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 join so that the original boundary 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.