The role of intergranular phases in silicon nitride and silicon carbide direct joining

The direct joining of non-oxide ceramics is strongly influenced by the nature of the intergranular phase and consequently by the specific sintering additive system, as this affects the type, amount and viscosity of the intergranular phase. Adhesion between two ceramic pieces depends on the tendency of the intergranular phase to move toward the interface and to fill in the surface-roughness, i.e., it depends on the mobility of the atoms within the grain boundaries-and triple junctions. Low mobility atoms and intergranular phase with high viscosity characterized by poor wetting of the ceramic grains will cause lower adhesion at the interface. Microstructural evolution and final properties of the joints were studied for two different systems: hot pressed Si3N4 with Al2O3-Y2O3 Or Y2O3-La2O3 as sintering additives and SiC with Al2O3-Y2O3; pressures up to 40 MPa and temperatures up to 1750 degrees C were used for the joining processes. The microstructure that forms at the interface is related to the mechanisms governing the mass transport of the involved phases and is affected by capillary forces, applied pressure, refractoriness of the intergranular phase and by the chemical composition of the principal phase. Therefore these features must be carefully evaluated and the joining conditions selected in order to produce successful joints with flexural strength values approaching those of the as-sintered material.