Grain Boundary Modification During Long-Term Creep in Silicon Nitride

Abstract

The microstructure changes in the commercial silicon nitride SN88M after tensile creep at the temperatures 1250°C-1400°C under the stresses of 140 - 430 MPa and lasting up to 10 000 hours were investigated. Creep induced changes in the grain boundary phases were studied by X-ray diffraction (XRD), high resolution transmission electron microscopy (HREM) a. nd scanning transmission electron microscopy with electron energy loss spectroscopy (STEM/EELS). XRD revealed the transformation of the ytterbium oxynitride crystalline phases at the multigrain junctions in the as-received materials into disilicates in the whole volume during creep tests. This change was confirmed by EELS. Epitaxially grown layers of silicon nitride were observed at the junctions of silicon nitride grains and transformed phase due to the Yb-rich precipitates segregated at the original facets. The reaction of the oxynitride with amorphous silica leading to the reduction of the amount of silica at the grain boundaries and formation of new silicon nitride epitaxially reprecipitated on the original grains is proposed as the mechanism for the observed grain growth and phase changes during creep.