Microstructure Characterization in Superplastically Deformed Silicon Nitride

Abstract

Fully dense silicon nitride (Si 3 N 4 ) has been produced by hot isostatically pressing α‐Si 3 N 4 powder at 1740°C under 160 MPa, with 0.5 wt% Y 2 O 3 and 0.5 wt% Al 2 O 3 as sintering additives. The sintered material was composed of very fine (0.5 μm) and equiaxed grains, as required for superplasticity. Before deformation, a very small amount of intergranular glassy silicate‐based film was detected by transmission electron microscopy at the two‐grain and triple‐point junctions. Compression tests with a strain value of up to }0.5 were conducted in nitrogen in the temperature range of 1600°–1700°C. The observation of a shear‐thickening phenomenon and the presence of a transition from a mild to a strong strain hardening at 20 MPa were attributed to the occurrence of rigid contacts between the grains. The angular distribution of the observed strain whorls was used to evidence the increase of rigid contacts between the grains, under the local expulsion of the wetting liquid film, with increases in compressive stress.