Compositions and Thicknesses of Grain Boundary Films in Ca-Doped Silicon Nitride Ceramics

Abstract

Nanobeam analytical electron microscopy and high-resolution electron microscopy have been used to characterize both the local composition as well as the thickness of the amorphous intergranular silicate-rich film in high-purity Si3N4 ceramics doped with different levels of Ca impurities. Grain boundary films could be detected in all ceramics. The films always contained Si (cation) and O and N (anions). Ca was detected at both grain boundary junctions and triple junctions for all materials doped with different levels of Ca. The thickness h of the intergranular films depends sensitively on the Ca content. In undoped materials h is 1.0±0.1 nm. With increasing Ca content the thickness h decreased for a dopant of 80 ppm Ca but increased with further additions of Ca. Variations in film thickness and composition can be semi-quantitatively understood in terms of different long-range interatomic forces acting normal to the film.