Electron spectroscopic imaging and fine probe EDX analysis of liquid phase sintered ceramics

Abstract

This paper is focussed on high resolution imaging and microanalysis of two classes of liquid phase sintered ceramic materials, α and duplex α/β sialons and α-SiC ceramics. The microstructures were characterized in a FEGTEM equipped with surrounding interactive instrumentation for EDX and EELS analysis and electron energy filtering, and special attention was paid to the intergranular microstructure and the variation in local chemical composition. The α and duplex α/β sialon ceramics had been fabricated from balanced starting powder mixtures corresponding to the α-sialon composition R 0·4Si 10·2Al 1·8O 0·6N 15·4 (R=Sm, Dy or Yb) and the β-sialon composition Si 5·4Al 0·6O 0·6N 7·4. The α-SiC ceramics had been pressureless sintered or hot isostatically pressed with smaller additions of Al 2O 3 and Y 2O 3. Combined high resolution analytical and spatial information was obtained from electron spectroscopic images recorded around the C K, N K, O K, Al L 2,3 and rare earth element N 4,5 edges in the electron energy loss spectrum. Residual glassy grain boundary films rich in O and cations originating from the metal oxide/nitride additives were present at all characterized grain boundaries. High resolution imaging and elemental maps computed from the electron energy filtered images showed intergranular film thicknesses in the range 1·5 to 2·3 nm. The results imply that the intergranular film thickness in the sialon microstructures is dependent upon the particular grain boundary and the local chemistry as well as the α′ stabilizing cation. Elemental concentration profiles obtained by stepwise fine probe EDX analysis across α′/α′, α′/β′ and β′/β′ sialon grain boundaries revealed significant variations in the local α′ and β′ substitution levels, both within and between analysed grains. Concentration gradients within the sialon and α-SiC grains, associated with the different grain boundaries, were not detected.