Transient liquid phase bonding of silicon nitride ceramics

Transient liquid-phase bonding describes a process using oxynitride glass compositions which melt and wet Si3N4 when heated and then, on further reaction, either disappear into the bulk or form other desired refractory phases. Such reactions occur between Si3N4 powder particles and intergranular liquids during densification and are the basis for all silicon nitride hot pressing and sintering. Since the bonding composition is very similar to that already present intergranularly in Si3N4, it is possible in principle to homogenize the joint so that the original bounary disappears. Alternatively, the sealing composition can be adjusted to crystallize specific refractory oxynitride compounds in the boundary. Reaction-bonded, hot-pressed and sintered silicon-nitride have been joined using this technique. Some butt-sealed specimens tested by four point bending show breaking strengths greater than those reported in the literature for other joining methods. Furthermore, fracture in those specimens occurred in the Si3N4 and not in the boundary. Microscopic examination of the interface region shows the boundary is diffuse, in accord with the explanation advanced above