The effects of N₂ pressure (0.1-50 MPa) during sintering on the thickness of the microstructural heterogeneous layer (MHL) formed near the surface of the compact, transverse-rupture strength, etc., were investigated for Si₃N₄-(10-20) mo1%MgO-5.5 mo1%Al₂O₃ ceramics. The sintering temperature and time were 1973 K and 3.6 ks, respectively. The N₂ gas was introduced into the furnace at about 1273 K.
When the compacts were sintered under a definite N₂ pressure, for example, about 20 and 7 MPa for 10 and 15 mo1%MgO, respectively, the evolutions of N and Si were suppressed, the thickness of MHL became very small and at the same time the strength of the surface layer of the compact (which was normally less than that of the inside in the case of 0.1 MPa) became nearly the same value as that of the inside. At the higher pressure, the strength of both surface layer and the inside decreased considerably. Some discussion was made on these results.