Silicon Nitride Joining

Hot‐pressed Si3N4 was joined using an Mgo‐A12O3‐SiO2 glass composition chosen to approximate the oxide portion of the grain‐boundary phase in the ceramic. After it has been heated at 1550° to 1650°, the interface of the joined ceramic is an interlocking mixture of Si2N2O, β‐Si3N4, and a residual oxy‐nitride glass. The kinetics of reactions between Si3N4 and the molten joining composition were studied by X‐ray diffraction analysis of the phases present in Si3N4 powder‐glass mixtures quenched after varied heat treatments. Analytical transmission electron microscopy of the composition and micro‐structure of the reaction zone in joined specimens, together with the X‐ray diffraction results, suggests that the driving force for joining is the lowering of the Si3N4 interfacial energy when it is wet by the molten silicate, augmented by the negative Gibbs energy for the reaction SiO2(l) + Si3N4= 2Si2N2O.