Chemistry of grain boundaries in calcia doped silicon nitride studied by spatially resolved electron energy-loss spectroscopy

Spatially resolved electron energy-loss spectroscopy has been used to interrogate the chemistry and bonding at grain boundaries in hot isostatically pressed (HIPed) high purity silicon nitride ceramics nominally doped with either 0, 100, 300 or 500 at. ppm calcium. Analysis of the nanometer thick glassy films between grains, shows that there is a monotonic increase in impurity segregation with dopant concentration. The predominant constituents of the grain boundary films are Si, O and N, forming a silicon oxynitride phase ranging in composition from SiO2 to Si2N2O. The energy-loss near-edge structure indicates the structure of the glass is composed predominantly of tetrahedra but also suggests the presence of a small concentration of molecular N2, possibly as sub-microscopic bubbles. There is still no data on the composition gradients on a sub-nanometer level, though it is shown that in the detection limit set by counting statistics, near single atom sensitivity with a spatial resolution of 0.22 nm is achievable.