Abstract
The present work evaluates Si3N4 powder surfaces by transmission electron microscopy (TEM). Si3N4 particles are covered by oxidized layer. The surface impurity oxide phases of powders play important roles for good performance of structural Si3N4 ceramics. The Si3N4 powder surface was modified by heat treatment in air and washing treatment in Soxhlet extractor. The surface oxygen content increased by the heat treatment, or decreased by the aqueous washing process. The oxygen content was measured by a combustion type oxygen analyzer. The powder surface conditions were analyzed by high resolution TEM (HREM) images. X-ray diffraction analysis was carried out to identify the structure of the surface modified Si3N4 powders. HREM and X-ray diffraction analysis showed that the surface oxidized impurity layer on Si3N4 particle was thin amorphus layer. The surface oxidized impurity layer increased with increasing heat treatment time in air, and decreased by washing treatment. The thickness of the surface oxidized layer of the asreceived powder (1.42 mass% O) was about 1.0nm, and that of the surface oxidized powders was 1.5nm for 4h heat treatment (1.91mass% O), and 2.0nm for 8h heat treatment (2.46mass% O). On the other hand, the thickness was about 0.5nm for the washed powder (0.57mass% O). Assuming that the silicon oxide layer is uniform in thickness on the particle surfaces, and using the specific surface area of the raw powder and the density of silica glass, the surface oxidized layer can be calculated. The estimated values agreed well with the HREM observation. The surface oxidation occurs uniformly on the particle surfaces.
