High Temperature Creep Deformation of Silicon Nitride Ceramics

Abstract

The compressive creep behaviour 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 these materials. The stress exponents for creep of the two materials indicate that they have different creep mechanisms during the second stage of creep. Cavitation during creep was determined by measuring the density change before and after creep using a water­-displacement method. The Ba doped material exhibited an obvious density decrease, indicating cavitation during creep, whereas the undoped material exhibited no cavitation. This is consistent with TEM observations. The microstructure of the materials, especially the amorphous grain-boundary phase was investigated for both as-sintered and crept specimans by means of transmission electron microscopy (TEM). Statistical analysis of a number of grain-­boundary films indicates that the film thickness is confined to a narrow range (standard deviation less than 0.15 nm) in the as-sintered materials. The average film thickness depends on film chemistry, increasing from 1.0 nm to 1.4 nm when Ba is added. The standard deviation of the film thickness of a given material after creep, however, is considerably larger than before (0.30 nm ~ 0.59 nm). This suggests that the grain-boundary glass phase is redistributed during creep. Viscous flow of the glass phase is proposed as die mechanism responsible for the first stage of creep. The data are compared with a model for viscous creep, yielding good correlation.