STATISTICAL-ANALYSIS OF THE INTERGRANULAR FILM THICKNESS IN SILICON-NITRIDE CERAMICS

Silicon nitride materials typically reveal thin amorphous intergranular films along grain boundaries, with only the exception of special boundaries. It is known that such grain-boundary films strongly affect the high-temperature properties of the bulk material. High-resolution electron microscopy (HREM) was used to study these amorphous films in different Si3N4 ceramics. The observed film thicknesses at grain boundaries in these materials varied between 5 and 15 angstrom. It was shown that the grain-boundary film thickness strongly depends on film chemistry. Careful inspections of film-thickness measurements across grain boundaries in a given material suggest that the film widths vary on the order of 1 angstrom. Therefore, a quantitative evaluation should allow for the determination of the standard deviation of the film thickness. The amorphous film widths along grain boundaries in four materials were measured over the entire length (up to 1 mum) of the grain boundary between two triple points. Forty to fifty data points were evaluated for each boundary, giving a Gaussian-like distribution of the film thickness around a median value, which corresponded well with the film width measured from single HREM micrographs. The accuracy achieved by the statistical method was better than +/- 1 angstrom.