SILICON-NITRIDE - FROM SINTERING TO JOINING

In earlier work, silicon nitride (Si3N4) powders were sintered with a mixture of rare-earth oxide (RE2O3) and silica (SiO2). When the chemis try and sintering schedule is carefully controlled, the intergranular phase is a crystalline RE2Si2O7 phase, which imparts improved mechanic al and oxidation properties to the Si3N4. Based on the premise that a material that is successful in joining individual grains will also be suitable for joining bulk pieces, the process has been extended to joi n bulk Si3N4. This method of joining causes the elimination of the joi nt, as the mechanism of joining is the same as that by which Si3N4 pow der sinters. By analogy with the results for sintered compacts, it is predicted that the rare-earth oxide joined parts may also exhibit impr oved high temperature performance. (C) 1998 Acta Metallurgica Inc.