The contribution of viscous flow to creep deformation of silicon nitride

For silicon nitride ceramics, in which a less refractory grain boundary amorphous film bonds more deformation-resistant Si3N4 grains, one of the possible creep mechanisms is the redistribution of this amorphous film. Although this mechanism has been confirmed by measuring the film thickness distribution before and after creep, it is not clear whether viscous flow occurs only in the initial stage of creep or throughout the creep process. In this paper, compression creep tests were conducted and the film thickness distributions at different strain values were examined using high-resolution electron microscopy. A bimodal distribution of the film widths was observed after creep for all the strains examined, with the peak positions essentially unchanged after creep ranging from 7to 200 h at 1400°C. This demonstrates that viscous flow contributes principally to the initial stage of creep. In general, creep continues at a much lower rate after this cavity-free viscous flow process, entering the second stage. However, once cavities have nucleated, a local film thickness change may occur in the neighbouring areas by viscous flow. Therefore, viscous flow due to cavitation and its contribution to creep deformation are also discussed.