DELAY OF ALPHA-TRANSFORMATION TO BETA-TRANSFORMATION OF SI3N4 CONTACTING WITH CARBON DURING HOT-PRESSING AND PECS

In the hot-pressing and pulse electric current sintering (PECS) of Si3 N4, alpha to beta transformation of Si3N4 was delayed in the surface o f the sintered body. The thickness of the surface layer depended on th e sintering conditions and was less than 0.5 mm in this experiment by X-ray analysis. Thin surface layers of 5-20 mu m including no nitrogen and 60-100 mu m layers in which the oxygen content changes gradually were observed by EPMA. The no nitrogen layer was thought to be formed by the decomposition of Si3N4 resulted from the reaction of Si3N4 with carbon or CO gas. The mechanism of the delayed alpha to beta transfor mation was speculated as follows: A low oxygen partial pressure in the atmosphere vaporized oxygen from the grain boundary liquid phase. The decreased oxygen content in the grain boundary liquid phase made incr ease the viscosity of the phase. The increased viscosity of the liquid phase might decreased the dissolution/precipitation rate, that is alp ha to beta transformation rate, of Si3N4 in the surface region.