Interfacial forces in Si3N4- and SiC-based systems and their influence on the joining process

High-temperature interactions between solid ceramic grains and liquid glass result in final microstructures in which the grain-to-grain distance is go verned by the specific equilibrium among the interfacial forces. Useful inf ormation for many practical applications is obtained by treating the system of interest as a high-temperature colloidal suspension and studying intera ctions among the constituents. Si3N4 and SiC silicate systems are investiga ted in the present study as dispersed suspensions, after liquid-penetration experiments and long oxidation treatment at high temperature. Dispersed sy stems by Si3N4 or SiC as solid particles and a silicate glass formed as a l iquid phase are studied at high temperature. Particle interactions are desc ribed in terms of the surface tensions of the solid-liquid interface (gamma (sl)) of the grain boundary, both with an intergranular phase (gamma(gb)*) and without (Y(gb)degrees). Agglomerations of a few particles form In both systems as a result of attractive forces; the needlelike shape of beta-Si3N 4 particles partially inhibits their mutual interaction. The specific equil ibrium among the interfacial forces also drives microstructural evolution d uring penetration experiments of liquid silicates in dense Si3N4 and SiC at high temperature. In fact, at the original ceramic-glass interface, beta-S i3N4 grains move easily, and grain boundaries thicken; in contrast, a strai ght line between the ceramic and the glass characterizes the SiC-silicate i nterface, When the same dense materials are heat-treated in air, a glass la yer forms on top of them, as a result of Si3N4 and SiC oxidation, The inter face between the so-formed glass and the original ceramic is similar to tho se found after the penetration experiments. Finally, knowledge of the speci fic high-temperature behavior of these systems is used to produce Si3N4-Si3 N4 and SiC-SiC joints both with and without a glass interlayer, Direct join ts with an average strength of 716 MPa (76% of the strength of the as-sinte red material) are obtained.