Effect of Microstructural Degradation on the Strength of Sintered Silicon Nitride after High Temperature Exposure
Microstructural degradation in two kinds of sintered silicon nitride due to high temperature exposure was examined on specimens after flexure static fatigue tests and room-temperature flexure tests of heated specimens. Surface oxidation layer was formed due to the reaction of silica formed by the oxidation of β-Si3N4 and sintering additives segregating from the interior grain boundary phase. Corresponding to the oxidation layer, a degraded layer containing numerous micro-pores resulting from the segregation of the sintering additives was formed just beneath the surface oxidation layer. The thickness of the surface oxidation layer, which was measured nondestructively by means of X-ray diffraction method, had increased along with the heating. Consequently, the room temperature strength of specimens after heating was considered to be related to the thicknesses of surface oxidation layer and surface degraded layer.
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