Determination of Grain-Boundary Film Thickness by the Fresnel Fringe Imaging Technique

The defocus Fresnel fringe imaging (FFI) technique is applied to the determination of nanoscale amorphous grain-boundary film thickness in a silicon nitride ceramic. By using only the transmitted beam and imaging out of focus, an amorphous grain-boundary film can be detected. The thickness of the amorphous film is determined by extrapolating the fringe spacing data obtained from a series of defocus images to zero defocus. High-resolution electron microscopy (HREM) has been suggested as the method capable of providing the most accurate measurements of film thickness (±0·1nm). Thus, the thickness of a grain-boundary film obtained by FFI is compared with that measured by HREM. It is found that FFI can provide a reliable value for grain-boundary film thickness, with an accuracy of ±0·15nm. Additionally, FFI is easier to use, and is applicable to any grain boundary of interest whereas the HREM method has some limitations. This make it more amenable to gathering statistical information on grain boundary thickness. It is suggested that FFI is a valid and useful method for determining the thin intergranular film thickness.