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

Silicon nitride materials typically reveal thin amorphous intergranula r films along grain boundaries, with only the exception of special bou ndaries. It is known that such grain-boundary films strongly affect th e high-temperature properties of the bulk material. High-resolution el ectron microscopy (HREM) was used to study these amorphous films in di fferent Si3N4 ceramics. The observed film thicknesses at grain boundar ies in these materials varied between 5 and 15 angstrom. It was shown that the grain-boundary film thickness strongly depends on film chemis try. Careful inspections of film-thickness measurements across grain b oundaries in a given material suggest that the film widths vary on the order of 1 angstrom. Therefore, a quantitative evaluation should allo w for the determination of the standard deviation of the film thicknes s. The amorphous film widths along grain boundaries in four materials were measured over the entire length (up to 1 mum) of the grain bounda ry between two triple points. Forty to fifty data points were evaluate d for each boundary, giving a Gaussian-like distribution of the film t hickness 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.