Analytical Electron Microscopy and Creep Deformation of Sintered Silicon Nitride

Abstract

<p>Silicon nitride is sintered with the use of additives, such as Y₂O₃ and Al₂O₃, in order to enhance densification. After sintering, these additives along with SiO₂, present on the starting Si₃N₄ powder, form an intergranular amorphous phase. The presence of a glassy phase is generally thought to be detrimental to the high temperature creep properties. In the material analyzed, Kyocera SN220, this intergranular glassy phase partially devitrifies upon annealing. Thus the effect of a fully amorphous or a partially crystalline intergranular phase on the creep properties could be assessed. The creep resistance of the material is only modestly affected by partial grain, boundary devitrification in both flexural and compressive creep. However the creep life is reduced significantly.</p> <p>Extensive analytical microscopy was done on the amorphous and partially crystalline material in order to determine what changes were occurring due to anneling and due to creep deformation. A quantitative methodology was developed for electron energy loss spectroscopy to analyze the intergranular phase composition. Using this technique concentrations of light elements, such as oxygen and nitrogen, and heavier elements could be determined. Different crystalline grain boundary products were found near the surface of samples annealed in air compared to the centre of these and throughout samples annealed in an inert atmosphere. However the residual amorphous phase composition was the same regardless of annealing atmosphere or location. In addition, phases present after devitrification did not depend on the stress state. Extensive cavitation, a commonly observed effect of creep, did not occur in samples containing the maximum obtainable strain in flexure of 2.7%.</p> <p>The observed microstructural information and creep data was taken into consideration in developing a creep model. This model describes creep due to non-linear viscous flow of an amorphous intergranular phase around a hexagonal array of grains. An initial constant strain rate is predicted at low strains, followed by a decrease in strain rate as the intergranular phase is squeezed out from between grains. This decrease occurs at smaller strains in compression than in tension or flexure. The creep behaviour observed experimentally corresponds well with that predicted theoretically.</p>