Electron microscopy study on the high-temperature oxidation of Si 3N 4–TiN ceramics: in situ and ex situ investigations

Abstract

The high-temperature oxidation of Si 3N 4–TiN particulate composites with different amounts of the glass forming sinter additives Al 2O 3 and Y 2O 3 has been studied in order to reveal the oxidation mechanism with its different reaction steps and kinetics and especially identify the role of the glass phase in the course of oxidation. The initial stages of oxidation have been observed in situ in an environmental scanning electron microscope while exposing the materials to dry or humid oxidation environment at temperatures between 600 and 1100 °C. For the characterization of the later oxidation stages, materials were oxidized ex situ for longer times. The oxidation scales were characterized by X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy. Oxidation of the composites starts at 650 °C, when TiN surface particles begin to oxidize and form on their exposed surface islands of nanocrystalline TiO 2. At around 950 °C, the glass transition temperature of the intergranular glass phase, these nanocrystals start to grow laterally on the surface. At the same time, oxidation progresses into the depth of the material, forming thereby several distinguished oxidation subscales. The intergranular glass plays a crucial role for the oxidation in the temperature range between 950 and 1100 °C. Depending on the glass quantity in presence, different reaction mechanisms dominate; the oxidation kinetics are strongly controlled by the transport within the intergranular glass.