Microstructure and tensile creep mechanisms of an in situ reinforced silicon nitride

The creep behaviour of an acicular-grained Si3N4 in the temperature range of 1260-1400 degrees C under 50-350 MPa tensile stress was investigated. The viscous flow of an amorphous grain boundary phase and uninhibited grain boundary sliding resulted in a fast initial creep regime characterized by a stress exponent = 1.8 and an activation energy = 509 kJ mol(-1), which increased to 3.2 and 1054 kJ mol(-1), respectively, in the secondary creep regime. Between these two regimes the creep rate dropped rapidly due to grain interlocking and loading of the acicular grains parallel to the tensile axis. An enhanced dislocation activity in the parallel grains and cavitation in the grain pockets were observed, but their roles in the creep deformation appear minor. It is proposed that the mechanism controlling the secondary creep regime is the nucleation and growth of surface steps on the atomically smooth (100) grain boundary plane through the solution/precipitation process. Copyright (C) 1996 Acta Metallurgica Inc.