Identification of intergranular phases in ceramic nanocomposites.
Analytical FEG-TEM was used for nanostructural and nanochemical characterization of Al2O3-TiN (composite I) and Si3N4-TiN (composite II) ceramic composite systems. The presence of vitreous intergranular phases in pockets at multiple grain junctions and in thin films ( approximately 0.8 nm thick) at grain boundaries was revealed by high resolution and Fresnel fringe imaging techniques. The existence of a Ti-rich thin intergranular film at alumina grain boundaries was revealed by EDS line-scanning across internal interfaces at the 1.5 nm lateral resolution level. Extracting interface specific information at subnanometre levels by means of quantitative spatial difference EELS allowed an identification of intergranular phases. Ti sub-oxide existed in thin films at Al2O3 and TiN grain boundaries, whereas a mixed Al-Ti-O-N glassy phase was observed in pockets at triple grain junctions in composite I. In composite II, residual siliceous oxide and oxynitride glass phases were identified in thin films at Si3N4 grain boundaries and multiple grain junctions, respectively. These observations indicated that the chemistry of the intergranular phase in thin grain boundary films is notably different from that in larger pockets at multiple grain junctions.