to 50 wt%. We propose the dovetail model which describes the reaction-layer growth behaviour with titanium. As the titanium content was increased, it tended to form a continuous thin reaction layer which greatly improved the wettability. From metallographic and XRD analyses, TiN and Ti suicide were found in the reaction layer. The thermodynamic reaction for TiN formation was suggested to be Si3N4(s) + 4Ti (1 − sol) = 4TiN(s) + 3Si(s). Ti-silicide might be formed during cooling by the reaction with Ti and Si which had been decomposed from Si3N4, diffused to and dissolved in the liquid Cu-rich alloy. The reaction layer growth was controlled by diffusion of nitrogen or titanium in the reaction layer according to the titanium concentration. The shear strength of Si3N4 to Si3N4 was affected by the morphology and thickness of the reaction layer rather than the wettability. As the titanium content increased, shear strength also increased rapidly up to 5 wt% and then slowly up to 50 wt%. As the reaction temperature and time were increased, shear strength was lowered due to the greater thickness of the reaction layer despite improved wettability.