Creep mechanisms in silicon nitride at elevated temperatures

Various mechanisms have been suggested to explain the creep of glass-containing ceramics such as sintered silicon nitride. These include dissolution-reprecipitation creep, dissolution-enhanced plasticity, and viscous flow. In this paper, the models for these processes are reviewed, with particular attention to some recent models for creep controlled by viscous flow of the intergranular glass phase. The models are then compared to data on the creep of silicon nitride. We find that viscous flow controls the creep process at 1200 to 1250°C. Since this is a transient process, the creep rate decreases dramatically above a critical strain. At 1325°C however, viscous flow at small strains is followed by a transition to a second mechanism, most probably dissolution-reprecipitation creep.