Redistribution of a Grain-Boundary Glass Phase during Creep of Silicon Nitride Ceramics

The compressive creep behavior of a high-purity silicon nitride ceramic with and without the addition of Ba was studied at 1400°C. Two distinct creep stages were observed during high-temperature deformation of both materials. Transmission electron microscopy (TEM) has been used to characterize the intergranular glass film thickness. Statistical analysis of a number of grain-boundary films indicates that the film thickness is confined to a narrow range in the as-sintered materials. However, the mean thickness is greater in the Ba-doped ceramic than in the undoped material. The standard deviation of the film thickness of a given material is considerably larger after creep than before. We conclude that the grain-boundary glass phase is redistributed during creep, suggesting that viscous flow of the glass phase is responsible for the first stage of the creep process.