Electron microscopy of oxidized silicon nitride

Abstract

The effect of oxidation at 1000 and 1400° C on the internal structure of reaction sintered silicon nitride has been examined by high voltage electron microscopy. The formation of a sheath region of amorphous silica around internal pores has been observed after oxidation at both temperatures. The frequency of occurrence of these regions is higher after oxidation at 1000° C, which is consistent with weight gain experiments. The effects of oxidation on strength are discussed. The main effect of the amorphous silica regions is probably in founding off internal pores, effectively increasing the surface energy and so increasing strength. Another factor is the formation at 1000 and 1400° C of an oxidized surface layer (containing crystalline silica) [1, 2] leading to an increase in room temperature strength after oxidation at 1000° C. Removal of this layer produces a further increase in strength (∼15%) showing that the oxidation has a greater beneficial effect on the internal structure.